Facile insertion of a cyclic alkyl(amino) carbene carbon into the B-B bond of diboron(4) reagents.

PubMed

Eichhorn, Antonius F; Kuehn, Laura; Marder, Todd B; Radius, Udo

2017-10-24

We report herein the room temperature insertion of the carbene carbon atom of the cyclic (alkyl)(amino) carbene cAAC Me into the B-B single bonds of the diboron(4) compounds B 2 pin 2 , B 2 cat 2 , B 2 neop 2 , and B 2 eg 2 (pin = pinacolato, cat = catecholato, neop = neopentylglycolato, eg = ethyleneglycolato).

An unprecedented chemospecific and stereoselective tandem nucleophilic addition/cycloaddition reaction of nucleophilic carbenes with ketenimines.

PubMed

Cheng, Ying; Ma, Yang-Guang; Wang, Xiao-Rong; Mo, Jun-Ming

2009-01-16

The first study of the reaction between nucleophilic carbenes and ketenimines is reported. The interaction of thiazole and benzothiazole carbenes with ketenimines proceeded in a chemospecific and stereoselective manner to produce thiazole- and benzothiazole-spiro-pyrrole derivatives generally in good yields. The reaction was proposed to proceed via a tandem nucleophilic addition of carbene to the C=N bond of ketenimine followed by a stepwise [3+2] cycloaddition of the 1,3-dipolar intermediate with the C=C bond of ketenimine. This reaction provides a powerful protocol for the construction of novel polyfunctional thiazole-spiro-pyrrole or benzothiazole-spiro-pyrrole compounds that are not readily accessible by other methods.

A Non-Diazo Approach to α-Oxo Gold Carbenes via Gold-Catalyzed Alkyne Oxidation

PubMed Central

2015-01-01

For the past dozen years, homogeneous gold catalysis has evolved from a little known topic in organic synthesis to a fully blown research field of significant importance to synthetic practitioners, due to its novel reactivities and reaction modes. Cationic gold(I) complexes are powerful soft Lewis acids that can activate alkynes and allenes toward efficient attack by nucleophiles, leading to the generation of alkenyl gold intermediates. Some of the most versatile aspects of gold catalysis involve the generation of gold carbene intermediates, which occurs through the approach of an electrophile to the distal end of the alkenyl gold moiety, and their diverse transformations thereafter. On the other hand, α-oxo metal carbene/carbenoids are highly versatile intermediates in organic synthesis and can undergo various synthetically challenging yet highly valuable transformations such as C–H insertion, ylide formation, and cyclopropanation reactions. Metal-catalyzed dediazotizations of diazo carbonyl compounds are the principle and most reliable strategy to access them. Unfortunately, the substrates contain a highly energetic diazo moiety and are potentially explosive. Moreover, chemists need to use energetic reagents to prepare them, putting further constrains on operational safety. In this Account, we show that the unique access to the gold carbene species in homogeneous gold catalysis offers an opportunity to generate α-oxo gold carbenes if both nucleophile and electrophile are oxygen. Hence, this approach would enable readily available and safer alkynes to replace hazardous α-diazo carbonyl compounds as precursors in the realm of gold carbene chemistry. For the past several years, we have demonstrated that alkynes can indeed effectively serve as precursors to versatile α-oxo gold carbenes. In our initial study, we showed that a tethered sulfoxide can be a suitable oxidant, which in some cases leads to the formation of α-oxo gold carbene intermediates. The intermolecular approach offers excellent synthetic flexibility because no tethering of the oxidant is required, and its reduced form is not tangled with the product. We were the first research group to develop this strategy, through the use of pyridine/quinolone N-oxides as the external oxidants. In this manner, we can effectively make a C–C triple bond a surrogate of an α-diazo carbonyl moiety in various gold catalyses. With terminal alkynes, we demonstrated that we can efficiently trap exclusively formed terminal carbene centers by internal nucleophiles en route to the formation of cyclic products, including strained oxetan-3-ones and azetidin-3-ones, and by external nucleophiles when a P,N-bidentate ligand is coordinated to gold. With internal alkynes, we generated synthetically useful regioselectivities in the generation of the α-oxo gold carbene moiety, which enables expedient formation of versatile enone products. Other research groups have also applied this strategy en route to versatile synthetic methods. The α-oxo gold carbenes appear to be more electrophilic than their Rh counterpart, which many chemists have focused on in a large array of excellent work on metal carbene chemistry. The ease of accessing the reactive gold carbenes opens up a vast area for developing new synthetic methods that would be distinctively different from the known Rh chemistry and promises to generate a new round of “gold rush”. PMID:24428596

A non-diazo approach to α-oxo gold carbenes via gold-catalyzed alkyne oxidation.

PubMed

Zhang, Liming

2014-03-18

For the past dozen years, homogeneous gold catalysis has evolved from a little known topic in organic synthesis to a fully blown research field of significant importance to synthetic practitioners, due to its novel reactivities and reaction modes. Cationic gold(I) complexes are powerful soft Lewis acids that can activate alkynes and allenes toward efficient attack by nucleophiles, leading to the generation of alkenyl gold intermediates. Some of the most versatile aspects of gold catalysis involve the generation of gold carbene intermediates, which occurs through the approach of an electrophile to the distal end of the alkenyl gold moiety, and their diverse transformations thereafter. On the other hand, α-oxo metal carbene/carbenoids are highly versatile intermediates in organic synthesis and can undergo various synthetically challenging yet highly valuable transformations such as C-H insertion, ylide formation, and cyclopropanation reactions. Metal-catalyzed dediazotizations of diazo carbonyl compounds are the principle and most reliable strategy to access them. Unfortunately, the substrates contain a highly energetic diazo moiety and are potentially explosive. Moreover, chemists need to use energetic reagents to prepare them, putting further constrains on operational safety. In this Account, we show that the unique access to the gold carbene species in homogeneous gold catalysis offers an opportunity to generate α-oxo gold carbenes if both nucleophile and electrophile are oxygen. Hence, this approach would enable readily available and safer alkynes to replace hazardous α-diazo carbonyl compounds as precursors in the realm of gold carbene chemistry. For the past several years, we have demonstrated that alkynes can indeed effectively serve as precursors to versatile α-oxo gold carbenes. In our initial study, we showed that a tethered sulfoxide can be a suitable oxidant, which in some cases leads to the formation of α-oxo gold carbene intermediates. The intermolecular approach offers excellent synthetic flexibility because no tethering of the oxidant is required, and its reduced form is not tangled with the product. We were the first research group to develop this strategy, through the use of pyridine/quinolone N-oxides as the external oxidants. In this manner, we can effectively make a C-C triple bond a surrogate of an α-diazo carbonyl moiety in various gold catalyses. With terminal alkynes, we demonstrated that we can efficiently trap exclusively formed terminal carbene centers by internal nucleophiles en route to the formation of cyclic products, including strained oxetan-3-ones and azetidin-3-ones, and by external nucleophiles when a P,N-bidentate ligand is coordinated to gold. With internal alkynes, we generated synthetically useful regioselectivities in the generation of the α-oxo gold carbene moiety, which enables expedient formation of versatile enone products. Other research groups have also applied this strategy en route to versatile synthetic methods. The α-oxo gold carbenes appear to be more electrophilic than their Rh counterpart, which many chemists have focused on in a large array of excellent work on metal carbene chemistry. The ease of accessing the reactive gold carbenes opens up a vast area for developing new synthetic methods that would be distinctively different from the known Rh chemistry and promises to generate a new round of "gold rush".

A dual Lewis base activation strategy for enantioselective carbene-catalyzed annulations.

PubMed

Izquierdo, Javier; Orue, Ane; Scheidt, Karl A

2013-07-24

A dual activation strategy integrating N-heterocyclic carbene (NHC) catalysis and a second Lewis base has been developed. NHC-bound homoenolate equivalents derived from α,β-unsaturated aldehydes combine with transient reactive o-quinone methides in an enantioselective formal [4 + 3] fashion to access 2-benzoxopinones. The overall approach provides a general blueprint for the integration of carbene catalysis with additional Lewis base activation modes.

Arylation of Rhodium(II) Azavinyl Carbenes with Boronic Acids

PubMed Central

Selander, Nicklas; Worrell, Brady T.; Chuprakov, Stepan; Velaparthi, Subash; Fokin, Valery V.

2013-01-01

A highly efficient and stereoselective arylation of in situ generated azavinyl carbenes affording 2,2-diaryl enamines at ambient temperatures has been developed. These transition metal carbenes are directly produced from readily available and stable 1-sulfonyl-1,2,3-triazoles in the presence of a rhodium carboxylate catalyst. In several cases, the enamines generated in this reaction can be cyclized into substituted indoles employing copper catalysts. PMID:22913576

A DFT and structural investigation of the conformations of Fischer carbene complexes

NASA Astrophysics Data System (ADS)

Landman, Marilé

2015-09-01

A set of different Fischer carbene complexes of group VI and VII metals, with varied heteroatom and heteroaromatic substituents on the carbene carbon atom, was studied. Density functional theory as well as single crystal diffraction techniques were employed to investigated the most stable conformation of these complexes. The complexes studied, [M(CO)4L{C(X)Z}], with L = PPh3 or CO, X = ethoxy (-OCH2CH3) or amino (-NH2 or NHCy) substituents as the heteroatom carbene substituents, Z = 2-furyl (-C4H3O), 2-thienyl (-C4H3S), 2-(N-methyl)pyrrolyl (-C4H3NCH3) as the second carbene substituent had their substituents varied systematically to give all the possible conformations of these complexes. The conformations of the complexes, in particular the relative orientations of the heteroatoms in the molecule (syn vs. anti), E/Z isomerism in the aminocarbene complexes and cis/trans isomerism in the ligand substituted complexes as well as various combinations of these aspects, were studied. In general, it was found that the most stable conformation theoretically as well as in the solid state for most of the complexes preferred the syn conformation. The Z-isomer is generally preferred over the E isomer while the cis is more predominant than the trans isomer. Using DFT and NBO calculations, explanations for the preferred conformations were explored. It was concluded that both steric and electronic factors influence the conformations of the carbene complexes, with the extent of contribution of these two factors varying for each of the different carbene substituents.

A Continuum of Progress: Applications of N-Hetereocyclic Carbene Catalysis in Total Synthesis

PubMed Central

Izquierdo, Javier; Hutson, Gerri E.; Cohen, Daniel T.; Scheidt, Karl A.

2013-01-01

N-Heterocyclic carbene (NHC) catalyzed transformations have emerged as powerful tactics for the construction of complex molecules. Since Stetter’s report in 1975 of the total synthesis of cis-jasmon and dihydrojasmon by using carbene catalysis, the use of NHCs in total synthesis has grown rapidly, particularly over the last decade. This renaissance is undoubtedly due to the recent developments in NHC-catalyzed reactions, including new benzoin, Stetter, homoenolate, and aroylation processes. These transformations employ typical as well as Umpolung types of bond disconnections and have served as the key step in several new total syntheses. This Minireview highlights these reports and captures the excitement and emerging synthetic utility of carbene catalysis in total synthesis. PMID:23074146

A LFER analysis of the singlet-triplet gap in a series of sixty-six carbenes

NASA Astrophysics Data System (ADS)

Alkorta, Ibon; Elguero, José

2018-01-01

Ab initio G4 calculations have been performed to investigate the singlet-triplet gap in a series of 66 simple carbenes. Energies and geometries were analyzed. An additive model has been explored that include four interaction terms. An abnormal behavior of the cyano group has been found. The 13C absolute shieldings of the carbenic carbon atom were calculated at the GIAO/B3LYP/6-311++G(d, p).

Metal-Diazo Radicals of α-Carbonyl Diazomethanes

PubMed Central

Li, Feifei; Xiao, Longqiang; Liu, Lijian

2016-01-01

Metal-diazo radicals of α-carbonyl diazomethanes are new members of the radical family and are precursors to metal-carbene radicals. Herein, using electron paramagnetic resonance spectroscopy with spin-trapping, we detect diazo radicals of α-carbonyl diazomethanes, induced by [RhICl(cod)]2, [CoII(por)] and PdCl2, at room temperature. The unique quintet signal of the Rh-diazo radical was observed in measurements of α-carbonyl diazomethane adducts of [RhICl(cod)]2 in the presence of 5,5-dimethyl-pyrroline-1-N-oxide (DMPO). DFT calculations indicated that 97.2% of spin density is localized on the diazo moiety. Co- and Pd-diazo radicals are EPR silent but were captured by DMPO to form spin adducts of DMPO-N∙ (triplet-of-sextets signal). The spin-trapping also provides a powerful tool for detection of metal-carbene radicals, as evidenced by the DMPO-trapped carbene radicals (DMPO-C∙, sextet signal) and 2-methyl-2-nitrosopropane-carbene adducts (MNP-C∙, doublet-of-triplets signal). The transformation of α-carbonyl diazomethanes to metal-carbene radicals was confirmed to be a two-step process via metal-diazo radicals. PMID:26960916

Metal-Diazo Radicals of α-Carbonyl Diazomethanes

NASA Astrophysics Data System (ADS)

Li, Feifei; Xiao, Longqiang; Liu, Lijian

2016-03-01

Metal-diazo radicals of α-carbonyl diazomethanes are new members of the radical family and are precursors to metal-carbene radicals. Herein, using electron paramagnetic resonance spectroscopy with spin-trapping, we detect diazo radicals of α-carbonyl diazomethanes, induced by [RhICl(cod)]2, [CoII(por)] and PdCl2, at room temperature. The unique quintet signal of the Rh-diazo radical was observed in measurements of α-carbonyl diazomethane adducts of [RhICl(cod)]2 in the presence of 5,5-dimethyl-pyrroline-1-N-oxide (DMPO). DFT calculations indicated that 97.2% of spin density is localized on the diazo moiety. Co- and Pd-diazo radicals are EPR silent but were captured by DMPO to form spin adducts of DMPO-N• (triplet-of-sextets signal). The spin-trapping also provides a powerful tool for detection of metal-carbene radicals, as evidenced by the DMPO-trapped carbene radicals (DMPO-C•, sextet signal) and 2-methyl-2-nitrosopropane-carbene adducts (MNP-C•, doublet-of-triplets signal). The transformation of α-carbonyl diazomethanes to metal-carbene radicals was confirmed to be a two-step process via metal-diazo radicals.

Metal-Diazo Radicals of α-Carbonyl Diazomethanes.

PubMed

Li, Feifei; Xiao, Longqiang; Liu, Lijian

2016-03-10

Metal-diazo radicals of α-carbonyl diazomethanes are new members of the radical family and are precursors to metal-carbene radicals. Herein, using electron paramagnetic resonance spectroscopy with spin-trapping, we detect diazo radicals of α-carbonyl diazomethanes, induced by [Rh(I)Cl(cod)]2, [Co(II)(por)] and PdCl2, at room temperature. The unique quintet signal of the Rh-diazo radical was observed in measurements of α-carbonyl diazomethane adducts of [Rh(I)Cl(cod)]2 in the presence of 5,5-dimethyl-pyrroline-1-N-oxide (DMPO). DFT calculations indicated that 97.2% of spin density is localized on the diazo moiety. Co- and Pd-diazo radicals are EPR silent but were captured by DMPO to form spin adducts of DMPO-N∙ (triplet-of-sextets signal). The spin-trapping also provides a powerful tool for detection of metal-carbene radicals, as evidenced by the DMPO-trapped carbene radicals (DMPO-C∙, sextet signal) and 2-methyl-2-nitrosopropane-carbene adducts (MNP-C∙, doublet-of-triplets signal). The transformation of α-carbonyl diazomethanes to metal-carbene radicals was confirmed to be a two-step process via metal-diazo radicals.

A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene.

PubMed

Mondal, Kartik Chandra; Roy, Sudipta; Dittrich, Birger; Andrada, Diego M; Frenking, Gernot; Roesky, Herbert W

2016-02-24

Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl4 (cAAC=cyclic alkyl(amino) carbene :C(CMe2)2 (CH2)N(2,6-iPr2C6H3) with potassium graphite produces stable (cAAC)3Si3, a carbene-stabilized triatomic silicon(0) molecule. The Si-Si bond lengths in (cAAC)3Si3 are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si-Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)3Si3 possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Quantum chemical studies on hypothetical Fischer type Mo(CO)5[C(OEt)Me] and Mo(CO)5[C(OMe)Et] carbene complexes

NASA Astrophysics Data System (ADS)

Gövdeli, Nezafet; Karakaş, Duran

2018-07-01

Quantum chemical calculations at B3LYP/LANL2DZ/6-31G(d) level were made on anti-eclipsed, anti-staggered, syn-eclipsed, syn-staggered conformers of hypothetical Fischer type Mo(CO)5[C(OEt)Me] and Mo(CO)5[C(OMe)Et] carbene complexes in the gas phase. The most stable conformer of the complexes was found to be anti-staggered according to the total energy values calculated at given level. Structural parameters, vibration spectra, charge distributions, molecular orbital energy diagrams, contour diagrams of frontier orbitals, molecular electrostatic potential maps and some electronic structure descriptors were obtained for the most stable conformers. NMR spectra of the most stable conformers were calculated at GIAO/B3LYP/LANL2DZ level. The most stable conformer geometry was found to be distorted octahedral. IR and NMR spectra of the complexes are consistent with their geometry. HOMOs of the complexes were found to be center-atomic character and LUMOs were carbene-carbon character. From the calculated charge analysis and molecular electrostatic potential maps, it is found that carbene-carbon acts as electrofil and metal center nucleophile. It is suggested that the catalytic properties of the carbene complexes may be due to the fact that the carbene-carbon behave as electrophile and metal center nucleophile. Some electronic structure descriptors of the complexes were calculated and the molecular properties were estimated.

Application of Chan-Lam cross coupling for the synthesis of N-heterocyclic carbene precursors bearing strong electron donating or withdrawing groups

NASA Astrophysics Data System (ADS)

Huang, Liliang; He, Chengxiang; Sun, Zhihua

2015-07-01

Chan-Lam cross coupling allowed efficient synthesis of N,N’-disubstituted ortho-phenylene diamines bearing strong electron donating or withdrawing groups, such as nitro or methoxy groups, with moderate to high yields. These diamines can then be turned into N-heterocyclic carbene precursors after condensation with trimethyl orthoformate. The same strategy can also be utilized for the synthesis of N-monosubstituted aniline derivatives containing a functionalized ortho-aminomethyl group as intermediates for chiral 6-membered ring carbene precursors.

Carbene Chemistry. I. Stereochemical Integrity at C Alpha in Ketone Tosylhydrazones. II. Hydrogen Migration in 2-Carbena-6,6-Dimethylnorbornane.

DTIC Science & Technology

1978-02-01

H20, 10% Na2 CO3 , H20, and dried over MqSO 4 . Yields were typically ca. 75%. "! 33 CARBENE CHEMISTRY PART II. HYDROGEN MIGRATION IN 2-CARBENA-6,6...any a delocaliza- tion. Thus if one assumes a single product determining intermediate, carbene 54 is classical in the usual sense of the word. It has...placed in a refrigerator. The crystalline product was re- crystallized from methanol-O-d/D20 yielding purified tosylhydrazone with mp 156-1580. 58

Sites of intermolecular crosslinking of fatty acyl chains in phospholipids carrying a photoactivable carbene precursor

PubMed Central

Gupta, Chhitar M.; Costello, Catherine E.; Khorana, H. Gobind

1979-01-01

Sonicated vesicles of 1-fatty acyl-2-ω-(2-diazo-3,3,3-trifluoropropionoxy) fatty acyl sn-glycero-3-phosphoryl-cholines were shown recently to form intermolecular crosslinks by insertion of the photogenerated carbene into a C—H bond of a neighboring hydrocarbon chain. We now report that photolysis of multilamellar dispersions gives a second series of products in which carbene insertion is accompanied by elimination of a molecule of hydrogen fluoride. The sites of crosslinking in the latter compounds have been studied by mass spectrometry using phospholipids with varying chain lengths of the fatty acyl groups carrying the carbene precursor. The patterns observed show that the point of maximum crosslinking is consistent with the recent conclusion that in phospholipids the sn-2 fatty acyl chain trails the sn-1 chain by 2-4 atoms. Images PMID:16592675

Coumaraz-2-on-4-ylidene: Ambiphilic N-heterocyclic Carbenes with a Fine-Tunable Electronic Structure.

PubMed

Song, Hayoung; Kim, Hyunho; Lee, Eunsung

2018-05-16

Herein, a coumaraz-2-on-4-ylidene (1) as a new example of ambiphilic N-heterocyclic carbenes with fine tunable electronic properties is reported. The N-carbamic and aryl groups on carbene carbon provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the 77Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine-tuned in quantitative and predictable way using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be further applicable to eliciting novel properties in main-group and transition metal chemistry. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Carbene complexes of rhodium and iridium from tripodal N-heterocyclic carbene ligands: synthesis and catalytic properties.

PubMed

Mas-Marzá, Elena; Poyatos, Macarena; Sanaú, Mercedes; Peris, Eduardo

2004-03-22

Two tripodal trisimidazolium ligand precursors have been tested in the synthesis of new N-heterocyclic carbene rhodium and iridium complexes. [Tris(3-methylbenzimidazolium-1-yl)]methane sulfate gave products with coordination of the decomposed precursor. [1,1,1-Tris(3-butylimidazolium-1-yl)methyl]ethane trichloride (TIMEH(3)(Bu)) coordinated to the metal in a chelate and bridged-chelate form, depending on the reaction conditions. The crystal structures of two of the products are described. The compounds resulting from the coordination with TIME(Bu) were tested in the catalytic hydrosilylation of terminal alkynes.

Facile synthesis of highly substituted 3-aminofurans from thiazolium salts, aldehydes, and dimethyl acetylenedicarboxylate.

PubMed

Ma, Cheng; Ding, Hanfeng; Wu, Guangming; Yang, Yewei

2005-10-28

[reaction: see text] A facile preparation of 3-aminofuran derivatives via multicomponent reactions of thiazole carbenes, aldehydes, and dimethyl acetylenedicarboxylate (DMAD) is reported. In this process, the thiazole carbenes, generated in situ from thiazolium salts, reacted with aldehydes and DMAD at -78 to 0 degree C in CH(2)Cl(2) to afford the substituted furans in moderate to good yields. Eight substituted thiazolium salts were employed as carbene precursors in the reaction. Besides aryl aldehydes, alpha,beta-unsaturated aldehydes, aliphatic aldehydes, and arenedial were also investigated and found to be applicable to this reaction.

Stereoselective 1,3-Insertions of Rhodium(II) Azavinyl Carbenes

PubMed Central

Chuprakov, Stepan; Worrell, Brady T.; Selander, Nicklas; Sit, Rakesh K.; Fokin, Valery V.

2014-01-01

Rhodium(II) azavinyl carbenes, conveniently generated from 1-sulfonyl-1,2,3-triazoles, undergo a facile, mild and convergent formal 1,3-insertion into N–H and O–H bonds of primary and secondary amides, various alcohols, and carboxylic acids to afford a wide range of vicinally bis-functionalized Z-olefins with perfect regio- and stereoselectively. Utilizing the distinctive functionality installed through these reactions, a number of subsequent rearrangements and cyclizations expand the repertoire of valuable organic building blocks constructed by reactions of transition metal carbene complexes, including α-allenyl ketones and amino-substituted heterocycles. PMID:24295389

Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled Through a Novel Bifunctional NHC-OH Ligand.

PubMed

Srivastava, Ravi; Moneuse, Raphaël; Petit, Julien; Pavard, Paul-Alexis; Dardun, Vincent; Rivat, Madleen; Schiltz, Pauline; Solari, Marius; Jeanneau, Erwann; Veyre, Laurent; Thieuleux, Chloé; Quadrelli, Elsje Alessandra; Camp, Clément

2018-03-20

The straightforward synthesis of a new unsymmetrical hydroxy-tethered N-heterocyclic carbene (NHC) ligand, HL, is presented. The free ligand exhibits an unusual OH-carbene hydrogen-bonding interaction. This OH-carbene motif was used to yield 1) the first tantalum complex displaying both a Fischer- and Schrock-type carbene ligand and 2) a unique NHC-based early/late heterobimetallic complex. More specifically, the protonolysis chemistry between the ligand's hydroxy group and imido-alkyl or alkylidene-alkyl tantalum precursor complexes yielded the rare monometallic tantalum-NHC complexes [Ta(XtBu)(L)(CH 2 tBu) 2 ] (X=N, CH), in which the alkoxy-carbene ligand acts as a chelate. In contrast, HL only binds to rhodium through the NHC unit in [Rh(HL)(cod)Cl] (cod=cycloocta-1,5-diene), the hydroxy pendant arm remaining unbound. This bifunctional ligand scaffold successfully promoted the assembly of rhodium/tantalum heterobimetallic complexes upon either 1) the insertion of [Rh(cod)Cl] 2 into the Ta-NHC bond in [Ta(NtBu)(L)(CH 2 tBu) 2 ] or 2) protonolysis between the free hydroxy group in [Rh(HL)(cod)Cl] and one alkyl group in [Ta(NtBu)(CH 2 tBu) 3 ]. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Pyrolysis of furan in a microreactor

NASA Astrophysics Data System (ADS)

Urness, Kimberly N.; Guan, Qi; Golan, Amir; Daily, John W.; Nimlos, Mark R.; Stanton, John F.; Ahmed, Musahid; Ellison, G. Barney

2013-09-01

A silicon carbide microtubular reactor has been used to measure branching ratios in the thermal decomposition of furan, C4H4O. The pyrolysis experiments are carried out by passing a dilute mixture of furan (approximately 0.01%) entrained in a stream of helium through the heated reactor. The SiC reactor (0.66 mm i.d., 2 mm o.d., 2.5 cm long) operates with continuous flow. Experiments were performed with a reactor inlet pressure of 100-300 Torr and a wall temperature between 1200 and 1600 K; characteristic residence times in the reactor are 60-150 μs. The unimolecular decomposition pathway of furan is confirmed to be: furan (+ M) rightleftharpoons α-carbene or β-carbene. The α-carbene fragments to CH2=C=O + HC≡CH while the β-carbene isomerizes to CH2=C=CHCHO. The formyl allene can isomerize to CO + CH3C≡CH or it can fragment to H + CO + HCCCH2. Tunable synchrotron radiation photoionization mass spectrometry is used to monitor the products and to measure the branching ratio of the two carbenes as well as the ratio of [HCCCH2]/[CH3C≡CH]. The results of these pyrolysis experiments demonstrate a preference for 80%-90% of furan decomposition to occur via the β-carbene. For reactor temperatures of 1200-1400 K, no propargyl radicals are formed. As the temperature rises to 1500-1600 K, at most 10% of the decomposition of CH2=C=CHCHO produces H + CO + HCCCH2 radicals. Thermodynamic conditions in the reactor have been modeled by computational fluid dynamics and the experimental results are compared to the predictions of three furan pyrolysis mechanisms. Uncertainty in the pressure-dependency of the initiation reaction rates is a possible a source of discrepancy between experimental results and theoretical predictions.

Heterobimetallic complexes with redox-active mesoionic carbenes as metalloligands: electrochemical properties, electronic structures and catalysis.

PubMed

Hettmanczyk, Lara; Manck, Sinja; Hoyer, Carolin; Hohloch, Stephan; Sarkar, Biprajit

2015-07-11

A mesoionic carbene with a ferrocene backbone is used as a metalloligand to generate the first example of their Fe-Au heterobimetallic complexes. The details of geometric and electronic structures in different redox states and preliminary catalytic results are presented.

N-Heterocyclic Carbene-Catalyzed Alcohol Acetylation: An Organic Experiment Using Organocatalysis

ERIC Educational Resources Information Center

Morgan, John P.; Shrimp, Jonathan H.

2014-01-01

Undergraduate students in the teaching laboratory have successfully used N-heterocyclic carbenes (NHCs) as organocatalysts for the acetylation of primary alcohols, despite the high water sensitivity of uncomplexed ("free") NHCs. The free NHC readily reacted with chloroform, resulting in an air- and moisture-stable adduct that liberates…

Stable singlet carbenes as mimics for transition metal centers

PubMed Central

Martin, David; Soleilhavoup, Michele

2011-01-01

This perspective summarizes recent results, which demonstrate that stable carbenes can activate small molecules (CO, H2, NH3 and P4) and stabilize highly reactive intermediates (main group elements in the zero oxidation state and paramagnetic species). These two tasks were previously exclusive for transition metal complexes. PMID:21743834

Cross enyne metathesis of para-substituted styrenes: a kinetic study of enyne metathesis.

PubMed

Giessert, Anthony J; Diver, Steven T

2005-01-20

[Reaction: see text] The intermolecular enyne metathesis between alkynes and styrene derivatives was developed to study electronic effects in enyne metathesis. A Hammett plot for the overall reaction, catalyst initiation and vinyl carbene turnover was determined with the second generation Grubbs ruthenium carbene catalyst.

Pd-N-Heterocyclic Carbene (NHC) Organic Silica: Synthesis and Application in Carbon-Carbon Coupling Reactions

EPA Science Inventory

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica was prepared using sol-gel method and its application in Heck and Suzuki reaction were demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wi...

Highly chemoselective intermolecular cross-benzoin reactions using an ad hoc designed novel N-heterocyclic carbene catalyst.

PubMed

Delany, Eoghan G; Connon, Stephen J

2018-01-31

The design of a novel N-heterocyclic carbene catalyst incorporating a bulky yet highly electron-deficient N-aryl substituent has allowed the development of an efficient protocol for the first highly chemoselective intermolecular benzoin condensations between two non-identical aromatic aldehydes.

Annulation of o-Quinodimethanes through N-Heterocyclic Carbene Catalysis for the Synthesis of 1-Isochromanones.

PubMed

Janssen-Müller, Daniel; Singha, Santanu; Olyschläger, Theresa; Daniliuc, Constantin G; Glorius, Frank

2016-09-02

The activation of 2-(bromomethyl)benzaldehydes using N-heterocyclic carbenes represents a novel approach to the generation of o-quinodimethane (o-QDM) intermediates. Coupling with ketones such as phenylglyoxylates, isatins, or trifluoromethyl ketones via [4 + 2] annulation gives access to functionalized 1-isochromanones.

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry.

PubMed

Wentrup, Curt

2018-05-29

There has been significant progress in the direct observation of carbenes, nitrenes, and many other reactive intermediates in recent years due to the application of matrix photolysis and flash vacuum pyrolysis linked with matrix isolation at cryogenic temperatures. Our understanding of singlet and triplet states has improved through the interplay of spectroscopy and computations. Bistable carbenes and nitrenes as well as many examples of tunneling have been discovered. Numerous rearrangements and fragmentations have been documented. This minireview aims to give an overview of some of these developments but will not generally cover laser flash photolysis and chemical reactions in liquid solution. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fischer and Schrock Carbene Complexes: A Molecular Modeling Exercise

ERIC Educational Resources Information Center

Montgomery, Craig D.

2015-01-01

An exercise in molecular modeling that demonstrates the distinctive features of Fischer and Schrock carbene complexes is presented. Semi-empirical calculations (PM3) demonstrate the singlet ground electronic state, restricted rotation about the C-Y bond, the positive charge on the carbon atom, and hence, the electrophilic nature of the Fischer…

Synthesis of Pd-N-heterocyclic carbene Pd-catalyst and its application in MW-assisted Heck and Suzuki reaction

EPA Science Inventory

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica is prepared using sol-gel method and its application in Heck and Suzuki reactions are demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wit...

Synthesis of axially chiral oxazoline-carbene ligands with an N-naphthyl framework and a study of their coordination with AuCl·SMe(2).

PubMed

Wang, Feijun; Li, Shengke; Qu, Mingliang; Zhao, Mei-Xin; Liu, Lian-Jun; Shi, Min

2012-01-01

Axially chiral oxazoline-carbene ligands with an N-naphthyl framework were successfully prepared, and their coordination behavior with AuCl·SMe(2) was also investigated, affording the corresponding Au(I) complexes in moderate to high yields.

N-heterocyclic carbene catalysed asymmetric cross-benzoin reactions of heteroaromatic aldehydes with trifluoromethyl ketones.

PubMed

Enders, Dieter; Grossmann, André; Fronert, Jeanne; Raabe, Gerhard

2010-09-14

A new triazolium salt derived N-heterocyclic carbene catalyses an asymmetric cross-benzoin-type reaction of heteroaromatic aldehydes and various trifluoromethyl ketones in good to excellent yields (69-96%) and moderate to good enantioselectivities (ee = 39-85%). Up to 99% ee can be achieved by recrystallisation.

Optimized Syntheses of Cyclopentadienyl Nickel Chloride Compounds Containing "N"-Heterocyclic Carbene Ligands for Short Laboratory Periods

ERIC Educational Resources Information Center

Cooke, Jason; Lightbody, Owen C.

2011-01-01

Experiments are described for the preparation of imidazolium chloride precursors to "N"-heterocyclic carbenes and their cyclopentadienyl nickel chloride derivatives. The syntheses have been optimized for second- and third-year undergraduate laboratories that have a maximum programmed length of three hours per week. The experiments are flexible and…

N-Heterocyclic carbene-catalyzed chemoselective cross-aza-benzoin reaction of enals with isatin-derived ketimines: access to chiral quaternary aminooxindoles.

PubMed

Xu, Jianfeng; Mou, Chengli; Zhu, Tingshun; Song, Bao-An; Chi, Yonggui Robin

2014-06-20

A chemo- and enantioselective cross-aza-benzoin reaction between enals and isatin-derived ketimines is disclosed. The high chemoselectivity (of the acyl anion reaction over enal α- and β-carbon reactions) is enabled by the electronic and steric properties of the N-heterocyclic carbene organocatalyst.

Identification of a Lead Candidate in the Search for Carbene-Stabilised Homoaromatics.

PubMed

Mattock, James D; Vargas, Alfredo; Dewhurst, Rian D

2015-11-16

The effect of carbenes as Lewis donor groups on the homoaromaticity of mono- and bicyclic organic molecules is surveyed. The search for viable carbene-stabilised homoaromatics resulted in a large amount of rejected candidates as well as nine promising candidates that are further analysed for their homoaromaticity by using a number of metrics. Of these, five appeared to show modest homoaromaticity, whereas another compound showed a level of homoaromaticity comparable with the homotropylium cation benchmark compound. Isoelectronic analogues and constitutional isomers of the lead compound were investigated, however, none of these showed comparable homoaromaticity. The implications of these calculations on the design of donor-stabilised homoaromatics are discussed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Evolution of structure and reactivity in a series of iconic carbenes.

PubMed

Zhang, Min; Moss, Robert A; Thompson, Jack; Krogh-Jespersen, Karsten

2012-01-20

We present experimental activation parameters for the reactions of six carbenes (CCl(2), CClF, CF(2), ClCOMe, FCOMe, and (MeO)(2)C) with six alkenes (tetramethylethylene, cyclohexene, 1-hexene, methyl acrylate, acrylonitrile, and α-chloroacrylonitrile). Activation energies range from -1 kcal/mol for the addition of CCl(2) to tetramethylethylene to 11 kcal/mol for the addition of FCOMe to acrylonitrile. A generally satisfactory analysis of major trends in the evolution of carbenic structure and reactivity is afforded by qualitative applications of frontier molecular orbital theory, although the observed entropies of activation appear to fall in a counterintuitive pattern. An analysis of computed cyclopropanation transition state parameters reveals significant nucleophilic selectivity of (MeO)(2)C toward α-chloroacrylonitrile.

Well-defined N-heterocyclic carbene silver halides of 1-cyclohexyl-3-arylmethylimidazolylidenes: synthesis, structure and catalysis in A3-reaction of aldehydes, amines and alkynes.

PubMed

Li, Yanbo; Chen, Xiaofeng; Song, Yin; Fang, Ling; Zou, Gang

2011-03-07

Structurally well-defined N-heterocyclic carbene silver chlorides and bromides supported by 1-cyclohexyl-3-benzylimidazolylidene (CyBn-NHC) or 1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene (CyNaph-NHC) were synthesized by reaction of the corresponding imidazolium halides with silver(I) oxide while cationic bis(CyBn-NHC) silver nitrate was isolated under similar conditions using imidazolium iodide in the presence of sodium nitrate. Single-crystal X-ray diffraction revealed a dimeric structure through a nonpolar weak-hydrogen-bond supported Ag-Ag bond for 1-cyclohexyl-3-benzylimidazolylidene silver halides [(CyBn-NHC)AgX](2) (X = Cl, 1; Br, 2) but a monomeric structure for N-heterocyclic carbene silver halides with the more sterically demanding 1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene ligand (CyNaph-NHC)AgX (X = Cl, 4; Br, 5). Cationic biscarbene silver nitrate [(CyBn-NHC)(2)Ag](+)NO(3)(-)3 assumed a cis orientation with respect to the two carbene ligands. The monomeric complexes (CyNaph-NHC)AgX 4 and 5 showed higher catalytic activity than the dimeric [(CyBn-NHC)AgX](2)1 and 2 as well as the cationic biscarbene silver nitrate 3 in the model three component reaction of 3-phenylpropionaldehyde, phenylacetylene and piperidine with chloride 4 performing best and giving product in almost quantitative yield within 2 h at 100 °C. An explanation for the structure-activity relationship in N-heterocyclic carbene silver halide catalyzed three component reaction is given based on a slightly modified mechanism from the one in literature.

Gold(I) Carbenoids: On‐Demand Access to Gold(I) Carbenes in Solution

PubMed Central

Sarria Toro, Juan M.; García‐Morales, Cristina; Raducan, Mihai; Smirnova, Ekaterina S.

2017-01-01

Abstract Chloromethylgold(I) complexes of phosphine, phosphite, and N‐heterocyclic carbene ligands are easily synthesized by reaction of trimethylsilyldiazomethane with the corresponding gold chloride precursors. Activation of these gold(I) carbenoids with a variety of chloride scavengers promotes reactivity typical of metallocarbenes in solution, namely homocoupling to ethylene, olefin cyclopropanation, and Buchner ring expansion of benzene. PMID:28090747

Straightforward Preparation Method for Complexes Bearing a Bidentate N-Heterocyclic Carbene to Introduce Undergraduate Students to Research Methodology

ERIC Educational Resources Information Center

Fernández, Alberto; López-Torres, Margarita; Fernández, Jesús J.; Vázquez-García, Digna; Marcos, Ismael

2017-01-01

A laboratory experiment for students in advanced inorganic chemistry is described. In this experiment, students prepare two metal complexes with a potentially bidentate-carbene ligand. The complexes are synthesized by reaction of a bisimidazolium salt with silver(I) oxide or palladium(II) acetate. Silver and palladium complexes are binuclear and…

Biscarbene palladium(II) complexes. reactivity of saturated versus unsaturated N-heterocyclic carbenes.

PubMed

Fu, Ching-Feng; Lee, Chun-Chin; Liu, Yi-Hung; Peng, Shie-Ming; Warsink, Stefan; Elsevier, Cornelis J; Chen, Jwu-Ting; Liu, Shiuh-Tzung

2010-03-15

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by (1)H and (13)C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C((saturated NHC)) is distinct from that of Pd-C((unsaturated NHC)). The Pd-C((saturated NHC)) bonds are fairly stable toward reagents such as CF(3)COOH, AgBF(4) and I(2), whereas Pd-C((unsaturated NHC)) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)(2)PdCl(2) > (sat-NHC)(unsat-NHC)PdCl(2 )> (unsat-NHC)(2)PdCl(2).

Late metal carbene complexes generated by multiple C-H activations: examining the continuum of M=C bond reactivity.

PubMed

Whited, Matthew T; Grubbs, Robert H

2009-10-20

Unactivated C(sp(3))-H bonds are ubiquitous in organic chemicals and hydrocarbon feedstocks. However, these resources remain largely untapped, and the development of efficient homogeneous methods for hydrocarbon functionalization by C-H activation is an attractive and unresolved challenge for synthetic chemists. Transition-metal catalysis offers an attractive possible means for achieving selective, catalytic C-H functionalization given the thermodynamically favorable nature of many desirable partial oxidation schemes and the propensity of transition-metal complexes to cleave C-H bonds. Selective C-H activation, typically by a single cleavage event to produce M-C(sp(3)) products, is possible through myriad reported transition-metal species. In contrast, several recent reports have shown that late transition metals may react with certain substrates to perform multiple C-H activations, generating M=C(sp(2)) complexes for further elaboration. In light of the rich reactivity of metal-bound carbenes, such a route could open a new manifold of reactivity for catalytic C-H functionalization, and we have targeted this strategy in our studies. In this Account, we highlight several early examples of late transition-metal complexes that have been shown to generate metal-bound carbenes by multiple C-H activations and briefly examine factors leading to the selective generation of metal carbenes through this route. Using these reports as a backdrop, we focus on the double C-H activation of ethers and amines at iridium complexes supported by Ozerov's amidophosphine PNP ligand (PNP = [N(2-P(i)Pr(2)-4-Me-C(6)H(3))(2)](-)), allowing isolation of unusual square-planar iridium(I) carbenes. These species exhibit reactivity that is distinct from the archetypal Fischer and Schrock designations. We present experimental and theoretical studies showing that, like the classical square-planar iridium(I) organometallics, these complexes are best described as nucleophilic at iridium. We discuss the classification of this reactivity in the context of a scheme originally delineated by Roper. These "Roper-type" carbenes perform a number of multiple-bond metatheses leading to atom and group transfer from electrophilic heterocumulene (e.g., CO(2), CS(2), PhNCS) and diazo (e.g., N(2)O, AdN(3)) reagents. In one instance, we have extended this methodology to a process for catalytic C-H functionalization by a double C-H activation-group transfer process. Although the scope of these reactions is currently limited, these new pathways may find broader utility as the reactivity of late-metal carbenes continues to be explored. Examination of alternative transition metals and supporting ligand sets will certainly be important. Nonetheless, our findings show that carbene generation by double C-H activation is a viable strategy for C-H functionalization, leading to products not accessible through traditional C(sp(3))-H activation pathways.

Synergistic N-Heterocyclic Carbene/Palladium-Catalyzed Reactions of Aldehyde Acyl Anions with either Diarylmethyl or Allylic Carbonates.

PubMed

Yasuda, Shigeo; Ishii, Takuya; Takemoto, Shunsuke; Haruki, Hiroki; Ohmiya, Hirohisa

2018-03-05

Benzylation and allylation of aldehyde acyl anions were enabled by the merger of a thiazolium N-heterocyclic carbene (NHC) catalyst and a palladium/bisphosphine catalyst in a synergistic manner. Owing to the mildness of the reaction conditions, various functional groups were tolerated in the substrates. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Preparation of a N-Heterocyclic Carbene Nickel(II) Complex: Synthetic Experiments in Current Organic and Organometallic Chemistry

ERIC Educational Resources Information Center

Ritleng, Vincent; Brenner, Eric; Chetcuti, Michael J.

2008-01-01

A four-part experiment that leads to the synthesis of a cyclopentadienyl chloro-nickel(II) complex bearing a N-heterocyclic carbene (NHC) ligand is presented. In the first part, the preparation of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride (IMes[middle dot]HCl) in a one-pot procedure by reaction of 2,4,6-trimethylaniline with…

Site-selective benzoin-type cyclization of unsymmetrical dialdoses catalyzed by N-heterocyclic carbenes for divergent cyclitol synthesis.

PubMed

Kang, Bubwoong; Wang, Yinli; Kuwano, Satoru; Yamaoka, Yousuke; Takasu, Kiyosei; Yamada, Ken-Ichi

2017-04-18

A highly site-selective N-heterocyclic carbene (NHC)-catalyzed benzoin-type cyclization of unsymmetrical dialdoses is developed to enable a divergent cyclitol synthesis. The choice of chiral NHCs and protecting groups affects the site-selectivity. The resulting inososes are converted into epi-, muco- and myo-inositols, and their chiral protected derivatives are formed in good yields.

NHC→SiCl4 : an ambivalent carbene-transfer reagent.

PubMed

Böttcher, Tobias; Steinhauer, Simon; Lewis-Alleyne, Lesley C; Neumann, Beate; Stammler, Hans-Georg; Bassil, Bassem S; Röschenthaler, Gerd-Volker; Hoge, Berthold

2015-01-07

The addition of BCl3 to the carbene-transfer reagent NHC→SiCl4 (NHC=1,3-dimethylimidazolidin-2-ylidene) gave the tetra- and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl3 ](+) and [(NHC)2 SiCl3 ](+) with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC→SiCl4 served as a transfer reagent for the NHC ligand. The addition of BF3 ⋅OEt2 , on the other hand, gave NHC→BF3 as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C2 F5 )2 SiCl2 was treated with NHC→SiCl4 . In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl3 ](+) and [(NHC)2 SiCl3 ](+) were detected with [(C2 F5 )SiCl3 ](-) as counterion. A similar result was already reported for the reaction of NHC→SiCl4 with (C2 F5 )2 SiH2 , which gave [(NHC)2 SiCl2 H][(C2 F5 )SiCl3 ]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC→Si(C2 F5 )2 Cl2 and NHC→Si(C2 F5 )2 ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron-poor silylenes. Furthermore, the reactivity of NHC→SiCl4 towards phosphines is discussed. The carbene complex NHC→PCl3 shows similar reactivity to NHC→SiCl4 , and may even serve as a carbene-transfer reagent as well. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Photochemistry of the Stilbenes in Methanol. Trapping the Common Phantom Singlet State.

PubMed

Saltiel, Jack; Gupta, Shipra

2018-06-21

A comparative study of the photochemistry of cis- and trans-stilbene in methanol shows that both isomers undergo methanol photoaddition giving similar yields of α-methoxybibenzyl in competition with cis-trans photoisomerization. Methanol addition occurs primarily following torsional relaxation of the lowest excited singlet states of each isomer, 1 c* and 1 t*, to a common twisted singlet excited state intermediate, 1 p*, initially called the phantom singlet state. The addition is consistent with the zwitterionic character of 1 p*. Ether forms by direct 1,2-addition of CH 3 OH to the central carbon atoms and by 1,1-addition following rearrangement to 1-benzyl-1-phenylcarbene. Use of CD 3 OD and GC/MS (gas chromatographic/mass spectroscopic) analysis of the ether products revealed that the ratio of carbene/direct addition pathways is higher starting from cis-stilbene. We conclude that 1 p* formed from 1 c* is hotter than 1 p* formed from 1 t*. Surprisingly, except for favoring the carbene pathway, the use of higher energy photons (254 vs 313 nm) does not affect the overall ether quantum yield starting from cis-stilbene, but significantly enhances both pathways starting from trans-stilbene. It appears that carbene formation and direct methanol addition to higher trans-stilbene excited state(s) compete with relaxation to S 1 . Substitution of D for the vinyl Hs of stilbene enhances the direct addition pathway more than two-fold and strongly suppresses the carbene insertion pathway, revealing a large, k pc d0 / k pc d2 = 6.3, primary deuterium isotope effect in the carbene rearrangement. The two-fold increase in the ether quantum yield is due primarily to a 2.75-fold increase in the lifetime of 1 p* on deuterium substitution of the vinyl hydrogens.

Photochemistry of furyl- and thienyldiazomethanes: spectroscopic characterization of triplet 3-thienylcarbene.

PubMed

Pharr, Caroline R; Kopff, Laura A; Bennett, Brian; Reid, Scott A; McMahon, Robert J

2012-04-11

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (1), matrix isolated in Ar or N(2) at 10 K, yields triplet 3-thienylcarbene (13) and α-thial-methylenecyclopropene (9). Carbene 13 was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (s-E and s-Z) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|D/hc| = 0.508 cm(-1), |E/hc| = 0.0554 cm(-1); |D/hc| = 0.579 cm(-1), |E/hc| = 0.0315 cm(-1)). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in D values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the s-Z rotamer of 13 and the antibonding σ orbital between sulfur and the neighboring carbon-an interaction that is not observed in the s-E rotamer of 13. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (13) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ(max) = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (2), 3-furyldiazomethane (3), or 2-furyldiazomethane (4) provided further insight into the photochemical interconversions among C(5)H(4)S or C(5)H(4)O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (11), 3-furylcarbene (23), or 2-furylcarbene (22), respectively). © 2012 American Chemical Society

Amino Acid Insertion Frequencies Arising from Photoproducts Generated Using Aliphatic Diazirines

NASA Astrophysics Data System (ADS)

Ziemianowicz, Daniel S.; Bomgarden, Ryan; Etienne, Chris; Schriemer, David C.

2017-10-01

Mapping proteins with chemical reagents and mass spectrometry can generate a measure of accessible surface area, which in turn can be used to support the modeling and refinement of protein structures. Photolytically generated carbenes are a promising class of reagent for this purpose. Substituent effects appear to influence surface mapping properties, allowing for a useful measure of design control. However, to use carbene labeling data in a quantitative manner for modeling activities, we require a better understanding of their inherent amino acid reactivity, so that incorporation data can be normalized. The current study presents an analysis of the amino acid insertion frequency of aliphatic carbenes generated by the photolysis of three different diazirines: 3,3'-azibutyl-1-ammonium, 3,3'-azibutan-1-ol, and 4,4'-azipentan-1-oate. Leveraging an improved photolysis system for single-shot labeling of sub-microliter frozen samples, we used EThCD to localize insertion products in a large population of labeled peptides. Counting statistics were drawn from data-dependent LC-MS2 experiments and used to estimate the frequencies of insertion as a function of amino acid. We observed labeling of all 20 amino acids over a remarkably narrow range of insertion frequencies. However, the nature of the substituent could influence relative insertion frequencies, within a general preference for larger polar amino acids. We confirm a large (6-fold) increase in labeling yield when carbenes were photogenerated in the solid phase (77 K) relative to the liquid phase (293 K), and we suggest that carbene labeling should always be conducted in the frozen state to avoid information loss in surface mapping experiments. [Figure not available: see fulltext.

Dimensioning the Term Carbenoid.

PubMed

Caballero, Ana; Pérez, Pedro J

2017-10-17

The current use of the term carbenoid is discussed, particularly in the context of carbene transfer reactions from diazo compounds, in which intermediates of type L n M=CR 1 R 2 , or one of its resonance forms, are tagged which such a name. We discuss this issue, on the basis of the data evidencing the metal-carbene nature of those intermediates, as well as the existence of carbenoids of type (N 2 )(M)CR 1 R 2 en route to the formation to L n M=CR 1 R 2' from diazo reagents. We propose the exclusive use of the carbenoid term to species of type (X)(M)CR 1 R 2 with a tetrasubstituted carbon center that upon loss of X afford an effective carbene transfer process. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

PubMed Central

Lavallo, Vincent; Frey, Guido D.; Kousar, Shazia; Donnadieu, Bruno; Bertrand, Guy

2007-01-01

Addition of a sterically demanding cyclic (alkyl)(amino)carbene (CAAC) to AuCl(SMe2) followed by treatment with [Et3Si(Tol)]+[B(C6F5)4]− in toluene affords the isolable [(CAAC)Au(η2-toluene)]+[B(C6F5)4]− complex. This cationic Au(I) complex efficiently mediates the catalytic coupling of enamines and terminal alkynes to yield allenes and not propargyl amines as observed with other catalysts. Mono-, di-, and tri-substituted enamines can be used, as well as aryl-, alkyl-, and trimethylsilyl-substituted terminal alkynes. The reaction tolerates sterically hindered substrates and is diastereoselective. This general catalytic protocol directly couples two unsaturated carbon centers to form the three-carbon allenic core. The reaction most probably proceeds through an unprecedented “carbene/vinylidene cross-coupling.” PMID:17698808

Carbene-catalysed reductive coupling of nitrobenzyl bromides and activated ketones or imines via single-electron-transfer process

NASA Astrophysics Data System (ADS)

Li, Bao-Sheng; Wang, Yuhuang; Proctor, Rupert S. J.; Zhang, Yuexia; Webster, Richard D.; Yang, Song; Song, Baoan; Chi, Yonggui Robin

2016-09-01

Benzyl bromides and related molecules are among the most common substrates in organic synthesis. They are typically used as electrophiles in nucleophilic substitution reactions. These molecules can also be activated via single-electron-transfer (SET) process for radical reactions. Representative recent progress includes α-carbon benzylation of ketones and aldehydes via photoredox catalysis. Here we disclose the generation of (nitro)benzyl radicals via N-heterocyclic carbene (NHC) catalysis under reductive conditions. The radical intermediates generated via NHC catalysis undergo formal 1,2-addition with ketones to eventually afford tertiary alcohol products. The overall process constitutes a formal polarity-inversion of benzyl bromide, allowing a direct coupling of two initially electrophilic carbons. Our study provides a new carbene-catalysed reaction mode that should enable unconventional transformation of (nitro)benzyl bromides under mild organocatalytic conditions.

Mechanistic switch in dual gold catalysis of diynes: C(sp(3))-H activation through bifurcation--vinylidene versus carbene pathways.

PubMed

Hansmann, Max M; Rudolph, Matthias; Rominger, Frank; Hashmi, A Stephen K

2013-02-25

The other side of the mountain: Changing the framework of diyne systems opens up new cyclization modes for dual gold catalysis. Instead of a 5-endo cyclization and gold vinylidenes a 6-endo cyclization gives rise to gold-stabilized carbenes as key intermediates for selective C-H insertions. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis of Well-Defined Copper "N"-Heterocyclic Carbene Complexes and Their Use as Catalysts for a "Click Reaction": A Multistep Experiment that Emphasizes the Role of Catalysis in Green Chemistry

ERIC Educational Resources Information Center

Ison, Elon A.; Ison, Ana

2012-01-01

A multistep experiment for an advanced synthesis lab course that incorporates topics in organic-inorganic synthesis and catalysis and highlights green chemistry principles was developed. Students synthesized two "N"-heterocyclic carbene ligands, used them to prepare two well-defined copper(I) complexes and subsequently utilized the complexes as…

"N"-Heterocyclic Carbene-Catalyzed Reaction of Chalcone and Cinnamaldehyde to Give 1,3,4-Triphenylcyclopentene Using Organocatalysis to Form a Homoenolate Equivalent

ERIC Educational Resources Information Center

Snider, Barry B.

2015-01-01

In this experiment, students carry out a modern organocatalytic reaction using IMes·HCl and NaOH to catalyze the formation of 1,3,4-triphenylcyclopentene from cinnamaldehyde and chalcone in water. Deprotonation of IMes·HCl with NaOH forms the "N"-heterocyclic carbene IMes that reacts with cinnamaldehyde to form a homoenolate equivalent…

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions.

PubMed

Menon, Rajeev S; Biju, Akkattu T; Nair, Vijay

2016-01-01

N-Heterocyclic carbenes (NHCs) have emerged as a powerful class of organocatalysts that mediate a variety of organic transformations. The Benzoin reaction constitutes one of the earliest known carbon-carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented.

N-heterocyclic carbene catalyzed regioselective oxo-acyloxylation of alkenes with aromatic aldehydes: a high yield synthesis of α-acyloxy ketones and esters.

PubMed

Reddi, Rambabu N; Malekar, Pushpa V; Sudalai, Arumugam

2013-10-14

An N-heterocyclic carbene (NHC)-catalyzed reaction of alkenes with aromatic aldehydes providing for a high yield synthesis of α-acyloxy ketones and esters has been described. This unprecedented regioselective oxidative process employs NBS and Et3N in stoichiometric amounts and O2 (1 atm) as an oxidant under ambient conditions in DMSO as a solvent.

Rare‐Earth‐ and Uranium‐Mesoionic Carbenes: A New Class of f‐Block Carbene Complex Derived from an N‐Heterocyclic Olefin

PubMed Central

Seed, John A.; Gregson, Matthew; Tuna, Floriana; Chilton, Nicholas F.; Wooles, Ashley J.; McInnes, Eric J. L.

2017-01-01

Abstract Neutral mesoionic carbenes (MICs) have emerged as an important class of carbene, however they are found in the free form or ligated to only a few d‐block ions. Unprecedented f‐block MIC complexes [M(N′′)3{CN(Me)C(Me)N(Me)CH}] (M=U, Y, La, Nd; N′′=N(SiMe3)2) are reported. These complexes were prepared by a formal 1,4‐proton migration reaction when the metal triamides [M(N′′)3] were treated with the N‐heterocyclic olefin H2C=C(NMeCH)2, which constitutes a new, general way to prepare MIC complexes. Quantum chemical calculations on the 5f3 uranium(III) complex suggest the presence of a U=C donor‐acceptor bond, composed of a MIC→U σ‐component and a U(5f)→MIC(2p) π‐back‐bond, but for the d0f0 Y and La and 4f3 Nd congeners only MIC→M σ‐bonding is found. Considering the generally negligible π‐acidity of MICs, this is surprising and highlights that greater consideration should possibly be given to recognizing MICs as potential π‐acid ligands when coordinated to strongly reducing metals. PMID:28719735

Synthesis of Backfunctionalized Imidazolinium Salts and NHC Carbene Complexes

DTIC Science & Technology

2017-04-02

at American Chemical Society National Meeting; San Francisco, CA, USA (02 April 2017) Prepared in collaboration with California Institute of...AFRL – Tenant of Edwards AFB since late ‘50s – Full scale testing of the Atlas rockets (Gemini missions) – Initial testing of the F-1 engine (Apollo...Force has an interest in NHC carbene precursors for a variety applications – Ionic liquid propellants and additives – Ligands for Supercritical Chemical

An electronic rationale for observed initiation rates in ruthenium-mediated olefin metathesis: charge donation in phosphine and N-heterocyclic carbene ligands.

PubMed

Getty, Kendra; Delgado-Jaime, Mario Ulises; Kennepohl, Pierre

2007-12-26

Ru K-edge XAS data indicate that second generation ruthenium-based olefin metathesis precatalysts (L = N-heterocyclic carbene) possess a more electron-deficient metal center than in the corresponding first generation species (L = tricyclohexylphosphine). This surprising effect is also observed from DFT calculations and provides a simple rationale for the slow phosphine dissociation kinetics previously noted for second-generation metathesis precatalysts.

N-Heterocyclic carbene-catalyzed direct cross-aza-benzoin reaction: Efficient synthesis of α-amino-β-keto esters.

PubMed

Uno, Takuya; Kobayashi, Yusuke; Takemoto, Yoshiji

2012-01-01

An efficient catalytic synthesis of α-amino-β-keto esters has been newly developed. Cross-coupling of various aldehydes with α-imino ester, catalyzed by N-heterocyclic carbene, leads chemoselectively to α-amino-β-keto esters in moderate to good yields with high atom efficiency. The reaction mechanism is discussed, and it is proposed that the α-amino-β-keto esters are formed under thermodynamic control.

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions

PubMed Central

Menon, Rajeev S; Biju, Akkattu T

2016-01-01

Summary N-Heterocyclic carbenes (NHCs) have emerged as a powerful class of organocatalysts that mediate a variety of organic transformations. The Benzoin reaction constitutes one of the earliest known carbon–carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented. PMID:27340440

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations.

PubMed

Li, Xiaoxun; Li, Hui; Song, Wangze; Tseng, Po-Sen; Liu, Lingyan; Guzei, Ilia A; Tang, Weiping

2015-10-26

Rhodium(I) carbenes were generated from propargylic alcohol derivatives as the result of a dehydrative indole annulation. Depending on the choice of the electron-withdrawing group on the aniline nitrogen nucleophile, either a cyclopropanation product or dimerization product was obtained chemoselectively. Intramolecular hydroamidation occurred for the same type of propargylic alcohol derivatives when other transition-metal catalysts were employed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Cyclometalated ruthenium(II) complexes with a bis-carbene CCC-pincer ligand.

PubMed

Zhang, You-Ming; Shao, Jiang-Yang; Yao, Chang-Jiang; Zhong, Yu-Wu

2012-08-21

The first series of cyclometalated ruthenium complexes with a CCC-pincer bis-carbene ligand have been obtained as bench-stable compounds. Single-crystal X-ray analysis of one of these complexes with 4'-di-p-anisylamino-2,2':6',2''-terpyridine is presented. The Ru(II/III) redox potentials and MLCT absorptions of these complexes can be varied by attaching an electron-donating or -withdrawing group on the noncyclometalating ligand.

Exploiting and engineering hemoproteins for abiological carbene and nitrene transfer reactions.

PubMed

Brandenberg, Oliver F; Fasan, Rudi; Arnold, Frances H

2017-10-01

The surge in reports of heme-dependent proteins as catalysts for abiotic, synthetically valuable carbene and nitrene transfer reactions dramatically illustrates the evolvability of the protein world and our nascent ability to exploit that for new enzyme chemistry. We highlight the latest additions to the hemoprotein-catalyzed reaction repertoire (including carbene Si-H and C-H insertions, Doyle-Kirmse reactions, aldehyde olefinations, azide-to-aldehyde conversions, and intermolecular nitrene C-H insertion) and show how different hemoprotein scaffolds offer varied reactivity and selectivity. Preparative-scale syntheses of pharmaceutically relevant compounds accomplished with these new catalysts are beginning to demonstrate their biotechnological relevance. Insights into the determinants of enzyme lifetime and product yield are providing generalizable cues for engineering heme-dependent proteins to further broaden the scope and utility of these non-natural activities. Copyright © 2017 Elsevier Ltd. All rights reserved.

Synthesis and crystal structure of the iridium(I) carbene complex with a pair of hydrogen wing tips

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, H.-Y.; Chen, Z.-M.; Wang, Y.

The iridium(I) cyclooctadiene complex with two (3-tert-butylimidazol-2-ylidene) ligands [(H-Im{sup t}Bu){sub 2}Ir(COD)]{sup +}PF{sub 6}{sup −} (C{sub 22}H{sub 32}PF{sub 6}IrN{sub 4}) has been prepared, and its crystal structure is determined by X-ray diffraction. Complex exhibits slightly distorted square planar configurations around the metal atom, which is coordinated by two H-Im{sup t}Bu ligands and one cyclooctadiene group. The new iridium carbene complex has a pair of hydrogen wing tips. The Ir−C{sub carbene} bond lengths are 2.066(5) and 2.052(5) Å, and the bond angle C−Ir−C between these bonds is 95.54(19)°. The dihedral angle between two imidazol-2-ylidene rings is 86.42°.

Surface hydrophobic modification of polyurethanes by diaryl carbene chemistry: Synthesis and characterization

NASA Astrophysics Data System (ADS)

Yang, Pengfei; Wang, Yongqing; Lu, Ling; Yu, Xi; Liu, Lian

2018-03-01

Dodecyl diaryl diazomethane was firstly synthesized from 4,4-dihydroxybenzophenone and 1-bromododecane by a series of reaction steps. Then water-borne polyurethane films with different amount of DMPA were prepared, as well as a type of solvent-borne polyurethane film for comparison. Finally, all these polyurethane films were modified by dodecyl diaryl diazomethane. The dodecyl diaryl carbene was generated from dodecyl diaryl diazomethane by strong solar light, which was very convenient to insert into the Xsbnd H bonds (X = C, N) on the surface of polyurethane films. The contact angle test was used to characterize these films and depict the surface property. DSC analysis and tensile test were used to investigate the physical properties of polyurethane films before and after modification. It was suggested that the hydrophobic modification protocol with carbene insertion was very useful and convenient to prepare water-proof coatings outdoors under direct solar-light exposure.

Carbene-catalysed reductive coupling of nitrobenzyl bromides and activated ketones or imines via single-electron-transfer process

PubMed Central

Li, Bao-Sheng; Wang, Yuhuang; Proctor, Rupert S. J.; Zhang, Yuexia; Webster, Richard D.; Yang, Song; Song, Baoan; Chi, Yonggui Robin

2016-01-01

Benzyl bromides and related molecules are among the most common substrates in organic synthesis. They are typically used as electrophiles in nucleophilic substitution reactions. These molecules can also be activated via single-electron-transfer (SET) process for radical reactions. Representative recent progress includes α-carbon benzylation of ketones and aldehydes via photoredox catalysis. Here we disclose the generation of (nitro)benzyl radicals via N-heterocyclic carbene (NHC) catalysis under reductive conditions. The radical intermediates generated via NHC catalysis undergo formal 1,2-addition with ketones to eventually afford tertiary alcohol products. The overall process constitutes a formal polarity-inversion of benzyl bromide, allowing a direct coupling of two initially electrophilic carbons. Our study provides a new carbene-catalysed reaction mode that should enable unconventional transformation of (nitro)benzyl bromides under mild organocatalytic conditions. PMID:27671606

A stable silicon(0) compound with a Si=Si double bond.

PubMed

Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong; King, R Bruce; Schaefer, Henry F; von R Schleyer, Paul; Robinson, Gregory H

2008-08-22

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 +/- 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si-Si(Cl):L and L:Si=Si:L.

Synthesis of a Simplified Version of Stable Bulky and Rigid Cyclic (Alkyl)(Amino)Carbenes (CAACs), and Catalytic Activity of the Ensuing Gold(I) Complex in the Three-Component Preparation of 1,2-Dihydroquinoline Derivatives

PubMed Central

Zeng, Xiaoming; Frey, Guido D.; Kinjo, Rei; Donnadieu, Bruno; Bertrand, Guy

2009-01-01

A 95/5 mixture of cis and trans 2,4-dimethyl-3-cyclohexenecarboxaldehyde (trivertal), a common fragrance and flavor material produced in bulk quantities, serves as the precursor for the synthesis of a stable spirocyclic (alkyl)(amino)carbene, in which the 2-methyl-substituted cyclohexenyl group provides steric protection to an ensuing metal. The efficiency of this carbene as ligand for transition metal based catalysts is first illustrated by the gold(I) catalyzed hydroamination of internal alkynes with secondary dialkyl amines, a process with little precedent. The feasibility of this reaction allows for significantly enlarging the scope of the one-pot three-component synthesis of 1,2-dihydroquinoline derivatives, and related nitrogen-containing heterocycles. Indeed, two different alkynes were used, which include an internal alkyne for the first step. PMID:19456108

Interfacial charge separation and photovoltaic efficiency in Fe(ii)-carbene sensitized solar cells.

PubMed

Pastore, Mariachiara; Duchanois, Thibaut; Liu, Li; Monari, Antonio; Assfeld, Xavier; Haacke, Stefan; Gros, Philippe C

2016-10-12

The first combined theoretical and photovoltaic characterization of both homoleptic and heteroleptic Fe(ii)-carbene sensitized photoanodes in working dye sensitized solar cells (DSSCs) has been performed. Three new heteroleptic Fe(ii)-NHC dye sensitizers have been synthesized, characterized and tested. Despite an improved interfacial charge separation in comparison to the homoleptic compounds, the heteroleptic complexes did not show boosted photovoltaic performances. The ab initio quantitative analysis of the interfacial electron and hole transfers and the measured photovoltaic data clearly evidenced fast recombination reactions for heteroleptics, even associated with un unfavorable directional electron flow, and hence slower injection rates, in the case of homoleptics. Notably, quantum mechanics calculations revealed that deprotonation of the not anchored carboxylic function in the homoleptic complex can effectively accelerate the electron injection rate and completely suppress the electron recombination to the oxidized dye. This result suggests that introduction of strong electron-donating substituents on the not-anchored carbene ligand in heteroleptic complexes, in such a way of mimicking the electronic effects of the carboxylate functionality, should yield markedly improved interfacial charge generation properties. The present results, providing for the first time a detailed understanding of the interfacial electron transfers and photovoltaic characterization in Fe(ii)-carbene sensitized solar cells, open the way to a rational molecular engineering of efficient iron-based dyes for photoelectrochemical applications.

On the organocatalytic activity of N-heterocyclic carbenes: role of sulfur in thiamine.

PubMed

Hollóczki, Oldamur; Kelemen, Zsolt; Nyulászi, László

2012-07-20

The reaction energy profiles of the benzoin condensation from three aldehydes catalyzed by imidazol-2-ylidene, triazol-3-ylidene, and thiazol-2-ylidene have been investigated computationally. The barriers for all steps of all investigated reactions have been found to be low enough to indicate the viability of the mechanism proposed by Breslow in the 1950s. The most remarkable difference in the catalytic cycles has been the increased stability of the Breslow intermediate in case of thiazol-2-ylidene (by ca. 10 kcal/mol) compared to the other two carbenes, which results in lower energy for the coupling of the second aldehyde molecule, thus, increasing the reversibility of the reaction. Since the analogous transketolase reaction, being involved in the carbohydrate metabolism of many organisms, requires an initial decoupling-a reverse benzoin condensation-this difference provides a reasonable explanation for the presence of a thiazolium ring in thiamine instead of the otherwise generally more available imidazole derivatives. The "resting intermediate" found by Berkessel and co-workers for a triazole-based catalyst was found more stable than the Breslow intermediate for all of the systems investigated. The (gas phase) proton affinities of several carbenes were compared, the relative trends being in agreement with the available (in aqueous solution) data. The hydrolytic ring-opening reaction of the thiazole-based carbene was shown to be different from that of imidazole-2-ylidenes.

C-H activation in Ir(III) and N-demethylation in Pt(II) complexes with mesoionic carbene ligands: examples of monometallic, homobimetallic and heterobimetallic complexes.

PubMed

Maity, Ramananda; Tichter, Tim; van der Meer, Margarethe; Sarkar, Biprajit

2015-11-14

Mononuclear Pt(II) and the first dinuclear Pt(II) complexes along with a cyclometalated heterobimetallic Ir(III)/Pd(II) complex bearing mesoionic carbene donor ligands are presented starting from the same bis-triazolium salt. The mononuclear Pt(II) complex possesses a free triazole moiety which is generated from the corresponding triazolium salt through an N-demethylation reaction, whereas the mononuclear Ir(III) complex features an unreacted triazolium unit.

Enantioselective N-Heterocyclic Carbene Catalysis via the Dienyl Acyl Azolium.

PubMed

Gillard, Rachel M; Fernando, Jared E M; Lupton, David W

2018-04-16

Herein we report the enantioselective N-heterocyclic carbene catalyzed (4+2) annulation of the dienyl acyl azolium with enolates. The reaction exploits readily accessible acyl fluorides and TMS enol ethers to give a range of highly enantio- and diastereo-enriched cyclohexenes (most >97:3 er and >20:1 dr). The reaction was found to require high nucleophilicity NHC catalysts with mechanistic studies supporting a stepwise 1,6-addition/β-lactonization. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Iodine-catalyzed diazo activation to access radical reactivity.

PubMed

Li, Pan; Zhao, Jingjing; Shi, Lijun; Wang, Jin; Shi, Xiaodong; Li, Fuwei

2018-05-17

Transition-metal-catalyzed diazo activation is a classical way to generate metal carbene, which are valuable intermediates in synthetic organic chemistry. An alternative iodine-catalyzed diazo activation is disclosed herein under either photo-initiated or thermal-initiated conditions, which represents an approach to enable carbene radical reactivity. This metal-free diazo activation strategy were successfully applied into olefin cyclopropanation and epoxidation, and applying this method to pyrrole synthesis under thermal-initiated conditions further demonstrates the unique reactivity using this method over typical metal-catalyzed conditions.

Rhodium-catalyzed NH insertion of pyridyl carbenes derived from pyridotriazoles: a general and efficient approach to 2-picolylamines and imidazo[1,5-a]pyridines.

PubMed

Shi, Yi; Gulevich, Anton V; Gevorgyan, Vladimir

2014-12-15

A general and efficient NH insertion reaction of rhodium pyridyl carbenes derived from pyridotriazoles was developed. Various NH-containing compounds, including amides, anilines, enamines, and aliphatic amines, smoothly underwent the NH insertion reaction to afford 2-picolylamine derivatives. The developed transformation was further utilized in a facile one-pot synthesis of imidazo[1,5-a]pyridines. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Oxidative enantioselective α-fluorination of aliphatic aldehydes enabled by N-heterocyclic carbene catalysis.

PubMed

Li, Fangyi; Wu, Zijun; Wang, Jian

2015-01-07

Described is the first study on oxidative enantioselective α-fluorination of simple aliphatic aldehydes enabled by N-heterocyclic carbene catalysis. N-fluorobis(phenyl)sulfonimide serves as a an oxidant and as an "F" source. The C-F bond formation occurs directly at the α position of simple aliphatic aldehydes, thus overcoming nontrivial challenges, such as competitive difluorination and nonfluorination, and proceeds with high to excellent enantioselectivities. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

PubMed Central

Melaimi, Mohand; Soleilhavoup, Michèle

2011-01-01

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made. PMID:20836099

Asymmetric intramolecular α-cyclopropanation of aldehydes using a donor/acceptor carbene mimetic

PubMed Central

Luo, Chaosheng; Wang, Zhen; Huang, Yong

2015-01-01

Enantioselective α-alkylation of carbonyl is considered as one of the most important processes for asymmetric synthesis. Common alkylation agents, that is, alkyl halides, are notorious substrates for both Lewis acids and organocatalysts. Recently, olefins emerged as a benign alkylating species via photo/radical mechanisms. However, examples of enantioselective alkylation of aldehydes/ketones are scarce and direct asymmetric dialkylation remains elusive. Here we report an intramolecular α-cyclopropanation reaction of olefinic aldehydes to form chiral cyclopropane aldehydes. We demonstrate that an α-iodo aldehyde can function as a donor/acceptor carbene equivalent, which engages in a formal [2+1] annulation with a tethered double bond. Privileged bicyclo[3.1.0]hexane-type scaffolds are prepared in good optical purity using a chiral amine. The synthetic utility of the products is demonstrated by versatile transformations of the bridgehead formyl functionality. We expect the concept of using α-iodo iminium as a donor/acceptor carbene surrogate will find wide applications in chemical reaction development. PMID:26644194

Palladium-catalyzed C-H functionalization of acyldiazomethane and tandem cross-coupling reactions.

PubMed

Ye, Fei; Qu, Shuanglin; Zhou, Lei; Peng, Cheng; Wang, Chengpeng; Cheng, Jiajia; Hossain, Mohammad Lokman; Liu, Yizhou; Zhang, Yan; Wang, Zhi-Xiang; Wang, Jianbo

2015-04-08

Palladium-catalyzed C-H functionalization of acyldiazomethanes with aryl iodides has been developed. This reaction is featured by the retention of the diazo functionality in the transformation, thus constituting a novel method for the introduction of diazo functionality to organic molecules. Consistent with the experimental results, the density functional theory (DFT) calculation indicates that the formation of Pd-carbene species in the catalytic cycle through dinitrogen extrusion from the palladium ethyl diazoacetate (Pd-EDA) complex is less favorable. The reaction instead proceeds through Ag2CO3 assisted deprotonation and subsequently reductive elimination to afford the products with diazo functionality remained. This C-H functionalization transformation can be further combined with the recently evolved palladium-catalyzed cross-coupling reaction of diazo compounds with aryl iodides to develop a tandem coupling process for the synthesis of α,α-diaryl esters. DFT calculation supports the involvement of Pd-carbene as reactive intermediate in the catalytic cycle, which goes through facile carbene migratory insertion with a low energy barrier (3.8 kcal/mol).

gem-Difluoroolefination of Diazo Compounds with TMSCF3 or TMSCF2Br: Transition-Metal-Free Cross-Coupling of Two Carbene Precursors.

PubMed

Hu, Mingyou; Ni, Chuanfa; Li, Lingchun; Han, Yongxin; Hu, Jinbo

2015-11-18

A new olefination protocol for transition-metal-free cross-coupling of two carbene fragments arising from two different sources, namely, a nonfluorinated carbene fragment resulting from a diazo compound and a difluorocarbene fragment derived from Ruppert-Prakash reagent (TMSCF3) or TMSCF2Br, has been developed. This gem-difluoroolefination proceeds through the direct nucleophilic addition of diazo compounds to difluorocarbene followed by elimination of N2. Compared to previously reported Cu-catalyzed gem-difluoroolefination of diazo compounds with TMSCF3, which possesses a narrow substrate scope due to a demanding requirement on the reactivity of diazo compounds and in-situ-generated CuCF3, this transition-metal-free protocol affords a general and efficient approach to various disubstituted 1,1-difluoroalkenes, including difluoroacrylates, diaryldifluoroolefins, as well as arylalkyldifluoroolefins. In view of the ready availability of diazo compounds and difluorocarbene reagents and versatile transformations of 1,1-difluoroalkenes, this new gem-difluoroolefination method is expected to find wide applications in organic synthesis.

C^C* cyclometalated platinum(II) N-heterocyclic carbene complexes with a sterically demanding β-diketonato ligand – synthesis, characterization and photophysical properties.

PubMed

Tenne, M; Metz, S; Wagenblast, G; Münster, Ingo; Strassner, T

2015-05-14

Neutral cyclometalated platinum(ii) N-heterocyclic carbene complexes [Pt(C^C*)(O^O)] with C^C* ligands based on 1-phenyl-1,2,4-triazol-5-ylidene and 4-phenyl-1,2,4-triazol-5-ylidene, as well as acetylacetonato (O^O = acac) and 1,3-bis(2,4,6-trimethylphenyl)propan-1,3-dionato (O^O = mesacac) ancillary ligands were synthesized and characterized. All complexes are emissive at room temperature in a poly(methyl methacrylate) (PMMA) matrix with emission maxima in the blue region of the spectrum. High quantum efficiencies and short decay times were observed for all complexes with mesacac ancillary ligands. The sterically demanding mesityl groups of the mesacac ligand effectively prevent molecular stacking. The emission behavior of these emitters is in general independent of the position of the nitrogen in the backbone of the N-heterocyclic carbene (NHC) unit and a variety of substituents in 4-position of the phenyl unit, meta to the cyclometalating bond.

Triazole-functionalized N-heterocyclic carbene complexes of palladium and platinum and efficient aqueous Suzuki-Miyaura coupling reaction.

PubMed

Gu, Shaojin; Xu, Hui; Zhang, Na; Chen, Wanzhi

2010-07-05

Imidazolium salts bearing triazole groups are synthesized via a copper catalyzed click reaction, and the silver, palladium, and platinum complexes of their N-heterocyclic carbenes are studied. [Ag(4)(L1)(4)](PF(6))(4), [Pd(L1)Cl](PF(6)), [Pt(L1)Cl](PF(6)) (L1=3-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)-1-(pyrimidin-2-yl)-1H-imidazolylidene), [Pd(2)(L2)(2)Cl(2)](PF(6))(2), and [Pd(L2)(2)](PF(6))(2) (L2=1-butyl-3-((1-(pyridin-2-yl)-1H-1,2,3-triazol-4-yl)methyl)imidazolylidene) have been synthesized and fully characterized by NMR, elemental analysis, and X-ray crystallography. The silver complex [Ag(4)(L1)(4)](PF(6))(4) consists of a Ag(4) zigzag chain. The complexes [Pd(L1)Cl](PF(6)) and [Pt(L1)Cl](PF(6)), containing a nonsymmetrical NCN' pincer ligand, are square planar with a chloride trans to the carbene donor. [Pd(2)(L2)(2)Cl(2)](PF(6))(2) consists of two palladium centers with CN(2)Cl coordination mode, whereas the palladium in [Pd(L2)(2)](PF(6))(2) is surrounded by two carbene and two triazole groups with two uncoordinated pyridines. The palladium compounds are highly active for Suzuki-Miyaura cross coupling reactions of aryl bromides and 1,1-dibromo-1-alkenes in neat water under an air atmosphere.

Cooperativity of anion⋯π and π⋯π interactions regulates the self-assembly of a series of carbene proligands: Towards quantitative analysis of intermolecular interactions with Hirshfeld surface

NASA Astrophysics Data System (ADS)

Samanta, Tapastaru; Dey, Lingaraj; Dinda, Joydev; Chattopadhyay, Shyamal Kumar; Seth, Saikat Kumar

2014-06-01

The cooperative effect of weak non-covalent forces between anions and electron deficient aromatics by π⋯π stacking of a series of carbene proligands (1-3) have been thoroughly explored by crystallographic studies. Structural analysis revealed that the anion⋯π and π⋯π interactions along with intermolecular hydrogen bonding mutually cooperate to facilitate the assembling of the supramolecular framework. The π⋯π and corresponding anion⋯π interactions have been investigated in the title carbene proligands despite their association with counter ions. The presence of the anion in the vicinity of the π-system leads to the formation of anion⋯π/π⋯π/π⋯anion network for an inductive stabilization of the assemblies. To assess the dimensionality of the supramolecular framework consolidated by cooperative anion⋯π/π⋯π interactions and hydrogen bonding, different substituent effects in the carbene backbone have been considered to tune these interactions. These facts show that the supramolecular framework based on these cooperative weak forces may be robust enough for application in molecular recognition. The investigation of close intermolecular interactions between the molecules via Hirshfeld surface analyses is presented in order to reveal subtle differences and similarities in the crystal structures. The decomposition of the fingerprint plot area provides a percentage of each intermolecular interaction, allowing for a quantified analysis of close contacts within each crystal.

Synthesis of a Benzodiazepine-derived Rhodium NHC Complex by C-H Bond Activation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bergman, Roberg G.; Gribble, Jr., Michael W.; Ellman, Jonathan A.

2008-01-30

The synthesis and characterization of a Rh(I)-NHC complex generated by C-H activation of 1,4-benzodiazepine heterocycle are reported. This complex constitutes a rare example of a carbene tautomer of a 1,4-benzodiazepine aldimine stabilized by transition metal coordination and demonstrates the ability of the catalytically relevant RhCl(PCy{sub 3}){sub 2} fragment to induce NHC-forming tautomerization of heterocycles possessing a single carbene-stabilizing heteroatom. Implications for the synthesis of benzodiazepines and related pharmacophores via C-H functionalization are discussed.

Building new discrete supramolecular assemblies through the interaction of iso-tellurazole N-oxides with Lewis acids and bases.

PubMed

Ho, Peter C; Jenkins, Hilary A; Britten, James F; Vargas-Baca, Ignacio

2017-10-13

The supramolecular macrocycles spontaneously assembled by iso-tellurazole N-oxides are stable towards Lewis bases as strong as N-heterocyclic carbenes (NHC) but readily react with Lewis acids such as BR 3 (R = Ph, F). The electron acceptor ability of the tellurium atom is greatly enhanced in the resulting O-bonded adducts, which consequently enables binding to a variety of Lewis bases that includes acetonitrile, 4-dimethylaminopyridine, 4,4'-bipyridine, triphenyl phosphine, a N-heterocyclic carbene and a second molecule of iso-tellurazole N-oxide.

Enantioselective copper catalysed intramolecular C-H insertion reactions of α-diazo-β-keto sulfones, α-diazo-β-keto phosphine oxides and 2-diazo-1,3-diketones; the influence of the carbene substituent.

PubMed

Shiely, Amy E; Slattery, Catherine N; Ford, Alan; Eccles, Kevin S; Lawrence, Simon E; Maguire, Anita R

2017-03-22

Enantioselectivities in C-H insertion reactions, employing the copper-bis(oxazoline)-NaBARF catalyst system, leading to cyclopentanones are highest with sulfonyl substituents on the carbene carbon, and furthermore, the impact is enhanced by increased steric demand on the sulfonyl substituent (up to 91%ee). Enantioselective intramolecular C-H insertion reactions of α-diazo-β-keto phosphine oxides and 2-diazo-1,3-diketones are reported for the first time.

Rate and Equilibrium Constants for the Addition of N-Heterocyclic Carbenes into Benzaldehydes: A Remarkable 2-Substituent Effect**

PubMed Central

Collett, Christopher J; Massey, Richard S; Taylor, James E; Maguire, Oliver R; O'Donoghue, AnnMarie C; Smith, Andrew D

2015-01-01

Rate and equilibrium constants for the reaction between N-aryl triazolium N-heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3-(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2-substituent in these reactions and provide insight into the chemoselectivity of cross-benzoin reactions. PMID:25908493

Rate and Equilibrium Constants for the Addition of N‐Heterocyclic Carbenes into Benzaldehydes: A Remarkable 2‐Substituent Effect†

PubMed Central

Collett, Christopher J.; Massey, Richard S.; Taylor, James E.; Maguire, Oliver R.

2015-01-01

Abstract Rate and equilibrium constants for the reaction between N‐aryl triazolium N‐heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3‐(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2‐substituent in these reactions and provide insight into the chemoselectivity of cross‐benzoin reactions. PMID:27478264

An ylide transformation of rhodium(I) carbene: enantioselective three-component reaction through trapping of rhodium(I)-associated ammonium ylides by β-nitroacrylates.

PubMed

Ma, Xiaochu; Jiang, Jun; Lv, Siying; Yao, Wenfeng; Yang, Yang; Liu, Shunying; Xia, Fei; Hu, Wenhao

2014-11-24

The chiral Rh(I)-diene-catalyzed asymmetric three-component reaction of aryldiazoacetates, aromatic amines, and β-nitroacrylates was achieved to obtain γ-nitro-α-amino-succinates in good yields and with high diastereo- and enantioselectivity. This reaction is proposed to proceed through the enantioselective trapping of Rh(I)-associated ammonium ylides by nitroacrylates. This new transformation represents the first example of Rh(I)-carbene-induced ylide transformation. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reactions of Persistent Carbenes with Hydrogen-Terminated Silicon Surfaces.

PubMed

Zhukhovitskiy, Aleksandr V; Mavros, Michael G; Queeney, K T; Wu, Tony; Voorhis, Troy Van; Johnson, Jeremiah A

2016-07-13

Surface passivation has enabled the development of silicon-based solar cells and microelectronics. However, a number of emerging applications require a paradigm shift from passivation to functionalization, wherein surface functionality is installed proximal to the silicon surface. To address this need, we report here the use of persistent aminocarbenes to functionalize hydrogen-terminated silicon surfaces via Si-H insertion reactions. Through the use of model compounds (H-Si(TMS)3 and H-Si(OTMS)3), nanoparticles (H-SiNPs), and planar Si(111) wafers (H-Si(111)), we demonstrate that among different classes of persistent carbenes, the more electrophilic and nucleophilic ones, in particular, a cyclic (alkyl)(amino)carbene (CAAC) and an acyclic diaminocarbene (ADAC), are able to undergo insertion into Si-H bonds at the silicon surface, forming persistent C-Si linkages and simultaneously installing amine or aminal functionality in proximity to the surface. The CAAC (6) is particularly notable for its clean insertion reactivity under mild conditions that produces monolayers with 21 ± 3% coverage of Si(111) atop sites, commensurate with the expected maximum of ∼20%. Atomic force and transmission electron microscopy, nuclear magnetic resonance, X-ray photoelectron, and infrared spectroscopy, and time-of-flight secondary ion mass spectrometry provided evidence for the surface Si-H insertion process. Furthermore, computational studies shed light on the reaction energetics and indicated that CAAC 6 should be particularly effective at binding to silicon dihydride, trihydride, and coupled monohyride motifs, as well as oxidized surface sites. Our results pave the way for the further development of persistent carbenes as universal ligands for silicon and potentially other nonmetallic substrates.

Finding Furfural Hydrogenation Catalysts via Predictive Modelling

PubMed Central

Strassberger, Zea; Mooijman, Maurice; Ruijter, Eelco; Alberts, Albert H; Maldonado, Ana G; Orru, Romano V A; Rothenberg, Gadi

2010-01-01

Abstract We combine multicomponent reactions, catalytic performance studies and predictive modelling to find transfer hydrogenation catalysts. An initial set of 18 ruthenium-carbene complexes were synthesized and screened in the transfer hydrogenation of furfural to furfurol with isopropyl alcohol complexes gave varied yields, from 62% up to >99.9%, with no obvious structure/activity correlations. Control experiments proved that the carbene ligand remains coordinated to the ruthenium centre throughout the reaction. Deuterium-labelling studies showed a secondary isotope effect (kH:kD=1.5). Further mechanistic studies showed that this transfer hydrogenation follows the so-called monohydride pathway. Using these data, we built a predictive model for 13 of the catalysts, based on 2D and 3D molecular descriptors. We tested and validated the model using the remaining five catalysts (cross-validation, R2=0.913). Then, with this model, the conversion and selectivity were predicted for four completely new ruthenium-carbene complexes. These four catalysts were then synthesized and tested. The results were within 3% of the model’s predictions, demonstrating the validity and value of predictive modelling in catalyst optimization. PMID:23193388

Singlet-triplet energy differences in divalent five membered cyclic conjugated Arduengo-type carbenes XC2HN2M (M = C, Si, Ge, Sn, and Pb; X = F, Cl, Br, and I)

NASA Astrophysics Data System (ADS)

Vessally, Esmail; Dehbandi, Behnam; Ahmadi, Elaheh

2016-09-01

Singlet-triplet energy differences in Arduengo-type carbenes XC2HN2C compared and contrasted with their sila, germa, stana and plumba analogues; at B3LYP/6-311++G** level of theory. Free Gibbs energy differences between triplet (t) and singlet (s) states (Δ G(t-s)) change in the following order: plumbylenes > stannylenes > germylenes > silylenes > carbenes. The singlet states in XC2HN2C are generally more stable when the electron withdrawing groups such as-F was used at β-position. However, the singlet states in XC2N2HM (M = Si, Ge, Sn, and Pb) are generally more stable when the withdrawing groups such as-F was placed. The puckering energy is investigated for each the singlet and triplet states. The DFT calculations found the linear correlation to size of the group 14 divalent element (M), the ∠N-M-N angle, and the Δ(LUMO-HOMO) of XC2HN2M.

Synthesis, structure and DFT conformation analysis of CpNiX(NHC) and NiX2(NHC)2 (X = SPh or Br) complexes

NASA Astrophysics Data System (ADS)

Malan, Frederick P.; Singleton, Eric; van Rooyen, Petrus H.; Conradie, Jeanet; Landman, Marilé

2017-11-01

The synthesis, density functional theory (DFT) conformational study and structure analysis of novel two-legged piano stool Ni N-heterocyclic carbene (NHC) complexes and square planar Ni bis-N-heterocyclic carbene complexes, all containing either bromido- or thiophenolato ligands, are described. [CpNi(SPh)(NHC)] complexes were obtained from the neutral 18-electron [CpNiBr(NHC)] complexes by substitution of a bromido ligand with SPh, using NEt3 as a base to abstract the proton of HSPh. The 16-electron biscarbene complexes [Ni(SPh)2{NHC}2] were isolated when an excess of HSPh was added to the reaction mixture. Biscarbene complexes of the type [NiBr2(NHC)2] were obtained in the reaction of NiCp2 with a slight excess of the specific imidazolium bromide salt. The molecular and electronic structures of the mono- and bis-N-heterocyclic carbene complexes have been analysed using single crystal diffraction and density functional theory (DFT) calculations, to give insight into their structural properties.

Synthesis and characterization of Pd(II)-methyl complexes with N-heterocyclic carbene-amine ligands.

PubMed

Warsink, Stefan; de Boer, Sandra Y; Jongens, Lianne M; Fu, Ching-Feng; Liu, Shiuh-Tzung; Chen, Jwu-Ting; Lutz, Martin; Spek, Anthony L; Elsevier, Cornelis J

2009-09-21

A number of palladium(ii) complexes with a heteroditopic NHC-amine ligand and their precursor silver(i) carbene complexes have been efficiently prepared and their structural features have been investigated. The heteroditopic coordination of this ligand class was unequivocally shown by NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere rapid reductive elimination of the acyl-imidazolium salt was observed.

Bimolecular reactions of carbenes: Proton transfer mechanism

NASA Astrophysics Data System (ADS)

Abu-Saleh, Abd Al-Aziz A.; Almatarneh, Mansour H.; Poirier, Raymond A.

2018-04-01

Here we report the bimolecular reaction of trifluoromethylhydroxycarbene conformers and the water-mediated mechanism of the 1,2-proton shift for the unimolecular trans-conformer by using quantum chemical calculations. The CCSD(T)/cc-pVTZ//MP2/cc-pVDZ potential-energy profile of the bimolecular reaction of cis- and trans-trifluoromethylhydroxycarbene, shows the lowest gas-phase barrier height of 13 kJ mol-1 compared to the recently reported value of 128 kJ mol-1 for the unimolecular reaction. We expect bimolecular reactions of carbene's stereoisomers will open a valuable field for new and useful synthetic strategies.

Versatile synthesis of cationic N-heterocyclic carbene-gold(i) complexes containing a second ancillary ligand. Design of heterobimetallic ruthenium-gold anticancer agents.

PubMed

Fernández-Gallardo, Jacob; Elie, Benelita T; Sanaú, Mercedes; Contel, María

2016-02-21

We describe a versatile and quick route to cationic gold(i) complexes containing N-heterocyclic carbenes and a second ancillary ligand (such as phosphanes, phosphites, arsines and amines) of interest for the synthesis of compounds with potential catalytic and medicinal applications. The general synthetic strategy has been applied in the preparation of novel cationic heterobimetallic ruthenium(ii)-gold(i) complexes that are highly cytotoxic to renal cancer Caki-1 and colon cancer HCT 116 cell lines while showing a synergistic effect and being more selective than their monometallic counterparts.

Aminomethylation of enals through carbene and acid cooperative catalysis: concise access to β(2)-amino acids.

PubMed

Xu, Jianfeng; Chen, Xingkuan; Wang, Ming; Zheng, Pengcheng; Song, Bao-An; Chi, Yonggui Robin

2015-04-20

A convergent, organocatalytic asymmetric aminomethylation of α,β-unsaturated aldehydes by N-heterocyclic carbene (NHC) and (in situ generated) Brønsted acid cooperative catalysis is disclosed. The catalytically generated conjugated acid from the base plays dual roles in promoting the formation of azolium enolate intermediate, formaldehyde-derived iminium ion (as an electrophilic reactant), and methanol (as a nucleophilic reactant). This redox-neutral strategy is suitable for the scalable synthesis of enantiomerically enriched β(2) -amino acids bearing various substituents. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Silica gel promotes reductions of aldehydes and ketones by N-heterocyclic carbene boranes.

PubMed

Taniguchi, Tsuyoshi; Curran, Dennis P

2012-09-07

N-Heterocyclic carbene boranes (NHC-boranes) such as 1,3-dimethylimidazol-2-ylidine trihydridoborane (diMe-Imd-BH(3)) serve as practical hydride donors for the reduction of aldehydes and ketones in the presence of silica gel. Primary and secondary alcohols are formed in good yields under ambient conditions. Aldehydes are selectively reduced in the presence of ketones. One, two, or even all three of the boron hydrides can be transferred. The process is attractive because all the components are stable and easy to handle and because both the reaction and isolation procedures are convenient.

Highly Unsaturated Platinum and Palladium Carbenes PtC 3 and PdC 3 Isolated and Characterized in the Gas Phase

DOE PAGES

Bittner, Dror M.; Zaleski, Daniel P.; Tew, David P.; ...

2016-02-16

Carbenes of platinum and palladium, PtC 3 and PdC 3 , were generated in the gas phase through laser vaporization of a metal target in the presence of a low concentration of a hydrocarbon precursor undergoing supersonic expansion. Rotational spectroscopy and abinitio calculations confirm that both molecules are linear. The geometry of PtC 3 was accurately determined by fitting to the experimental moments of inertia of twenty-six isotopologues. In conclusion, the results are consistent with the proposal of an autogenic isolobal relationship between O, Au + , and Ptatoms.

Coordination chemistry and catalytic activity of N-heterocyclic carbene iridium(I) complexes.

PubMed

Fu, Ching-Feng; Chang, Yung-Hung; Liu, Yi-Hong; Peng, Shei-Ming; Elsevier, Cornelis J; Chen, Jwu-Ting; Liu, Shiuh-Tzung

2009-09-21

Iridium complexes [(CO)2Ir(NHC-R)Cl] (R = Et-, 3a; PhCH2-, 3b; CH3OCH2CH2-, 3c; o-CH3OC6H4CH2-, 3d; NHC: N-heterocyclic carbene) are prepared via the carbene transfer from [(NHC-R)W(CO)5] to [Ir(COD)Cl]2. By using substitution with 13CO, we are able to estimate the activation energy (G) of the CO-exchange in 3a-d, which are in the range of 12-13 kcal mol-1, significantly higher than those for the phosphine analog [(CO)2Ir(PCy3)Cl]. Reactions of 3b and 3d with an equimolar amount of PPh3 result in the formation of the corresponding [(NHC-R)Ir(CO)(PPh3)Cl] with the phosphine and NHC in trans arrangement. In contrast, the analogous reaction of 3a or 3c with phosphine undergoes substitution followed by the anion metathesis to yield the corresponding di-substituted [(NHC-R)Ir(CO)(PPh3)2]BF4 (5) directly. Treatment of 3b or 3d with excess of PPh3 leads to the similar product of disubstitution 5b and 5d. The analysis for the IR data of carbonyliridium complexes provides the estimation of electron-donating power of NHCs versus phosphines. The NHC moiety on the iridium center cannot be replaced by phosphines, even 1,2-bis(diphenylphohino)ethane (dppe). All the carbene moieties on the iridium complexes are inert toward sulfur treatment, indicating a strong interaction between NHC and the iridium centers. Complexes 3a-c are active on the catalysis of the oxidative cyclization of 2-(o-aminophenyl)ethanol to yield the indole compound. The phosphine substituted complexes or analogs are less active.

Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetone

NASA Astrophysics Data System (ADS)

Lu, Xiu Hui; Yu, Hai Bin; Wu, Wei Rong; Xu, Yue Hua

Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second-order Møller-Plesset (MP2)/6-31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero-point energy (ZPE) and CCSD(T)/6-31G* single-point calculations. From the PES obtained with the CCSD(T)//MP2/6-31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four-membered ring intermediate, INT2, which is a barrier-free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four-membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier-free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier-free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol.

Long-lived and highly efficient green and blue phosphorescent emitters and device architectures for OLED displays

NASA Astrophysics Data System (ADS)

Eickhoff, Christian; Murer, Peter; Geßner, Thomas; Birnstock, Jan; Kröger, Michael; Choi, Zungsun; Watanabe, Soichi; May, Falk; Lennartz, Christian; Stengel, Ilona; Münster, Ingo; Kahle, Klaus; Wagenblast, Gerhard; Mangold, Hannah

2015-09-01

In this paper, two OLED device concepts are introduced. First, classical phosphorescent green carbene emitters with unsurpassed lifetime, combined with low voltage and high efficiency are presented and the associated optimized OLED stacks are explained. Second, a path towards highly efficient, long-lived deep blue systems is shown. The high efficiencies can be reached by having the charge-recombination on the phosphorescent carbene emitter while at the same time short emissive lifetimes are realized by fast energy transfer to the fluorescent emitter, which eventually allows for higher OLED stability in the deep blue. Device architectures, materials and performance data are presented showing that carbene type emitters have the potential to outperform established phosphorescent green emitters both in terms of lifetime and efficiency. The specific class of green emitters under investigation shows distinctly larger electron affinities (2.1 to 2.5 eV) and ionization potentials (5.6 to 5.8 eV) as compared to the "standard" emitter Ir(ppy)3 (5.0/1.6 eV). This difference in energy levels requires an adopted OLED design, in particular with respect to emitter hosts and blocking layers. Consequently, in the diode setup presented here, the emitter species is electron transporting or electron trapping. For said green carbene emitters, the typical peak wavelength is 525 nm yielding CIE color coordinates of (x = 0.33, y = 0.62). Device data of green OLEDs are shown with EQEs of 26 %. Driving voltage at 1000 cd/m2 is below 3 V. In an optimized stack, a device lifetime of LT95 > 15,000 h (1000 cd/m2) has been reached, thus fulfilling AMOLED display requirements.

Syntheses and structures of ruthenium(II) N,S-heterocyclic carbene diphosphine complexes and their catalytic activity towards transfer hydrogenation.

PubMed

Ding, Nini; Hor, T S Andy

2011-06-06

Phosphine exchange of [Ru(II) Br(MeCOO)(PPh(3))(2)(3-RBzTh)] (3-RBzTh=3-benzylbenzothiazol-2-ylidene) with a series of diphosphines (bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethylene (dppv), 1,1'-bis(diphenylphosphino)ferrocene (dppf), 1,4-bis(diphenylphosphino)butane (dppb), and 1,3-(diphenylphosphino)propane (dppp)) gave mononuclear and neutral octahedral complexes [RuBr(MeCOO)(η(2)-P(2))(3-RBzTh)] (P(2)=dppm (2), dppv (3), dppf (4), dppb (5), or dppp (6)), the coordination spheres of which contained four different ligands, namely, a chelating diphosphine, carboxylate, N,S-heterocyclic carbene (NSHC), and a bromide. Two geometric isomers of 6 (6a and 6b) have been isolated. The structures of these products, which have been elucidated by single-crystal X-ray crystallography, show two structural types, I and II, depending on the relative dispositions of the ligands. Type I structures contain a carbenic carbon atom trans to the oxygen atom, whereas two phosphorus atoms are trans to bromine and oxygen atoms. The type II system comprises a carbene carbon atom trans to one of the phosphorus atoms, whereas the other phosphorus is trans to the oxygen atom, with the bromine trans to the remaining oxygen atom. Complexes 2, 3, 4, and 6a belong to type I, whereas 5 and 6b are of type II. The kinetic product 6b eventually converts into 6a upon standing. These complexes are active towards catalytic reduction of para-methyl acetophenone by 2-propanol at 82 °C under 1% catalyst load giving the corresponding alcohols. The dppm complex 2 shows the good yields (91-97%) towards selected ketones. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Highly Unsaturated Platinum and Palladium Carbenes PtC3 and PdC3 Isolated and Characterized in the Gas Phase

PubMed Central

Bittner, Dror M.; Zaleski, Daniel P.; Tew, David P.

2016-01-01

Abstract Carbenes of platinum and palladium, PtC3 and PdC3, were generated in the gas phase through laser vaporization of a metal target in the presence of a low concentration of a hydrocarbon precursor undergoing supersonic expansion. Rotational spectroscopy and ab initio calculations confirm that both molecules are linear. The geometry of PtC3 was accurately determined by fitting to the experimental moments of inertia of twenty‐six isotopologues. The results are consistent with the proposal of an autogenic isolobal relationship between O, Au+, and Pt atoms. PMID:26879473

N-Heterocyclic carbene metal complexes: photoluminescence and applications.

PubMed

Visbal, Renso; Gimeno, M Concepción

2014-05-21

This review covers the advances made in the synthesis of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands. The presence of a high field strength ligand such as an NHC in the complexes gives rise to high energy emissions, and consequently, to the desired blue colour needed for OLED applications. Furthermore, the great versatility of NHC ligands for structural modifications, together with the use of other ancillary ligands in the complex, provides numerous possibilities for the synthesis of phosphorescent materials, with emission colours over the entire visible spectra and potential future applications in fields such as photochemical water-splitting, chemosensors, dye-sensitised solar cells, oxygen sensors, and medicine.

Mono- and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls.

PubMed

Giner, Elena A; Santiago, Alicia; Gómez-Gallego, Mar; Ramírez de Arellano, Carmen; Poulten, Rebecca C; Whittlesey, Michael K; Sierra, Miguel A

2015-06-01

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)═C═CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,β-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.

Organocatalytic activation of isocyanides: N-heterocyclic carbene-catalyzed enaminone synthesis from ketones† †Electronic supplementary information (ESI) available: Full data for reaction conditions optimizations, detailed experimental procedures, and full characterization of substrates and products. Crystallographic data for compound 3sa. CCDC 1503347. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc05266e Click here for additional data file. Click here for additional data file.

PubMed Central

Kim, Jungwon

2017-01-01

The first example of the use of an N-heterocyclic carbene (NHC) as an organocatalyst for the activation of isocyanides was demonstrated. On the basis of previous reports on the interaction between NHCs and isocyanides, we developed a catalytic cycle involving transient imidoyl intermediate. The reaction of ketones with isocyanides produced the corresponding enaminones with high efficiency. Control experiments suggested a novel role for the carbene in the activation of isocyanides, and a proton transfer process was found to be crucial for the generation of two activated species in the catalytic cycle. Various enaminones, some of which are not easily accessible by other methods, were synthesized in excellent yields. This study clearly demonstrates the potential of the nucleophilic activation of isocyanides in the expansion of their reactivity scope. PMID:28451346

Cyclometalated Iridium(III) Carbene Phosphors for Highly Efficient Blue Organic Light-Emitting Diodes.

PubMed

Chen, Zhao; Wang, Liqi; Su, Sikai; Zheng, Xingyu; Zhu, Nianyong; Ho, Cheuk-Lam; Chen, Shuming; Wong, Wai-Yeung

2017-11-22

Five deep blue carbene-based iridium(III) phosphors were synthesized and characterized. Interestingly, one of them can be fabricated into deep blue, sky blue and white organic light-emitting diodes (OLEDs) through changing the host materials and exciton blocking layers. These deep and sky blue devices exhibit Commission Internationale de l'Éclairage (CIE) coordinates of (0.145, 0.186) and (0.152, 0.277) with external quantum efficiency (EQE) of 15.2% and 9.6%, respectively. The EQE of the deep blue device can be further improved up to 19.0% by choosing a host with suitable energy level of its lowest unoccupied molecular orbital (LUMO).

Synthesis of novel synthetic intermediates from the reaction of benzimidazole and triazole carbenes with ketenimines and their application in the construction of spiro-pyrroles.

PubMed

Mo, Jun-Ming; Ma, Yang-Guang; Cheng, Ying

2009-12-07

2-(2-Alkoxycarbonyl-1-arylamino-1-propenyl)benzimidazolium and 5-(2-alkoxycarbonyl-1-arylamino-1-propenyl)triazolium salts were synthesized in good yields from the reaction of benzimidazole and triazole carbenes with ketenimines. Upon treatment with a base, both salts were converted into novel 1,3-dipoles which underwent [3+2] cycloaddition reactions with electron-deficient alkynes and allenes to produce benzimidazole-spiro-pyrroles or triazole-spiro-pyrroles. This work provides novel synthons for the construction of multifunctional spiro-pyrrole derivatives that are not easy accessible by other synthetic methods and are potentially amenable to further transformations.

A two-color fluorogenic carbene complex for tagging olefins via metathesis reaction

NASA Astrophysics Data System (ADS)

Wirtz, Marcel; Grüter, Andreas; Heib, Florian; Huch, Volker; Zapp, Josef; Herten, Dirk-Peter; Schmitt, Michael; Jung, Gregor

2015-12-01

We describe a fluorogenic ruthenium (II) carbene complex in which the chromophore is directly connected to the metal center. The compound introduces a boron dipyrromethene (BODIPY) moiety into target double bonds by metathesis. Tagging of terminal double bonds is demonstrated on immobilized styrene units on a glass surface. We also show that two compounds with distinguishable fluorescence properties are formed in the model reaction with styrene. The outcome of the metathesis reaction is characterized by 19F-NMR, optical spectroscopy, and, finally, single-molecule trajectories. This labeling scheme, in our perception, is of particular interest in the fields of interfacial science and biorthogonal ligation in combination with super-resolution imaging.

Hydrofluorination of Alkynes Catalysed by Gold Bifluorides.

PubMed

Nahra, Fady; Patrick, Scott R; Bello, Davide; Brill, Marcel; Obled, Alan; Cordes, David B; Slawin, Alexandra M Z; O'Hagan, David; Nolan, Steven P

2015-01-01

We report the synthesis of nine new N -heterocyclic carbene gold bifluoride complexes starting from the corresponding N -heterocyclic carbene gold hydroxides. A new methodology to access N,N' -bis(2,6-diisopropylphenyl)imidazol-2-ylidene gold(I) fluoride starting from N,N' -bis(2,6-diisopropylphenyl)imidazol-2-ylidene gold(I) hydroxide and readily available potassium bifluoride is also reported. These gold bifluorides were shown to be efficient catalysts in the hydrofluorination of symmetrical and unsymmetrical alkynes, thus affording fluorinated stilbene analogues and fluorovinyl thioethers in good to excellent yields with high stereo- and regioselectivity. The method is exploited further to access a fluorinated combretastatin analogue selectively in two steps starting from commercially available reagents.

Remote modulation of singlet-triplet gaps in carbenes

NASA Astrophysics Data System (ADS)

Alkorta, Ibon; Montero-Campillo, M. Merced; Elguero, José

2018-02-01

The modulation of the singlet-triplet (S/T) gap of phenyl-carbene derivatives by hydrogen bond formation has been studied using the G4(MP2) computational method. The complexation of the aromatic ring substituents (-NH2, -OH, -PH2, -SH) in meta- and para-positions with water and the protonation or deprotonation of such groups have a remarkable influence on the S/T gaps, reaching S/T gap variations from 25.7 to 93.7 kJ mol-1. This variation is linearly related to the binding energy difference of the S/T configurations. Importantly, the triplet and singlet electronic configurations are systematically favored in the protonated and deprotonated forms, respectively, in all cases.

Tandem enyne metathesis-Diels-Alder reaction for construction of natural product frameworks.

PubMed

Rosillo, Marta; Domínguez, Gema; Casarrubios, Luis; Amador, Ulises; Pérez-Castells, Javier

2004-03-19

Enynes connected through aromatic rings are used as substrates for metathesis reactions. The reactivity of three ruthenium carbene complexes is compared. The resulting 1,3-dienes are suitable precursors of polycyclic structures via a Diels-Alder process. Some domino RCM-Diels-Alder reactions are performed, suggesting a possible beneficial effect of the ruthenium catalyst in the cycloaddition process. Other examples require Lewis acid cocatalyst. When applied to aromatic ynamines or enamines, a new synthesis of vinylindoles is achieved. Monitorization of several metathesis reactions with NMR shows the different behavior for ruthenium catalysts. New carbenic species are detected in some reactions with an important dependence on the solvent used.

N-Heterocyclic Carbenes as Promotors for the Rearrangement of Phosphaketenes to Phosphaheteroallenes: A Case Study for OCP to OPC Constitutional Isomerism.

PubMed

Li, Zhongshu; Chen, Xiaodan; Benkő, Zoltán; Liu, Liu; Ruiz, David A; Peltier, Jesse L; Bertrand, Guy; Su, Chen-Yong; Grützmacher, Hansjörg

2016-05-10

The concept of isomerism is essential to chemistry and allows defining molecules with an identical composition but different connectivity (bonds) between their atoms (constitutional isomers) and/or a different arrangement in space (stereoisomers). The reaction of phosphanyl ketenes, (NHP)-P=C=O (NHP=N-heterocyclic phosphenium) with N-heterocyclic carbenes (NHCs) leads to phosphaheteroallenes (NHP)-O-P=C=NHC in which the PCO unit has been isomerized to OPC. Based on the isolation of several intermediates and DFT calculations, a mechanism for this fundamental isomerisation process is proposed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nucleophilic addition of amines to ruthenium carbenes: ortho-(alkynyloxy)benzylamine cyclizations towards 1,3-benzoxazines.

PubMed

González-Rodríguez, Carlos; Suárez, José Ramón; Varela, Jesús A; Saá, Carlos

2015-02-23

A new ruthenium-catalyzed cyclization of ortho-(alkynyloxy)benzylamines to dihydro-1,3-benzoxazines is reported. The cyclization is thought to take place via the vinyl ruthenium carbene intermediates which are easily formed from [Cp*RuCl(cod)] and N2 CHSiMe3 . The mild reaction conditions and the efficiency of the procedure allow the easy preparation of a broad range of new 2-vinyl-2-substituted 1,3-benzoxazine derivatives. Rearrangement of an internal C(sp) in the starting material into a tetrasubstituted C(sp(3) ) atom in the final 1,3-benzoxazine is highly remarkable. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

N-heterocyclic carbene metal complexes as bio-organometallic antimicrobial and anticancer drugs.

PubMed

Patil, Siddappa A; Patil, Shivaputra A; Patil, Renukadevi; Keri, Rangappa S; Budagumpi, Srinivasa; Balakrishna, Geetha R; Tacke, Matthias

2015-01-01

Late transition metal complexes that bear N-heterocyclic carbene (NHC) ligands have seen a speedy growth in their use as both, metal-based drug candidates and potentially active homogeneous catalysts in a plethora of C-C and C-N bond forming reactions. This review article focuses on the recent developments and advances in preparation and characterization of NHC-metal complexes (metal: silver, gold, copper, palladium, nickel and ruthenium) and their biomedical applications. Their design, syntheses and characterization have been reviewed and correlated to their antimicrobial and anticancer efficacies. All these initial discoveries help validate the great potential of NHC-metal derivatives as a class of effective antimicrobial and anticancer agents.

Structure and spectroscopic properties of the dimeric copper(I) N-heterocyclic carbene complex [Cu₂(CNC(t-Bu))₂](PF₆)₂.

PubMed

Riener, Korbinian; Pöthig, Alexander; Cokoja, Mirza; Herrmann, Wolfgang A; Kühn, Fritz E

2015-08-01

In recent years, the use of copper N-heterocyclic carbene (NHC) complexes has expanded to fields besides catalysis, namely medicinal chemistry and luminescence applications. In the latter case, multinuclear copper NHC compounds have attracted interest, however, the number of these complexes in the literature is still quite limited. Bis[μ-1,3-bis(3-tert-butylimidazolin-2-yliden-1-yl)pyridine]-1κ(4)C(2),N:N,C(2');2κ(4)C(2),N:N,C(2')-dicopper(I) bis(hexafluoridophosphate), [Cu2(C19H25N5)2](PF6)2, is a dimeric copper(I) complex bridged by two CNC, i.e. bis(N-heterocyclic carbene)pyridine, ligands. Each Cu(I) atom is almost linearly coordinated by two NHC ligands and interactions are observed between the pyridine N atoms and the metal centres, while no cuprophilic interactions were observed. Very strong absorption bands are evident in the UV-Vis spectrum at 236 and 274 nm, and an emission band is observed at 450 nm. The reported complex is a new example of a multinuclear copper NHC complex and a member of a compound class which has only rarely been reported.

Oriented and covalent immobilization of target molecules to solid supports: synthesis and application of a light-activatable and thiol-reactive cross-linking reagent.

PubMed

Collioud, A; Clémence, J F; Sänger, M; Sigrist, H

1993-01-01

Light-dependent oriented and covalent immobilization of target molecules has been achieved by combining two modification procedures: light-dependent coupling of target molecules to inert surfaces and thiol-selective reactions occurring at macromolecule or substrate surfaces. For immobilization purposes the heterobifunctional reagent N-[m-[3-(trifluoromethyl)diazirin-3-yl]phenyl]-4-maleimidobutyr amide was synthesized and chemically characterized. The photosensitivity of the carbene-generating reagent and its reactivity toward thiols were ascertained. Light-induced cross-linking properties of the reagent were documented (i) by reacting first the maleimide function with a thiolated surface, followed by carbene insertion into applied target molecules, (ii) by photochemical coupling of the reagent to an inert support followed by thermochemical reactions with thiol functions, and (iii) by thermochemical modification of target molecules prior to carbene-mediated insertion into surface materials. Procedures mentioned led to light-dependent covalent immobilization of target molecules including amino acids, a synthetic peptide, and antibody-derived F(ab') fragments. Topically selective, light-dependent immobilization was attained with the bifunctional reagent by irradiation of coated surfaces through patterned masks. Glass and polystyrene served as substrates. Molecular orientation is asserted by inherently available or selectively introduced terminal thiol functions in F(ab') fragments and synthetic polypeptides, respectively.

In Silico Olefin Metathesis with Ru-Based Catalysts Containing N-Heterocyclic Carbenes Bearing C60 Fullerenes.

PubMed

Martínez, Juan Pablo; Vummaleti, Sai Vikrama Chaitanya; Falivene, Laura; Nolan, Steven P; Cavallo, Luigi; Solà, Miquel; Poater, Albert

2016-05-04

Density functional theory calculations have been used to explore the potential of Ru-based complexes with 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) ligand backbone (A) being modified in silico by the insertion of a C60 molecule (B and C), as olefin metathesis catalysts. To this end, we investigated the olefin metathesis reaction catalyzed by complexes A, B, and C using ethylene as the substrate, focusing mainly on the thermodynamic stability of all possible reaction intermediates. Our results suggest that complex B bearing an electron-withdrawing N-heterocyclic carbene improves the performance of unannulated complex A. The efficiency of complex B is only surpassed by complex A when the backbone of the N-heterocyclic carbene of complex A is substituted by two amino groups. The particular performance of complexes B and C has to be attributed to electronic factors, that is, the electronic-donating capacity of modified SIMes ligand rather than steric effects, because the latter are predicted to be almost identical for complexes B and C when compared to those of A. Overall, this study indicates that such Ru-based complexes B and C might have the potential to be effective olefin metathesis catalysts. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Quantum and Statistical Mechanics Applied to Singlet Carbenes, Pericyclic Reactions, and Condensed Phase Phenomena

NASA Astrophysics Data System (ADS)

Evanseck, Jeffrey Donald

The completed research covers a broad range of theoretical applications in organic chemistry. It is divided into three chapters which covers the chemistry of singlet carbenes (Chapter 1), substituent effects in pericyclic rearrangements (Chapter 2), and the effects of solvent on the reactivity of organic reactions (Chapter 3). The selectivity between 1,2- and 1,4-intramolecular additions to restricted diene systems has been investigated. A decrease in activation energy for the intramolecular cycloaddition is noted for systems which approach the idealized geometry found with intermolecular addition of carbenes to olefins. Direct substitution at the carbene site dramatically effects the predicted activation barriers for 1,2-hydrogen shifts. An excellent correlation between the activation energy and a substituents sigma_sp {rm R}{rm o} parameters has been demonstrated. The long standing problem of orbital alignment influences on the selectivity of 1,2-hydrogen arrangements shows significant geometric distortions, yet has little influence on the rates of singlet alkylcarbene rearrangements. The exo-selectivities observed for 1,2-shifts in rigid systems are explained by torsional and steric interactions which develop in the transition structures. Substituent effects on pericyclic reactions have been computed for several conrotatory and disrotatory electrocyclizations. The six-electron disrotatory electrocyclization of 1-substituted hexatrienes displays a strong electronic component in determining stereoselectivity, despite incredible steric interference. The eight-electron conrotatory electrocyclization transition structure of 1-substituted octatetraene has an unusual helical transition structure which does not differentiate between substituent position. The effects of solvents on the acidity differences between E and Z esters has supplemented earlier ab initio quantum mechanical results on the enhanced acidity of Meldrum's acid. Monte Carlo simulations predict a preferential stabilization of the E isomer in both acetonitrile and aqueous solutions. The rates of intramolecular Diels-Alder reactions are compared to recent experimental work and predictions of different solvent systems are made.

Diazo compounds and N-tosylhydrazones: novel cross-coupling partners in transition-metal-catalyzed reactions.

PubMed

Xiao, Qing; Zhang, Yan; Wang, Jianbo

2013-02-19

Transition-metal-catalyzed carbene transformations and cross-couplings represent two major reaction types in organometallic chemistry and organic synthesis. However, for a long period of time, these two important areas have evolved separately, with essentially no overlap or integration. Thus, an intriguing question has emerged: can cross-coupling and metal carbene transformations be merged into a single reaction cycle? Such a combination could facilitate the development of novel carbon-carbon bond-forming methodologies. Although this concept was first explored about 10 years ago, rapid developments inthis area have been achieved recently. Palladium catalysts can be used to couple diazo compounds with a wide variety of organic halides. Under oxidative coupling conditions, diazo compounds can also react with arylboronic acids and terminal alkynes. Both of these coupling reactions form carbon-carbon double bonds. As the key step in these catalytic processes, Pd carbene migratory insertion plays a vital role in merging the elementary steps of Pd intermediates, leading to novel carbon-carbon bond formations. Because the diazo substrates can be generated in situ from N-tosylhydrazones in the presence of base, the N-tosylhydrazones can be used as reaction partners, making this type of cross-coupling reaction practical in organic synthesis. N-Tosylhydrazones are easily derived from the corresponding aldehydes or ketones. The Pd-catalyzed cross-coupling of N-tosylhydrazones is considered a complementary reaction to the classic Shapiro reaction for converting carbonyl functionalities into carbon-carbon double bonds. It can also serve as an alternative approach for the Pd-catalyzed cross-coupling of carbonyl compounds, which is usually achieved via triflates. The combination of carbene formation and cross-coupling in a single catalytic cycle is not limited to Pd-catalyzed reactions. Recent studies of Cu-, Rh-, Ni-, and Co-catalyzed cross-coupling reactions with diazo compounds or N-tosylhydrazones show that these transformations also work with other transition metals, demonstrating the generality of the diazo compounds as new cross-coupling partners in transition-metal-catalyzed coupling reactions.

Theoretical investigations on the mechanism of benzoin condensation catalyzed by pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene.

PubMed

He, Yunqing; Xue, Ying

2011-03-03

A new annulated N-heterocyclic carbene (NHC), pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene, has been synthesized and its good catalytic activity for benzoin condensation has been experimentally determined by You and co-workers recently [ Ma , Y. J. , Wei , S. P. , Lan , J. B. , Wang , J. Z. , Xie , R. G. , and You , J. S. J. Org. Chem. 2008 , 73 , 8256 ]. In this work, the mechanism of the title reaction has been intensively studied computationally by employing the density functional theory (B3LYP) method in conjunction with 6-31+G(d) and 6-311+G(2d,p) basis sets. Our results indicate that path A (in which a sequence of intermolecular proton transfers between two carbene/benzaldehyde coupling intermediates affords enamine) and path B (in which a t-BuOH assisted hydrogen transfer generates enamine) proposed on the basis of the Breslow mechanism are competitive for their similar barriers. In path A, the first intermolecular proton transfer between two N-heterocyclic carbene/benzaldehyde coupled intermediates to form tertiary alcohol and enolate anion is theoretically the rate-determining step with corresponding barrier (30.93 kcal/mol), while the t-BuOH assisted hydrogen transfer generating Breslow enamine is the rate-determining step with corresponding barrier (28.84 kcal/mol) in path B. The coupling of carbene and benzaldehyde, and the coupling of enamine and another benzaldehyde to form a C-C bond are partially rate-determining for their relatively significant barriers (24.06 and 26.95 kcal/mol, respectively), being the same in both paths A and B. Our results are in nice agreement with the experimental result in a kinetic investigation of thiazolium ion-catalyzed benzoin condensation performed by White and Leeper in 2001.

Synthesis and characterization of new N-heterocyclic carbene ligands: 1,3-Bis(acetamide)imidazol-3-ium bromide and 3-(acetamide)-1-(3-aminopropyl)-1H-imidazol-3-ium bromide

NASA Astrophysics Data System (ADS)

Turkyilmaz, Murat; Uluçam, Gühergül; Aktaş, Şaban; Okan, S. Erol

2017-05-01

Two new pincer type N-heterocyclic carbene ligands were synthesized. The compounds were characterized by FTIR, NMR (1H, 13C) GC-MS and elemental analyses. They were also both modelled by DFT calculations as the crystal structure of 1,3-bis(acetamide)imidazol-3-ium bromide was determined by XRD which is an orthorhombic system with space group P21212. The structural analyses in gas phase were realized by comparing the experimental NMR and IR spectra with those of the theoretical calculations. In vitro biological activities of the molecules were determined and found that one of them exhibits significant cytotoxic activity.

Synthesis of palladium(0) and -(II) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation.

PubMed

Sluijter, Soraya N; Warsink, Stefan; Lutz, Martin; Elsevier, Cornelis J

2013-05-28

A transmetallation route, using silver(I) precursors, to several zero- and di-valent palladium complexes with chelating bis(N-heterocyclic carbene) ligands bearing various N-substituents has been established. The resulting complexes have been characterized by NMR and mass spectroscopy. In addition, the structure of a representative compound, [Pd(0)(bis-(Mes)NHC)(η(2)-ma)] (3a), was confirmed by X-ray crystal structure determination. In contrast to the transfer semihydrogenation, in which only low activity was observed, complex 3a showed activity (TOF = 49 mol(sub) mol(cat)(-1) h(-1)) and selectivity comparable to its monodentate counterparts in the semihydrogenation of 1-phenyl-1-propyne with molecular hydrogen.

N-Heterocyclic-Carbene-Treated Gold Surfaces in Pentacene Organic Field-Effect Transistors: Improved Stability and Contact at the Interface.

PubMed

Lv, Aifeng; Freitag, Matthias; Chepiga, Kathryn M; Schäfer, Andreas H; Glorius, Frank; Chi, Lifeng

2018-04-16

N-Heterocyclic carbenes (NHCs), which react with the surface of Au electrodes, have been successfully applied in pentacene transistors. With the application of NHCs, the charge-carrier mobility of pentacene transistors increased by five times, while the contact resistance at the pentacene-Au interface was reduced by 85 %. Even after annealing the NHC-Au electrodes at 200 °C for 2 h before pentacene deposition, the charge-carrier mobility of the pentacene transistors did not decrease. The distinguished performance makes NHCs as excellent alternatives to thiols as metal modifiers for the application in organic field-effect transistors (OFETs). © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

A multi-target caffeine derived rhodium(i) N-heterocyclic carbene complex: evaluation of the mechanism of action.

PubMed

Zhang, Jing-Jing; Muenzner, Julienne K; Abu El Maaty, Mohamed A; Karge, Bianka; Schobert, Rainer; Wölfl, Stefan; Ott, Ingo

2016-08-16

A rhodium(i) and a ruthenium(ii) complex with a caffeine derived N-heterocyclic carbene (NHC) ligand were biologically investigated as organometallic conjugates consisting of a metal center and a naturally occurring moiety. While the ruthenium(ii) complex was largely inactive, the rhodium(i) NHC complex displayed selective cytotoxicity and significant anti-metastatic and in vivo anti-vascular activities and acted as both a mammalian and an E. coli thioredoxin reductase inhibitor. In HCT-116 cells it increased the reactive oxygen species level, leading to DNA damage, and it induced cell cycle arrest, decreased the mitochondrial membrane potential, and triggered apoptosis. This rhodium(i) NHC derivative thus represents a multi-target compound with promising anti-cancer potential.

A general access to organogold(iii) complexes by oxidative addition of diazonium salts.

PubMed

Huang, Long; Rominger, Frank; Rudolph, Matthias; Hashmi, A Stephen K

2016-05-11

At room temperature under mild photochemical conditions, namely irradiation with a simple blue light LED, gold(i) chloro complexes of both phosphane and carbene ligands in combination with aryldiazonium salts afford arylgold(iii) complexes. With chelating P,N-ligands cationic six- or five-membered chelate complexes were isolated in the form of salts with weakly coordinating counter anions that were brought in from the diazonium salt. With monodentate P ligands or N-heterocyclic carbene ligands and diazonium chlorides neutral arylgold(iii) dichloro complexes were obtained. The coordination geometry was determined by X-ray crystal structure analyses of representative compounds, a cis arrangement of the aryl and the phosphane ligand at the square planar gold(iii) center is observed.

Ruthenium(II) carbonyl complexes bearing CCC-pincer bis-(carbene) ligands: synthesis, structures and activities toward recycle transfer hydrogenation reactions.

PubMed

Naziruddin, Abbas Raja; Huang, Zhao-Jiunn; Lai, Wei-Chih; Lin, Wan-Jung; Hwang, Wen-Shu

2013-09-28

A new series of ruthenium(II) carbonyl complexes with benzene-based CCC-pincer bis-(carbene) ligands, [((R)CCC(R))Ru(CO)2(X)](0/+) and [((R)CCC(R))Ru(CO)(NN)](+) ((R)CCC(R) = 2,6-bis-(1-alkylimidazolylidene)benzene, R = Me or (n)Bu; X = I, Br, CH3CN, or 6-(aminomethyl)pyridine (ampy); NN = 2·CH3CN, or chelating ampy or bipyridine), was synthesized and fully characterized. X-Ray structure determinations revealed that these eight complexes have pseudo-octahedral configurations around the ruthenium center with the pincer ligand occupying three meridional sites. These complexes prove to be efficient precatalysts demonstrating very good activity and reusability for the transfer hydrogenation of ketones.

Mechanism of UVA-dependent DNA damage induced by an antitumor drug dacarbazine in relation to its photogenotoxicity.

PubMed

Iwamoto, Takuya; Hiraku, Yusuke; Okuda, Masahiro; Kawanishi, Shosuke

2008-03-01

It has been reported that dacarbazine (DTIC) is photogenotoxic. The purpose of this study is to clarify the mechanism of photogenotoxicity induced by DTIC. We examined DNA damage induced by UVA-irradiated DTIC using 32P-5'-end-labeled DNA fragments obtained from human genes. Formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in calf thymus DNA was measured by high performance liquid chromatograph with an electrochemical detector. Electron spin resonance (ESR) spin-trapping experiments were performed to detect radical species generated from UVA-irradiated DTIC. UVA-irradiated DTIC caused DNA damage at guanine residues, especially at the 5'-GGT-3' sequence in the presence of Cu(II) and also induced 8-oxodG generation in calf thymus DNA. DTIC-induced photodamage to DNA fragments was partially inhibited by catalase, whereas 8-oxodG formation was significantly increased by catalase. NaN3, a carbene scavenger, inhibited DNA damage and 8-oxodG formation in a dose-dependent manner, suggesting that carbene intermediates are involved. The ESR spin-trapping experiments demonstrated the generation of aryl radicals in the process of photodegradation of DTIC. Photoactivated DTIC generates the carbene and aryl radicals, which may induce both DNA adduct and 8-oxodG formation, resulting in photogenotoxicity. This study could provide an insight into the safe usage of DTIC.

On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene.

PubMed

Holzmeier, Fabian; Fischer, Ingo; Kiendl, Benjamin; Krueger, Anke; Bodi, Andras; Hemberger, Patrick

2016-04-07

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C3H2, and the heat of formation of the C3H radical and ion derived by the dissociative ionization of the carbene. Vacuum ultraviolet (VUV) synchrotron radiation as provided by the Swiss Light Source and imaging photoelectron photoion coincidence (iPEPICO) were employed. Cyclopropenylidene was generated by pyrolysis of a quadricyclane precursor in a 1 : 1 ratio with benzene, which enabled us to derive the carbene's near threshold absolute photoionization cross section from the photoionization yield of the two pyrolysis products and the known cross section of benzene. The cross section at 9.5 eV, for example, was determined to be 4.5 ± 1.4 Mb. Upon dissociative ionization the carbene decomposes by hydrogen atom loss to the linear isomer of C3H(+). The appearance energy for this process was determined to be AE(0K)(c-C3H2; l-C3H(+)) = 13.67 ± 0.10 eV. The heat of formation of neutral and cationic C3H was derived from this value via a thermochemical cycle as Δ(f)H(0K)(C3H) = 725 ± 25 kJ mol(-1) and Δ(f)H(0K)(C3H(+)) = 1604 ± 19 kJ mol(-1), using a previously reported ionization energy of C3H.

Coordination chemistry of highly hemilabile bidentate sulfoxide N-heterocyclic carbenes with palladium(II).

PubMed

Yu, Kuo-Hsuan; Wang, Chia-Ching; Chang, I-Hsin; Liu, Yi-Hung; Wang, Yu; Elsevier, Cornelis J; Liu, Shiuh-Tzung; Chen, Jwu-Ting

2014-12-01

Imidazolium salts, [RS(O)-CH2 (C3 H3 N2 )Mes]Cl (R=Me (L1a), Ph (L1b)); Mes=mesityl), make convenient carbene precursors. Palladation of L1a affords the monodentate dinuclear complex, [(PdCl2 {MeS(O)CH2 (C3 H2 N2 )Mes})2 ] (2a), which is converted into trans-[PdCl2 (NHC)2] (trans-4a; N-heterocyclic carbene) with two rotamers in anti and syn configurations. Complex trans-4a can isomerize into cis-4a(anti) at reflux in acetonitrile. Abstraction of chlorides from 4a or 4b leads to the formation of a new dication: trans-[Pd{RS(O)CH2(C3H2N2)Mes}2](PF6)2 (R=Me (5a), Ph (5b)). The X-ray structure of 5a provides evidence that the two bidentate SO-NHC ligands at palladium(II) are in square-planar geometry. Two sulfoxides are sulfur- and oxygen-bound, and constitute five- and six-membered chelate rings with the metal center, respectively. In acetonitrile, complexes 5a or 5b spontaneously transform into cis-[Pd(NHC)2(NCMe)2](PF6)2. Similar studies of thioether-NHCs have also been examined for comparison. The results indicate that sulfoxides are more labile than thioethers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

IR spectroscopy as a source of data on bond strengths

NASA Astrophysics Data System (ADS)

Finkelshtein, E. I.; Shamsiev, R. S.

2018-02-01

The aim of this work is the estimation of double bond strength, namely Cdbnd O bonds in ketones and aldehydes and Cdbnd C bonds in various compounds. By the breaking of these bonds one or both fragments formed are carbenes, for which experimental data on the enthalpies of formation (ΔHf298) are scarce. Thus for the estimation of ΔHf298 of the corresponding carbenes, the empirical equations were proposed based on different approximations. In addition, a quantum chemical calculations of the ΔHf298 values of carbenes were performed, and the data obtained were compared with experimental values and the results of earlier calculations. Equations for the calculation of Cdbnd O bond strengths of different ketones and aldehydes from the corresponding stretching frequencies ν(Cdbnd O) were derived. Using the proposed equations, the strengths of Cdbnd O bonds of 25 ketones and 12 conjugated aldehydes, as well as Cdbnd C bonds of 13 hydrocarbons and 7 conjugated aldehydes were estimated for the first time. Linear correlations of Cdbnd C and Cdbnd O bond strengths with the bond lengths were established, and the equations permitting the estimation of the double bond strengths and lengths with acceptable accuracy were obtained. Also, the strength of central Cdbnd C bond of stilbene was calculated for the first time. The uncertainty of the strengths of double bonds obtained may be regarded as accurate ±10-15 kJ/mol.

Breslow Intermediates from Aromatic N-Heterocyclic Carbenes (Benzimidazolin-2-ylidenes, Thiazolin-2-ylidenes).

PubMed

Berkessel, Albrecht; Paul, Mathias; Sudkaow, Panyapon; Wessels, Alina; Schlörer, Nils E; Neudörfl, Jörg M

2018-04-12

We report the first generation and characterization of the elusive Breslow intermediates derived from aromatic N-heterocyclic carbenes (NHCs), namely benzimidazolin-2-ylidenes (NMR, X-ray) and thiazolin-2-ylidenes (NMR). In the former case, the diaminoenols were generated by reaction of the free N,N-bis-Dipp- and N,N-bis-Mes-benzimidazolin-2-ylidenes with aldehydes, while the dimer of 3,4,5-trimethylthiazolin-2-ylidene served as the starting material in the latter case. The unambiguous NMR-identification of the first thiazolin-2-ylidene based Breslow intermediate rests on double 13C labeling of both the NHC and the aldehyde component. Acyl anion reactivity was proven by benzoin formation with excess aldehyde. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis of Water-Soluble Palladium Nanoparticles Stabilized by Sulfonated N-Heterocyclic Carbenes.

PubMed

Asensio, Juan M; Tricard, Simon; Coppel, Yannick; Andrés, Román; Chaudret, Bruno; de Jesús, Ernesto

2017-09-27

A strategy involving the decomposition of palladium(II) organometallic complexes with sulfonated N-heterocyclic carbene ligands leads to the formation of stable and water-soluble Pd nanoparticles. Three different methodologies (thermal decomposition, reduction under 13 CO atmosphere, and reduction with H 2 ) gave particles with different shapes and sizes, ranging from 1.5 to 7 nm. The structures of the organometallic intermediates and organic decomposition products were elucidated by NMR spectroscopy. To check the accessibility of the surface, the nanoparticles were tested as catalysts for the chemoselective hydrogenation of styrene in water. An effect of the particle size on the catalyst activity was observed. The aqueous phase was recycled up to ten times without any precipitation of metallic palladium. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Cyclic alkyl amino carbene (CAAC) ruthenium complexes as remarkably active catalysts for ethenolysis

DOE PAGES

Marx, Vanessa M.; Sullivan, Alexandra H.; Melaimi, Mohand; ...

2014-12-17

In this paper, an expanded family of ruthenium-based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed-oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100,000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340 000, at a catalyst loading of only 1 ppm. Finally, this is the first time a series of metathesis catalysts has exhibited such high performance in cross-metathesis reactions employing ethylene gas, withmore » activities sufficient to render ethenolysis applicable to the industrial-scale production of linear α-olefins (LAOs) and other terminal-olefin products.« less

The antimicrobial efficacy of sustained release silver–carbene complex-loaded l-tyrosine polyphosphate nanoparticles: Characterization, in vitro and in vivo studies

PubMed Central

Hindi, Khadijah M.; Ditto, Andrew J.; Panzner, Matthew J.; Medvetz, Douglas A.; Han, Daniel S.; Hovis, Christine E.; Hilliard, Julia K.; Taylor, Jane B.; Yun, Yang H.; Cannon, Carolyn L.; Youngs, Wiley J.

2009-01-01

The pressing need to treat multi-drug resistant bacteria in the chronically infected lungs of cystic fibrosis (CF) patients has given rise to novel nebulized antimicrobials. We have synthesized a silver–carbene complex (SCC10) active against a variety of bacterial strains associated with CF and chronic lung infections. Our studies have demonstrated that SCC10-loaded into l-tyrosine polyphosphate nanoparticles (LTP NPs) exhibits excellent antimicrobial activity in vitro and in vivo against the CF relevant bacteria Pseudomonas aeruginosa. Encapsulation of SCC10 in LTP NPs provides sustained release of the antimicrobial over the course of several days translating into efficacious results in vivo with only two administered doses over a 72 h period. PMID:19395021

Theoretical investigation on the chemoselective N-heterocyclic carbene-catalyzed cross-benzoin reactions.

PubMed

Liu, Tao; Han, Shu-Min; Han, Ling-Li; Wang, Lu; Cui, Xiang-Yang; Du, Chong-Yang; Bi, Siwei

2015-03-28

A density functional theory study was performed to understand the detailed mechanisms of the cross-benzoin reactions catalyzed by N-heterocyclic carbene (NHC) species. Our theoretical study predicted that the first H-transfer operates with water in solution as a mediator, and the second H-transfer undergoes a concerted mechanism rather than a stepwise one. In addition, the chemoselectivity of the reactions studied in this work has been explored. P1 was obtained as a major product mainly due to the more stable intermediate formed by reaction of NHC with reactant R1. Different steric effects resulting from the fused six-membered ring in transition state TS7 and the fused five-membered ring in transition state TS13 are the origin leading to the chemoselectivity.

Computational Investigations of Potential Energy Function Development for Metal--Organic Framework Simulations, Metal Carbenes, and Chemical Warfare Agents

NASA Astrophysics Data System (ADS)

Cioce, Christian R.

Metal-Organic Frameworks (MOFs) are three-dimensional porous nanomaterials with a variety of applications, including catalysis, gas storage and separation, and sustainable energy. Their potential as air filtration systems is of interest for designer carbon capture materials. The chemical constituents (i.e. organic ligands) can be functionalized to create rationally designed CO2 sequestration platforms, for example. Hardware and software alike at the bleeding edge of supercomputing are utilized for designing first principles-based molecular models for the simulation of gas sorption in these frameworks. The classical potentials developed herein are named PHAST --- Potentials with High Accuracy, Speed, and Transferability, and thus are designed via a "bottom-up" approach. Specifically, models for N2 and CH4 are constructed and presented. Extensive verification and validation leads to insights and range of applicability. Through this experience, the PHAST models are improved upon further to be more applicable in heterogeneous environments. Given this, the models are applied to reproducing high level ab initio energies for gas sorption trajectories of helium atoms in a variety of rare-gas clusters, the geometries of which being representative of sorption-like environments commonly encountered in a porous nanomaterial. This work seeks to push forward the state of classical and first principles materials modeling. Additionally, the characterization of a new type of tunable radical metal---carbene is presented. Here, a cobalt(II)---porphyrin complex, [Co(Por)], was investigated to understand its role as an effective catalyst in stereoselective cyclopropanation of a diazoacetate reagent. Density functional theory along with natural bond order analysis and charge decomposition analysis gave insight into the electronics of the catalytic intermediate. The bonding pattern unveiled a new class of radical metal---carbene complex, with a doublet cobalt into which a triplet carbene sigma donates, and subsequent back-bonding occurs into a pi* antibonding orbital. This is a different type of interaction not seen in the three existing classes of metal-carbene complexes, namely Fischer, Schrock, and Grubbs. Finally, the virtual engineering of enhanced chemical warfare agent (CWA) detection systems is discussed. As part of a U.S. Department of Defense supported research project, in silico chemical modifications to a previously synthesized zinc-porphyrin, ZnCS1, were made to attempt to achieve preferential binding of the nerve agent sarin versus its simulant, DIMP (diisopropyl methylphosphonate). Upon modification, a combination of steric effects and induced hydrogen bonding allowed for the selective binding of sarin. The success of this work demonstrates the role that high performance computing can play in national security research, without the associated costs and high security required for experimentation.

Oxygenation of ruthenium carbene complexes containing naphthothiophene or naphthofuran: spectroscopic and DFT studies.

PubMed

Tsai, Fu-Yuan; Lo, Ji-Xian; Hsu, Hsin-Tzu; Lin, Ying-Chih; Huang, Shou-Ling; Wang, Ju-Chun; Liu, Yi-Hong

2013-11-01

The aryl propargylic alcohol 1-[2-(thiophen-3-yl)phenyl]prop-2-yn-1-ol (1a) is readily prepared from 2-(thiophen-3-yl)benzaldehyde. In the presence of visible light, treatment of 1a with one-half mole equivalent of [Ru]Cl ([Ru]=Cp(dppe)Ru) (dppe=1,2-bis(diphenylphosphino)ethane) and NH4PF6 in O2 affords the naphtha[2,1-b]thiophene-4-carbaldehyde (4a) in high yields. The cyclization reaction of 1a proceeds through the formation of the carbene complex 2a that contains the naphtha[2,1-b]thiophene ring, which is isolated in a 1:1 stoichiometric reaction. The C-C bond formation between the inner carbon of the terminal triple bond and the heterocyclic ring is confirmed by structure determination of 2a using single-crystal X-ray diffraction analysis. Facile oxygenation of 2a by O2 yields the aldehyde product 4a accompanied by the formation of phosphine oxide of dppe. Oxygen is most likely activated by coordination to the ruthenium center when one PPh2 unit of the dppe ligand dissociates. This dissociated PPh2 unit then reacts with the coordinated oxygen nearby to generate half-oxidized dppe ligand and an unobserved oxo-carbene intermediate. Coupling of the oxo/carbene ligands followed by demetalation then yields 4a. Presumably the resulting complex with the half-oxidized dppe ligand continuously promotes cyclization/oxygenation of 1a to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of 4a and the corresponding esters 5a or 5a'. Four other aryl propargylic alcohols 1b-e, which contain thiophen-2-yl, isopropenyl, fur-3-yl, and fur-2-yl, respectively, on the aryl ring are also prepared. Analogous aldehydes 4b-e are similarly prepared from 1b-e, respectively. For oxygenations of 1b, 1d, and 1e in alcohol, mixtures of aldehyde 4, ester 5, and acetal 8 are obtained. The carbene complex 2b obtained from 1b was also characterized by single-crystal X-ray diffraction analysis. The UV/Vis spectra of 2a and 2b consist of absorption bands with a high extinction coefficient. From DFT calculations on 2a and 2b, the visible light is found to populate the LUMO antibonding orbital of mainly Ru=C bonds, thereby weakening the Ru=C bond and promoting the oxygenation/demetalation reactions of 2. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A Perspective on Physical Organic Chemistry

PubMed Central

2015-01-01

A perspective on the development of mechanistic carbene chemistry is presented. The author will point out questions that have been answered, and a next generation of questions will be proposed. PMID:24571434

N‐Heterocyclic Carbene Self‐assembled Monolayers on Copper and Gold: Dramatic Effect of Wingtip Groups on Binding, Orientation and Assembly

PubMed Central

Larrea, Christian R.; Narouz, Mina R.; Mosey, Nicholas J.; Horton, J. Hugh; Crudden, Cathleen M.

2017-01-01

Abstract Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of E des=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing. PMID:28960768

Surface-active ionic liquids for palladium-catalysed cross coupling in water: effect of ionic liquid concentration on the catalytically active species† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra07757b

PubMed Central

Taskin, Meltem; Cognigni, Alice; Zirbs, Ronald; Reimhult, Erik

2017-01-01

We report the design and synthesis of surface-active ionic liquids for application in palladium-catalyzed cross coupling reactions. A series of dodecylimidazolium-based ionic liquids were applied as additives in the Heck reaction of ethyl acrylate and iodobenzene, and high yields of >90% could be obtained in water without the addition of further ligands. Our results indicate that the ionic liquid concentration in water is the key factor affecting the formation of the catalytically active species and hence the yield. Moreover, imidazolium-based ionic liquids that are able to form a carbene species differ significantly from conventional cationic surfactants, as a concentration dependent formation of the N-heterocyclic carbene complex was observed. PMID:29308189

The microwave spectrum of a triplet carbene: HCCN in the X 3Sigma - state

NASA Astrophysics Data System (ADS)

Saito, Shuji; Endo, Yasuki; Hirota, Eizi

1984-02-01

A simple carbene, the HCCN radical, has been identified in the gas phase using a microwave spectroscopic method. The HCCN molecule was generated in a free space absorption cell by the reaction of CH3CN with the microwave discharge products of CF4. Five rotational transitions, each split into three fine structure components, were observed in the region of 110 to 198 GHz. No hyperfine structure was resolved, although some of the observed lines showed broadening. The rotational constant, the centrifugal distortion constant, the spin-spin coupling constant, and the spin-rotation coupling constant were determined with good precision. The observed spectrum is completely consistent with that expected for a linear molecule in a 3Σ state, in agreement with an earlier matrix EPR study of Bernheim et al. [J. Chem. Phys. 43, 196 (1965)].

NHC carbene supported half-sandwich hydridosilyl complexes of ruthenium: the impact of supporting ligands on Si∙∙∙H interligand interactions.

PubMed

Mai, Van Hung; Kuzmina, Lyudmila G; Churakov, Andrei V; Korobkov, Ilia; Howard, Judith A K; Nikonov, Georgii I

2016-01-07

Reaction of complex [CpRu(pyr)3][PF6] (3) with the NHC carbene IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) results in the NHC complex [Cp(IPr)Ru(pyr)2][PF6] (4), which was studied by NMR specroscopy and X-ray diffraction analysis. Reaction of [Cp(IPr)Ru(pyr)2][PF6] (4) with LiAlH4 leads to the trihydride Cp(IPr)RuH3 (5) characterised by spectroscopic methods. Heating compound 5 with hydrosilanes gives the dihydrido silyl derivatives Cp(IPr)RuH2(SiR3) (6). Systematic X-ray diffraction studies suggest that complexes 6 have stronger interligand Si∙∙∙H interactions than the isolobal phosphine complexes Cp(Pr3P)RuH2(SiR3).

Carbene based photochemical molecular assemblies for solar driven hydrogen generation.

PubMed

Peuntinger, Katrin; Pilz, T David; Staehle, Robert; Schaub, Markus; Kaufhold, Simon; Petermann, Lydia; Wunderlin, Markus; Görls, Helmar; Heinemann, Frank W; Li, Jing; Drewello, Thomas; Vos, Johannes G; Guldi, Dirk M; Rau, Sven

2014-09-28

Novel photocatalysts based on ruthenium complexes with NHC (N-heterocyclic carbene)-type bridging ligands have been prepared and structurally and photophysically characterised. The identity of the NHC-unit of the bridging ligand was established unambiguously by means of X-ray structural analysis of a heterodinuclear ruthenium-silver complex. The photophysical data indicate ultrafast intersystem crossing into an emissive and a non-emissive triplet excited state after excitation of the ruthenium centre. Exceptionally high luminescence quantum yields of up to 39% and long lifetimes of up to 2 μs are some of the triplet excited state characteristics. Preliminary studies into the visible light driven photocatalytic hydrogen formation show no induction phase and constant turnover frequencies that are independent on the concentration of the photocatalyst. In conclusion this supports the notion of a stable assembly under photocatalytic conditions.

Cu/Mn bimetallic catalysis enables carbonylative Suzuki-Miyaura coupling with unactivated alkyl electrophiles.

PubMed

Pye, Dominic R; Cheng, Li-Jie; Mankad, Neal P

2017-07-01

A bimetallic system consisting of Cu-carbene and Mn-carbonyl co-catalysts was employed for carbonylative C-C coupling of arylboronic esters with alkyl halides, allowing for the convergent synthesis of ketones. The system operates under mild conditions and exhibits complementary reactivity to Pd catalysis. The method is compatible with a wide range of arylboronic ester nucleophiles and proceeds smoothly for both primary and secondary alkyl iodide electrophiles. Preliminary mechanistic experiments corroborate a hypothetical catalytic mechanism consisting of co-dependent cycles wherein the Cu-carbene co-catalyst engages in transmetallation to generate an organocopper nucleophile, while the Mn-carbonyl co-catalyst activates the alkyl halide electrophile by single-electron transfer and then undergoes reversible carbonylation to generate an acylmanganese electrophile. The two cycles then intersect with a heterobimetallic, product-releasing C-C coupling step.

Bimetallic d10 -Metal Complexes of a Bipyridine Substituted N-Heterocyclic Carbene.

PubMed

Kaub, Christoph; Lebedkin, Sergei; Li, Alina; Kruppa, Sebastian V; Strebert, Patrick H; Kappes, Manfred M; Riehn, Christoph; Roesky, Peter W

2018-04-20

The hybrid ligand 3-(2,2'-bipyridine-6-ylmethyl)-1-mesityl-1H-imidazolylidene (NHC Bipy ) featuring both carbene and N-donor sites, was selectively complexed with various d 10 metal cations in order to examine its coordination behavior with regard to homo and heterometallic structures. Respective silver complexes can be obtained by the silver oxide route and are suitable transmetallation reagents for the synthesis of gold(I) compounds. Starting from the mononuclear complexes [(NHC Bipy )AuCl], [(NHC Bipy )Au(C 6 F 5 )] and [(NHC Bipy ) 2 Au][ClO 4 ], open-chain as well as cyclic heterobimetallic complexes containing Cu + , Ag + , Zn 2+ , Cd 2+ , and Hg 2+ were synthesized. Furthermore, the homobimetallic species [(NHC Bipy ) 2 M 2 ][ClO 4 ] 2 (M=Cu, Ag) were obtained. All bimetallic compounds were fully characterized including single-crystal X-ray analysis. Their photoluminescence (PL) properties were investigated in the solid state at temperatures between 15 and 295 K and compared with those of the mononuclear species. There is a clear difference in PL properties between the open chain and the cyclic heterobimetallic complexes. The latter species show different PL properties, depending on the metals involved. In addition, collision-induced dissociation (CID) experiments were performed on electrosprayed cations of the cyclic heterobimetallic compounds, to compare the metal binding at the carbene and N-donor sites. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Water-soluble platinum nanoparticles stabilized by sulfonated N-heterocyclic carbenes: influence of the synthetic approach.

PubMed

Baquero, Edwin A; Tricard, Simon; Coppel, Yannick; Flores, Juan C; Chaudret, Bruno; de Jesús, Ernesto

2018-03-28

The synthesis of metal nanoparticles (NPs) under controlled conditions in water remains a challenge in nanochemistry. Two different approaches to obtain platinum NPs, which involve the treatment of aqueous solutions of preformed sulfonated (NHC)Pt(ii) dimethyl complexes with carbon monoxide, and of (NHC)Pt(0) diolefin complexes with dihydrogen (NHC = N-heterocyclic carbene), are disclosed here. The resulting NPs were found to be highly stable in water under air for an indefinite time period. Coordination of the NHC ligands to the platinum surface via the carbenic carbon was monitored by solid-state NMR spectroscopy, and the presence of a platinum-carbon bond was unambiguously evidenced by the determination of a 13 C- 195 Pt coupling constant (1106 and 1050 Hz for NPs containing 13 C labeled-NHC ligands and prepared under CO and H 2 , respectively). The coordination of CO to the (NHC)Pt(ii) precursors prior to formation of the NPs was confirmed by NMR spectroscopy. When using a disulfonated NHC ligand, a second coordination sphere containing bis(NHC)Pt(ii) complexes is described. Under CO, the formation of NPs was found to be slower than in a previously reported thermal method (Angew. Chem., Int. Ed., 2014, 53, 13220-13224), but led to NPs of similar sizes, whereas under H 2 , the synthesis of platinum NPs progressed even more slowly and produced larger NPs. In addition to the influence of the synthetic approach, the present study highlights the importance of ligand design for NP stabilization.

Bond Energies and Thermochemical Properties of Ring-Opened Diradicals and Carbenes of exo-Tricyclo[5.2.1.0(2,6)]decane.

PubMed

Hudzik, Jason M; Castillo, Álvaro; Bozzelli, Joseph W

2015-09-24

Exo-tricyclo[5.2.1.0(2,6)]decane (TCD) or exo-tetrahydrodicyclopentadiene is an interesting strained ring compound and the single-component high-energy density hydrocarbon fuel known as JP-10. Important initial reactions of TCD at high temperatures could cleave a strained carbon-carbon (C-C) bond in the ring system creating diradicals also constrained by the remaining ring system. This study determines the thermochemical properties of these diradicals (TCD-H2 mJ-nJ where m and n correspond to the cleaved carbons sites) including the carbon-carbon bond dissociation energy (C-C BDE) corresponding to the cleaved TCD site. Thermochemical properties including enthalpies (ΔH°f298), entropies (S(T)), heat capacities (Cp(T)), and C-H and C-C BDEs for the parent (TCD-H2 m-n), radical (TCD-H2 mJ-n and m-nJ), diradical (TCD-H2 mJ-nJ), and carbene (TCD-H2 mJJ-n and m-nJJ) species are determined. Structures, vibrational frequencies, moments of inertia, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) level of theory. Standard enthalpies of formation in the gas phase for the TCD-H2 m-n parent and radical species are determined using the B3LYP density functional theory and the higher level G3MP2B3 and CBS-QB3 composite methods. For singlet and triplet TCD diradicals and carbenes, M06-2X, ωB97X-D, and CCSD(T) methods are included in the analysis to determine ΔH°f298 values. The C-C BDEs are further calculated using CASMP2(2,2)/aug-cc-pvtz//CASSCF(2,2)/cc-pvtz and with the CASMP2 energies extrapolated to the complete basis set limit. The bond energies calculated with these methods are shown to be comparable to the other calculation methods. Isodesmic work reactions are used for enthalpy analysis of these compounds for effective cancelation of systematic errors arising from ring strain. C-C BDEs range from 77.4 to 84.6 kcal mol(-1) for TCD diradical singlet species. C-H BDEs for the parent TCD-H2 m-n carbon sites range from 93 to 101 kcal mol(-1) with a similar range seen for loss of the second hydrogen to generate the diradical singlet species. A wider range for C-C BDEs is seen for the carbenes from about 77 to 100 kcal mol(-1) as compared to the diradicals. Results from the DFT methods for the parents, radicals, diradicals, and carbenes are in good agreement with results from the composite methods using our sets of work reactions.

Synthesis of the first radiolabeled 188Re N-heterocyclic carbene complex and initial studies on its potential use in radiopharmaceutical applications

PubMed Central

Wagner, Thomas; Zeglis, Brian M.; Groveman, Sam; Hille, Claudia; Pöthig, Alexander; Francesconi, Lynn C.; Herrmann, Wolfgang A.; Kühn, Fritz E.; Reiner, Thomas

2015-01-01

A novel approach towards the synthesis of radiolabeled organometallic rhenium complexes is presented. We successfully synthesized and analyzed the first 188Re-labeled N-heterocyclic biscarbene complex, trans-dioxobis(1,1′-methylene-bis(3,3′-diisopropylimidazolium-2-ylidene))188rhenium(V) hexafluorophosphate (188Re-4) via transmetalation using an air-stable and moisture-stable silver(I) biscarbene complex. In order to assess the viability of this complex as a potential lead structure for in vivo applications, the stability of the 188Re-NHC complex was tested in physiologically relevant media. Ultimately, our studies illustrate that the complex we synthesized dissociates rapidly and is therefore unsuitable for use in radiopharmaceuticals. However, it is clear that the transmetalation approach we have developed is a rapid, robust, and mild method for the synthesis of new 188Re-labeled carbene complexes. PMID:24889257

Ballbot-type motion of N-heterocyclic carbenes on gold surfaces

NASA Astrophysics Data System (ADS)

Wang, Gaoqiang; Rühling, Andreas; Amirjalayer, Saeed; Knor, Marek; Ernst, Johannes Bruno; Richter, Christian; Gao, Hong-Jun; Timmer, Alexander; Gao, Hong-Ying; Doltsinis, Nikos L.; Glorius, Frank; Fuchs, Harald

2017-02-01

Recently, N-heterocyclic carbenes (NHCs) were introduced as alternative anchors for surface modifications and so offered many attractive features, which might render them superior to thiol-based systems. However, little effort has been made to investigate the self-organization process of NHCs on surfaces, an important aspect for the formation of self-assembled monolayers (SAMs), which requires molecular mobility. Based on investigations with scanning tunnelling microscopy and first-principles calculations, we provide an understanding of the microscopic mechanism behind the high mobility observed for NHCs. These NHCs extract a gold atom from the surface, which leads to the formation of an NHC-gold adatom complex that displays a high surface mobility by a ballbot-type motion. Together with their high desorption barrier this enables the formation of ordered and strongly bound SAMs. In addition, this mechanism allows a complementary surface-assisted synthesis of dimeric and hitherto unknown trimeric NHC gold complexes on the surface.

Ballbot-type motion of N-heterocyclic carbenes on gold surfaces.

PubMed

Wang, Gaoqiang; Rühling, Andreas; Amirjalayer, Saeed; Knor, Marek; Ernst, Johannes Bruno; Richter, Christian; Gao, Hong-Jun; Timmer, Alexander; Gao, Hong-Ying; Doltsinis, Nikos L; Glorius, Frank; Fuchs, Harald

2017-02-01

Recently, N-heterocyclic carbenes (NHCs) were introduced as alternative anchors for surface modifications and so offered many attractive features, which might render them superior to thiol-based systems. However, little effort has been made to investigate the self-organization process of NHCs on surfaces, an important aspect for the formation of self-assembled monolayers (SAMs), which requires molecular mobility. Based on investigations with scanning tunnelling microscopy and first-principles calculations, we provide an understanding of the microscopic mechanism behind the high mobility observed for NHCs. These NHCs extract a gold atom from the surface, which leads to the formation of an NHC-gold adatom complex that displays a high surface mobility by a ballbot-type motion. Together with their high desorption barrier this enables the formation of ordered and strongly bound SAMs. In addition, this mechanism allows a complementary surface-assisted synthesis of dimeric and hitherto unknown trimeric NHC gold complexes on the surface.

Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons

DOE PAGES

Lu, Haipeng; Brutchey, Richard L.

2017-01-23

Here we present a new toolset of precursors for semiconductor nanocrystal synthesis, N-heterocyclic carbene (NHC)-metal halide complexes, which enables a tunable molecular platform for the preparation of coinage metal chalcogenide quantum dots (QDs). Phase-pure and highly monodisperse coinage metal chalcogenide (Ag 2E, Cu 2-xE; E = S, Se) QDs are readily synthesized from the direct reaction of an NHC-MBr synthon (where M = Ag, Cu) with alkylsilyl chalcogenide reagents at room temperature. We demonstrate that the size of the resulting QDs is well tailored by the electron-donating ability of the L-type NHC ligands, which are further confirmed to be themore » only organic capping ligands on the QD surface, imparting excellent colloidal stability. Local superstructures of the NHC-capped Ag 2S QDs are observed by TEM, further demonstrating their potential for synthesizing monodisperse ensembles and mediating self-assembly.« less

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism

PubMed Central

Bai, Da-Chang; Yu, Fei-Le; Wang, Wan-Ying; Chen, Di; Li, Hao; Liu, Qing-Rong; Ding, Chang-Hua; Chen, Bo; Hou, Xue-Long

2016-01-01

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of β-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a (syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. Mechanistic studies by both experiments and density functional theory (DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism—nucleophilic attack of enolate oxygen on Palladium followed by C–C bond-forming [3,3']-reductive elimination. PMID:27283477

Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lu, Haipeng; Brutchey, Richard L.

Here we present a new toolset of precursors for semiconductor nanocrystal synthesis, N-heterocyclic carbene (NHC)-metal halide complexes, which enables a tunable molecular platform for the preparation of coinage metal chalcogenide quantum dots (QDs). Phase-pure and highly monodisperse coinage metal chalcogenide (Ag 2E, Cu 2-xE; E = S, Se) QDs are readily synthesized from the direct reaction of an NHC-MBr synthon (where M = Ag, Cu) with alkylsilyl chalcogenide reagents at room temperature. We demonstrate that the size of the resulting QDs is well tailored by the electron-donating ability of the L-type NHC ligands, which are further confirmed to be themore » only organic capping ligands on the QD surface, imparting excellent colloidal stability. Local superstructures of the NHC-capped Ag 2S QDs are observed by TEM, further demonstrating their potential for synthesizing monodisperse ensembles and mediating self-assembly.« less

Carbene footprinting accurately maps binding sites in protein-ligand and protein-protein interactions

NASA Astrophysics Data System (ADS)

Manzi, Lucio; Barrow, Andrew S.; Scott, Daniel; Layfield, Robert; Wright, Timothy G.; Moses, John E.; Oldham, Neil J.

2016-11-01

Specific interactions between proteins and their binding partners are fundamental to life processes. The ability to detect protein complexes, and map their sites of binding, is crucial to understanding basic biology at the molecular level. Methods that employ sensitive analytical techniques such as mass spectrometry have the potential to provide valuable insights with very little material and on short time scales. Here we present a differential protein footprinting technique employing an efficient photo-activated probe for use with mass spectrometry. Using this methodology the location of a carbohydrate substrate was accurately mapped to the binding cleft of lysozyme, and in a more complex example, the interactions between a 100 kDa, multi-domain deubiquitinating enzyme, USP5 and a diubiquitin substrate were located to different functional domains. The much improved properties of this probe make carbene footprinting a viable method for rapid and accurate identification of protein binding sites utilizing benign, near-UV photoactivation.

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bai, Da -Chang; Yu, Fei -Le; Wang, Wan -Ying

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of beta-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a ( syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. As a result, mechanisticmore » studies by both experiments and density functional theory ( DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism-nucleophilic attack of enolate oxygen on Palladium followed by C-C bond-forming [3,3']-reductive elimination.« less

Palladium/N-heterocyclic carbene catalysed regio and diastereoselective reaction of ketones with allyl reagents via inner-sphere mechanism

DOE PAGES

Bai, Da -Chang; Yu, Fei -Le; Wang, Wan -Ying; ...

2016-06-10

The palladium-catalysed allylic substitution reaction is one of the most important reactions in transition-metal catalysis and has been well-studied in the past decades. Most of the reactions proceed through an outer-sphere mechanism, affording linear products when monosubstituted allyl reagents are used. Here, we report an efficient Palladium-catalysed protocol for reactions of beta-substituted ketones with monosubstituted allyl substrates, simply by using N-heterocyclic carbene as ligand, leading to branched products with up to three contiguous stereocentres in a ( syn, anti)-mode with excellent regio and diastereoselectivities. The scope of the protocol in organic synthesis has been examined preliminarily. As a result, mechanisticmore » studies by both experiments and density functional theory ( DFT) calculations reveal that the reaction proceeds via an inner-sphere mechanism-nucleophilic attack of enolate oxygen on Palladium followed by C-C bond-forming [3,3']-reductive elimination.« less

Selenophene transition metal complexes

DOE Office of Scientific and Technical Information (OSTI.GOV)

White, Carter James

1994-07-27

This research shows that selenophene transition metal complexes have a chemistry that is similar to their thiophene analogs. Selenophene coordination has been demonstrated and confirmed by molecular structure in both the η 5- and the η 1(Se)-coordination modes. The reaction chemistry of selenophene complexes closely resembles that of the analogous thiophene complexes. One major difference, however, is that selenophene is a better donor ligand than thiophene making the selenophene complexes more stable than the corresponding thiophene complexes. The 77Se NMR chemical shift values for selenophene complexes fall within distinct regions primarily depending on the coordination mode of the selenophene ligand.more » In the final paper, the C-H bond activation of η 1(S)-bound thiophenes, η 1(S)-benzothiophene and η 1(Se)-bound selenophenes has been demonstrated. The deprotonation and rearrangement of the η 1(E)-bound ligand to the carbon bound L-yl complex readily occurs in the presence of base. Reprotonation with a strong acid gives a carbene complex that is unreactive towards nucleophilic attack at the carbene carbon and is stable towards exposure to air. The molecular structure of [Cp(NO)(PPh 3)Re(2-benzothioenylcarbene)]O 3SCF 3 was determined and contains a Re-C bond with substantial double bond character. Methyl substitution for the thienylcarbene or selenylcarbene gives a carbene that rearranges thermally to give back the η 1(E)-bound complex. Based on these model reactions, a new mechanism for the H/D exchange of thiophene over the hydrodesulfurization catalyst has been proposed.« less

Nitrenes, carbenes, diradicals, and ylides. Interconversions of reactive intermediates.

PubMed

Wentrup, Curt

2011-06-21

Rearrangements of aromatic and heteroaromatic nitrenes and carbenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene or carbene to a seven-membered ring ketenimine, carbodiimide, or allene (that is, a cycloheptatetraene or an azacycloheptatetraene when a nitrogen is involved). Over the last few years, we have found that two types of ring opening take place as well. Type I is an ylidic ring opening that yields nitrile ylides or diazo compounds as transient intermediates. Type II ring opening produces either dienylnitrenes (for example, from 2-pyridylnitrenes) or 1,7-(1,5)-diradicals (such as those formed from 2-quinoxalinylnitrenes), depending on which of these species is better stabilized by resonance. In this Account, we describe our achievements in elucidating the nature of the ring-opened species and unraveling the connections between the various reactive intermediates. Both of these ring-opening reactions are found, at least in some cases, to dominate the subsequent chemistry. Examples include the formation of ring-opened ketenimines and carbodiimides, as well as the ring contraction reactions that form five-membered ring nitriles (such as 2- and 3-cyanopyrroles from pyridylnitrenes, N-cyanoimidazoles from 2-pyrazinyl and 4-pyrimidinylnitrenes, N-cyanopyrazoles from 2-pyrimidinylnitrenes and 3-pyridazinylnitrenes, and so forth). The mechanisms of formation of the open-chain and ring-contraction products were unknown at the onset of this study. In the course of our investigation, several reactions with three or more consecutive reactive intermediates have been unraveled, such as nitrene, seven-membered cyclic carbodiimide, and open-chain nitrile ylide. It has been possible in some cases to observe them all and determine their interrelationships by means of a combination of matrix-isolation spectroscopy, photochemistry, flash vacuum thermolysis, and computational chemistry. These studies have led to a deeper understanding of the nature of reactive intermediates and chemical reactivity. Moreover, the results indicate new directions for further exploration: ring-opening reactions of carbenes, nitrenes, and cyclic cumulenes can be expected in many other systems.

Towards a global model of spin-orbit coupling in the halocarbenes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nyambo, Silver; Karshenas, Cyrus; Reid, Scott A., E-mail: scott.reid@marquette.edu, E-mail: dawesr@mst.edu

We report a global analysis of spin-orbit coupling in the mono-halocarbenes, CH(D)X, where X = Cl, Br, and I. These are model systems for examining carbene singlet-triplet energy gaps and spin-orbit coupling. Over the past decade, rich data sets collected using single vibronic level emission spectroscopy and stimulated emission pumping spectroscopy have yielded much information on the ground vibrational level structure and clearly demonstrated the presence of perturbations involving the low-lying triplet state. To model these interactions globally, we compare two approaches. First, we employ a diabatic treatment of the spin-orbit coupling, where the coupling matrix elements are written inmore » terms of a purely electronic spin-orbit matrix element which is independent of nuclear coordinates, and an integral representing the overlap of the singlet and triplet vibrational wavefunctions. In this way, the structures, harmonic frequencies, and normal mode displacements from ab initio calculations were used to calculate the vibrational overlaps of the singlet and triplet state levels, including the full effects of Duschinsky mixing. These calculations have allowed many new assignments to be made, particularly for CHI, and provided spin-orbit coupling parameters and values for the singlet-triplet gaps. In a second approach, we have computed and fit full geometry dependent spin-orbit coupling surfaces and used them to compute matrix elements without the product form approximation. Those matrix elements were used in similar fits varying the anharmonic constants and singlet-triplet gap to reproduce the experimental levels. The derived spin-orbit parameters for carbenes CHX (X = Cl, Br, and I) show an excellent linear correlation with the atomic spin-orbit constant of the corresponding halogen, indicating that the spin-orbit coupling in the carbenes is consistently around 14% of the atomic value.« less

Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel N-Heterocyclic Carbene-Isoquinoline Complex

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thoi, VanSara; Kornienko, Nick; Margarit, C

2013-06-07

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene amine ligands that exhibit high selectivity and activity formore » the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni(Prbimiq1)]2+ (1c, where Prbimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (Ecat = 1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s1, respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion.« less

N-Heterocyclic Carbene Capture by Cytochrome P450 3A4

PubMed Central

Jennings, Gareth K.; Ritchie, Caroline M.; Shock, Lisa S.; Lyons, Charles E.

2016-01-01

Cytochrome P450 3A4 (CYP3A4) is the dominant P450 enzyme involved in human drug metabolism, and its inhibition may result in adverse interactions or, conversely, favorably reduce the systemic elimination rates of poorly bioavailable drugs. Herein we describe a spectroscopic investigation of the interaction of CYP3A4 with N-methylritonavir, an analog of ritonavir, widely used as a pharmacoenhancer. In contrast to ritonavir, the binding affinity of N-methylritonavir for CYP3A4 is pH-dependent. At pH <7.4, the spectra are definitively type I, whereas at pH ≥7.4 the spectra have split Soret bands, including a red-shifted component characteristic of a P450-carbene complex. Variable-pH UV-visible spectroscopy binding studies with molecular fragments narrows the source of this pH dependence to its N-methylthiazolium fragment. The C2 proton of this group is acidic, and variable-pH resonance Raman spectroscopy tentatively assigns it a pKa of 7.4. Hence, this fragment of N-methylritonavir is expected to be readily deprotonated under physiologic conditions to yield a thiazol-2-ylidene, which is an N-heterocyclic carbene that has high-affinity for and is presumed to be subsequently captured by the heme iron. This mechanism is supported by time-dependent density functional theory with an active site model that accurately reproduces distinguishing features of the experimental UV-visible spectra of N-methylritonavir bound to CYP3A4. Finally, density functional theory calculations support that this novel interaction is as strong as the tightest-binding azaheterocycles found in P450 inhibitors and could offer new avenues for inhibitor development. PMID:27126611

Efficient Covalent Bond Formation in Gas-Phase Peptide-Peptide Ion Complexes with the Photoleucine Stapler

NASA Astrophysics Data System (ADS)

Shaffer, Christopher J.; Andrikopoulos, Prokopis C.; Řezáč, Jan; Rulíšek, Lubomír; Tureček, František

2016-04-01

Noncovalent complexes of hydrophobic peptides GLLLG and GLLLK with photoleucine (L*) tagged peptides G(L* n L m )K (n = 1,3, m = 2,0) were generated as singly charged ions in the gas phase and probed by photodissociation at 355 nm. Carbene intermediates produced by photodissociative loss of N2 from the L* diazirine rings underwent insertion into X-H bonds of the target peptide moiety, forming covalent adducts with yields reaching 30%. Gas-phase sequencing of the covalent adducts revealed preferred bond formation at the C-terminal residue of the target peptide. Site-selective carbene insertion was achieved by placing the L* residue in different positions along the photopeptide chain, and the residues in the target peptide undergoing carbene insertion were identified by gas-phase ion sequencing that was aided by specific 13C labeling. Density functional theory calculations indicated that noncovalent binding to GL*L*L*K resulted in substantial changes of the (GLLLK + H)+ ground state conformation. The peptide moieties in [GL*L*LK + GLLLK + H]+ ion complexes were held together by hydrogen bonds, whereas dispersion interactions of the nonpolar groups were only secondary in ground-state 0 K structures. Born-Oppenheimer molecular dynamics for 100 ps trajectories of several different conformers at the 310 K laboratory temperature showed that noncovalent complexes developed multiple, residue-specific contacts between the diazirine carbons and GLLLK residues. The calculations pointed to the substantial fluidity of the nonpolar side chains in the complexes. Diazirine photochemistry in combination with Born-Oppenheimer molecular dynamics is a promising tool for investigations of peptide-peptide ion interactions in the gas phase.

Heterogeneous substitution effects in chlorocyanomethyl radical and chlorocyanocarbene.

PubMed

Khuseynov, Dmitry; Dixon, Andrew R; Goebbert, Daniel J; Sanov, Andrei

2013-10-17

We report a photoelectron-imaging investigation of the chlorocyanomethyl radical (CHClCN) and the corresponding carbene (CClCN). The results are discussed in comparison with the corresponding dichloro- and dicyano-substituted species, focusing on the divergent effects of the halogen and pseudohalogen (CN) substitutions. A cooperative (captodative) interaction of the π-donor Cl and π-acceptor cyano groups favors the increased stability of the CHClCN radical, but a competition of the two substituents is observed in the singlet-triplet splitting of the carbene. The vertical detachment energy (VDE) of CHClCN(-) is determined to be 2.39 ± 0.04 eV, with the broad photoelectron band consistent with the significant geometry change predicted by theory for the detachment transition. The adiabatic electron affinity of CHClCN, EA = 1.86 ± 0.08 eV, is estimated on the basis of the experimental VDE and the computed difference between the VDE and EA values. This result allows the calculation of the bond dissociation energy of chloroacetonitrile, DH298(H-CHClCN) = 87.0 ± 2.7 kcal/mol. Photoelectron imaging of CClCN(-) reveals two main transitions, assigned to the singlet ((1)A') and triplet ((3)A″) states of the CClCN carbene. The respective VDEs are 2.76 ± 0.05 and 3.25 ± 0.05 eV. The experimental results are in good agreement with the theoretically predicted singlet-triplet vertical energy gap at the anion geometry, but inconclusive with regard to the adiabatic singlet-triplet splitting in CClCN. Consistent with the experimental findings, ab initio calculations using the spin-flip approach in combination with the coupled-cluster theory, indicate that the (1)A' and (3)A″ states are nearly degenerate, with the singlet state lying adiabatically only ∼0.01 eV below the triplet.

Cu/Mn bimetallic catalysis enables carbonylative Suzuki–Miyaura coupling with unactivated alkyl electrophiles† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sc01170a Click here for additional data file.

PubMed Central

Pye, Dominic R.; Cheng, Li-Jie

2017-01-01

A bimetallic system consisting of Cu-carbene and Mn-carbonyl co-catalysts was employed for carbonylative C–C coupling of arylboronic esters with alkyl halides, allowing for the convergent synthesis of ketones. The system operates under mild conditions and exhibits complementary reactivity to Pd catalysis. The method is compatible with a wide range of arylboronic ester nucleophiles and proceeds smoothly for both primary and secondary alkyl iodide electrophiles. Preliminary mechanistic experiments corroborate a hypothetical catalytic mechanism consisting of co-dependent cycles wherein the Cu-carbene co-catalyst engages in transmetallation to generate an organocopper nucleophile, while the Mn-carbonyl co-catalyst activates the alkyl halide electrophile by single-electron transfer and then undergoes reversible carbonylation to generate an acylmanganese electrophile. The two cycles then intersect with a heterobimetallic, product-releasing C–C coupling step. PMID:28966784

Anti-trypanosomal activity of cationic N-heterocyclic carbene gold(I) complexes.

PubMed

Winter, Isabel; Lockhauserbäumer, Julia; Lallinger-Kube, Gertrud; Schobert, Rainer; Ersfeld, Klaus; Biersack, Bernhard

2017-06-01

Two gold(I) N-heterocyclic carbene complexes 1a and 1b were tested for their anti-trypanosomal activity against Trypanosoma brucei parasites. Both gold compounds exhibited excellent anti-trypanosomal activity (IC 50 =0.9-3.0nM). The effects of the gold complexes 1a and 1b on the T. b. brucei cytoskeleton were evaluated. Rapid detachment of the flagellum from the cell body occurred after treatment with the gold complexes. In addition, a quick and complete degeneration of the parasitic cytoskeleton was induced by the gold complexes, only the microtubules of the detached flagellum remained intact. Both gold compounds 1a and 1b feature selective anti-trypanosomal agents and were distinctly more active against T. b. brucei cells than against human HeLa cells. Thus, the gold complexes 1a and 1b feature promising drug candidates for the treatment of trypanosome infections such as sleeping sickness (human African Trypanosomiasis caused by Trypanosoma brucei parasites). Copyright © 2017 Elsevier B.V. All rights reserved.

Highly Reactive, General and Long-Lived Catalysts for Palladium-Catalyzed Amination of Heteroaryl and Aryl Chlorides, Bromides and Iodides: Scope and Structure-Activity Relationships

PubMed Central

Shen, Qilong; Ogata, Tokutaro; Hartwig, John F.

2010-01-01

We describe a systematic study of the scope and relationship between ligand structure and activity for a highly efficient and selective class of catalysts for the amination of heteroaryl and aryl chlorides, bromides and iodides containing sterically hindered chelating alkylphosphines. In the presence of this catalyst, aryl and heteroaryl chlorides, bromides and iodides react with many primary amines in high yields with part-per-million quantities of palladium precursor and ligand. Many reactions of primary amines with both heteroaryl and aryl chlorides, bromides and iodides occur to completion with 0.0005-0.05 mol % catalysts. A comparison of the reactivity of this catalyst for coupling of primary amines at these loadings is made with catalysts generated from hindered monophosphines and carbenes, and these data illustrate the benefits of chelation. Thus, these complexes constitute a fourth-generation catalyst for the amination of aryl halides, whose activity complements catalysts based on monophosphines and carbenes. PMID:18444639

The effect of substituted moiety on the optoelectronic and photophysical properties of tris (phenylbenzimidazolinato) Ir (III) carbene complexes and the OLED performance: a theoretical study

NASA Astrophysics Data System (ADS)

Srivastava, Ruby

2015-06-01

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are used to analyse theoretically the optoelectronic, photophysical properties and organic light-emitting diode performance of a series of fac-mer blue-emitting Iridium (III) carbene complexes. Swain-Lupton constant is used to discuss the substituents effect. 5d-orbital splitting and d-d* transitions are calculated to assess the efficiency of the studied complexes. The reorganisation energies (λ), transfer integrals, mobilities, radiative decay rate (kr), and triplet exciton generation fraction (χT) are also calculated. Due to the higher χT of these complexes, the formation of triplet exciton will be more and it will cause a faster intersystem crossing. Two host materials are proposed and host-guest match (Dexter-Förster energy) is also discussed. We hope that this unified work will surely help to design new blue-emitting phosphorescent materials in future.

In Situ Generated Ruthenium-Arene Catalyst for Photoactivated Ring-Opening Metathesis Polymerization through Photolatent N-Heterocyclic Carbene Ligand.

PubMed

Pinaud, Julien; Trinh, Thi Kim Hoang; Sauvanier, David; Placet, Emeline; Songsee, Sriprapai; Lacroix-Desmazes, Patrick; Becht, Jean-Michel; Tarablsi, Bassam; Lalevée, Jacques; Pichavant, Loïc; Héroguez, Valérie; Chemtob, Abraham

2018-01-09

1,3-Bis(mesityl)imidazolium tetraphenylborate (IMesH + BPh 4 - ) can be synthesized in one step by anion metathesis between the corresponding imidazolium chloride and sodium tetraphenylborate. In the presence of 2-isopropylthioxanthone (sensitizer), an IMes N-heterocyclic carbene (NHC) ligand can be photogenerated under irradiation at 365 nm through coupled electron/proton transfer reactions. By combining this tandem NHC photogenerator system with metathesis inactive [RuCl 2 (p-cymene)] 2 precatalyst, the highly active RuCl 2 (p-cymene)(IMes) complex can be formed in situ, enabling a complete ring-opening metathesis polymerization (ROMP) of norbornene in the matter of minutes at room temperature. To the best of our knowledge, this is the first example of a photogenerated NHC. Its exploitation in photoROMP has resulted in a simplified process compared to current photocatalysts, because only stable commercial or easily synthesized reagents are required. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

A theoretical study on the mechanistic highlights behind the Brønsted-acid dependent mer-fac isomerization of homoleptic carbenic iridium complexes for PhOLEDs.

PubMed

Setzer, Tobias; Lennartz, Christian; Dreuw, Andreas

2017-06-06

Recently, a successful Brønsted-acid mediated geometric isomerization of the meridional homoleptic carbenic iridium(iii) complexes tris-(N-phenyl,N-methyl-benzimidazol-2-yl)iridium(iii) (1) and tris-(N-phenyl,N-benzyl-benzimidazol-2-yl)iridium(iii) (2) into their facial form has been reported. In the present work the pronounced acid-dependency of this particular isomerization procedure is revisited and additional mechanistic pathways are taken into account. Moreover, the acid-induced material decomposition is addressed. All calculations are carried out using density functional theory (DFT) while the environmental effects in solution are accounted for by the COSMO-RS model. The simulated results clearly reveal the outstanding importance of the complex interplay between acid strength, coordinating power of the corresponding base and the steric influence of the ligand system in contrast to the plain calculation of minimum energy pathways for selected complexes. Eventually, general rules to enhance the material-specific reaction yields are provided.

Synthesis, crystal, and biological activity of a novel carbene silver(I) complex with imidazole derivative

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jiu-Fu, Lu, E-mail: jiufulu@163.com; Hong-Guang, Ge; Juan, Shi

2015-12-15

Reaction of 2-(1-methyl-1,2-dihydroimidazol-3-yl)acetonitrile tetrafluoroborate with silver oxide in dichloromethane readily yields [Ag(DIM){sub 2}]BF{sub 4}, where DIM is 2-(1-methyl-1, 2-dihydroimidazol-3-yl)acetonitrile, representing a carbene organic ligand. The title compound was characterized by elemental analysis, IR, MS and single crystal X-ray diffraction. The crystal is of monoclinic system, space group C2/c with a = 14.010(18), b = 8.303(11), c = 14.936(20) Å, β = 93.910(4)°, V = 1639(4) Å{sup 3}, Z = 4, D{sub x} = 1.771 g/cm{sup 3}, F (000) = 864, µ(MoK{sub α}) = 1.278 mm{sup –1}. The final R{sup 1} = 0.0711 and wR{sup 2} = 0.1903 for reflections withmore » I > 2σ(I). In addition, the preliminary biological test showed that the title compound had anti-fungus yeast activity.« less

Complementary Strategies for Directed C(sp3 )-H Functionalization: A Comparison of Transition-Metal-Catalyzed Activation, Hydrogen Atom Transfer, and Carbene/Nitrene Transfer.

PubMed

Chu, John C K; Rovis, Tomislav

2018-01-02

The functionalization of C(sp 3 )-H bonds streamlines chemical synthesis by allowing the use of simple molecules and providing novel synthetic disconnections. Intensive recent efforts in the development of new reactions based on C-H functionalization have led to its wider adoption across a range of research areas. This Review discusses the strengths and weaknesses of three main approaches: transition-metal-catalyzed C-H activation, 1,n-hydrogen atom transfer, and transition-metal-catalyzed carbene/nitrene transfer, for the directed functionalization of unactivated C(sp 3 )-H bonds. For each strategy, the scope, the reactivity of different C-H bonds, the position of the reacting C-H bonds relative to the directing group, and stereochemical outcomes are illustrated with examples in the literature. The aim of this Review is to provide guidance for the use of C-H functionalization reactions and inspire future research in this area. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis of water-soluble mono- and ditopic imidazoliums for carbene ligands

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anstey, Mitchell; Murtagh, Dustin; Cordaro, Joseph Gabriel

2015-09-01

Synthesis of ditopic imidazoliums was achieved using a modular step-wise procedure. The procedure itself is amenable to a wide array of functional groups that can be incorporated into the imidazolium architecture. The resulting compounds range from ditopic zwitterions to highly-soluble dicationic aromatics

Asymmetric Desymmetrization of 1,3-Diketones via Intramolecular Benzoin Reaction.

PubMed

Li, Yuanzhen; Yang, Shuang; Wen, Genfa; Lin, Qiqiao; Zhang, Guoxiang; Qiu, Lin; Zhang, Xiaoyan; Du, Guangfen; Fang, Xinqiang

2016-04-01

A general method for the asymmetric desymmetrization of 1,3-diketone substrates via chiral N-heterocyclic carbene catalyzed intramolecular benzoin reactions was developed. Five- and six-membered cyclic ketones bearing two contiguous fully substituted stereocenters were generated with excellent diastereoselectivities and moderate to excellent enantioselectivities.

[CNN]-pincer nickel(II) complexes of N-heterocyclic carbene (NHC): synthesis and catalysis of the Kumada reaction of unactivated C-Cl bonds.

PubMed

Sun, Yunqiang; Li, Xiaoyan; Sun, Hongjian

2014-07-07

Three novel [CNN]-pincer nickel(ii) complexes with NHC-amine arms were synthesized in three steps. Complex was proven to be an efficient catalyst for the Kumada coupling of aryl chlorides or aryl dichlorides under mild conditions.

Access to unusual polycyclic spiro-enones from 2,2'-bis(allyloxy)-1,1'-binaphthyls using Grubbs' catalysts: an unprecedented one-pot RCM/Claisen sequence.

PubMed

Piedra, Estefanía; Francos, Javier; Nebra, Noel; Suárez, Francisco J; Díez, Josefina; Cadierno, Victorio

2011-07-21

Treatment of 2,2'-bis(allyloxy)-1,1'-binaphthyls with the first-generation Grubbs' carbene under MW-irradiation results in the formation of new polycyclic spiro-enones through an unprecedented RCM/Claisen sequence.

A tritopic carbanionic N-heterocyclic dicarbene and its homo- and heterometallic coinage metal complexes.

PubMed

Zhang, Fan; Cao, Xiao-Ming; Wang, Jiwei; Jiao, Jiajun; Huang, Yongming; Shi, Min; Braunstein, Pierre; Zhang, Jun

2018-05-21

Homo (Au3)- and heterotrinuclear coinage metal complexes (Au2Ag and Au2Cu) ligated by the first tritopic carbanionic N-heterocyclic carbene (NHC) have been prepared by deprotonation of ditopic NHC digold complexes and structurally characterized by single-crystal X-ray diffraction.

N-heterocyclic carbene-catalyzed tandem aza-benzoin/Michael reactions: on site reversal of the reactivity of N-Boc imines.

PubMed

Wu, Ke-Jia; Li, Gong-Qiang; Li, Yi; Dai, Li-Xin; You, Shu-Li

2011-01-07

A tandem NHC-catalyzed aza-benzoin/Michael reaction has been developed as a method to efficiently produce dihydroindenones and pyrrolidinone-containing tricycles. The novel reaction pattern involves tert-butyl aryl(tosyl)methylcarbamates reacting as both electrophile and nucleophile on the same carbon.

Hydrogen Production and Storage on a Formic Acid/Bicarbonate Platform using Water-Soluble N-Heterocyclic Carbene Complexes of Late Transition Metals.

PubMed

Jantke, Dominik; Pardatscher, Lorenz; Drees, Markus; Cokoja, Mirza; Herrmann, Wolfgang A; Kühn, Fritz E

2016-10-06

The synthesis and characterization of two water-soluble bis-N-heterocyclic carbene (NHC) complexes of rhodium and iridium is presented. Both compounds are active in H 2 generation from formic acid and in hydrogenation of bicarbonate to formate. The rhodium derivative is most active in both reactions, reaching a TOF of 39 000 h -1 and a TON of 449 000 for H 2 production. The catalytic hydrogenation reactions were carried out in an autoclave system and analyzed using the integrated peak areas in the 1 H NMR spectra. Decomposition of formic acid was investigated using a Fisher-Porter bottle equipped with a pressure transducer. Long-term stability for hydrogen evolution was tested by surveillance of the gas flow rate. The procedure does not require any additives like amines or inert gas conditions. Density functional theory calculations in agreement with experimental results suggest a bicarbonate reduction mechanism involving a second catalyst molecule, which provides an external hydride acting as reducing agent. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes

NASA Astrophysics Data System (ADS)

Gregson, Matthew; Lu, Erli; Mills, David P.; Tuna, Floriana; McInnes, Eric J. L.; Hennig, Christoph; Scheinost, Andreas C.; McMaster, Jonathan; Lewis, William; Blake, Alexander J.; Kerridge, Andrew; Liddle, Stephen T.

2017-02-01

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal-ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

Palladium complexes of N-heterocyclic carbenes as catalysts for cross-coupling reactions--a synthetic chemist's perspective.

PubMed

Kantchev, Eric Assen B; O'Brien, Christopher J; Organ, Michael G

2007-01-01

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

N-heterocyclic carbene gold(I) and silver(I) complexes bearing functional groups for bio-conjugation

PubMed Central

Garner, Mary E.; Niu, Weijia; Chen, Xigao; Ghiviriga, Ion; Tan, Weihong; Veige, Adam S.

2015-01-01

This work describes several synthetic approaches to append organic functional groups to gold and silver N-heterocyclic carbene (NHC) complexes suitable for applications in biomolecule conjugation. Carboxylate appended NHC ligands (3) lead to unstable AuI complexes that convert into bis-NHC species (4). A benzyl protected carboxylate NHC-AuI complex 2 was synthesized but deprotection to produce the carboxylic acid functionality could not be achieved. A small library of new alkyne functionalized NHC proligands were synthesized and used for subsequent silver and gold metalation reactions. The alkyne appended NHC gold complex 13 readily react with benzyl azide in a copper catalyzed azide-alkyne cycloaddition reaction to form the triazole appended NHC gold complex 14. Cell cytotoxicity studies were performed on DLD-1 (colorectal adenocarcinoma), Hep-G2 (hepatocellular carcinoma), MCF-7 (breast adenocarcinoma), CCRF-CEM (human T-Cell leukemia), and HEK (human embryonic kidney). Complete spectroscopic characterization of the ligands and complexes was achieved using 1H and 13C NMR, gHMBC, ESI-MS, and combustion analysis. PMID:25490699

Carbene supported dimer of heavier ketenimine analogue with p and si atoms.

PubMed

Roy, Sudipta; Dittrich, Birger; Mondal, Totan; Koley, Debasis; Stückl, A Claudia; Schwederski, Brigitte; Kaim, Wolfgang; John, Michael; Vasa, Suresh Kumar; Linser, Rasmus; Roesky, Herbert W

2015-05-20

A cyclic alkyl(amino) carbene (cAAC) stabilized dimer [(cAAC)Si(P-Tip)]2 (2) (Tip = 2,4,6-triisopropylphenyl) is reported. 2 can be considered as a dimer of the heavier ketenimine (R2C═C═N-R) analogue. The dark-red rod-shaped crystals of 2 were synthesized by reduction of the precursor, cAAC-dichlorosilylene-stabilized phosphinidene (cAAC)SiCl2→P-Tip with sodium napthalenide. The crystals of 2 are storable at room temperature for several months and stable up to 215 °C under an inert atmosphere. X-ray single-crystal diffraction revealed that 2 contains a cyclic nonplanar four-membered SiPSiP ring. Magnetic susceptibility measurements confirmed the singlet spin ground state of 2. Cyclic voltammetry of 2 showed a quasi-reversible one-electron reduction indicating the formation of the corresponding radical anion 2(•-), which was further characterized by EPR measurements in solution. The electronic structure and bonding of 2 and 2(•-) were studied by theoretical calculations. The experimentally obtained data are in good agreement with the calculated values.

Copper- and copper–N-heterocyclic carbene-catalyzed C─H activating carboxylation of terminal alkynes with CO2 at ambient conditions

PubMed Central

Yu, Dingyi; Zhang, Yugen

2010-01-01

The use of carbon dioxide as a renewable and environmentally friendly source of carbon in organic synthesis is a highly attractive approach, but its real world applications remain a great challenge. The major obstacles for commercialization of most current protocols are their low catalytic performances, harsh reaction conditions, and limited substrate scope. It is important to develop new reactions and new protocols for CO2 transformations at mild conditions and in cost-efficient ways. Herein, a copper-catalyzed and copper–N-heterocyclic carbene-cocatalyzed transformation of CO2 to carboxylic acids via C─H bond activation of terminal alkynes with or without base additives is reported. Various propiolic acids were synthesized in good to excellent yields under ambient conditions without consumption of any organometallic or organic reagent additives. This system has a wide scope of substrates and functional group tolerances and provides a powerful tool for the synthesis of highly functionalized propiolic acids. This catalytic system is a simple and economically viable protocol with great potential in practical applications. PMID:21059950

The Mesomeric Effect of Thiazolium on non-Kekulé Diradicals in Pichia stipitis Transketolase.

PubMed

Hsu, Ning-Shian; Wang, Yung-Lin; Lin, Kuan-Hung; Chang, Chi-Fon; Lyu, Syue-Yi; Hsu, Li-Jen; Liu, Yu-Chen; Chang, Chin-Yuan; Wu, Chang-Jer; Li, Tsung-Lin

2018-02-12

It is theoretically plausible that thiazolium mesomerizes to congeners other than carbene in a low effective dielectric binding site; especially given the energetics and uneven electronegativity of carbene groups. However, such a phenomenon has never been reported. Nine crystal structures of transketolase obtained from Pichia stipitis (TKps) are reported with subatomic resolution, where thiazolium displays an extraordinary ring-bending effect. The bent thiazolium congeners correlate with non-Kekulé diradicals because there is no gain or loss of electrons. In conjunction with biophysical and biochemical analyses, it is concluded that ring bending is a result of tautomerization of thiazolium with its non- Kekulé diradicals, exclusively in the binding site of TKps. The chemophysical properties of these thiazolium mesomers may account for the great variety of reactivities carried out by thiamine-diphosphate-containing (ThDP) enzymes. The stability of ThDP in living systems can be regulated by the levels of substrates, and hydration and dehydration, as well as diradical-mediated oxidative degradation. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes.

PubMed

Gregson, Matthew; Lu, Erli; Mills, David P; Tuna, Floriana; McInnes, Eric J L; Hennig, Christoph; Scheinost, Andreas C; McMaster, Jonathan; Lewis, William; Blake, Alexander J; Kerridge, Andrew; Liddle, Stephen T

2017-02-03

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal-ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

From the N-Heterocyclic Carbene-Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates.

PubMed

Ottou, Winnie Nzahou; Bourichon, Damien; Vignolle, Joan; Wirotius, Anne-Laure; Robert, Fredéric; Landais, Yannick; Sotiropoulos, Jean-Marc; Miqueu, Karinne; Taton, Daniel

2015-06-22

Among various N-heterocyclic carbenes (NHCs) tested, only 1,3-bis(tert-butyl)imidazol-2-ylidene (NHC(tBu) ) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC-catalyzed 1,4-addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well-defined α-alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]0/[ROH]0 molar ratio, were ultimately obtained in N,N-dimethylformamide at 25 °C. A hydroxyl-terminated poly(ethylene oxide) (PEO-OH) macro-initiator was also employed to directly access PEO-b-PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC(tBu) . © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Alkali-Metal-Mediated Magnesiations of an N-Heterocyclic Carbene: Normal, Abnormal, and "Paranormal" Reactivity in a Single Tritopic Molecule.

PubMed

Martínez-Martínez, Antonio J; Fuentes, M Ángeles; Hernán-Gómez, Alberto; Hevia, Eva; Kennedy, Alan R; Mulvey, Robert E; O'Hara, Charles T

2015-11-16

Herein the sodium alkylmagnesium amide [Na4Mg2(TMP)6(nBu)2] (TMP=2,2,6,6-tetramethylpiperidide), a template base as its deprotonating action is dictated primarily by its 12 atom ring structure, is studied with the common N-heterocyclic carbene (NHC) IPr [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Remarkably, magnesiation of IPr occurs at the para-position of an aryl substituent, sodiation occurs at the abnormal C4 position, and a dative bond occurs between normal C2 and sodium, all within a 20 atom ring structure accommodating two IPr(2-). Studies with different K/Mg and Na/Mg bimetallic bases led to two other magnesiated NHC structures containing two or three IPr(-) monoanions bound to Mg through abnormal C4 sites. Synergistic in that magnesiation can only work through alkali-metal mediation, these reactions add magnesium to the small cartel of metals capable of directly metalating a NHC. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Antibacterial and DNA cleavage activity of carbonyl functionalized N-heterocyclic carbene-silver(I) and selenium compounds

NASA Astrophysics Data System (ADS)

Haque, Rosenani A.; Iqbal, Muhammad Adnan; Mohamad, Faisal; Razali, Mohd R.

2018-03-01

The article describes syntheses and characterizations of carbonyl functionalized benzimidazolium salts, I-IV. While salts I-III are unstable at room temperature, salt IV remained stable and was further utilised to form N-heterocyclic carbene (NHC) compounds of silver(I), V and VI, and selenium compound, VII respectively. Compounds IV-VII were tested for their antibacterial potential against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Salt IV shows a very low inhibition potential (minimum inhibitory concentration, MIC 500 μg/mL) compared to the respective silver(I)-NHC, V and VI (MIC 31.25 μg/mL against both, E. coli and S. aureus) and selenium compound, VII (MIC 125 μg/mL against E. coli and 62.50 μg/mL against S. aureus). In DNA cleavage abilities, all the test compounds cleave DNA in which the VII cleaves the DNA at the faster rate. Meanwhile, the silver(I)-NHC complexes V and VI act at the same mode and pattern of DNA cleavage while VII is similar to IV.

A chemical-biological evaluation of rhodium(I) N-heterocyclic carbene complexes as prospective anticancer drugs.

PubMed

Oehninger, Luciano; Küster, Laura Nadine; Schmidt, Claudia; Muñoz-Castro, Alvaro; Prokop, Aram; Ott, Ingo

2013-12-23

Rhodium(I) complexes bearing N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry, but there are very few reports of biological properties of these organometallics. A series of Rh(I)-NHC derivatives with 1,5-cyclooctadiene and CO as secondary ligands were synthesized, characterized, and biologically investigated as prospective antitumor drug candidates. Pronounced antiproliferative effects were noted for all complexes, along with moderate inhibitory activity of thioredoxin reductase (TrxR) and efficient binding to biomolecules (DNA, albumin). Biodistribution studies showed that the presence of albumin lowered the cellular uptake and confirmed the transport of rhodium into the nuclei. Changes in the mitochondrial membrane potential (MMP) were observed as well as DNA fragmentation in wild-type and daunorubicin- or vincristine-resistant Nalm-6 leukemia cells. Overall, these studies indicated that Rh(I)-NHC fragments could be used as partial structures of new antitumor agents, in particular in those drugs designed to address resistant malignant tissues. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes

PubMed Central

Gregson, Matthew; Lu, Erli; Mills, David P.; Tuna, Floriana; McInnes, Eric J. L.; Hennig, Christoph; Scheinost, Andreas C.; McMaster, Jonathan; Lewis, William; Blake, Alexander J.; Kerridge, Andrew; Liddle, Stephen T.

2017-01-01

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal–ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle. PMID:28155857

Magnetic properties of 1 : 4 complexes of CoCl2 and pyridines carrying carbenes (S(0) = 4/2, 6/2, and 8/2) in diluted frozen solution; influence of carbene multiplicity on heterospin single-molecule magnets.

PubMed

Karasawa, Satoru; Nakano, Kimihiro; Tanokashira, Jun-ichi; Yamamoto, Noriko; Yoshizaki, Takahito; Koga, Noboru

2012-11-28

The microcrystalline sample of a parent complex, [CoCl(2)(py)(4)], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, U(eff)/k(B), of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2-4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1 : 4 complexes, CoCl(2)(DYpy)(4); Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, S(total) = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, H(c), at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give U(eff)/k(B) values of 94, 92, 93, and 87 K for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl(2)(CYpy)(4); Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5-1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl(2)(CYpy)(4) including [CoCl(2)(py)(4)] and CoCl(2)(C1py)(4) suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, τ(q), due to the spin quantum tunneling magnetization, which became larger with increasing S(total) of the complex.

NHC-catalyzed cleavage of vicinal diketones and triketones followed by insertion of enones and ynones.

PubMed

Takaki, Ken; Hino, Makoto; Ohno, Akira; Komeyama, Kimihiro; Yoshida, Hiroto; Fukuoka, Hiroshi

2017-01-01

Thiazolium carbene-catalyzed reactions of 1,2-diketones and 1,2,3-triketones with enones and ynones have been investigated. The diketones gave α,β-double acylation products via unique Breslow intermediates isolable as acid salts, whereas the triketones formed stable adducts with the NHC instead of the coupling products.

Asymmetric NHC-catalyzed redox α-amination of α-aroyloxyaldehydes.

PubMed

Taylor, James E; Daniels, David S B; Smith, Andrew D

2013-12-06

Asymmetric α-amination through an N-heterocyclic carbene (NHC)-catalyzed redox reaction of α-aroyloxyaldehydes with N-aryl-N-aroyldiazenes to form α-hydrazino esters with high enantioselectivity (up to 99% ee) is reported. The hydrazide products are readily converted into enantioenriched N-aryl amino esters through samarium(II) iodide mediated N-N bond cleavage.

NHC-catalyzed cleavage of vicinal diketones and triketones followed by insertion of enones and ynones

PubMed Central

Hino, Makoto; Ohno, Akira; Komeyama, Kimihiro; Yoshida, Hiroto; Fukuoka, Hiroshi

2017-01-01

Thiazolium carbene-catalyzed reactions of 1,2-diketones and 1,2,3-triketones with enones and ynones have been investigated. The diketones gave α,β-double acylation products via unique Breslow intermediates isolable as acid salts, whereas the triketones formed stable adducts with the NHC instead of the coupling products. PMID:28904625

Ring Expansion and Rearrangements of Rhodium(II) Azavinyl Carbenes

PubMed Central

Selander, Nicklas; Worrell, Brady T.

2013-01-01

An efficient, regioselective and convergent method for the ring expansion and rearrangement of 1-sulfonyl-1,2,3-triazoles under rhodium(II)-catalyzed conditions is described. These denitrogenative reactions form substituted enaminone and olefin-based products, which in the former case can be further functionalized to unique products rendering the sulfonyl triazole traceless. PMID:23161725

Copper-Catalyzed SN2'-Selective Allylic Substitution Reaction of gem-Diborylalkanes.

PubMed

Zhang, Zhen-Qi; Zhang, Ben; Lu, Xi; Liu, Jing-Hui; Lu, Xiao-Yu; Xiao, Bin; Fu, Yao

2016-03-04

A Cu/(NHC)-catalyzed SN2'-selective substitution reaction of allylic electrophiles with gem-diborylalkanes is reported. Different substituted gem-diborylalkanes and allylic electrophiles can be employed in this reaction, and various synthetic valuable functional groups can be tolerated. The asymmetric version of this reaction was initially researched with chiral N-heterocyclic carbene (NHC) ligands.

Catalytic carbene transfer allows the direct customization of cyclic purine dinucleotides.

PubMed

Fei, Na; Häussinger, Daniel; Blümli, Seraina; Laventie, Benoît-Joseph; Bizzini, Lorenzo D; Zimmermann, Kaspar; Jenal, Urs; Gillingham, Dennis

2014-08-11

We describe a simple method for the direct modification of nucleobases in cyclic purine dinucleotides, important signalling molecules in both prokaryotes and eukaryotes. The method tolerates all members of the cyclic dinucleotide family and could be used to modulate their function or introduce useful side-chains such as fluorophores and photo-crosslinking groups.

Hydrophenoxylation of internal alkynes catalysed with a heterobimetallic Cu-NHC/Au-NHC system.

PubMed

Lazreg, Faïma; Guidone, Stefano; Gómez-Herrera, Alberto; Nahra, Fady; Cazin, Catherine S J

2017-02-21

A straightforward method for the hydrophenoxylation of internal alkynes, using N-heterocyclic carbene-based copper(i) and gold(i) complexes, is described. The heterobimetallic catalytic system proceeds via dual activation of the substrates to afford the desired vinylether derivatives. This methodology is shown to be highly efficient and tolerates a wide range of substituted phenols and alkynes.

IR and visible luminescence studies in the infrared multiphoton dissociation of 1,2-dibromo-1,1-difluoroethane

NASA Astrophysics Data System (ADS)

Pushpa, K. K.; Kumar, Awadhesh; Vatsa, R. K.; Naik, P. D.; Annaji Rao, K.; Mittal, J. P.; Parthasarathy, V.; Sarkar, S. K.

1995-07-01

The infrared multiphoton dissociation of 1,2-dibromo-1,1-difluoroethane gives rise to IR and visible luminescence. Vibrationally excited parent molecules dissociate via two primary channels yielding bromine and vibrationally excited HBr. The strong visible emission observed between 350 to 750 nm has been assigned to electronically excited carbene CF 2Br CH.

Unraveling the synthesis of homoleptic [Ag(N,N-diaryl-NHC)2]Y (Y = BF4, PF6) complexes by ball-milling.

PubMed

Beillard, Audrey; Bantreil, Xavier; Métro, Thomas-Xavier; Martinez, Jean; Lamaty, Frédéric

2016-11-28

A user-friendly and general mechanochemical method was developed to access rarely described NHC (N-heterocyclic carbene) silver(i) complexes featuring N,N-diarylimidazol(idin)ene ligands and non-coordinating tetrafluoroborate or hexafluorophosphate counter anions. Comparison with syntheses in solution clearly demonstrated the superiority of the ball-milling conditions.

Stereoselective Synthesis of Tetrasubstituted Furylalkenes via Gold-Catalyzed Cross-Coupling of Enynones with Diazo Compounds.

PubMed

Liu, Pei; Sun, Jiangtao

2017-07-07

A stereoselective, gold-catalyzed, cross-coupling reaction of enynones with diazo compounds has been developed, affording 2-alkenylfurans in moderate to good yields with excellent E-stereoselectivity. Upon using diazo compounds as nucleophiles to trap the in situ formed gold furyl carbene, this protocol provides a novel path toward the formation of unsymmetrical tetrasubstituted alkenes.

The Twentieth International Symposium on Molten Salts and Ionic Liquids

DTIC Science & Technology

2016-11-29

Heterocyclic Carbene Involved?" by Hyung Kim "Carbon Dioxide Absorption Behavior and Cabronate Ion Transport of Lithium Orthosilicate/Molten Carbonate...K. Gemmell, K. Johnson, A. East 575 Lithium Ion Conduction in Single Lithium Perfluorosulfonylamides K. Kubota, H. Matsumoto 585...energy applications (e.g., batteries , fuel cells, semiconductors, photovoltaics, and phase change energy storage); (3) rare earth and nuclear chemistry

Rhenium(i) complexes of N-heterocyclic carbene ligands that bind to amyloid plaques of Alzheimer's disease.

PubMed

Chan, Chung Ying; Noor, Asif; McLean, Catriona A; Donnelly, Paul S; Barnard, Peter J

2017-02-16

A series of [Re(i)L(CO) 3 ] + complexes (where L is a bifunctional bis(NHC)-amine ligand) that are analogues of potential Tc-99m diagnostic imaging agents for Alzheimer's disease have been synthesised. One of the complexes bound to amyloid plaques in human frontal cortex brain tissue from subjects with Alzheimer's disease.

A monotopic aluminum telluride with an Al=Te double bond stabilized by N-heterocyclic carbenes

PubMed Central

Franz, Daniel; Szilvási, Tibor; Irran, Elisabeth; Inoue, Shigeyoshi

2015-01-01

Aluminum chalcogenides are mostly encountered in the form of bulk aluminum oxides that are structurally diverse but typically consist of networks with high lattice energy in which the chalcogen atoms bridge the metal centres. This makes their molecular congeners difficult to synthesize because of a pronounced tendency for oligomerization. Here we describe the isolation of the monotopic aluminum chalcogenide (LDipN)AlTe(LEt)2 (LDip=1,3-(2,6-diisopropylphenyl)-imidazolin-2-imine, LEt=1,3-diethyl-4,5-dimethyl-imidazolin-2-ylidene). Unique features of (LDipN)AlTe(LEt)2 are the terminal position of the tellurium atom, the shortest aluminum–tellurium distance hitherto reported for a molecular complex and the highest bond order reported for an interaction between these elements, to the best of our knowledge. At elevated temperature (LDipN)AlTe(LEt)2 equilibrates with dimeric {(LDipN)AlTe(LEt)}2 in which the chalcogen atoms assume their common role as bridges between the metal centres. These findings demonstrate that (LDipN)AlTe(LEt)2 comprises the elusive Al=Te double bond in the form of an N-heterocyclic carbene-stabilized species. PMID:26612781

Ruthenium(II) Complexes Containing Lutidine-Derived Pincer CNC Ligands: Synthesis, Structure, and Catalytic Hydrogenation of C-N bonds.

PubMed

Hernández-Juárez, Martín; López-Serrano, Joaquín; Lara, Patricia; Morales-Cerón, Judith P; Vaquero, Mónica; Álvarez, Eleuterio; Salazar, Verónica; Suárez, Andrés

2015-05-11

A series of Ru complexes containing lutidine-derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver-carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N-heterocyclic carbene fragments. In the presence of tBuOK, the Ru-CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand-assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru-CNC complex 5 e(BF4 ) is able to add aldimines to the metal-ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer-sphere stepwise hydrogen transfer to the C-N bond assisted either by the pincer ligand or a second coordinated H2 molecule. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tautomerism and thermal decomposition of tetrazole: high-level ab initio study.

PubMed

Kiselev, Vitaly G; Cheblakov, Pavel B; Gritsan, Nina P

2011-03-10

The mutual interconversion and decomposition reactions of four tetrazole isomers (1H-TZ, 2H-TZ, 5H-TZ, and an N-heterocyclic carbene 14H) have been studied theoretically using the W1 high-level procedure. Computations allowed resolution of the existing discrepancies in the mechanism and key intermediates of TZ thermolysis. The tautomeric equilibria between 1H-TZ, 2H-TZ, and 14H turned out to play a very important role in the mechanism of thermal decomposition. Although the barriers of monomolecular tautomeric transformations were found to be high (∼50-70 kcal/mol), the concerted double H atom transfer reactions in the H-bonded complexes of TZ tautomers have profoundly lower barriers (∼18-28 kcal/mol). These reactions lead to fast interconversion between 1H-TZ, 2H-TZ, and 14H. The carbene 14H has never been considered before; however, it was predicted to be a key intermediate in the mechanism of thermal decomposition of TZ. For all species considered, the unimolecular reactions of N(2) elimination were predicted to dominate over the elimination of hydrazoic acid. In agreement with existing experimental data, the effective activation energy of thermolysis was calculated to be 36.2 kcal/mol.

Self-Assembled N-Heterocyclic Carbene-Based Carboxymethylated Dextran Monolayers on Gold as a Tunable Platform for Designing Affinity-Capture Biosensor Surfaces.

PubMed

Li, Zhijun; Munro, Kim; Narouz, Mina R; Lau, Andrew; Hao, Hongxia; Crudden, Cathleen M; Horton, J Hugh

2018-05-30

Sensor surfaces play a predominant role in the development of optical biosensor technologies for the analysis of biomolecular interactions. Thiol-based self-assembled monolayers (SAMs) on gold have been widely used as linker layers for sensor surfaces. However, the degradation of the thiol-gold bond can limit the performance and durability of such surfaces, directly impacting their performance and cost-effectiveness. To this end, a new family of materials based on N-heterocyclic carbenes (NHCs) has emerged as an alternative for surface modification, capable of self-assembling onto a gold surface with higher affinity and superior stability as compared to the thiol-based systems. Here we demonstrate three applications of NHC SAMs supporting a dextran layer as a tunable platform for developing various affinity-capture biosensor surfaces. We describe the development and testing of NHC-based dextran biosensor surfaces modified with each of streptavidin, nitrilotriacetic acid, and recombinant Protein A. These affinity-capture sensor surfaces enable oriented binding of ligands for optimal performance in biomolecular assays. Together, the intrinsic high stability and flexible design of the NHC biosensing platforms show great promise and open up exciting possibilities for future biosensing applications.

Arrested α-hydride migration activates a phosphido ligand for C–H insertion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hickey, Anne K.; Muñoz, Salvador B.; Lutz, Sean A.

Bulky tris(carbene)borate ligands provide access to high spin iron(II) phosphido complexes. The complex PhB(MesIm) 3FeP(H)Ph is thermally unstable, and we observed [PPh] group insertion into a C–H bond of the supporting ligand. An arrested α-hydride migration mechanism suggests increased nucleophilicity of the phosphorus atom facilitates [PPh] group transfer reactivity.

Application of the π-accepting ability parameter of N-heterocyclic carbene ligands in iridium complexes for signal amplification by reversible exchange (SABRE).

PubMed

van Weerdenburg, Bram J A; Eshuis, Nan; Tessari, Marco; Rutjes, Floris P J T; Feiters, Martin C

2015-09-21

The new π-accepting ability parameter (PAAP) appears to be the best tool to analyse the electronic properties of NHC ligands in [Ir(H)2(NHC)(Py)3](+) complexes for SABRE. Together with the buried volume, the efficiency of hyperpolarisation transfer in SABRE, depending on the exchange rate of pyridine, can be described.

Activation of C-H bonds by rare-earth metallocene-butyl complexes.

PubMed

Grindell, Richard; Day, Benjamin M; Guo, Fu-Sheng; Pugh, Thomas; Layfield, Richard A

2017-09-05

The stable metallocene-butyl complexes [(Cp Me ) 2 M( n Bu)] 2 (M = Y, Dy) were synthesized and their reactivity towards to ferrocene and bulky N-heterocyclic carbenes investigated. Selective mono-deprotonation of ferrocene and a benzylic methyl group of IMes were observed, whereas a control reaction of (Cp Me ) 3 M with IMes resulted in a normal-to-abnormal NHC rearrangement.

Synthesis and characterization of nitrile functionalized silver(I)-N-heterocyclic carbene complexes: DNA binding, cleavage studies, antibacterial properties and mosquitocidal activity against the dengue vector, Aedes albopictus.

PubMed

Asekunowo, Patrick O; Haque, Rosenani A; Razali, Mohd R; Avicor, Silas W; Wajidi, Mustafa F F

2018-04-25

A series of four benzimidazolium based nitrile-functionalized mononuclear-Ag(I)-N-heterocyclic carbene and binuclear-Ag(I)-N-heterocyclic carbene (Ag(I)-NHC) hexafluorophosphate complexes (5b-8b) were synthesized by reacting the corresponding hexafluorophosphate salts (1b-4b) with Ag 2 O in acetonitrile, respectively. These compounds were characterized by 1 H NMR, 13 C NMR, IR, UV-visible spectroscopic techniques, elemental analyses and molar conductivity. Additionally, 8b was structurally characterized by single crystal X-ray diffraction technique. Preliminary in vitro antibacterial evaluation was conducted for all the compounds against two standard bacteria; gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacterial strains. Most of the Ag(I)-NHC complexes (5b-8b) showed moderate to good antibacterial activity with MIC values in the range of 12.5-100 μg/mL. Especially, compound 8b exhibited promising anti-Staphylococcus aureus activity with a low MIC value (12.5 μg/mL). However, all the hexafluorophosphate salts (1b-4b) were inactive against the bacteria strains. The preliminary interactive investigation revealed that the most active compound, 8b, could effectively intercalate into DNA to form 8b-DNA complex which shows a better binding ability for DNA (K b  = 3.627 × 10 6 ) than the complexes 5b-7b (2.177 × 10 6 , 8.672 × 10 5 and 6.665 × 10 5 , respectively). Nuclease activity of the complexes on plasmid DNA and Aedes albopictus genomic DNA was time-dependent, although minimal. The complexes were larvicidal to the mosquito, with 5b, 6b and 8b being highly active. Developmental progression from the larval to the adult stage was affected by the complexes, progressively being toxic to the insect's development with increasing concentration. These indicate the potential use of these complexes as control agents against bacteria and the dengue mosquito Ae. albopictus. Copyright © 2018 Elsevier Masson SAS. All rights reserved.

Easy access to silicon(0) and silicon(II) compounds.

PubMed

Mondal, Kartik Chandra; Samuel, Prinson P; Tretiakov, Mykyta; Singh, Amit Pratap; Roesky, Herbert W; Stückl, A Claudia; Niepötter, Benedikt; Carl, Elena; Wolf, Hilke; Herbst-Irmer, Regine; Stalke, Dietmar

2013-04-15

Two different synthetic methodologies of silicon dihalide bridged biradicals of the general formula (L(n)•)2SiX2 (n = 1, 2) have been developed. First, the metathesis reaction between NHC:SiX2 and L(n): (L(n): = cyclic akyl(amino) carbene in a 1:3 molar ratio leads to the products 2 (n = 1, X = Cl), 4 (n = 2, X = Cl), 6 (n = 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce coupled NHCs (3, 5) under C-C bond formation. The formation of the coupled NHCs (L(m) = cyclic alkyl(amino) carbene substituted N-heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster during the metathesis reaction between NHC:SiBr2 and L(n): when compared with that of NHC:SiCl2. Second, the reaction of L(1):SiCl4 (8) (L(1): =:C(CH2)(CMe2)2N-2,6-iPr2C6H3) with a non-nucleophilic base LiN(iPr)2 in a 1:1 molar ratio shows an unprecedented methodology for the synthesis of the biradical (L(1)•)2SiCl2 (2). The blue blocks of silicon dichloride bridged biradicals (2, 4) are stable for more than six months under an inert atmosphere and in air for one week. Compounds 2 and 4 melt in the temperature range of 185 to 195 °C. The dibromide (6, 7) analogue is more prone to decomposition in the solution but comparatively more stable in the solid state than in the solution. Decomposition of the products has been observed in the UV-vis spectra. Moreover, compounds 2 and 4 were further converted to stable singlet biradicaloid dicarbene-coordinated (L(n):)2Si(0) (n = 1 (9), 2 (10)) under KC8 reduction. Compounds 2 and 4 were also reduced to dehalogenated products 9 and 10, respectively when treated with RLi (R = Ph, Me, tBu). Cyclic voltametry measurements show that 10 can irreversibly undergo both one electron oxidation and reduction.

From Solvolysis to Self-Assembly*

PubMed Central

Stang, Peter J.

2009-01-01

My sojourn from classical physical-organic chemistry and solvolysis to self-assembly and supramolecular chemistry, over the last forty years, is described. My contributions to unsaturated reactive intermediates, namely vinyl cations and unsaturated carbenes, along with my decade long involvement with polyvalent iodine chemistry, especially alkynyliodonium salts, as well as my more recent research with metal-ligand, coordination driven and directed self-assembly of finite supramolecular ensembles are discussed. PMID:19111062

Catalytic Enantioselective Aza-Benzoin Reactions of Aldehydes with 2H-Azirines.

PubMed

Peng, Qiupeng; Guo, Donghui; Bie, Jianbo; Wang, Jian

2018-03-26

The unprecedented enantioselective aza-benzoin reaction of aldehydes with 2H-azirines was developed by utilizing a chiral N-heterocyclic carbene as the catalyst. A wide range of corresponding aziridines can be obtained in good yields with high enantioselectivities. The obtained optically active aziridines should be useful in the synthesis of other valuable molecules. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ruthenium(η⁶,η¹-arene-CH₂-NHC) Catalysts for Direct Arylation of 2-Phenylpyridine with (Hetero)Aryl Chlorides in Water.

PubMed

Kaloğlu, Nazan; Özdemir, İsmail; Gürbüz, Nevin; Arslan, Hakan; Dixneuf, Pierre H

2018-03-13

A series of new benzimidazolium halides were synthesized in good yields as unsymmetrical N -heterocyclic carbene (NHC) precursors containing the N-CH₂-arene group. The benzimidazolium halides were readily converted into ruthenium(II)-NHC complexes with the general formula [RuCl₂(η⁶,η¹-arene-CH₂-NHC)]. The structures of all new compounds were characterized by ¹H NMR (Nuclear Magnetic Resonance), 13 C NMR, FT-IR (Fourier Transform Infrared) spectroscopy and elemental analysis techniques. The single crystal structure of one benzimidazole ruthenium complex, 2b , was determined. The complex is best thought of as containing an octahedrally coordinated Ru center with the arene residue occupying three sites, the remaining sites being occupied by a (carbene)C-Ru bond and two Ru-Cl bonds. The catalytic activity of [RuCl₂(η⁶,η ¹ -arene-CH₂-NHC)] complexes was evaluated in the direct (hetero)arylation of 2-phenylpyridine with (hetero)aryl chlorides in water as the nontoxic reaction medium. These results show that catalysts 2a and 2b were the best for monoarylation with simple phenyl and tolyl chlorides. For functional aryl chlorides, 2d , 2e , and 2c appeared to be the most efficient.

Synthesis, photophysical properties, and computational studies of four-coordinate copper(I) complexes based on benzimidazolylidene N-heterocyclic carbene (NHC) ligands bearing aryl substituents

NASA Astrophysics Data System (ADS)

Xu, Shengxian; Wang, Jinglan; Liu, Shaobo; Zhao, Feng; Xia, Hongying; Wang, Yibo

2018-02-01

Three four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes, [Cu(Ph-BenIm-Py)(POP)]PF6 (1), [Cu(Naph-BenIm-Py)(POP)]PF6 (2), and [Cu(Anthr-BenIm-Py)(POP)]PF6 (3) (Ph-BenIm-Py = 3-benzyl-1-(pyridin-2-yl)-1H-benzimidazolylidene, Naph-BenIm-Py = 3-(naphthalen-2-yl-1-(pyridin-2-yl)-1H- benzimidazolylidene, Anthr-BenIm-Py = 3-(anthracen-9-yl)-1-(pyridin-2-yl)-1H-benzimidazolylidene, and POP = bis[2-diphenylphosphino]-phenyl)ether) have been synthesized and characterized. The different aryl substituents (phenyl, naphthyl, and anthracyl groups) were introduced into NHC ligands and the corresponding photophysical properties of the complexes were systematically investigated. The absorption spectra of all NHCsbnd Cu(I) complexes show a characteristic feature of metal-to-ligand charge transfer (MLCT) in the lower-energy region. Complex 1 exhibited good photoluminescence (PL) properties companying with the high quantum yields and long excited-state lifetimes, whereas 2 and 3 with naphthyl and anthracyl groups show the low PL efficiency caused by the strong π-π stacking interactions. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations were employed to rationalize the photophysical properties of the NHCsbnd Cu(I) complexes.

Dinuclear NHC-palladium complexes containing phosphine spacers: synthesis, X-ray structures and their catalytic activities towards the Hiyama coupling reaction.

PubMed

Yang, Jin; Li, Pinhua; Zhang, Yicheng; Wang, Lei

2014-05-21

Six dinuclear N-heterocyclic carbene (NHC) palladium complexes, [PdCl2(IMes)]2(μ-dppe) (1), [PdCl2(IPr)]2(μ-dppe) (2), [PdCl2(IMes)]2(μ-dppb) (3), [PdCl2(IPr)]2(μ-dppb) (4), [PdCl2(IMes)]2(μ-dpph) (5), and [PdCl2(IPr)]2(μ-dpph) (6) [IMes = N,N'-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr = N,N'-bis-(2,6-di(iso-propyl)phenyl)imidazol-2-ylidene; dppe = 1,2-bis(diphenylphosphino)ethane, dppb = 1,4-bis(diphenylphosphino)butane; and dpph = 1,6-bis(diphenylphosphino)hexane], have been synthesized through bridge-cleavage reactions of chloro-bridged dimeric compounds, [Pd(μ-Cl)(Cl)(NHC)]2, with the corresponding diphosphine ligands. The obtained compounds were fully characterized by (1)H NMR, (13)C NMR and (31)P NMR spectroscopy, FT-IR, elemental analysis and single-crystal X-ray crystallography. Moreover, further explorations of the catalytic potential of the dinuclear carbene palladium complexes as catalysts for the Pd-catalyzed transformations have been performed under microwave irradiation conditions, and the complexes exhibited moderate to good catalytic activity in the Hiyama coupling reaction of trimethoxyphenylsilane with aryl chlorides.

High-spatial-resolution mapping of catalytic reactions on single particles

DOE PAGES

Wu, Chung-Yeh; Wolf, William J.; Levartovsky, Yehonatan; ...

2017-01-26

We report the critical role in surface reactions and heterogeneous catalysis of metal atoms with low coordination numbers, such as found at atomic steps and surface defects, is firmly established. But despite the growing availability of tools that enable detailed in situ characterization, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resolution, and catalytic conversion can be tracked with a clear chemical signature; however, the combination of the two, which would enable high-spatial-resolution detection of reactions on catalytic surfaces, has rarely been achieved. Single-molecule fluorescence spectroscopy has beenmore » used to image and characterize single turnover sites at catalytic surfaces, but is restricted to reactions that generate highly fluorescing product molecules. Herein the chemical conversion of N-heterocyclic carbene molecules attached to catalytic particles is mapped using synchrotron-radiation-based infrared nanospectroscopy with a spatial resolution of 25 nanometres, which enabled particle regions that differ in reactivity to be distinguished. Lastly, these observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles-which contain metal atoms with low coordination numbers-are more active in catalysing oxidation and reduction of chemically active groups in surface-anchored N-heterocyclic carbene molecules.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Chung-Yeh; Wolf, William J.; Levartovsky, Yehonatan

We report the critical role in surface reactions and heterogeneous catalysis of metal atoms with low coordination numbers, such as found at atomic steps and surface defects, is firmly established. But despite the growing availability of tools that enable detailed in situ characterization, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resolution, and catalytic conversion can be tracked with a clear chemical signature; however, the combination of the two, which would enable high-spatial-resolution detection of reactions on catalytic surfaces, has rarely been achieved. Single-molecule fluorescence spectroscopy has beenmore » used to image and characterize single turnover sites at catalytic surfaces, but is restricted to reactions that generate highly fluorescing product molecules. Herein the chemical conversion of N-heterocyclic carbene molecules attached to catalytic particles is mapped using synchrotron-radiation-based infrared nanospectroscopy with a spatial resolution of 25 nanometres, which enabled particle regions that differ in reactivity to be distinguished. Lastly, these observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles-which contain metal atoms with low coordination numbers-are more active in catalysing oxidation and reduction of chemically active groups in surface-anchored N-heterocyclic carbene molecules.« less

Laser Spectroscopy of Radicals, Carbenes, and Ions in Superfluid Helium Droplets

NASA Astrophysics Data System (ADS)

Douberly, Gary E.

2015-06-01

The first beam of helium droplets was reported in the 1961 paper Strahlen aus kondensiertem Helium im Hochvakuum by Von E. W. Becker and co-workers. However, molecular spectroscopy of helium-solvated dopants wasn't realized until 30 years later in the laboratories of Scoles and Toennies. It has now been two decades since this early, seminal work on doped helium droplets, yet the field of helium droplet spectroscopy is still fresh with vast potential. Analogous in many ways to cryogenic matrix isolation spectroscopy, the helium droplet is an ideal environment to spectroscopically probe difficult to prepare molecular species, such as radicals, carbenes and ions. The quantum nature of helium at 0.35 K often results in molecular spectra that are sufficiently resolved to evoke an analysis of line shapes and fine-structure that is worthy of the International Symposium on Molecular Spectroscopy. The present talk will focus on our recent successful attempts to efficiently dope the title molecular species into helium droplets and probe their properties with infrared laser Stark and Zeeman spectroscopies. E. W. Becker, R. Klingelhöfer, P. Lohse, Z. Naturforsch. A 16A, 1259 (1961). S. Goyal, D. L. Schutt, G. Scoles, Phys. Rev. Lett. 69, 933 (1992). M. Hartmann, R. E. Miller, J. P. Toennies, A. F. Vilesov, Phys. Rev. Lett. 75, 1566, (1995).

Homobimetallic Ruthenium-N-Heterocyclic Carbene Complexes For Olefin Metathesis

NASA Astrophysics Data System (ADS)

Sauvage, Xavier; Demonceau, Albert; Delaude, Lionel

In this chapter, the synthesis and catalytic activity towards olefin metathesis of homobimetallic ruthenium (Ru)-alkylidene, -cyclodiene or -arene complexes bearing phosphine or N-heterocyclic carbene (NHC) ligands are reviewed. Emphasis is placed on the last category of bimetallic compounds. Three representatives of this new type of molecular scaffold were investigated. Thus, [(p-cymene)Ru(m-Cl)3RuCl (h2-C2H4)(L)] complexes with L = PCy3 (15a), IMes (16a), or IMesCl2 (16b) were prepared. They served as catalyst precursors for cross-metathesis (CM) of various styrene derivatives. These experiments revealed the outstanding aptitude of complex 16a (and to a lesser extent of 16b) to catalyze olefin metathesis reactions. Contrary to monometallic Ru-arene complexes of the [RuCl2(p-cymene)(L)] type, the new homobimetallic species did not require the addition of a diazo compound nor visible light illumination to initiate the ring-opening metathesis of norbornene or cyclooctene. When diethyl 2,2-diallylmalonate and N,N-diallyltosylamide were exposed to 16a,b, a mixture of cycloisomerization and ring-closing metathesis (RCM) products was obtained in a nonselective way. Addition of phenylacetylene enhanced the metathetical activity while completely repressing the cycloisomerization process.

Insights into the Competing Mechanisms and Origin of Enantioselectivity for N-Heterocyclic Carbene-Catalyzed Reaction of Aldehyde with Enamide

NASA Astrophysics Data System (ADS)

Qiao, Yan; Chen, Xinhuan; Wei, Donghui; Chang, Junbiao

2016-12-01

Hydroacylation reactions and aza-benzoin reactions have attracted considerable attention from experimental chemists. Recently, Wang et al. reported an interesting reaction of N-heterocyclic carbene (NHC)-catalyzed addition of aldehyde to enamide, in which both hydroacylation and aza-benzoin reactions may be involved. Thus, understanding the competing relationship between them is of great interest. Now, density functional theory (DFT) investigation was performed to elucidate this issue. Our results reveal that enamide can tautomerize to its imine isomer with the assistance of HCO3-. The addition of NHC to aldehydes formed Breslow intermediate, which can go through cross-coupling with enamide via hydroacylation reaction or its imine isomer via aza-benzoin reaction. The aza-benzoin reaction requires relatively lower free energy barrier than the hydroacylation reaction. The more polar characteristic of C=N group in the imine isomers, and the more advantageous stereoelectronic effect in the carbon-carbon bond forming transition states in aza-benzoin pathway were identified to determine that the imine isomer can react with the Breslow intermediate more easily. Furthermore, the origin of enantioselectivities for the reaction was explored and reasonably explained by structural analyses on key transition states. The work should provide valuable insights for rational design of switchable NHC-catalyzed hydroacylation and aza-benzoin reactions with high stereoselectivity.

Organocatalysis by bimacrocyclic NHCs: unexpected formation of a cyclic hemiacetal instead of a gamma-butyrolactone.

PubMed

Winkelmann, Ole; Näther, Christian; Lüning, Ulrich

2009-02-07

Two bimacrocyclic imidazolinium salts of different size, precursors to respective NHCs (N-heterocyclic carbenes), were tested as precatalysts in the reaction of aromatic aldehydes or ketones with enals. The expected lactones were produced in most cases, but in the reaction of methyl 4-formylbenzoate with cinnamaldehyde, the larger bimacrocycle led to the formation of a cyclic hemiacetal, while the smaller bimacrocycle gave the anticipated lactone.

Reaction of Diazo Compounds with Difluorocarbene: An Efficient Approach towards 1,1-Difluoroolefins.

PubMed

Zhang, Zhikun; Yu, Weizhi; Wu, Chenggui; Wang, Chengpeng; Zhang, Yan; Wang, Jianbo

2016-01-04

A transition-metal-free difluoromethylenation of diazo compounds that proceeds under mild conditions has been developed and is based on the use of TMSCF2 Br as the difluoromethylene source and tetrabutylammonium bromide (TBAB) as the promoter. The chemoselective formal carbene dimerization reaction is achieved owing to the electronic properties and the relative stability of the difluorocarbene intermediate. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Solvent-Free Benzoin and Stetter Reactions with a Small Amount of NHC Catalyst in the Liquid or Semisolid State.

PubMed

Ema, Tadashi; Nanjo, Yoshiko; Shiratori, Sho; Terao, Yuta; Kimura, Ryo

2016-11-04

The intermolecular or intramolecular asymmetric benzoin reaction was catalyzed by a small amount of N-heterocyclic carbene (NHC) (0.2-1 mol %) under solvent-free conditions. The solvent-free intramolecular asymmetric Stetter reaction also proceeded efficiently with NHC (0.2-1 mol %). In some cases, even solid-to-solid or solid-to-liquid conversions took place with low catalyst loading (0.2-1 mol %).

Chemo- and Diastereoselective N-Heterocyclic Carbene-Catalyzed Cross-Benzoin Reactions Using N-Boc-α-amino Aldehydes.

PubMed

Haghshenas, Pouyan; Gravel, Michel

2016-09-16

N-Boc-α-amino aldehydes are shown to be excellent partners in cross-benzoin reactions with aliphatic or heteroaromatic aldehydes. The chemoselectivity of the reaction and the facial selectivity on the amino aldehyde allow cross-benzoin products to be obtained in good yields and good diastereomeric ratios. The developed method is utilized as the key step in a concise total synthesis of d-arabino-phytosphingosine.

Efficient Domino Hydroformylation/Benzoin Condensation: Highly Selective Synthesis of α-Hydroxy Ketones.

PubMed

Dong, Kaiwu; Sang, Rui; Soule, Jean-Francois; Bruneau, Christian; Franke, Robert; Jackstell, Ralf; Beller, Matthias

2015-12-07

An improved domino hydroformylation/benzoin condensation to give α-hydroxy ketones has been developed. Easily available olefins are smoothly converted into the corresponding α-hydroxy ketones in high yields with excellent regioselectivities. Key to success is the use of a specific catalytic system consisting of a rhodium/phosphine complex and the CO2 adduct of an N-heterocyclic carbene. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chemoselective N-heterocyclic carbene-catalyzed cross-benzoin reactions: importance of the fused ring in triazolium salts.

PubMed

Langdon, Steven M; Wilde, Myron M D; Thai, Karen; Gravel, Michel

2014-05-28

Morpholinone- and piperidinone-derived triazolium salts are shown to catalyze highly chemoselective cross-benzoin reactions between aliphatic and aromatic aldehydes. The reaction scope includes ortho-, meta-, and para-substituted benzaldehyde derivatives with a range of electron-donating and -withdrawing groups as well as branched and unbranched aliphatic aldehydes. Catalytic loadings as low as 5 mol % give excellent yields in these reactions (up to 99%).

Stereoselective Synthesis of Tetrahydroindolizines through the Catalytic Formation of Pyridinium Ylides from Diazo Compounds.

PubMed

Day, Jonathan; McKeever-Abbas, Ben; Dowden, James

2016-05-04

Commercially available iron(III) and copper(I) complexes catalyzed multicomponent cycloaddition reactions between diazo compounds, pyridines, and electrophilic alkenes to give alkaloid-inspired tetrahydroindolizidines in high yield with high diastereoselectivity. Hitherto, the catalytic formation of versatile pyridinium ylides from metal carbenes has been poorly developed; the broad utility demonstrated herein sets the stage for the invention of further multicomponent reactions in future. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Regioselective syntheses of 1,2-benzothiazines by rhodium-catalyzed annulation reactions.

PubMed

Cheng, Ying; Bolm, Carsten

2015-10-12

Rhodium-catalyzed directed carbene insertions into aromatic CH bonds of S-aryl sulfoximines lead to intermediates, which upon dehydration provide 1,2-benzothiazines in excellent yields. The domino-type process is regioselective and shows a high functional-group tolerance. It is scalable, and the only by-products are dinitrogen and water. Three illustrative transformations underscore the synthetic value of the products. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Developing effective electronic-only coupled-cluster and Møller-Plesset perturbation theories for the muonic molecules.

PubMed

Goli, Mohammad; Shahbazian, Shant

2018-06-20

Recently we have proposed an effective Hartree-Fock (EHF) theory for the electrons of the muonic molecules that is formally equivalent to the HF theory within the context of the nuclear-electronic orbital theory [Phys. Chem. Chem. Phys., 2018, 20, 4466]. In the present report we extend the muon-specific effective electronic structure theory beyond the EHF level by introducing the effective second order Møller-Plesset perturbation theory (EMP2) and the effective coupled-cluster theory at single and double excitation levels (ECCSD) as well as an improved version including perturbative triple excitations (ECCSD(T)). These theories incorporate electron-electron correlation into the effective paradigm and through their computational implementation, a diverse set of small muonic species is considered as a benchmark at these post-EHF levels. A comparative computational study on this set demonstrates that the muonic bond length is in general non-negligibly longer than corresponding hydrogenic analogs. Next, the developed post-EHF theories are applied for the muoniated N-heterocyclic carbene/silylene/germylene and the muoniated triazolium cation revealing the relative stability of the sticking sites of the muon in each species. The computational results, in line with previously reported experimental data demonstrate that the muon generally prefers to attach to the divalent atom with carbeneic nature. A detailed comparison of these muonic adducts with the corresponding hydrogenic adducts reveals subtle differences that have already been overlooked.

Experimental charge density analysis of a gallium(I) N-heterocyclic carbene analogue.

PubMed

Overgaard, Jacob; Jones, Cameron; Dange, Deepak; Platts, James A

2011-09-05

The experimental electron density of the only known example of a four-membered Ga(I) N-heterocyclic carbene analogue has been determined by multipole modeling of 90 K X-ray diffraction data and compared to theoretical data. In order to obtain a satisfactory model, it is necessary to modify the radial dependency of the core electrons of Ga using two separate scaling parameters for s,p- and d-electrons. Evidence for significant lone-pair density on Ga is found in the electron density and derived properties despite the partial positive charge of this atom. Static deformation density and molecular electrostatic potential clearly show a directional lone pair on Ga, whereas the Laplacian of the total electron density does not; this feature is, however, present in the Laplacian of the valence-only density. The Ga center also acts as an acceptor in four intramolecular C-H···Ga contacts, whose nature is probed by density properties. Substantial covalent character is apparent in the Ga-N bonds, but no sign of donation from filled N p-orbitals to empty Ga p-orbitals is found, whereas π-delocalization over the organic ligand is evident. This study highlights the utility of experimental charge density analysis as a technique to investigate the unusual bonding and electronic characteristics of low oxidation state/low coordinate p-block complexes.

Biomedical properties of a series of ruthenium-N-heterocyclic carbene complexes based on oxidant activity in vitro and assessment in vivo of biosafety in zebrafish embryos.

PubMed

Alfaro, Juan M; Prades, Amparo; del Carmen Ramos, María; Peris, Eduardo; Ripoll-Gómez, Jorge; Poyatos, Macarena; Burgos, Javier S

2010-03-01

N-Heterocyclic carbene (NHC) ligands have attracted great interest over the last decade for their use in the design of homogenous catalysts. NHC-based metal complexes have interesting potential biomedical applications, such as in antimicrobial and cancer therapy, which are beginning to be explored more fully. We have studied here the oxidant activities of a series of Ru(II) complexes in vitro and zebrafish (Danio rerio) have been used as a model in vivo to investigate and characterize the toxicity of some of these compounds. Dual behavior was observed for the NHC-based complexes as they behaved as antioxidants at low concentrations but showed pro-oxidant capacity at higher concentrations. Zebrafish embryos were exposed to Ru(II) complexes under several different conditions (0 or 24 h postfertilization, with or without the chorion) and various parameters, such as viability, edema, heart rate, blood coagulation, pigmentation, scoliosis, malformation, and hatching, were tested. In general, zebrafish embryos were not harmed by exposure to Ru(II) complexes whatever the experimental conditions. Several toxicity profiles were observed depending upon the chemical structure of the compound in question. Their characteristics as pro-oxidant and/or antioxidant agents together with their biosafety may point to their having biomedical applications as antitumoral or neuroprotective drugs.

Insights into the Competing Mechanisms and Origin of Enantioselectivity for N-Heterocyclic Carbene-Catalyzed Reaction of Aldehyde with Enamide

PubMed Central

Qiao, Yan; Chen, Xinhuan; Wei, Donghui; Chang, Junbiao

2016-01-01

Hydroacylation reactions and aza-benzoin reactions have attracted considerable attention from experimental chemists. Recently, Wang et al. reported an interesting reaction of N-heterocyclic carbene (NHC)-catalyzed addition of aldehyde to enamide, in which both hydroacylation and aza-benzoin reactions may be involved. Thus, understanding the competing relationship between them is of great interest. Now, density functional theory (DFT) investigation was performed to elucidate this issue. Our results reveal that enamide can tautomerize to its imine isomer with the assistance of HCO3−. The addition of NHC to aldehydes formed Breslow intermediate, which can go through cross-coupling with enamide via hydroacylation reaction or its imine isomer via aza-benzoin reaction. The aza-benzoin reaction requires relatively lower free energy barrier than the hydroacylation reaction. The more polar characteristic of C=N group in the imine isomers, and the more advantageous stereoelectronic effect in the carbon-carbon bond forming transition states in aza-benzoin pathway were identified to determine that the imine isomer can react with the Breslow intermediate more easily. Furthermore, the origin of enantioselectivities for the reaction was explored and reasonably explained by structural analyses on key transition states. The work should provide valuable insights for rational design of switchable NHC-catalyzed hydroacylation and aza-benzoin reactions with high stereoselectivity. PMID:27905524

Targeting antioxidant pathways with ferrocenylated N-heterocyclic carbene supported gold(i) complexes in A549 lung cancer cells† †Electronic supplementary information (ESI) available. CCDC 1419940 and 1419941. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc03519h

PubMed Central

McCall, R.; Sidoran, K. J.; Magda, D.; Mitchell, N. A.; Bielawski, C. W.; Lynch, V. M.; Sessler, J. L.

2016-01-01

Ferrocene containing N-heterocyclic carbene (NHC) ligated gold(i) complexes of the type [Au(NHC)2]+ were prepared and found to be capable of regulating the formation of reactive oxygen species (ROS) via multiple mechanisms. Single crystal X-ray analysis of bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)-gold(i) chloride (5) and bis(1,3-di(ferrocenylmethyl)imidazol-2-ylidene)-gold(i) chloride (6) revealed a quasi-linear geometry around the gold(i) centers (i.e., the C–Au–C bond angle were measured to be ∼177° and all the Au–Ccarbene bonds distances were in the range of 2.00 (7)–2.03 (1) Å). A series of cell studies indicated that cell proliferation inhibition and ROS generation were directly proportional to the amount of ferrocene contained within the [Au(NHC)2]+ complexes (IC50 of 6 < 5 < bis(1-benzyl-3-mesitylimidazol-2-ylidene)-gold(i) chloride (4)). Complexes 4–6 were also confirmed to inhibit thioredoxin reductase as inferred from lipoate reduction assays and increased chelatable intracellular zinc concentrations. RNA microarray gene expression assays revealed that 6 induces endoplasmic reticulum stress response pathways as a result of ROS increase. PMID:26918111

In situ Generated Ruthenium Catalyst Systems Bearing Diverse N-Heterocyclic Carbene Precursors for Atom-Economic Amide Synthesis from Alcohols and Amines.

PubMed

Cheng, Hua; Xiong, Mao-Qian; Cheng, Chuan-Xiang; Wang, Hua-Jing; Lu, Qiang; Liu, Hong-Fu; Yao, Fu-Bin; Chen, Cheng; Verpoort, Francis

2018-02-16

The transition-metal-catalyzed direct synthesis of amides from alcohols and amines is herein demonstrated as a highly environmentally benign and atom-economic process. Among various catalyst systems, in situ generated N-heterocyclic carbene (NHC)-based ruthenium (Ru) halide catalyst systems have been proven to be active for this transformation. However, these existing catalyst systems usually require an additional ligand to achieve satisfactory results. In this work, through extensive screening of a diverse variety of NHC precursors, we discovered an active in situ catalyst system for efficient amide synthesis without any additional ligand. Notably, this catalyst system was found to be insensitive to the electronic effects of the substrates, and various electron-deficient substrates, which were not highly reactive with our previous catalyst systems, could be employed to afford the corresponding amides efficiently. Furthermore, mechanistic investigations were performed to provide a rationale for the high activity of the optimized catalyst system. NMR-scale reactions indicated that the rapid formation of a Ru hydride intermediate (signal at δ=-7.8 ppm in the 1 H NMR spectrum) after the addition of the alcohol substrate should be pivotal in establishing the high catalyst activity. Besides, HRMS analysis provided possible structures of the in situ generated catalyst system. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Visible luminescence studies in the infrared multiphoton dissociation of 1,2-dichloro-1,1-difluoroethane

NASA Astrophysics Data System (ADS)

Pushpa, K. K.; Kumar, Awadesh; Naik, P. D.; Annaji Rao, K.; Parthasarathy, V.; Sarkar, S. K.; Mittal, J. P.

1997-11-01

A strong visible luminescence was observed in the CO 2 laser induced infrared multiphoton dissociation of 1,2-dichloro-1,1-difluoroethane. The emission observed between 350-750 nm is attributed to electronically excited carbene CF 2ClCH. The temporal profile of this luminescence was studied as a function of laser pulse duration, pulse energy, excitation frequency and substrate pressure. A suitable dissociation mechanism is presented considering various channels of IRMPD of this molecule.

Molecular Catalysis at Polarized Interfaces Created by Ferroelectric BaTiO3 (Postprint)

DTIC Science & Technology

2017-02-06

product ratio to 4.5 : 1.0. Next, 4b was attached to various ferroelectric and non-ferroelectric oxides studied . Reactions performed with 2 mM 1 and...interfaces without applying a voltage. We studied the effects of ferroelectric oxides on the selectivity of an Rh porphyrin-catalyzed carbene...rearrangement. The addition of ferroelectric BaTiO3 nanoparticles to the reaction solution changed the product ratio in the same direction and by a similar

Synthesis of isocoumarins through three-component couplings of arynes, terminal alkynes, and carbon dioxide catalyzed by an NHC-copper complex.

PubMed

Yoo, Woo-Jin; Nguyen, Thanh V Q; Kobayashi, Shū

2014-09-15

A copper-catalyzed multicomponent coupling reaction between in situ generated ortho-arynes, terminal alkynes, and carbon dioxide was developed to access isocoumarins in moderate to good yields. The key to this CO2-incorporating reaction was the use of a versatile N-heterocyclic carbene/copper complex that was able to catalyze multiple transformations within the three-component reaction. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Total Synthesis of the Post-translationally Modified Polyazole Peptide Antibiotic Goadsporin.

PubMed

Dexter, Hannah L; Williams, Huw E L; Lewis, William; Moody, Christopher J

2017-03-06

The structurally unique polyazole antibiotic goadsporin contains six heteroaromatic oxazole and thiazole rings integrated into a linear array of amino acids that also contains two dehydroalanine residues. An efficient total synthesis of goadsporin is reported in which the key steps are the use of rhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate the four oxazole rings, which demonstrates the power of rhodium carbene chemistry in organic chemical synthesis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gold(I)-catalyzed diazo coupling: strategy towards alkene formation and tandem benzannulation.

PubMed

Zhang, Daming; Xu, Guangyang; Ding, Dong; Zhu, Chenghao; Li, Jian; Sun, Jiangtao

2014-10-06

A gold(I)-catalyzed cross-coupling of diazo compounds to afford tetrasubstituted alkenes has been developed by taking advantage of a trivial electronic difference between two diazo substrates. A N-heterocyclic-carbene-derived gold complex is the most effective catalyst for this transformation. Based on this new strategy, a gold(I)-initiated benzannulation has been achieved through a tandem reaction involving a diazo cross-coupling, 6π electrocyclization, and oxidative aromatization. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Alkyl halide-free heteroatom alkylation and epoxidation facilitated by a recyclable polymer-supported oxidant for the in-flow preparation of diazo compounds.

PubMed

Nicolle, Simon M; Hayes, Christopher J; Moody, Christopher J

2015-03-16

Highly reactive metal carbenes, generated from simple ketones via diazo compounds, including diazo-amides and -phosphonates, using a recyclable reagent in-flow, are transient but versatile electrophiles for heteroatom alkylation reactions and for epoxide formation. The method produces no organic waste, with the only by-products being water, KI and nitrogen, without the attendant hazards of isolation of intermediate diazo compounds. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Rhodium(III)-Catalyzed [3+2]/[5+2] Annulation of 4-Aryl 1,2,3-Triazoles with Internal Alkynes through Dual C(sp2)-H Functionalization.

PubMed

Yang, Yuan; Zhou, Ming-Bo; Ouyang, Xuan-Hui; Pi, Rui; Song, Ren-Jie; Li, Jin-Heng

2015-05-26

A rhodium(III)-catalyzed [3+2]/[5+2] annulation of 4-aryl 1-tosyl-1,2,3-triazoles with internal alkynes is presented. This transformation provides straightforward access to indeno[1,7-cd]azepine architectures through a sequence involving the formation of a rhodium(III) azavinyl carbene, dual C(sp(2))-H functionalization, and [3+2]/[5+2] annulation. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Antibacterial Drug Releasing Materials by Post-Polymerization Surface Modification

NASA Astrophysics Data System (ADS)

Chng, Shuyun; Moloney, Mark G.; Wu, Linda Y. L.

Functional materials are available by the post-polymerization surface modification of diverse polymers in a three-step process mediated, firstly, by carbene insertion chemistry, secondly, by diazonium coupling, and thirdly by modification with a remotely tethered spiropyran unit, and these materials may be used for the reversible binding and release of Penicillin V. Surface loading densities of up to 0.19mmol/g polymer are achievable, leading to materials with higher loading densities and release behavior relative to unmodified controls, and observable antibacterial biocidal activity.

Organocatalytic C-H bond arylation of aldehydes to bis-heteroaryl ketones.

PubMed

Toh, Qiao Yan; McNally, Andrew; Vera, Silvia; Erdmann, Nico; Gaunt, Matthew J

2013-03-13

An organocatalytic aldehyde C-H bond arylation process for the synthesis of complex heteroaryl ketones has been developed. By exploiting the inherent electrophilicity of diaryliodonium salts, we have found that a commercial N-heterocyclic carbene catalyst promotes the union of heteroaryl aldehydes and these heteroaromatic electrophile equivalents in good yields. This straightforward catalytic protocol offers access to ketones bearing a diverse array of arene and heteroarene substituents that can subsequently be converted into molecules displaying structural motifs commonly found in medicinal agents.

Physical Properties and CO2 Reaction Pathway of 1-Ethyl-3-Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seo, S; DeSilva, MA; Brennecke, JF

2014-12-25

Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO2 capture technologies. In this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim](+)) cations were synthesized, and their physical properties (density, viscosity, and ionic conductivity) were measured. In addition, CO2 solubility in each IL was determined at room temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic anions that are capable of reacting stoichiometrically with CO2 to form carbamate species. An interesting CO2 uptake isotherm behavior was observed, and this may be attributed to a parallel, equilibrium proton exchange processmore » between the imidazolium cation and the basic AHA in the presence of CO2, followed by the formation of "transient" carbene species that react rapidly with CO2. The presence of the imidazolium-carboxylate species and carbamate anion species was verified using H-1 and C-13 NMR spectroscopy. While the reaction between CO2 and the proposed transient carbene resulted in cation-CO2 binding that is stronger than the anion-CO2 reaction, the reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 degrees C with nitrogen purging. The presence of water decreased the CO2 uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed.« less

Aerosolized Antimicrobial Agents Based on Degradable Dextran Nanoparticles Loaded with Silver Carbene Complexes

PubMed Central

Ornelas-Megiatto, Cátia; Shah, Parth N.; Wich, Peter R.; Cohen, Jessica L.; Tagaev, Jasur A.; Smolen, Justin A.; Wright, Brian D.; Panzner, Matthew J.; Youngs, Wiley J.; Fréchet, Jean M. J.; Cannon, Carolyn L.

2012-01-01

Degradable acetalated dextran (Ac-DEX) nanoparticles were prepared and loaded with a hydrophobic silver carbene complex (SCC) by a single-emulsion process. The resulting particles were characterized for morphology and size distribution using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The average particle size and particle size distribution were found to be a function of the ratio of the organic phase to the surfactant containing aqueous phase with a 1:5 volume ratio of Ac-DEX CH2Cl2 (organic): PBS (aqueous) being optimal for the formulation of nanoparticles with an average size of 100 ± 40 nm and a low polydispersity. The SCC loading was found to increase with an increase in the SCC quantity in the initial feed used during particle formulation up to 30% (w/w); however, the encapsulation efficiency was observed to be the best at a feed ratio of 20% (w/w). In vitro efficacy testing of the SCC loaded Ac-DEX nanoparticles demonstrated their activity against both Gram-negative and Gram-positive bacteria; the nanoparticles inhibited the growth of every bacterial species tested. As expected, a higher concentration of drug was required to inhibit bacterial growth when the drug was encapsulated within the nanoparticle formulations compared with the free drug illustrating the desired depot release. Compared with free drug, the Ac-DEX nanoparticles were much more readily suspended in an aqueous phase and subsequently aerosolized, thus providing an effective method of pulmonary drug delivery. PMID:23025592

Starbon/High-Amylose Corn Starch-Supported N-Heterocyclic Carbene-Iron(III) Catalyst for Conversion of Fructose into 5-Hydroxymethylfurfural.

PubMed

Matharu, Avtar S; Ahmed, Suleiman; Almonthery, Badriya; Macquarrie, Duncan J; Lee, Yoon-Sik; Kim, Yohan

2018-02-22

Iron-N-heterocyclic carbene complexes (Fe-NHCs) have come to prominence because of their applicability in diverse catalytic reactions, ranging from C-C cross-coupling and C-X bond formation to substitution, reduction, polymerization, and dehydration reactions. The detailed synthesis, characterization, and application of novel heterogeneous Fe-NHC catalysts immobilized on mesoporous expanded high-amylose corn starch (HACS) and Starbon 350 (S350) for facile fructose conversion into 5-hydroxymethylfurfural (HMF) is reported. Both catalyst types showed good performance for the dehydration of fructose to HMF when the reaction was tested at 100 °C with varying time (10 min, 20 min, 0.5 h, 1 h, 3 h and 6 h). For Fe-NHC/S350, the highest HMF yield was 81.7 % (t=0.5 h), with a TOF of 169 h -1 , fructose conversion of 95 %, and HMF selectivity of 85.7 %, whereas for Fe-NHC/expanded HACS, the highest yield was 86 % (t=0.5 h), with a TOF of 206 h -1 , fructose conversion of 87 %, and HMF selectivity of 99 %. Iron loadings of 0.26 and 0.30 mmol g -1 were achieved for Fe-NHC/expanded starch and Fe-NHC/S350, respectively. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Rhenium complexes of bidentate, bis-bidentate and tridentate N-heterocyclic carbene ligands.

PubMed

Chan, Chung Ying; Barnard, Peter J

2015-11-28

A series of eight Rhenium(I)-N-heterocyclic carbene (NHC) complexes of the general form [ReCl(CO)3(C^C)] (where C^C is a bis(NHC) bidentate ligand), [ReCl(CO)3(C^C)]2 (where C^C is a bis-bidentate tetra-NHC ligand) and [Re(CO)3(C^N^C)](+)[X](-) (where C^N^C is a bis(NHC)-amine ligand and the counter ion X is either the ReO4(-) or PF6(-)) have been synthesised using a Ag2O transmetallation protocol. The novel precursor imidazolium salts and Re(I) complexes were characterized by elemental analysis, (1)H and (13)C NMR spectroscopy and the molecular structures for two imidazolium salt and six Re(I) complexes were determined by single crystal X-ray diffraction. These NHC ligand systems are of interest for possible applications in the development of Tc-99m or Re-186/188 radiopharmaceuticals and as such the stability of two complexes of the form [ReCl(CO)3(C^C)] and [Re(CO)3(C^N^C)][ReO4] were evaluated in ligand challenge experiments using the metal binding amino acids L-histidine or L-cysteine. These studies showed that the former was unstable, with the chloride ligand being replaced by either cysteine or histidine, while no evidence for transchelation was observed for the latter suggesting that bis(NHC)-amine ligands of this type may be suitable for biological applications.

Metal-free catalytic enantioselective C-B bond formation: (pinacolato)boron conjugate additions to α,β-unsaturated ketones, esters, Weinreb amides, and aldehydes promoted by chiral N-heterocyclic carbenes.

PubMed

Wu, Hao; Radomkit, Suttipol; O'Brien, Jeannette M; Hoveyda, Amir H

2012-05-16

The first broadly applicable metal-free enantioselective method for boron conjugate addition (BCA) to α,β-unsaturated carbonyls is presented. The C-B bond forming reactions are promoted in the presence of 2.5-7.5 mol % of a readily accessible C(1)-symmetric chiral imidazolinium salt, which is converted, in situ, to the catalytically active diastereo- and enantiomerically pure N-heterocyclic carbene (NHC) by the common organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu). In addition to the commercially available bis(pinacolato)diboron [B(2)(pin)(2)], and in contrast to reactions with the less sterically demanding achiral NHCs, the presence of MeOH is required for high efficiency. Acyclic and cyclic α,β-unsaturated ketones, as well as acyclic esters, Weinreb amides, and aldehydes, can serve as suitable substrates; the desired β-boryl carbonyls are isolated in up to 94% yield and >98:2 enantiomer ratio (er). Transformations are often carried out at ambient temperature. In certain cases, such as when the relatively less reactive unsaturated amides are used, elevated temperatures are required (50-66 °C); nonetheless, reactions remain highly enantioselective. The utility of the NHC-catalyzed method is demonstrated through comparison with the alternative Cu-catalyzed protocols; in cases involving a polyfunctional substrate, unique profiles in chemoselectivity are exhibited by the metal-free approach (e.g., conjugate addition vs reaction with an alkyne, allene, or aldehyde).

Aerosolized antimicrobial agents based on degradable dextran nanoparticles loaded with silver carbene complexes.

PubMed

Ornelas-Megiatto, Cátia; Shah, Parth N; Wich, Peter R; Cohen, Jessica L; Tagaev, Jasur A; Smolen, Justin A; Wright, Brian D; Panzner, Matthew J; Youngs, Wiley J; Fréchet, Jean M J; Cannon, Carolyn L

2012-11-05

Degradable acetalated dextran (Ac-DEX) nanoparticles were prepared and loaded with a hydrophobic silver carbene complex (SCC) by a single-emulsion process. The resulting particles were characterized for morphology and size distribution using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The average particle size and particle size distribution were found to be a function of the ratio of the organic phase to the surfactant containing aqueous phase with a 1:5 volume ratio of Ac-DEX CH(2)Cl(2) (organic):PBS (aqueous) being optimal for the formulation of nanoparticles with an average size of 100 ± 40 nm and a low polydispersity. The SCC loading was found to increase with an increase in the SCC quantity in the initial feed used during particle formulation up to 30% (w/w); however, the encapsulation efficiency was observed to be the best at a feed ratio of 20% (w/w). In vitro efficacy testing of the SCC loaded Ac-DEX nanoparticles demonstrated their activity against both Gram-negative and Gram-positive bacteria; the nanoparticles inhibited the growth of every bacterial species tested. As expected, a higher concentration of drug was required to inhibit bacterial growth when the drug was encapsulated within the nanoparticle formulations compared with the free drug illustrating the desired depot release. Compared with free drug, the Ac-DEX nanoparticles were much more readily suspended in an aqueous phase and subsequently aerosolized, thus providing an effective method of pulmonary drug delivery.

Gold(I) and Gold(III) Complexes of Cyclic (Alkyl)(amino)carbenes

PubMed Central

2016-01-01

The chemistry of Au(I) complexes with two types of cyclic (alkyl)(amino)carbene (CAAC) ligands has been explored, using the sterically less demanding dimethyl derivative Me2CAAC and the 2-adamantyl ligand AdCAAC. The conversion of (AdCAAC)AuCl into (AdCAAC)AuOH by treatment with KOH is significantly accelerated by the addition of tBuOH. (AdCAAC)AuOH is a convenient starting material for the high-yield syntheses of (AdCAAC)AuX complexes by acid/base and C–H activation reactions (X = OAryl, CF3CO2, N(Tf)2, C2Ph, C6F5, C6HF4, C6H2F3, CH2C(O)C6H4OMe, CH(Ph)C(O)Ph, CH2SO2Ph), while the cationic complexes [(AdCAAC)AuL]+ (L = CO, CNtBu) and (AdCAAC)AuCN were obtained by chloride substitution from (AdCAAC)AuCl. The reactivity toward variously substituted fluoroarenes suggests that (AdCAAC)AuOH is able to react with C–H bonds with pKa values lower than about 31.5. This, together with the spectroscopic data, confirm the somewhat stronger electron-donor properties of CAAC ligands in comparison to imidazolylidene-type N-heterocyclic carbenes (NHCs). In spite of this, the oxidation of Me2CAAC and AdCAAC gold compounds is much less facile. Oxidations proceed with C–Au cleavage by halogens unless light is strictly excluded. The oxidation of (AdCAAC)AuCl with PhICl2 in the dark gives near-quantitative yields of (AdCAAC)AuCl3, while [Au(Me2CAAC)2]Cl leads to trans-[AuCl2(Me2CAAC)2]Cl. In contrast to the chemistry of imidazolylidene-type gold NHC complexes, oxidation products containing Au–Br or Au–I bonds could not be obtained; whereas the reaction with CsBr3 cleaves the Au–C bond to give mixtures of [AdCAAC-Br]+[AuBr2]− and [(AdCAAC-Br)]+ [AuBr4]−, the oxidation of (AdCAAC)AuI with I2 leads to the adduct (AdCAAC)AuI·I2. Irrespective of the steric demands of the CAAC ligands, their gold complexes proved more resistant to oxidation and more prone to halogen cleavage of the Au–C bonds than gold(I) complexes of imidazole-based NHC ligands. PMID:26146436

Optical spectra, electronic structure and aromaticity of benzannulated N-heterocyclic carbene and its analogues of the type C6H4(NR)2E: (E = Si, Ge, Sn, Pb).

PubMed

Aysin, Rinat R; Bukalov, Sergey S; Leites, Larissa A; Zabula, Alexander V

2017-07-11

A series of benzannulated N-heterocyclic compounds containing divalent 14 group atoms, C 6 H 4 (NR) 2 E II , E = C, Si, Ge, Sn, Pb, have been studied by various experimental (vibrational and UV-vis spectroscopy) and theoretical (NICS, ISE, ACID) techniques. The methods used confirm 10 π-electron delocalization (aromaticity) in these heterocycles, however, the aromaticity sequences estimated by the criteria based on different physical properties do not coincide.

Rh-Catalyzed Intermolecular Reactions of α-Alkyl-α-Diazo Carbonyl Compounds with Selectivity over β-Hydride Migration

PubMed Central

DeAngelis, Andrew; Panish, Robert; Fox, Joseph M.

2016-01-01

CONSPECTUS Rh-carbenes derived from α-diazocarbonyl compounds have found broad utility across a remarkable range of reactivity, including cyclopropanation, cyclopropenation, C–H insertions, heteroatom–H insertions, and ylide forming reactions. However, in contrast to α-aryl or α-vinyl-α-diazocarbonyl compounds, the utility of α-alkyl-α-diazocarbonyl compounds had been moderated by the propensity of such compounds to undergo intramolecular β-hydride migration to give alkene products. Especially challenging had been intermolecular reactions involving α-alkyl-α-diazocarbonyl compounds. PMID:26689221

E-Selective Semi-Hydrogenation of Alkynes by Heterobimetallic Catalysis.

PubMed

Karunananda, Malkanthi K; Mankad, Neal P

2015-11-25

A unique cooperative H2 activation reaction by heterobimetallic (NHC)M'-MCp(CO)2 complexes (NHC = N-heterocyclic carbene, M' = Cu or Ag, M = Fe or Ru) has been leveraged to develop a catalytic alkyne semi-hydrogenation transformation. The optimal Ag-Ru catalyst gives high selectivity for converting alkynes to E-alkenes, a rare selectivity mode for reduction reactions with H2. The transformation is tolerant of many reducible functional groups. Computational analysis of H2 activation thermodynamics guided rational catalyst development. Bimetallic alkyne hydrogenation and alkene isomerization mechanisms are proposed.

N-Heterocyclic Carbene Complexes in Dehalogenation Reactions

NASA Astrophysics Data System (ADS)

Mas-Marzá, Elena; Page, Michael J.; Whittlesey, Michael K.

Catalytic dehalogenation represents an underdeveloped transformation in M-NHC chemistry with a small number of reports detailing the reactivity of Co, Ru, Ni and Pd catalysts. In situ generated nickel and palladium NHC complexes catalyse the hydrodechlorination of aryl chlorides. Lower coordinate Ni complexes are proposed to operate in the hydrodefluorination of mono- and poly-fluorinated substrates. The single example of Ru-NHC catalysed hydrodefluorination of fully and partially fluorinated aromatic substrates is characterised by an unusual regioselectivity. The highly regioselective dehydrohalogenation of relatively unreactive alkyl halide substrates is achieved with a cobalt NHC catalyst.

Selective synthesis of cis- and trans-[(NHC(Me))2PtCl2] and [NHC(Me)Pt(cod)Cl][NHC(Me)PtCl3] using NHC(Me)SiCl4.

PubMed

Lewis-Alleyne, Lesley C; Bassil, Bassem S; Böttcher, Tobias; Röschenthaler, Gerd-Volker

2014-11-14

NHC(Me)SiCl4 (NHC(Me) = 1,3-dimethylimidazolidin-2-ylidene) was used to synthesise novel NHC(Me)-Pt(ii) complexes. An atypical trans-cis isomerisation process was also achieved for [(NHC(Me))2PtCl2], while the synthesis of the unique double-complex salt [(NHC(Me))Pt(cod)Cl] [(NHC(Me))PtCl3] (cod = 1,5-cyclooctadiene) revealed the first-ever N-heterocyclic carbene analogue of the Cossa's salt anion.

Dynamic kinetic asymmetric cross-benzoin additions of β-stereogenic α-keto esters.

PubMed

Goodman, C Guy; Johnson, Jeffrey S

2014-10-22

The dynamic kinetic resolution of β-halo α-keto esters via an asymmetric cross-benzoin reaction is described. A chiral N-heterocyclic carbene catalyzes the umpolung addition of aldehydes to racemic α-keto esters. The resulting fully substituted β-halo glycolic ester products are obtained with high levels of enantio- and diastereocontrol. The high chemoselectivity observed is a result of greater electrophilicity of the α-keto ester toward the Breslow intermediate. The reaction products are shown to undergo highly diastereoselective substrate-controlled reduction to give highly functionalized stereotriads.

Dynamic Kinetic Asymmetric Cross-Benzoin Additions of β-Stereogenic α-Keto Esters

PubMed Central

2015-01-01

The dynamic kinetic resolution of β-halo α-keto esters via an asymmetric cross-benzoin reaction is described. A chiral N-heterocyclic carbene catalyzes the umpolung addition of aldehydes to racemic α-keto esters. The resulting fully substituted β-halo glycolic ester products are obtained with high levels of enantio- and diastereocontrol. The high chemoselectivity observed is a result of greater electrophilicity of the α-keto ester toward the Breslow intermediate. The reaction products are shown to undergo highly diastereoselective substrate-controlled reduction to give highly functionalized stereotriads. PMID:25299730

Nature Inspired Strategies for New Organic Materials

DTIC Science & Technology

2007-01-14

8217/. Compound Side-products: lir- NOH Ar AH ÷ R IS Ar • R Ph Ph 65 3a Ar benzoin product Ph NHDF 04 Ar-1A- bnzinpodcNaH/DMF Ph Ph H 61 3b 1 2 up to 65% yield...NHC-Catalyzed Acylatlon0 process is depicted in Scheme 2. The major N 0 side products of the reaction were identified o as benzoin -type products...derived from the Ph 3C\\NJM Ar H addition of intermediate IV to the starting carbene aldehyde) and more importantly, the acylated benzoin adduct 4

Rhodium(II)-Catalyzed and Thermally Induced Intramolecular Migration of N-Sulfonyl-1,2,3-triazoles: New Approaches to 1,2-Dihydroisoquinolines and 1-Indanones.

PubMed

Sun, Run; Jiang, Yu; Tang, Xiang-Ying; Shi, Min

2016-04-11

New rhodium(II)-catalyzed or thermally induced intramolecular alkoxy group migration of N-sulfonyl-1,2,3-triazoles has been developed, affording divergent synthesis of 1,2-dihydroisoquinoline and 1-indanone derivatives according to different conditions. N-Sulfonyl keteneimine is the key intermediate for the synthesis of dihydroisoquinoline, whereas the aza-vinyl carbene intermediate results in the formation of 1-indanone. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Catalytic Activation of Nitrogen Dioxide for Selective Synthesis of Nitroorganics

DTIC Science & Technology

2015-01-15

observed, with C-P and C-C reductive elimination of ancillary phosphine ligands taking place preferentially, though traces of nitroarene are...reductive elimination) as well as some phosphine oxide. Thermolysis of (Ph3P)Pd(IPr)(C6H4CH3)(NO2) gives unusual C–C reductive elimination of the...N-heterocyclic carbene to give the imidazolium salt [CH3C6H4–IPr]NO2. Only in the case of the bis(diphenylphosphino) ferrocene complex [Fe(C5H4PPh2

Oxidation-promoted activation of a ferrocene C-H bond by a rhodium complex.

PubMed

Labande, Agnès; Debono, Nathalie; Sournia-Saquet, Alix; Daran, Jean-Claude; Poli, Rinaldo

2013-05-14

The oxidation of a rhodium(I) complex containing a ferrocene-based heterodifunctional phosphine N-heterocyclic carbene (NHC) ligand produces a stable, planar chiral rhodium(III) complex with an unexpected C-H activation on ferrocene. The oxidation of rhodium(I) to rhodium(III) may be accomplished by initial oxidation of ferrocene to ferrocenium and subsequent electron transfer from rhodium to ferrocenium. Preliminary catalytic tests showed that the rhodium(III) complex is active for the Grignard-type arylation of 4-nitrobenzaldehyde via C-H activation of 2-phenylpyridine.

Probing protein surface with a solvent mimetic carbene coupled to detection by mass spectrometry.

PubMed

Gómez, Gabriela E; Mundo, Mariana R; Craig, Patricio O; Delfino, José M

2012-01-01

Much knowledge into protein folding, ligand binding, and complex formation can be derived from the examination of the nature and size of the accessible surface area (SASA) of the polypeptide chain, a key parameter in protein science not directly measurable in an experimental fashion. To this end, an ideal chemical approach should aim at exerting solvent mimicry and achieving minimal selectivity to probe the protein surface regardless of its chemical nature. The choice of the photoreagent diazirine to fulfill these goals arises from its size comparable to water and from being a convenient source of the extremely reactive methylene carbene (:CH(2)). The ensuing methylation depends primarily on the solvent accessibility of the polypeptide chain, turning it into a valuable signal to address experimentally the measurement of SASA in proteins. The superb sensitivity and high resolution of modern mass spectrometry techniques allows us to derive a quantitative signal proportional to the extent of modification (EM) of the sample. Thus, diazirine labeling coupled to electrospray mass spectrometry (ESI-MS) detection can shed light on conformational features of the native as well as non-native states, not easily addressable by other methods. Enzymatic fragmentation of the polypeptide chain at the level of small peptides allows us to locate the covalent tag along the amino acid sequence, therefore enabling the construction of a map of solvent accessibility. Moreover, by subsequent MS/MS analysis of peptides, we demonstrate here the feasibility of attaining amino acid resolution in defining the target sites. © American Society for Mass Spectrometry, 2011

Solvent mimicry with methylene carbene to probe protein topography.

PubMed

Gómez, Gabriela Elena; Monti, José Luis E; Mundo, Mariana Rocío; Delfino, José María

2015-10-06

The solvent accessible surface area (SASA) of the polypeptide chain plays a key role in protein folding, conformational change, and interaction. This fundamental biophysical parameter is elusive in experimental measurement. Our approach to this problem relies on the reaction of the minimal photochemical reagent diazirine (DZN) with polypeptides. This reagent (i) exerts solvent mimicry because its size is comparable to water and (ii) shows scant chemical selectivity because it generates extremely reactive methylene carbene. Methylation gives rise to the EM (extent of modification) signal, which is useful for scrutinizing the conformational change triggered by Ca(2+) binding to calmodulin (CaM). The increased EM observed for the full protein is dominated by the enhanced exposure of hydrophobic area in Ca(2+)-CaM. Fragmentation allowed us to quantify the methylene incorporation at specific sites. Peptide 91-106 reveals a major reorganization around the calcium 151 binding site, resulting in local ordering and a greater exposure of the hydrophobic surface. Additionally, this technique shows a high sensitivity to probe recognition between CaM and melittin (Mel). The large decrease in EM indicates the occlusion of a significant hydrophobic area upon complexation. Protection from labeling reveals a larger involvement of the N-terminal and central regions of CaM in this interaction. Despite its smaller size, Mel's differential exposure can also be quantified. Moreover, MS/MS fragmentation realizes the goal of extending the resolution of labeled sites at the amino acid level. Overall, DZN labeling emerges as a useful footprinting method capable of shedding light on physiological conformational changes and interactions.

Photodissociative Cross-Linking of Non-covalent Peptide-Peptide Ion Complexes in the Gas Phase

NASA Astrophysics Data System (ADS)

Nguyen, Huong T. H.; Andrikopoulos, Prokopis C.; Rulíšek, Lubomír; Shaffer, Christopher J.; Tureček, František

2018-05-01

We report a gas-phase UV photodissociation study investigating non-covalent interactions between neutral hydrophobic pentapeptides and peptide ions incorporating a diazirine-tagged photoleucine residue. Phenylalanine (Phe) and proline (Pro) were chosen as the conformation-affecting residues that were incorporated into a small library of neutral pentapeptides. Gas-phase ion-molecule complexes of these peptides with photo-labeled pentapeptides were subjected to photodissociation. Selective photocleavage of the diazirine ring at 355 nm formed short-lived carbene intermediates that underwent cross-linking by insertion into H-X bonds of the target peptide. The cross-link positions were established from collision-induced dissociation tandem mass spectra (CID-MS3) providing sequence information on the covalent adducts. Effects of the amino acid residue (Pro or Phe) and its position in the target peptide sequence were evaluated. For proline-containing peptides, interactions resulting in covalent cross-links in these complexes became more prominent as proline was moved towards the C-terminus of the target peptide sequence. The photocross-linking yields of phenylalanine-containing peptides depended on the position of both phenylalanine and photoleucine. Density functional theory calculations were used to assign structures of low-energy conformers of the (GLPMG + GLL*LK + H)+ complex. Born-Oppenheimer molecular dynamics trajectory calculations were used to capture the thermal motion in the complexes within 100 ps and determine close contacts between the incipient carbene and the H-X bonds in the target peptide. This provided atomic-level resolution of potential cross-links that aided spectra interpretation and was in agreement with experimental data. [Figure not available: see fulltext.

Metal-Free Catalytic Enantioselective C–B Bond Formation: (Pinacolato)boron Conjugate Additions to α,β-Unsaturated Ketones, Esters, Weinreb Amides and Aldehydes Promoted by Chiral N-Heterocyclic Carbenes

PubMed Central

Wu, Hao; Radomkit, Suttipol; O’Brien, Jeannette M.; Hoveyda, Amir H.

2012-01-01

The first broadly applicable metal-free enantioselective method for boron conjugate addition (BCA) to α,β-unsaturated carbonyls is presented. The C–B bond forming reactions are promoted in the presence of 2.5–7.5 mol % of a readily accessible C1-symmetric chiral imidazolinium salt, which is converted, in situ, to the catalytically active diastereo- and enantiomerically pure N-heterocyclic carbene (NHC) by the common organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu). In addition to the commercially available bis(pinacolato)diboron [B2(pin)2], and in contrast to reactions with the less sterically demanding achiral NHCs, the presence of MeOH is required for high efficiency. Acyclic and cyclic α,β-unsaturated ketones, as well as acyclic esters, Weinreb amides and aldehydes can serve as suitable substrates; the desired β-boryl carbonyls are isolated in up to 94% yield and >98:2 enantiomer ratio (er). Transformations are often carried out at ambient temperature. In certain cases, such as when the relatively less reactive unsaturated amides are used, elevated temperatures are required (50–66 °C); nonetheless, reactions remain highly enantioselective. The utility of the NHC-catalyzed method is demonstrated through comparison with the alternative Cu-catalyzed protocols; in cases involving a polyfunctional substrate, unique profiles in chemoselectivity are exhibited by the metal-free approach (e.g., conjugate addition vs reaction with an alkyne, allene or aldehyde). PMID:22559866

Exceptionally Strong Electron-Donating Ability of Bora-Ylide Substituent vis-à-vis Silylene and Silylium Ion.

PubMed

Rosas-Sánchez, Alfredo; Alvarado-Beltran, Isabel; Baceiredo, Antoine; Saffon-Merceron, Nathalie; Massou, Stéphane; Branchadell, Vicenç; Kato, Tsuyoshi

2017-08-21

Electropositive boron-based substituent (phosphonium bora-ylide) with an exceptionally strong π- and σ-electron donating character dramatically increases the stability of a new type of N-heterocyclic silylene 2 featuring amino- and bora-ylide-substituents. Moreover, the related silylium ion 4 and transition-metal-silylene complexes, with trigonal-planar geometries around the silicon center, are also well stabilized. Therefore, the N,B-heterocyclic silylene 2 can be used as a strongly electron-donating innocent ligand in coordination chemistry similarly to N-heterocyclic carbenes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

General and mild Ni(0)-catalyzed α-arylation of ketones using aryl chlorides.

PubMed

Fernández-Salas, José A; Marelli, Enrico; Cordes, David B; Slawin, Alexandra M Z; Nolan, Steven P

2015-03-02

A general methodology for the α-arylation of ketones using a nickel catalyst has been developed. The new well-defined [Ni(IPr*)(cin)Cl] (1 c) pre-catalyst showed great efficiency for this transformation, allowing the coupling of a wide range of ketones, including acetophenone derivatives, with various functionalised aryl chlorides. This cinnamyl-based Ni-N-heterocyclic carbene (NHC) complex has demonstrated a different behaviour to previously reported NHC-Ni catalysts. Preliminary mechanistic studies suggest a Ni(0)/Ni(II) catalytic cycle to be at play. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Functionalization of P4 through Direct P-C Bond Formation.

PubMed

Borger, Jaap E; Ehlers, Andreas W; Slootweg, J Chris; Lammertsma, Koop

2017-09-04

Research on chlorine-free conversions of P 4 into organophosphorus compounds (OPCs) has a long track record, but methods that allow desirable, direct P-C bond formations have only recently emerged. These include the use of metal organyls, carbenes, carboradicals, and photochemical approaches. The versatile product scope enables the preparation of both industrially relevant organophosphorus compounds, as well as a broad range of intriguing new compound classes. Herein we provide a concise overview of recent breakthroughs and outline the acquired fundamental insights to aid future developments. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

A well-defined Pd hybrid material for the Z-selective semihydrogenation of alkynes characterized at the molecular level by DNP SENS.

PubMed

Conley, Matthew P; Drost, Ruben M; Baffert, Mathieu; Gajan, David; Elsevier, Cornelis; Franks, W Trent; Oschkinat, Hartmut; Veyre, Laurent; Zagdoun, Alexandre; Rossini, Aaron; Lelli, Moreno; Lesage, Anne; Casano, Gilles; Ouari, Olivier; Tordo, Paul; Emsley, Lyndon; Copéret, Christophe; Thieuleux, Chloé

2013-09-09

Direct evidence of the conformation of a Pd-N heterocyclic carbene (NHC) moiety imbedded in a hybrid material and of the Pd-NHC bond were obtained by dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) at natural abundance in short experimental times (hours). Overall, this silica-based hybrid material containing well-defined Pd-NHC sites in a uniform environment displays high activity and selectivity in the semihydrogenation of alkynes into Z-alkenes (see figure). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis and Reactivity of Backfluorinated NHC Carbene Complexes (Briefing Charts)

DTIC Science & Technology

2015-07-01

Distribution A: Approved for public release 13 Preparation of Perfluoroalkyl Grignard Reagents F(CF2CF2)nCH2CH2I Mg, Et2O ref lux F(CF2CF2)nCH2CH2MgI F...Synthetic Method #1 Grignard Addition/Reduction/Cyclization • Very general reaction pathway: suitable for all Grignard reagents • Attempts at...dialkylation unsuccessful • Slight excess of Grignard reagent ensures complete monoalkylation N N MesMes N N MesMes F2 C MgX Et2O, rt N N MesMes CF2 MgX 2

Co-complexes derived from alkene insertion to alkyne-dicobaltpentacarbonyl complexes: insight into the regioselectivity of pauson-khand reactions of cyclopropenes.

PubMed

Pallerla, Mahesh K; Yap, Glenn P A; Fox, Joseph M

2008-08-15

Described are the X-ray crystallographic and spectral properties of Co-complexes that were isolated from two Pauson-Khand reactions of chiral cyclopropenes. These are the first examples of isolated Co-complexes derived from the putative alkene-insertion intermediates of Pauson-Khand reactions. The binuclear Co-complexes are coordinated to mu-bonded, five-carbon "flyover" carbene ligands. It is proposed that the complexes result from cyclopropane fragmentation subsequent to alkene insertion. The observation of these metal complexes provides a rationale for the origin of regioselectivity in Pauson-Khand reactions of cyclopropenes.

Photochemical transformations of diazocarbonyl compounds: expected and novel reactions

NASA Astrophysics Data System (ADS)

Galkina, O. S.; Rodina, L. L.

2016-05-01

Photochemical reactions of diazocarbonyl compounds are well positioned in synthetic practice as an efficient method for ring contraction and homologation of carboxylic acids and as a carbene generation method. However, interpretation of the observed transformations of diazo compounds in electronically excited states is incomplete and requires a careful study of the fine mechanisms of these processes specific to different excited states of diazo compounds resorting to modern methods of investigation, including laser technology. The review is devoted to analysis of new data in the chemistry of excited states of diazocarbonyl compounds. The bibliography includes 155 references.

Reactions of monodithiolene tungsten(VI) sulfido complexes with copper(I) in relation to the structure of the active site of carbon monoxide dehydrogenase.

PubMed

Groysman, Stanislav; Majumdar, Amit; Zheng, Shao-Liang; Holm, R H

2010-02-01

Reactions directed at the synthesis of structural analogues of the active site of molybdenum-containing carbon monoxide dehydrogenase have been investigated utilizing [WO(2)S(bdt)](2-) (1) and [WOS(2)(bdt)](2-) (2) and sterically hindered [Cu(R)L] or [Cu(SSiR'(3))(2)](-) as reactants. All successful reactions of 2 afford the binuclear W(VI)/Cu(I) products [WO(bdt)(mu(2)-S)(2)Cu(L)](2-/-) with L = carbene (3), Ar*S (4), Ar* (7), SSiR(3) (R = Ph (5), Pr(i) (6)). Similarly, [W(bdt)(OSiPh(3))S(2)](-) leads to [W(bdt)(OSiPh(3))(mu(2)-S)(2)Cu(SAr*)](-) (8). These complexes, with apical oxo and basal dithiolato and sulfido coordination (excluding 8), terminal thiolate ligation at Cu(I) (4-6, 8), and W-(mu(2)-S)-Cu bridging, bear a structural resemblance to the enzyme site. Differences include two bridges instead of one and the absence of basal oxo/hydroxo ligation. Complex 8 differs from the others by utilizing apical and basal sulfido ligands in bridge formation. Related reaction systems based on 1 gave 4 in small yield or product mixtures in which the desired monobridged complex [WO(2)(bdt)(mu(2)-S)Cu(R)](2-) was not detected. Mass spectrometric analysis of the reaction system with L = carbene suggests that any monobridged species forms may converted to the dibridged form by disproportionation. In these experiments, the use of W(VI) preserves the structural integrity of Mo(VI), whose analogues of 1 and 2 have not been isolated. (Ar* = 2,6-bis(2,4,6-triisopropylphenyl)phenyl, bdt = benzene-1,2-dithiolate(2-)).

Role of four-membered rings in C32 fullerene stability and mechanisms of generalized Stone-Wales transformation: a density functional theory investigation.

PubMed

Wang, Weiwei; Dang, Jingshuang; Zhao, Xiang

2011-08-28

Density functional theory (DFT) methods have been applied to study C(32) fullerenes built from four-, five-, and six-membered rings. The relative energies of pure C(32) fullerenes have been evaluated to locate three most stable structures, 32:D(4d) with two squares, 1:D(3) without square and 5:C(s) with one square. Structural analysis reveals that there is a rearrangement pathway between the lowest energy classical isomer 1:D(3) and the lowest energy non-classical isomer 32:D(4d), and 5:C(s) behaves just as an intermediate between them. The kinetic processes of generalized Stone-Wales transformation (GSWT) with four-membered rings have been explored and two distinct reaction mechanisms are determined by all the transition states and intrinsic reaction coordinates with PBE1PBE/6-31G(d) approach for the first time. One mechanism is the concerted reaction with a rotating dimer closed to the cage surface and another is the stepwise reaction with a carbene-like sp(3) structure, whereas the latter is sorted into two paths based on four-membered ring vanishing before or after the formation of the carbene-like structure. It is indicated that there is no absolute preference for any mechanism, which depends on the adaptability of different reactants on the diverse mechanisms. Furthermore, it's found that the interconversion process with the participation of squares is more reactive than the rearrangement between C(60)_I(h) and C(60)_C(2v), implying some potential importance of non-classical small fullerenes in the fullerene isomerization.

Nonthrombogenic Hydrogel Coatings with Carbene-Cross-Linking Bioadhesives.

PubMed

Nanda, Himansu Sekhar; Shah, Ankur Harish; Wicaksono, Gautama; Pokholenko, Oleksandr; Gao, Feng; Djordjevic, Ivan; Steele, Terry W J

2018-05-14

Bioadhesives are a current unmet clinical need for mending of blood contacting soft tissues without inducing thrombosis. Recent development of carbene precursor bioadhesives with the advantages of on-demand curing, tuneable modulus, and wet adhesion have been synthesized by grafting diazirine onto poly (amidoamine) (PAMAM-G5) dendrimers. Herein, the structure activity relationships of platelet adhesion and activation is evaluated for the first time on the cured PAMAM-g-diazirine bioadhesives. Three strategies were employed to prevent healthy human donor platelets from adhering and activating on light-cured bioadhesive surfaces: (1) Attenuation of cationic surface charge, (2) antifouling composites by incorporating heparin and alginate in uncured formulation, and (3) heparin wash of cured bioadhesive surface. Topographical imaging of cured and ethanol dehydrated bioadhesive surfaces was used to quantify the adhered and activated platelets with scanning electron microscopy, whose resolution allowed identification of round senescent, short dendritic, and long dendritic platelets. Cured surfaces of PAMAM-g-diazirine (15%) had 10300 ± 500 adhered platelets mm -2 with 99.7% activation into short/long dendritic cells. Reduction of primary amines by higher degree of diazirine grafting or capping of free amines by acetylation reduces platelet adherence (2400 ± 200 vs 3000 ± 300, respectively). Physical incorporation of heparin and alginate in the formulations reduced the activated platelet; 1300 ± 300 and 300 ± 50, activated platelets mm -2 , in comparison with additive free adhesive formulation. Similarly, heparin rinse of the surface of additive free bioadhesive reduced the activated platelet to platelets of heparin composites at 600 ± 100 platelets mm -2 . PAMAM-g-diazirine (15%) bioadhesive retained the photocured mechanical properties and lap shear adhesion despite the addition of heparin and alginate additives.

N-alkyl functionalised expanded ring N-heterocyclic carbene complexes of rhodium(I) and iridium(I): structural investigations and preliminary catalytic evaluation.

PubMed

Dunsford, Jay J; Tromp, Dorette S; Cavell, Kingsley J; Elsevier, Cornelis J; Kariuki, Benson M

2013-05-28

A series of new N-alkyl functionalised 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) pro-ligands 3-6 and their corresponding complexes of rhodium(I) and iridium(I), [M(NHC)(COD)Cl] 7-14 and [M(NHC)(CO)2Cl] 15-22 are described. The complexes have been characterised by (1)H and (13)C{(1)H} NMR, mass spectrometry, IR and X-ray diffraction. It is noted from X-ray diffraction studies that the N-alkyl substituents are found to orientate themselves away from the metal centre due to unfavourable steric interactions resulting in low percent buried volume (%V(bur)) values in the solid state. The heterocycle ring size is also found to dictate the spatial orientation of the N-alkyl substituents in the neopentyl functionalised derivatives 10 and 14. The 7-membered derivative 14 allows for a conformational 'twist' of the heterocycle ring with the N-alkyl substituents adopting a mutually trans configuration with respect to each other, while the more rigid 6-membered system 10 does not allow for this conformational 'twist' and consequently the N-alkyl substituents adopt a mutually cis configuration. The σ-donor function of this new class of expanded ring NHC ligand has also been probed by measured IR stretching frequencies of the [M(NHC)(CO)2Cl] complexes 15-22. A preliminary catalytic survey of the hydrogenation of functionalised alkenes with molecular hydrogen under mild conditions has also been undertaken with complex , affording an insight into the application of large ring NHC ancillary ligands bearing N-alkyl substituents in hydrogenation transformations.

1. Medicinal chemistry of a small molecule drug lead: Tamoxilog 2. Electronic communication through ruthenium nanoparticles: Synthesis of custom ligands and nanoparticles

NASA Astrophysics Data System (ADS)

Zuckerman, Nathaniel Benjamin

1. Compound NSC-670224, previously shown to be toxic to Saccharomyces cerevisiae at low micromolar concentrations, potentially acts via a mechanism of action related to that of tamoxifen (NSC 180973), a widely utilized breast cancer drug. The structure of NSC-670224, previously thought to be a 2,4-dichloro arene, was established as the 3,4-dichloro arene, and a focused library of analogues were synthesized and biologically evaluated in conjunction with the UCSC Chemical Screening Center. The synthesis of a biotinylated affinity probe was also completed in order to extract the protein target(s) of NSC-670224 from yeast and human cell lines in collaboration with the Hartzog lab (UCSC MCD Biology) 2. Stabilization of ruthenium nanoparticles (Ru NPs) through carbene bound ligands has led to a simple and effective means to generate new materials with unique optoelectronic properties. The affinity of freshly prepared Ru NPs to diazo compounds, specifically octyl diazoacetate (ODA), provides a robust nanostructure that can be further functionalized via metathesis of terminal olefins to generate these unique materials. Carbene-stabilized Ru NPs have provided insights into the nature of extended conjugation and intraparticle charge delocalization through covalently bound probes (e.g., ferrocene and pyrene). The growing interest to study electronic communication through Ru NPs has lead to collaborative, multidisciplinary efforts between analytical (Shaowei Chen lab, UCSC), theoretical (Haobin Wang Lab, NMSU), and synthetic organic chemists (Konopelski Lab, UCSC). With this powerful collaboration, new methods to generate stabilized Ru NPs, testing theory with experiment, and efficient means to functionalize NPs have been investigated. The syntheses of custom ligands and their applications to nanoparticle-mediated electronic communication are reported.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Turner, Andrew M.; Abplanalp, Matthew J.; Kaiser, Ralf I.

Phosphine, which has now been confirmed around the carbon-rich star IRC+10216, provides the first example of a phosphorus-containing single bond in interstellar or circumstellar media. While four compounds containing both phosphorus and carbon have been discovered, none contain a carbon–phosphorus single bond. Here, we show that this moiety is plausible from the reaction of phosphine with methane in electron-irradiated interstellar ice analogues. Fractional sublimation allows for detection of individual products at distinct temperatures using reflectron time-of-flight mass spectrometry (ReTOF) coupled with vacuum ultraviolet photoionization. This method produced phosphanes and methylphosphanes as large as P{sub 8}H{sub 10} and CH{sub 3}P{sub 8}H{submore » 9}, which demonstrates that a phosphorus–carbon bond can readily form and that methylphosphanes sublime at 12–17 K higher temperatures than the non-organic phosphanes. Also, irradiated ices of phosphine with deuterated-methane untangle the reaction pathways through which these methylphosphanes were formed and identified radical recombination to be preferred over carbene/phosphinidene insertion reactions. In addition, these ReTOF results confirm that CH{sub 3}PH{sub 2} and CH{sub 6}P{sub 2} can form via insertion of carbene and phosphinidene and that the methylenediphosphine (PH{sub 2}CH{sub 2}PH{sub 2}) isomer forms in the ices, although methylphosphine (CH{sub 3}P{sub 2}H{sub 3}) is likely the more abundant isomer and that phosphanes and organophosphanes preferentially fragment via the loss of a phosphino group when photoionized. While the formation of methylphosphine is overall endoergic, the intermediates produced by interactions with energetic electrons proceed toward methylphosphine favorably and barrierlessly and provide plausible mechanisms toward hitherto unidentified interstellar compounds.« less

Lipophilicity-dependent ruthenium N-heterocyclic carbene complexes as potential anticancer agents.

PubMed

Lv, Gaochao; Guo, Liubin; Qiu, Ling; Yang, Hui; Wang, Tengfei; Liu, Hong; Lin, Jianguo

2015-04-28

Five Ru(II)-N-heterocyclic carbenes (NHC) (1-5) were synthesized by reacting the appropriately substituted imidazolium chlorides with Ag2O, forming the NHC-silver chloride in situ followed by transmetalation with dimeric p-cymene ruthenium(II) dichloride. All the complexes were characterized by NMR and ESI-MS, and complex 1 was also characterized by single-crystal X-ray diffraction. The IC50 values of these five complexes were determined by the MTT-based assay on four human cancer cell lines, SKOV-3 (ovarian), PC-3 (prostate), MDA-MB-231 (breast) and EC109 (esophagus). The cytotoxicities of these complexes changed from a moderate effect to a fine one, corresponding to the increasing lipophilicity order of the complex of 2 < 1 < 3 < 4 < 5 (0.91, 0.88, 1.36, 1.85 and 2.62 for 1–5 respectively). Complex 5 showed the most cytotoxicity with the IC50 values 10.3 ± 0.3 μM for SKOV-3, 2.9 ± 0.1 μM for PC-3, 8.2 ± 0.6 μM for MDA-MB-231, 6.4 ± 0.2 μM for EC109 cell lines. Due to the superior cytotoxicity of complex 5 against the PC-3 cell lines, further biological evaluations were carried out to elucidate its action mechanism. The morphologic changes and cell cycle analysis showed that complex 5 can inhibit PC-3 cell lines by inducing cell cycle arrest at the G2/M phase. The DNA binding experiments further demonstrate that complex 5 has a better binding ability for DNA (Kb = 2.2 × 10(6) M(-1)) than complexes 1-4 (3.8 × 10(5), 7.0 × 10(5), 5.7 × 10(5), and 1.9 × 10(5) respectively).

Synthesis of precursors to and spectroscopic characterization of highly unsaturated carbenes and diradicals and development and implementation of a web-based stereochemistry tutorial

NASA Astrophysics Data System (ADS)

Burrmann, Nicola Jean

A variety of substituted diynols, diynals, and diynones have been prepared en route to the synthesis of precursors to dialkynyl carbenes (R1--C≡C--C--C≡C--R 2). In light of the unique reactivity associated with these simple systems, several strategies were required to assemble the carbon backbones (Chapter I). Tosylhydrazone and trisylhydrazone precursors to 2-diazo-3-pentyne ( 1), 2-diazo-3-butyne (2), and Idiazo-2-butyne ( 3) were synthesized and then converted into their respective diazo compounds. Various attempts to study these diazo compounds using matrix isolation IR and EPR spectroscopy were made and proved to be unsuccessful. Computations were done to characterize the C5H6 potential energy surface, as well as to determine the IR vibrational frequencies of the isomers on this surface (Chapter 2). A web-based Stereochemistry Tutorial that details the core definitions and structural representations relevant to organic stereochemistry was designed and implemented into several introductory-level organic chemistry classes. This tutorial also allows for students to select their preferred structural representation and method for making stereochemical comparisons between molecules. The tutorial was evaluated, either qualitatively, quantitatively, or both, by students in three different introductory organic chemistry courses at the University of Wisconsin---Madison. The data show that students did use a variety of different methods for making stereochemical comparisons between molecules, and that prior exposure to lectures on stereochemistry by the course professor strongly influenced these choices. Furthermore, the level of improvement in stereochemical knowledge as a result of using only the tutorial was comparable to, or higher than, that achieved by students who were only exposed to lectures by the course professor, regardless of the method chosen for making stereochemical comparisons between molecules (Chapter 3)

Silver-free activation of ligated gold(I) chlorides: the use of [Me3NB12Cl11]- as a weakly coordinating anion in homogeneous gold catalysis.

PubMed

Wegener, Michael; Huber, Florian; Bolli, Christoph; Jenne, Carsten; Kirsch, Stefan F

2015-01-12

Phosphane and N-heterocyclic carbene ligated gold(I) chlorides can be effectively activated by Na[Me3NB12Cl11] (1) under silver-free conditions. This activation method with a weakly coordinating closo-dodecaborate anion was shown to be suitable for a large variety of reactions known to be catalyzed by homogeneous gold species, ranging from carbocyclizations to heterocyclizations. Additionally, the capability of 1 in a previously unknown conversion of 5-silyloxy-1,6-allenynes was demonstrated. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ab initio predictions on the rotational spectra of carbon-chain carbene molecules.

PubMed

Maluendes, S A; McLean, A D

1992-12-18

We predict rotational constants for the carbon-chain molecules H2C=(C=)nC, n=3-8, using ab initio computations, observed values for the earlier members in the series, H2CCC and H2CCCC with n=1 and 2, and empirical geometry corrections derived from comparison of computation and experiment on related molecules. H2CCC and H2CCCC have already been observed by radioastronomy; higher members in the series, because of their large dipole moments, which we have calculated, are candidates for astronomical searches. Our predictions can guide searches and assist in both astronomical and laboratory detection.

Ab initio predictions on the rotational spectra of carbon-chain carbene molecules

NASA Technical Reports Server (NTRS)

Maluendes, S. A.; McLean, A. D.; Loew, G. H. (Principal Investigator)

1992-01-01

We predict rotational constants for the carbon-chain molecules H2C=(C=)nC, n=3-8, using ab initio computations, observed values for the earlier members in the series, H2CCC and H2CCCC with n=1 and 2, and empirical geometry corrections derived from comparison of computation and experiment on related molecules. H2CCC and H2CCCC have already been observed by radioastronomy; higher members in the series, because of their large dipole moments, which we have calculated, are candidates for astronomical searches. Our predictions can guide searches and assist in both astronomical and laboratory detection.

Intervalence transfer of ferrocene moieties adsorbed on electrode surfaces by a conjugated linkage

NASA Astrophysics Data System (ADS)

Chen, Wei; Brown, Lauren E.; Konopelski, Joseph P.; Chen, Shaowei

2009-03-01

Effective intervalence transfer occurred between the metal centers of ferrocene moieties that were adsorbed onto a ruthenium thin film surface by ruthenium-carbene π bonds, a direct verification of Hush's four-decade-old prediction. Electrochemical measurements showed two pairs of voltammetric peaks where the separation of the formal potentials suggested a Class II behavior. Additionally, the potential spacing increased with increasing ferrocene surface coverage, most probably as a consequence of the enhanced contribution from through-space electronic interactions between the metal centers. In contrast, the incorporation of a sp 3 carbon spacer into the ferrocene-ruthenium linkage led to the diminishment of interfacial electronic communication.

Rapid Construction of a Benzo-Fused Indoxamycin Core Enabled by Site-Selective C-H Functionalizations.

PubMed

Bedell, T Aaron; Hone, Graham A B; Valette, Damien; Yu, Jin-Quan; Davies, Huw M L; Sorensen, Erik J

2016-07-11

Methods for functionalizing carbon-hydrogen bonds are featured in a new synthesis of the tricyclic core architecture that characterizes the indoxamycin family of secondary metabolites. A unique collaboration between three laboratories has engendered a design for synthesis featuring two sequential C-H functionalization reactions, namely a diastereoselective dirhodium carbene insertion followed by an ester-directed oxidative Heck cyclization, to rapidly assemble the congested tricyclic core of the indoxamycins. This project exemplifies how multi-laboratory collaborations can foster conceptually novel approaches to challenging problems in chemical synthesis. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Group transfer and electron transfer reactions of organometallic complexes

NASA Astrophysics Data System (ADS)

Atwood, Jim D.

During 1994, despite the disruptions, the authors have made progress in several aspects of their research on electron transfer reactions between organometallic complexes. This summary covers three areas that are relatively complete: (1) reactions between metal carbonyl anions and metal carbonyl halides, (2) reactions of hydrido- and alkyl-containing anions (RFe(CO)4(-) and RW(CO)5(-) with metal carbonyl cations; and (3) reactions of a seventeen-electron complex (Cp* Cr(CO)3*) with metal carbonyl derivatives. Two areas of examination that have just begun (possible carbene transfer and the possible role of metal carbonyl anions in carbon-hydrogen bond activation) will also be described.

C(2) symmetric chiral NHC ligand for asymmetric quaternary carbon constructing copper-catalyzed conjugate addition of grignard reagents to 3-substituted cyclohexenones.

PubMed

Matsumoto, Yasumasa; Yamada, Ken-ichi; Tomioka, Kiyoshi

2008-06-20

The asymmetric construction of quaternary carbon centers by conjugate addition of Grignard reagents to 3-methyl- and 3-ethylcyclohexenones was realized in a maximum enantioselectivity of 80% by using a C 2 symmetric chiral N-heterocyclic carbene (NHC)-copper catalyst, generated from (4 S,5 S)-1,3-bis(2-methoxyphenyl)-4,5-diphenyl-4,5-dihydro-1 H-imidazol-3-ium tetrafluoroborate and copper(II) triflate. The stereostructures of the NHC-Au complexes were analyzed by X-ray crystallography, which rationalized the good stereocontrolling ability of N-aryl NHCs.

Co-complexes Derived from Alkene Insertion to Alkyne-dicobaltpentacarbonyl complexes: Insight into the Regioselectivity of Pauson-Khand Reactions of Cyclopropenes

PubMed Central

Pallerla, Mahesh K.; Yap, Glenn P. A.; Fox, Joseph M.

2009-01-01

Described are the X-ray crystallographic and spectral properties of Co-complexes that were isolated from two Pauson-Khand reactions of chiral cyclopropenes. These are the first examples of isolated Co-complexes derived from the putative alkene-insertion intermediates of Pauson-Khand reactions. The binuclear Co-complexes are coordinated to μ-bonded, five-carbon “flyover” carbene ligands. It is proposed that the complexes result from cyclopropane fragmentation subsequent to alkene insertion. The observation of these metal complexes provides a rationale for the origin of regioselectivity in Pauson-Khand reactions of cyclopropenes. PMID:18637694

Synthesis, molecular structure, and C-C coupling reactions of carbeneruthenium(II) complexes with C5H5Ru(=CRR') and C5Me5Ru(=CRR') as molecular units.

PubMed

Braun, Thomas; Münch, Gerhard; Windmüller, Bettina; Gevert, Olaf; Laubender, Matthias; Werner, Helmut

2003-06-06

The ethene derivatives [(eta(5)-C(5)R(5))RuX(C(2)H(4))(PPh(3))] with R=H and Me, which have been prepared from the eta(3)-allylic compounds [(eta(5)-C(5)R(5))Ru(eta(3)-2-MeC(3)H(4))(PPh(3))] (1, 2) and acids HX under an ethene atmosphere, are excellent starting materials for the synthesis of a series of new halfsandwich-type ruthenium(II) complexes. The olefinic ligand is replaced not only by CO and pyridine, but also by internal and terminal alkynes to give (for X=Cl) alkyne, vinylidene, and allene compounds of the general composition [(eta(5)-C(5)R(5))RuCl(L)(PPh(3))] with L=C(2)(CO(2)Me)(2), Me(3)SiC(2)CO(2)Et, C=CHCO(2)R, and C(3)H(4). The allenylidene complex [(eta(5)-C(5)H(5))RuCl(=C=C=CPh(2))(PPh(3))] is directly accessible from 1 (R=H) in two steps with the propargylic alcohol HC triple bond CC(OH)Ph(2) as the precursor. The reactions of the ethene derivatives [(eta(5)-C(5)H(5))RuX(C(2)H(4))(PPh(3))] (X=Cl, CF(3)CO(2)) with diazo compounds RR'CN(2) yield the corresponding carbene complexes [(eta(5)-C(5)R(5))RuX(=CRR')(PPh(3))], while with ethyl diazoacetate (for X=Cl) the diethyl maleate compound [(eta(5)-C(5)H(5))RuCl[eta(2)-Z-C(2)H(2)(CO(2)Et)(2)](PPh(3))] is obtained. Halfsandwich-type ruthenium(II) complexes [(eta(5)-C(5)R(5))RuCl(=CHR')(PPh(3))] with secondary carbenes as ligands, as well as cationic species [(eta(5)-C(5)H(5))Ru(=CPh(2))(L)(PPh(3))]X with L=CO and CNtBu and X=AlCl(4) and PF(6), have also been prepared. The neutral compounds [(eta(5)-C(5)H(5))RuCl(=CRR')(PPh(3))] react with phenyllithium, methyllithium, and the vinyl Grignard reagent CH(2)=CHMgBr by displacement of the chloride and subsequent C-C coupling to generate halfsandwich-type ruthenium(II) complexes with eta(3)-benzyl, eta(3)-allyl, and substituted olefins as ligands. Protolytic cleavage of the metal-allylic bond in [(eta(5)-C(5)H(5))Ru(eta(3)-CH(2)CHCR(2))(PPh(3))] with acetic acid affords the corresponding olefins R(2)C=CHCH(3). The by-product of this process is the acetato derivative [(eta(5)-C(5)H(5))Ru(kappa(2)-O(2)CCH(3))(PPh(3))], which can be reconverted to the carbene complexes [(eta(5)-C(5)H(5))RuCl(=CR(2))(PPh(3))] in a one-pot reaction with R(2)CN(2) and Et(3)NHCl.

Direct Functionalization of Nitrogen Heterocycles via Rh-Catalyzed C-H Bond Activation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lewis, Jared; Bergman, Robert; Ellman, Jonathan

2008-02-04

Nitrogen heterocycles are present in many compounds of enormous practical importance, ranging from pharmaceutical agents and biological probes to electroactive materials. Direct funtionalization of nitrogen heterocycles through C-H bond activation constitutes a powerful means of regioselectively introducing a variety of substituents with diverse functional groups onto the heterocycle scaffold. Working together, our two groups have developed a family of Rh-catalyzed heterocycle alkylation and arylation reactions that are notable for their high level of functional-group compatibility. This Account describes their work in this area, emphasizing the relevant mechanistic insights that enabled synthetic advances and distinguished the resulting transformations from other methods.more » They initially discovered an intramolecular Rh-catalyzed C-2-alkylation of azoles by alkenyl groups. That reaction provided access to a number of di-, tri-, and tetracyclic azole derivatives. They then developed conditions that exploited microwave heating to expedite these reactions. While investigating the mechanism of this transformation, they discovered that a novel substrate-derived Rh-N-heterocyclic carbene (NHC) complex was involved as an intermediate. They then synthesized analogous Rh-NHC complexes directly by treating precursors to the intermediate [RhCl(PCy{sub 3}){sub 2}] with N-methylbenzimidazole, 3-methyl-3,4-dihydroquinazolein, and 1-methyl-1,4-benzodiazepine-2-one. Extensive kinetic analysis and DFT calculations supported a mechanism for carbene formation in which the catalytically active RhCl(PCy{sub 3}){sub 2} fragment coordinates to the heterocycle before intramolecular activation of the C-H bond occurs. The resulting Rh-H intermediate ultimately tautomerizes to the observed carbene complex. With this mechanistic information and the discovery that acid co-catalysts accelerate the alkylation, they developed conditions that efficiently and intermolecularly alkylate a variety of heterocycles, including azoles, azolines, dihydroquinazolines, pyridines, and quinolines, with a wide range of functionalized olefins. They demonstrated the utility of this methodology in the synthesis of natural products, drug candidates, and other biologically active molecules. In addition, they developed conditions to directly arylate these heterocycles with aryl halides. The initial conditions that used PCy{sub 3} as a ligand were successful only for aryl iodides. However, efforts designed to avoid catalyst decomposition led to the development of ligands based on 9-phosphabicyclo[4.2.1]nonane (Phoban) that also facilitated the coupling of aryl bromides. They then replicated the unique coordination environment, stability, and catalytic activity of this complex using the much simpler tetrahydrophosphepine ligands and developed conditions that coupled aryl bromides bearing diverse functional groups without the use of a glovebox or purified reagents. With further mechanistic inquiry, they anticipate that researchers will better understand the details of the aforementioned Rh-catalyzed C-H bond functionalization reactions, resulting in the design of more efficient and robust catalysts, expanded substrate scope, and new transformations.« less

Coordination behavior of bis-phenolate saturated and unsaturated N-heterocyclic carbene ligands to zirconium: reactivity and activity in the copolymerization of cyclohexene oxide with CO2.

PubMed

Lalrempuia, Ralte; Breivik, Frida; Törnroos, Karl W; Le Roux, Erwan

2017-06-27

Tetravalent zirconium complexes supported by tridentate bis-phenolate imidazolidin-2-ylidene (L1), imidazol-2-ylidene (L2) and benzimidazol-2-ylidene (L3) NHC ligands were synthesized and evaluated as precursors for the copolymerization of cyclohexene oxide (CHO) with CO 2 . While the reactivity of the imidazolidinium [H 3 L1] chloride salt with Zr(OiPr) 4 (HOiPr), and subsequent ligand exchanges with either (CH 3 ) 3 SiCl or LiOiPr lead to a series of heteroleptic compounds (κ 3 -O,C,O-L1)Zr(X) 2 (THF) (X = Cl, OiPr), both imidazolium [H 3 L2] and benzimidazolium [H 3 L3] chloride salts give a mixture of homoleptic (κ 3 -O,C,O-NHC) 2 Zr and zwitterionic (κ 2 -O,O-HL)ZrCl 2 (OiPr) compounds along with traces or the absence of the heteroleptic intermediate (κ 3 -O,C,O-NHC)Zr(Cl)(OiPr)(THF). Such dissimilar reactivity between the unsaturated and saturated NHC ligands is predominantly ascribed to the increased acidity of azolium salts along with the π-donor strength of the C carbene in L2 and L3-Zr moieties. The reactivity with the more acidic azolium salts (H 3 L2/3) and the destabilized Zr-X trans to NHC carbene bond results in a significant increase in the amount of homoleptic compounds generating HCl. The released HCl reacts preferentially with the heteroleptic intermediates having non-planar NHC ligands (i.e. L2/3) promoting the formation of zwitterionic complexes. The in situ deprotonation of the isolated zwitterionic (κ 2 -O,O-HL3)ZrCl 2 (OiPr) compound by using Ag 2 O gives the homoleptic complex as the major component along with a bimetallic hydroxo-bridged [(κ 3 -O,C,O-L3)Zr(μ-OH)(OiPr)] 2 compound. Of particular interest is that only the heteroleptic NHC-Zr(iv) complexes were identified to be active and highly selective towards the copolymerization of CHO with CO 2 independently of the co-catalysts used (both anionic and neutral) under mild conditions (P CO 2 < 1 bar, T = 60 °C), and gave atactic and completely alternating copolymers in a controlled manner (M w /M n ≈ 1.3-1.8). In contrast, the isolated homoleptic, zwitterionic and bimetallic zirconium species were found to be inactive under similar reaction conditions. Although the activity found for NHC-Zr(iv) complexes is nearly of the same order of magnitude as that of the NHC-Ti(iv) analogues, these results are the first examples of tetravalent zirconium complexes achieving high selectivity (99% in PCHC) in the catalyzed copolymerization of CHO with CO 2 .

Rate constant calculations in the dimerization of diaminocarbene: a direct dynamics study

NASA Astrophysics Data System (ADS)

Oliva, Josep M.

1999-03-01

Generalized transition state theory calculations are performed on the dimerization of diaminocarbene [(H 2N) 2C:] ( 1) to tetrakis(amino)ethene [(H 2N) 2CC(NH 2) 2] ( 2). This process involves the formation of a double bond from two carbenes and therefore inclusion of correlation energy is vital. The density functionals BPW91, B3LYP and the QCISD(T)//MP2 model are used in the electronic structure calculations of reactants, transition states and products. The goal of this work is to gain insight into the mechanism of dimerization of the diaminocarbenes [(R 2N) 2C:] (R=H, Me), where experimental activation parameters are already available for R=Me.

Rh(II)-catalyzed Reactions of Diazoesters with Organozinc Reagents

PubMed Central

Panish, Robert; Selvaraj, Ramajeyam; Fox, Joseph M.

2015-01-01

Rh(II)-catalyzed reactions of diazoesters with organozinc reagents are described. Diorganozinc reagents participate in reactions with diazo compounds by two distinct, catalyst-dependent mechanisms. With bulky diisopropylethylacetate ligands, the reaction mechanism is proposed to involve initial formation of a Rh-carbene and subsequent carbozincation to give a zinc enolate. With Rh2(OAc)4, it is proposed that initial formation of an azine precedes 1,2-addition by an organozinc reagent. This straightforward route to the hydrazone products provides a useful method for preparing chiral quaternary α-aminoesters or pyrazoles via the Paul-Knorr condensation with 1,3-diketones. Crossover and deuterium labeling experiments provide evidence for the mechanisms proposed. PMID:26241081

Rh(II)-Catalyzed Reactions of Diazoesters with Organozinc Reagents.

PubMed

Panish, Robert; Selvaraj, Ramajeyam; Fox, Joseph M

2015-08-21

Rh(II)-catalyzed reactions of diazoesters with organozinc reagents are described. Diorganozinc reagents participate in reactions with diazo compounds by two distinct, catalyst-dependent mechanisms. With bulky diisopropylethyl acetate ligands, the reaction mechanism is proposed to involve initial formation of a Rh-carbene and subsequent carbozincation to give a zinc enolate. With Rh2(OAc)4, it is proposed that initial formation of an azine precedes 1,2-addition by an organozinc reagent. This straightforward route to the hydrazone products provides a useful method for preparing chiral quaternary α-aminoesters or pyrazoles via the Paul-Knorr condensation with 1,3-diketones. Crossover and deuterium labeling experiments provide evidence for the mechanisms proposed.

Recent advances in the chemical modification of unsaturated polymers

NASA Technical Reports Server (NTRS)

Schulz, D. N.; Turner, S. R.; Golub, M. A.

1982-01-01

The present discussion has the objective to update the most comprehensive reviews on the considered subject and to fill in the gaps of less complete, but more modern treatments. Only simple chemical functionalization or structural modification of unsaturated polymers are covered, and the literature of diene polymer modification since 1974 is emphasized. Attention is given to hydrogenation, halogenation and hydrohalogenation, cyclization, cis-trans isomerization, epoxidation, ene and other cycloaddition reactions, sulfonation, carboxylation, phosphonylation, sulfenyl chloride addition, carbene addition, metalation, and silylation. It is pointed out that modern synthetic reagents and catalysts have been advantageously employed to improve process and/or product quality. Synthetic techniques have been refined to allow the selective modification of specific polymer microstructures or blocks.

A Low Spin Manganese(IV) Nitride Single Molecule Magnet

PubMed Central

Ding, Mei; Cutsail, George E.; Aravena, Daniel; Amoza, Martín; Rouzières, Mathieu; Dechambenoit, Pierre; Losovyj, Yaroslav; Pink, Maren

2016-01-01

Structural, spectroscopic and magnetic methods have been used to characterize the tris(carbene)borate compound PhB(MesIm)3Mn≡N as a four-coordinate manganese(IV) complex with a low spin (S = 1/2) configuration. The slow relaxation of the magnetization in this complex, i.e. its single-molecule magnet (SMM) properties, is revealed under an applied dc field. Multireference quantum mechanical calculations indicate that this SMM behavior originates from an anisotropic ground doublet stabilized by spin-orbit coupling. Consistent theoretical and experiment data show that the resulting magnetization dynamics in this system is dominated by ground state quantum tunneling, while its temperature dependence is influenced by Raman relaxation. PMID:27746891

Selenium containing imidazolium salt in designing single source precursors for silver bromide and selenide nano-particles.

PubMed

Joshi, Hemant; Sharma, Kamal Nayan; Singh, Ved Vati; Singh, Pradhumn; Singh, Ajai Kumar

2013-02-21

The AgBr and Ag(2)Se nanoparticles (NPs) have been synthesized for the first time from two single source precursors ([Ag(2)(L)(2)Br(2)] (1) and [Ag(L-HBr)(2)]BF(4) (2) respectively) designed using the same ligand 3-benzyl-1-(2-phenylselanyl-ethyl)-3H-imidazolium bromide (L). The ODE-ODA-OA (1 : 1 : 2) and TOP-OA (1 : 2) are most suitable solvents for thermolysis of 1 and 2 respectively, resulting in the NPs. The composition of the solvent used in thermolysis affects the purity of NPs. The bonding of L in 1 is unique, as it has a pre-carbene site intact.

Iridium-Catalysed ortho-Directed Deuterium Labelling of Aromatic Esters--An Experimental and Theoretical Study on Directing Group Chemoselectivity.

PubMed

Devlin, Jennifer; Kerr, William J; Lindsay, David M; McCabe, Timothy J D; Reid, Marc; Tuttle, Tell

2015-06-25

Herein we report a combined experimental and theoretical study on the deuterium labelling of benzoate ester derivatives, utilizing our developed iridium N-heterocyclic carbene/phosphine catalysts. A range of benzoate esters were screened, including derivatives with electron-donating and -withdrawing groups in the para- position. The substrate scope, in terms of the alkoxy group, was studied and the nature of the catalyst counter-ion was shown to have a profound effect on the efficiency of isotope exchange. Finally, the observed chemoselectivity was rationalized by rate studies and theoretical calculations, and this insight was applied to the selective labelling of benzoate esters bearing a second directing group.

Divergent Synthesis of Multisubstituted Tetrahydrofurans and Pyrrolidines via Intramolecular Aldol-type Trapping of Onium Ylide Intermediates.

PubMed

Jing, Changcheng; Xing, Dong; Gao, Lixin; Li, Jia; Hu, Wenhao

2015-12-21

This paper reports a divergent strategy for the synthesis of multisubstituted tetrahydrofurans and pyrrolidines, starting from easily accessible β-hydroxyketones or β-aminoketones to react with diazo compounds. Under Rh(II) catalysis, this transformation is proposed to proceed through a metal-carbene-induced oxonium ylide or ammonium ylide formation followed by an intramolecular aldol-type trapping of these active intermediates. A series of highly substituted tetrahydrofurans and pyrrolidines are synthesized in high yields with good to excellent diastereoselectivities. Preliminary biological evaluations revealed that both types of heterocycles show good PTP1B inhibitory activities. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gold-Catalyzed Formal [4+1]/[4+3] Cycloadditions of Diazo Esters with Triazines.

PubMed

Zhu, Chenghao; Xu, Guangyang; Sun, Jiangtao

2016-09-19

Reported herein is the unprecedented gold-catalyzed formal [4+1]/[4+3] cycloadditions of diazo esters with hexahydro-1,3,4-triazines, thus providing five- and seven-membered heterocycles in moderate to high yields under mild reaction conditions. These reactions feature the use of a gold complex to accomplish the diverse annulations and the first example of the involvement of a gold metallo-enolcarbene in a cycloaddition. It is also the first utilization of stable triazines as formal dipolar adducts in the carbene-involved cycloadditions. Mechanistic investigations reveal that the triazines reacted directly, rather than as formaldimine precursors, in the reaction process. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Template Synthesis, Metalation, and Self-Assembly of Protic Gold(I)/(NHC)2 Tectons Driven by Metallophilic Interactions.

PubMed

Ruiz, Javier; García, Lucía; Sol, Daniel; Vivanco, Marilín

2016-07-11

A new protocol for the synthesis of protic bis(N-heterocyclic carbene) complexes of Au(I) by a stepwise metal-controlled coupling of isocyanide and propargylamine is described. They are used as tectons for the construction of supramolecular architectures through metalation and self-assembly. Notably a unique polymeric chain of Cu(I) with alternate Au(I) /bis(imidazolate) bridging scaffolds and strong unsupported Cu(I) -Cu(I) interactions has been generated, as well as a 28-metal-atoms cluster containing a nanopiece of Cu2 O trapped by peripheral Au(I) /bis(imidazolate) moieties. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Catalytic SN2'- and Enantioselective Allylic Substitution with a Diborylmethane Reagent and Application in Synthesis.

PubMed

Shi, Ying; Hoveyda, Amir H

2016-03-01

A catalytic method for the site- and enantioselective addition of commercially available di-B(pin)-methane to allylic phosphates is introduced (pin=pinacolato). Transformations may be facilitated by an NHC-Cu complex (NHC=N-heterocyclic carbene) and products obtained in 63-95 % yield, 88:12 to >98:2 S(N)2'/S(N)2 selectivity, and 85:15-99:1 enantiomeric ratio. The utility of the approach, entailing the involvement of different catalytic cross-coupling processes, is highlighted by its application to the formal synthesis of the cytotoxic natural product rhopaloic acid A. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hg(ii) and Pd(ii) complexes with a new selenoether bridged biscarbene ligand: efficient mono- and bis-arylation of methyl acrylate with a pincer biscarbene Pd(ii) precatalyst.

PubMed

Rishu; Prashanth, Billa; Bawari, Deependra; Mandal, Ushnish; Verma, Aditya; Choudhury, Angshuman Roy; Singh, Sanjay

2017-05-16

Two equivalents of 1-benzyl-3-bromoethylbenzimidazolium bromide couple with Na 2 Se to produce the first selenoether bridged bis-benzimidazolium salt (LH 2 )Br 2 . The nitrate (LH 2 )(NO 3 ) 2 and tetrafluoroborate (LH 2 )(BF 4 ) 2 salts were also synthesized from (LH 2 )Br 2 . The reaction of Hg(OAc) 2 with (LH 2 )Br 2 gave the first pseudo pincer carbene mercury complex, [Hg(L-κ 2 C)][HgBr 4 ] (C1). Different complexes of Pd(ii) with selenoether bridged carbene were obtained using (LH 2 )Br 2 and (LH 2 )(NO 3 ) 2 . Syntheses of these complexes were dependent on the counter anion and the temperature. Thus, the pincer type ionic complex [PdBr(L-κ 3 CSeC)]Br (C2) was isolated at 80 °C and the pseudo pincer type neutral complex cis-[PdBr 2 (L-κ 2 C)] (C3) was isolated at room temperature from (LH 2 )Br 2 and Pd(OAc) 2 in DMSO. The nitrate precursor (LH 2 )(NO 3 ) 2 on palladation with Pd(OAc) 2 afforded [Pd(L-κ 4 C Bz CSeC)]NO 3 (C4) showing an unprecedented intramolecular metallation at the ortho position of the benzyl wingtip of the benzimidazole moiety. The ligand salts and metal complexes have been characterized using HRMS, heteronuclear NMR and IR spectroscopy. Single crystal X-ray structures of the ligand salts (LH 2 )Br 2 and (LH 2 )(BF 4 ) 2 and complexes C1-C4 have also been elucidated. Complex C2 showed good activity for C-C coupling in the mono-Heck reaction of methyl acrylate and arylbromides. Interestingly, the less common bis-arylation was also observed with deactivated arylbromides as the result of double-Heck coupling.

Effects of histidin-2-ylidene vs. imidazol-2-ylidene ligands on the anticancer and antivascular activity of complexes of ruthenium, iridium, platinum, and gold.

PubMed

Schmitt, Florian; Donnelly, Kate; Muenzner, Julienne K; Rehm, Tobias; Novohradsky, Vojtech; Brabec, Viktor; Kasparkova, Jana; Albrecht, Martin; Schobert, Rainer; Mueller, Thomas

2016-10-01

Couples of N-heterocyclic carbene complexes of ruthenium, iridium, platinum, and gold, each differing only in the carbene ligand being either 1,3-dimethylimidazol-2-ylidene (IM) or 1,3-dimethyl-N-boc-O-methylhistidin-2-ylidene (HIS), were assessed for their antiproliferative effect on seven cancer cell lines, their interaction with DNA, their cell cycle interference, and their vascular disrupting properties. In MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays only the platinum complexes were cytotoxic at single-digit micromolar IC 50 concentrations with the (HIS)Pt complex being on average twice as active as the (IM)Pt complex. The former was highly efficacious against cisplatin-resistant HT-29 colon carcinoma cells where the latter had no effect. Both Pt complexes were accumulated by cancer cells and bound to double-helical DNA equally well. Only the (HIS)Pt complex modified the electrophoretic mobility of circular DNA in vitro due to the HIS ligand causing greater morphological changes to the DNA. Both platinum complexes induced accumulation of 518A2 melanoma cells in G2/M and S phase of the cell cycle. A disruption of blood vessels in the chorioallantoic membrane of fertilized chicken eggs was observed for both platinum complexes and the (IM)gold complex. The (HIS)platinum complex was as active as cisplatin in tumor xenografted mice while being tolerated better. We found that the HIS ligand may augment the cytotoxicity of certain antitumoral metal fragments in two ways: by acting as a transmembrane carrier increasing the cellular accumulation of the complex, and by initiating a pronounced distortion and unwinding of DNA. We identified a new (HIS)platinum complex which was highly cytotoxic against cancer cells including cisplatin-resistant ones. Copyright © 2016 Elsevier Inc. All rights reserved.

Transition-metal phosphors with cyclometalating ligands: fundamentals and applications.

PubMed

Chi, Yun; Chou, Pi-Tai

2010-02-01

One goal of this critical review is to provide advanced methodologies for systematic preparation of transition-metal based phosphors that show latent applications in the field of organic light emitting diodes (OLEDs). We are therefore reviewing various types of cyclometalating chelates for which the favorable metal-chelate bonding interaction, on the one hand, makes the resulting phosphorescent complexes highly emissive in both fluid and solid states at room temperature. On the other hand, fine adjustment of ligand-centered pi-pi* electronic transitions allows tuning of emission wavelength across the whole visible spectrum. The cyclometalating chelates are then classified according to types of cyclometalating groups, i.e. either aromatic C-H or azolic N-H fragment, and the adjacent donor fragment involved in the formation of metallacycles; the latter is an N-containing heterocycle, N-heterocyclic (NHC) carbene fragment or even diphenylphosphino group. These cyclometalating ligands are capable to react with heavy transition-metal elements, namely: Ru(II), Os(II), Ir(III) and Pt(II), to afford a variety of highly emissive phosphors, for which the photophysical properties as a function of chelate or metal characteristics are systematically discussed. Using Ir(III) complexes as examples, the C--N chelates possessing both C-H site and N-heterocyclic donor group are essential for obtaining phosphors with emission ranging from sky-blue to saturated red, while the N--N chelates such as 2-pyridyl-C-linked azolates are found useful for serving as true-blue chromophores due to their increased ligand-centered pi-pi* energy gap. Lastly, the remaining NHC carbene and benzyl phosphine chelates are highly desirable to serve as ancillary chelates in localizing the electronic transition between the metal and remaining lower energy chromophoric chelates. As for the potential opto-electronic applications, many of them exhibit remarkable performance data, which are convincing to pave a broad avenue for further development of all types of phosphorescent displays and illumination devices (94 references).

Redox-Active Bis(phenolate) N-Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States.

PubMed

Harris, Caleb F; Bayless, Michael B; van Leest, Nicolaas P; Bruch, Quinton J; Livesay, Brooke N; Bacsa, John; Hardcastle, Kenneth I; Shores, Matthew P; de Bruin, Bas; Soper, Jake D

2017-10-16

A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO] 2- dianions, of the general formula [(OCO)Co II L] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)Co II L] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc + /Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [( S OCO)Co III (THF) 2 ] + containing a closed-shell [ S OCO] 2- diphenolate ligand bound to a S = 1 Co(III) center, or [( S OCO • )Co II (THF) 2 ] + with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [ S OCO • ] - containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [( S OCO 0 )Co II (THF) 3 ] 2+ , with a single unpaired electron localized on the d 7 Co(II) center and a doubly oxidized, charge-neutral, closed-shell S OCO 0 ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.

Hydrogenation of fluoroarenes: Direct access to all-cis-(multi)fluorinated cycloalkanes.

PubMed

Wiesenfeldt, Mario P; Nairoukh, Zackaria; Li, Wei; Glorius, Frank

2017-09-01

All-c is -multifluorinated cycloalkanes exhibit intriguing electronic properties. In particular, they display extremely high dipole moments perpendicular to the aliphatic ring, making them highly desired motifs in material science. Very few such motifs have been prepared, as their syntheses require multistep sequences from diastereoselectively prefunctionalized precursors. Herein we report a synthetic strategy to access these valuable materials via the rhodium-cyclic (alkyl)(amino)carbene (CAAC)-catalyzed hydrogenation of readily available fluorinated arenes in hexane. This route enables the scalable single-step preparation of an abundance of multisubstituted and multifluorinated cycloalkanes, including all- cis -1,2,3,4,5,6-hexafluorocyclohexane as well as cis-configured fluorinated aliphatic heterocycles. Copyright © 2017, American Association for the Advancement of Science.

Double stabilization of nanocrystalline silicon: a bonus from solvent

NASA Astrophysics Data System (ADS)

Kolyagin, Y. G.; Zakharov, V. N.; Yatsenko, A. V.; Paseshnichenko, K. A.; Savilov, S. V.; Aslanov, L. A.

2016-01-01

Double stabilization of the silicon nanocrystals was observed for the first time by 29Si and 13C MAS NMR spectroscopy. The role of solvent, 1,2-dimethoxyethane (glyme), in formation and stabilization of silicon nanocrystals as well as mechanism of modification of the surface of silicon nanocrystals by nitrogen-heterocyclic carbene (NHC) was studied in this research. It was shown that silicon nanocrystals were stabilized by the products of cleavage of the C-O bonds in ethers and similar compounds. The fact of stabilization of silicon nanoparticles with NHC ligands in glyme was experimentally detected. It was demonstrated that MAS NMR spectroscopy is rather informative for study of the surface of silicon nanoparticles but it needs very pure samples.

From furfural to fuel: synthesis of furoins by organocatalysis and their hydrodeoxygenation by cascade catalysis.

PubMed

Wegenhart, Benjamin L; Yang, Linan; Kwan, Soon Cheong; Harris, Remi; Kenttämaa, Hilkka I; Abu-Omar, Mahdi M

2014-09-01

The synthesis of furoins from biomass-derived furfural and 2-methylfurfural is demonstrated in high yields in green and renewable solvents using N-heterocyclic carbene organocatalysts. The resulting furoin molecules are used as precursors for fuels using cascade catalysis, first by using Pd/C with acidic co-catalysts under very mild conditions to yield oxygenated C12 molecules. Two main products were formed, which we identified as 1,2-bis(5-methyltetrahydrofuran-2-yl)ethane and 1-(5-methyltetrahydrofuran-2-yl)heptanol. The use of a Pd/Zeolite-β catalyst under more extreme conditions resulted in the complete hydrodeoxygenation of 5,5'-dimethylfuroin to dodecanes in high yields (76%) and exceptional selectivity (94%) for n-dodecane. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Rhodium-Catalyzed Acyloxy Migration of Propargylic Esters in Cycloadditions, Inspiration from Recent “Gold Rush”

PubMed Central

Shu, Xing-Zhong; Shu, Dongxu; Schienebeck, Casi M.

2012-01-01

Transition metal-catalyzed acyloxy migration of propargylic esters offers versatile entries to allene and vinyl carbene intermediates for various fascinating subsequent transformations. Most π-acidic metals (e.g. gold and platinum) are capable of facilitating these acyloxy migration events. However, very few of these processes involve redox chemistry, which are well-known for most other transition metals such as rhodium. The coupling of acyloxy migration of propargylic esters with oxidative addition, migratory insertion, and reductive elimination may lead to ample new opportunities for the design of new reactions. This tutorial review summarizes recent developments in Rh-catalyzed 1,3- and 1,2-acyloxy migration of propargylic esters in a number of cycloaddition reactions. Related Au- and Pt-catalyzed cycloadditions involving acyloxy migration are also discussed. PMID:22895533

Base-stabilized silaimine and its donor-free dimer derived from the reaction of NHC-supported silylene with SiCl4.

PubMed

Cui, Haiyan; Cui, Chunming

2015-12-21

Reaction of the N-heterocyclic carbene (NHC)-stabilized silylene ArN(SiMe3)Si(IiPr)Cl (1, Ar = 2,6-iPr2C6H3, IiPr = 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene) with SiCl4 resulted in the formation of three different products NHC-stabilized dichlorosilaimine ArN=Si(IiPr)Cl2 (2), aminotrichlorosilane ArN(SiMe3)SiCl3 (3) and a silaimine dimer (ArNSiCl2)2 (4) under different conditions. The products can be controlled by reaction conditions. Compound 2 is the first example of a less bulky NHC-supported silaimine via the reaction of a silylene with SiCl4.

Chemiluminescence in cryogenic matrices

NASA Astrophysics Data System (ADS)

Lotnik, S. V.; Kazakov, Valeri P.

1989-04-01

The literature data on chemiluminescence (CL) in cryogenic matrices have been classified and correlated for the first time. The role of studies on phosphorescence and CL at low temperatures in the development of cryochemistry is shown. The features of low-temperature CL in matrices of nitrogen and inert gases (fine structure of spectra, matrix effects) and the data on the mobility and reactivity of atoms and radicals at very low temperatures are examined. The trends in the development of studies on CL in cryogenic matrices, such as the search for systems involving polyatomic molecules and extending the forms of CL reactions, are followed. The reactions of active nitrogen with hydrocarbons that are accompanied by light emission and CL in the oxidation of carbenes at T >= 77 K are examined. The bibliography includes 112 references.

Crystal structure of bis-(1,3-di-meth-oxy-imidazolin-2-yl-idene)silver(I) hexa-fluorido-phosphate, N-heterocyclic carbene (NHC) complex.

PubMed

Rietzler, Barbara; Laus, Gerhard; Kahlenberg, Volker; Schottenberger, Herwig

2015-12-01

The title salt, [Ag(C5H8N2O2)2]PF6, was obtained by deprotonation and metalation of 1,3-di-meth-oxy-imidazolium hexa-fluorido-phosphate using silver(I) oxide in methanol. The C-Ag-C angle in the cation is 178.1 (2)°, and the N-C-N angles are 101.1 (4) and 100.5 (4)°. The meth-oxy groups adopt an anti conformation. In the crystal, anions (A) are sandwiched between cations (C) in a layered arrangement {C…A…C} n stacked along [001]. Within a C…A…C layer, the hexafluoridophosphate anions accept several C-H⋯F hydrogen bonds from the cationic complex.

Designing Stability into Thermally Reactive Plumbylenes.

PubMed

Bačić, Goran; Zanders, David; Mallick, Bert; Devi, Anjana; Barry, Seán T

2018-06-26

Lead analogues of N-heterocyclic carbenes (NHPbs) are the least understood members of this increasingly important class of compounds. Here we report the design, preparation, isolation, structure, volatility, and decomposition pathways of a novel aliphatic NHPb: rac- N  2 , N  3 -di- tert-butylbutane-2,3-diamido lead(II) (1Pb). The large steric bulk of the tert-butylamido moieties and rac-butane backbone successfully hinder redox decomposition pathways observed for diamidoethylene and -ethane backbone analogues, pushing the onset of thermal decomposition from below 0 °C to above 150 °C. With an exceptionally high vapor pressure of 1 Torr at 94 ± 2 °C and excellent thermal stability among Pb(II) complexes, 1Pb is a promising precursor for the chemical vapor deposition (CVD) and atomic layer deposition (ALD) of functional lead-containing materials.

On the behaviour of biradicaloid [P(μ-NTer)]2 towards Lewis acids and bases.

PubMed

Hinz, Alexander; Schulz, Axel; Villinger, Alexander

2016-05-07

The well-known diphosphadiazane-1,3-diyl [P(μ-NTer)]2 (Ter = 2,6-bis(2,4,6-trimethyl-phenyl)-phenyl) was treated with Lewis bases such as N-heterocyclic carbenes and Lewis acids e.g. gold(i) chloride complexes. In the reaction with the Lewis base, fragmentation of the P2N2 framework was observed, yielding a salt of the type [(NHC)2P](+)[(TerN)2P](-) in a clean reaction. The reaction of [P(μ-NTer)]2 with gold(i) chloride afforded 1 : 1 and 1 : 2 complexes. The dinuclear complex [(ClAu)2P(μ-NTer)2P] displays a bridging P atom between both gold centers, as has been observed for P based zwitterions.

Water-Soluble N-Heterocyclic Carbene-Protected Gold Nanoparticles: Size-Controlled Synthesis, Stability, and Optical Properties.

PubMed

Salorinne, Kirsi; Man, Renee W Y; Li, Chien-Hung; Taki, Masayasu; Nambo, Masakazu; Crudden, Cathleen M

2017-05-22

NHC-Au I complexes were used to prepare stable, water-soluble, NHC-protected gold nanoparticles. The water-soluble, charged nature of the nanoparticles permitted analysis by polyacrylamide gel electrophoresis (PAGE), which showed that the nanoparticles were highly monodisperse, with tunable core diameters between 2.0 and 3.3 nm depending on the synthesis conditions. Temporal, thermal, and chemical stability of the nanoparticles were determined to be high. Treatment with thiols caused etching of the particles after 24 h; however larger plasmonic particles showed greater resistance to thiol treatment. These water-soluble, bio-compatible nanoparticles are promising candidates for use in photoacoustic imaging, with even the smallest nanoparticles giving reliable photoacoustic signals. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis of functionalized imidazolidine-2-thiones via NHC/base-promoted aza-benzoin/aza-acetalization domino reactions.

PubMed

Di Carmine, Graziano; Ragno, Daniele; De Risi, Carmela; Bortolini, Olga; Giovannini, Pier Paolo; Fantin, Giancarlo; Massi, Alessandro

2017-10-25

A strategy for the synthesis of biologically relevant 5-hydroxy-imidazolidine-2-thione derivatives is presented. A novel class of α-sulfonylamines have been suitably prepared (46-81% yield) as precursors of formal benzylidenethiourea acceptors; these are generated in situ and intercepted by N-heterocyclic carbene (NHC)-activated aldehydes affording open-chain aza-benzoin-type adducts, which in turn undergo an intramolecular aza-acetalization reaction in a one-pot fashion. A thiazolium salt/triethylamine couple proved to be the more effective system to trigger the domino sequence giving the target heterocycles in good yields (45-97%) and diastereoselectivities (up to 99 : 1 dr). The multigram scale synthesis and elaboration of a selected 5-hydroxy-imidazolidine-2-thione compound is also described.

Synthesis and crystal structure of the rhodium(I) cyclooctadiene complex with bis(3-tert-butylimidazol-2-ylidene)borate ligand

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, F.; Shao, K.-J.; Xiao, Y.-C.

2015-12-15

The rhodium(I) cyclooctadiene complex with the bis(3-tert-butylimidazol-2-ylidene)borate ligand [H{sub 2}B(Im{sup t}Bu){sup 2}]Rh(COD) C{sup 22}H{sup 36}BN{sup 4}Rh, has been prepared, and its crystal structure is determined by X-ray diffraction. Complex exhibits slightly distorted square planar configurations around the metal center, which is coordinated by the bidentate H{sup 2}B(Im{sup t}Bu){sub 2} and one cyclooctadiene group. The Rh–C{sub carbene} bond lengths are 2.043(4) and 2.074(4) Å, and the bond angle C–Rh1–C is 82.59°. The dihedral angle between two imidazol-2-ylidene rings is 67.30°.

Beyond Iron: Iridium-Containing P450 Enzymes for Selective Cyclopropanations of Structurally Diverse Alkenes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Key, Hanna M.; Dydio, Paweł; Liu, Zhennan

Enzymes catalyze organic transformations with exquisite levels of selectivity, including chemoselectivity, stereoselectivity, and substrate selectivity, but the types of reactions catalyzed by enzymes are more limited than those of chemical catalysts. Thus, the convergence of chemical catalysis and biocatalysis can enable enzymatic systems to catalyze abiological reactions with high selectivity. Recently, we disclosed artificial enzymes constructed from the apo form of heme proteins and iridium porphyrins that catalyze the insertion of carbenes into a C-H bond. Here, we postulated that the same type of Ir(Me)-PIX enzymes could catalyze the cyclopropanation of a broad range of alkenes with control of multiplemore » modes of selectivity. Here, we report the evolution of artificial enzymes that are highly active and highly stereoselective for the addition of carbenes to a wide range of alkenes. These enzymes catalyze the cyclopropanation of terminal and internal, activated and unactivated, electron-rich and electron-deficient, conjugated and nonconjugated alkenes. In particular, Ir(Me)-PIX enzymes derived from CYP119 catalyze highly enantio- and diastereoselective cyclopropanations of styrene with ±98% ee, > 70:1 dr, > 75% yield, and ~10,000 turnovers (TON), as well as 1,2-disubstituted styrenes with up to 99% ee, 35:1 dr, and 54% yield. Moreover, Ir(Me)-PIX enzymes catalyze cyclopropanation of internal, unactivated alkenes with up to 99% stereoselectivity, 76% yield, and 1300 TON. They also catalyze cyclopropanation of natural products with diastereoselectivities that are complementary to those attained with standard transition metal catalysts. Finally, Ir(Me)-PIX P450 variants react with substrate selectivity that is reminiscent of natural enzymes; they react preferentially with less reactive internal alkenes in the presence of more reactive terminal alkenes. Altogether, the studies reveal the suitability of Ir-containing P450s to combine the broad reactivity and substrate scope of transition metal catalysts with the exquisite selectivity of enzymes, generating catalysts that enable reactions to occur with levels and modes of activity and selectivity previously unattainable with natural enzymes or transition metal complexes alone.« less

Beyond Iron: Iridium-Containing P450 Enzymes for Selective Cyclopropanations of Structurally Diverse Alkenes

DOE PAGES

Key, Hanna M.; Dydio, Paweł; Liu, Zhennan; ...

2017-04-01

Enzymes catalyze organic transformations with exquisite levels of selectivity, including chemoselectivity, stereoselectivity, and substrate selectivity, but the types of reactions catalyzed by enzymes are more limited than those of chemical catalysts. Thus, the convergence of chemical catalysis and biocatalysis can enable enzymatic systems to catalyze abiological reactions with high selectivity. Recently, we disclosed artificial enzymes constructed from the apo form of heme proteins and iridium porphyrins that catalyze the insertion of carbenes into a C-H bond. Here, we postulated that the same type of Ir(Me)-PIX enzymes could catalyze the cyclopropanation of a broad range of alkenes with control of multiplemore » modes of selectivity. Here, we report the evolution of artificial enzymes that are highly active and highly stereoselective for the addition of carbenes to a wide range of alkenes. These enzymes catalyze the cyclopropanation of terminal and internal, activated and unactivated, electron-rich and electron-deficient, conjugated and nonconjugated alkenes. In particular, Ir(Me)-PIX enzymes derived from CYP119 catalyze highly enantio- and diastereoselective cyclopropanations of styrene with ±98% ee, > 70:1 dr, > 75% yield, and ~10,000 turnovers (TON), as well as 1,2-disubstituted styrenes with up to 99% ee, 35:1 dr, and 54% yield. Moreover, Ir(Me)-PIX enzymes catalyze cyclopropanation of internal, unactivated alkenes with up to 99% stereoselectivity, 76% yield, and 1300 TON. They also catalyze cyclopropanation of natural products with diastereoselectivities that are complementary to those attained with standard transition metal catalysts. Finally, Ir(Me)-PIX P450 variants react with substrate selectivity that is reminiscent of natural enzymes; they react preferentially with less reactive internal alkenes in the presence of more reactive terminal alkenes. Altogether, the studies reveal the suitability of Ir-containing P450s to combine the broad reactivity and substrate scope of transition metal catalysts with the exquisite selectivity of enzymes, generating catalysts that enable reactions to occur with levels and modes of activity and selectivity previously unattainable with natural enzymes or transition metal complexes alone.« less

Matrix-Isolation Spectroscopy of Reactive Organic Molecules of Relevance to Interstellar Space

NASA Astrophysics Data System (ADS)

Kopff, Laura A.; Nolan, Alex M.; Kreifels, Terese A.; Draxler, Thomas W.; Esselman, Brian J.; Burrmann, Nicola J.; McMahon, Robert J.

2010-11-01

Matrix isolation, the process of trapping a molecule in an inert gas at low temperature, provides a means for studying highly reactive intermediates, such as carbenes or radicals. Reactive species can be characterized by IR, UV-vis and/or EPR spectroscopy. Comparison of experimental and computed spectral data, as well as chemical reactivity, is used for structural assignment Triplet propynylidene is proposed to exist in the interstellar medium (ISM), due to the detection of a higher-energy isomers via rotational spectroscopy. Currently, we are exploring the structural and photochemical effects of varying substituents on the propynylidne system. A diazo precursor has been synthesized and photolyzed to produce dimethylpropynylidene in an argon matrix. A photochemical hydrogen shift to produce 1-penten-3-yne has been observed through infrared spectroscopy. Cyanocarbons are known to be abundant in the ISM and the atmosphere of Titan, however matrixisolation studies have not yet been carried out for a significant number of these compounds. Photolysis of 3-cyano-3-methyldiazirine should yield methylcyanocarbene, one of the simplest species in this family. Another molecule of interest is l-HC4N, which has been detected in the ISM, but has not yet been matrix-isolated and characterized. The study of arylcarbenes is vital to understanding the chemistry of carbon-rich environments, such as discharges, interstellar clouds, and circumstellar envelopes. The identification of small, sulfur containing molecules, and the identification of aromatics in the ISM make future thiophene and benzothiophene detections a real possibility. Studies on 2- and 3-diazomethyl substituted benzothiophenes are underway to assess their photochemical reactivity and potential for forming benzothiophene carbenes. Macrocylic polyynes are proposed to be involved in carbon condensation via the ring coalescence and annealing model to produce graphitic sheets or fullerenes. To simplify a complex system we are experimentally and computationally studying the series of ethynyl-substituted cyclobutadienes and their possible involvement in the build-up of larger carbon containing molecules in the ISM. The Bergman cyclization of cyclobutadiene has been explored computationally and the photochemical precursor is currently being synthesized.

High-performance light-emitting diodes based on carbene-metal-amides

NASA Astrophysics Data System (ADS)

Di, Dawei; Romanov, Alexander S.; Yang, Le; Richter, Johannes M.; Rivett, Jasmine P. H.; Jones, Saul; Thomas, Tudor H.; Abdi Jalebi, Mojtaba; Friend, Richard H.; Linnolahti, Mikko; Bochmann, Manfred; Credgington, Dan

2017-04-01

Organic light-emitting diodes (OLEDs) promise highly efficient lighting and display technologies. We introduce a new class of linear donor-bridge-acceptor light-emitting molecules, which enable solution-processed OLEDs with near-100% internal quantum efficiency at high brightness. Key to this performance is their rapid and efficient utilization of triplet states. Using time-resolved spectroscopy, we establish that luminescence via triplets occurs within 350 nanoseconds at ambient temperature, after reverse intersystem crossing to singlets. We find that molecular geometries exist at which the singlet-triplet energy gap (exchange energy) is close to zero, so that rapid interconversion is possible. Calculations indicate that exchange energy is tuned by relative rotation of the donor and acceptor moieties about the bridge. Unlike other systems with low exchange energy, substantial oscillator strength is sustained at the singlet-triplet degeneracy point.

Antiplasmodial activities of gold(I) complexes involving functionalized N-heterocyclic carbenes.

PubMed

Hemmert, Catherine; Ramadani, Arba Pramundita; Boselli, Luca; Fernández Álvarez, Álvaro; Paloque, Lucie; Augereau, Jean-Michel; Gornitzka, Heinz; Benoit-Vical, Françoise

2016-07-01

A series of twenty five molecules, including imidazolium salts functionalized by N-, O- or S-containing groups and their corresponding cationic, neutral or anionic gold(I) complexes were evaluated on Plasmodium falciparum in vitro and then on Vero cells to determine their selectivity. Among them, eight new compounds were synthesized and fully characterized by spectroscopic methods. The X-ray structures of three gold(I) complexes are presented. Except one complex (18), all the cationic gold(I) complexes show potent antiplasmodial activity with IC50 in the micro- and submicromolar range, correlated with their lipophilicity. Structure-activity relationships enable to evidence a lead-complex (21) displaying a good activity (IC50=210nM) close to the value obtained with chloroquine (IC50=514nM) and a weak cytotoxicity. Copyright © 2016 Elsevier Ltd. All rights reserved.

The nature of chemical innovation: new enzymes by evolution.

PubMed

Arnold, Frances H

2015-11-01

I describe how we direct the evolution of non-natural enzyme activities, using chemical intuition and information on structure and mechanism to guide us to the most promising reaction/enzyme systems. With synthetic reagents to generate new reactive intermediates and just a few amino acid substitutions to tune the active site, a cytochrome P450 can catalyze a variety of carbene and nitrene transfer reactions. The cyclopropanation, N-H insertion, C-H amination, sulfimidation, and aziridination reactions now demonstrated are all well known in chemical catalysis but have no counterparts in nature. The new enzymes are fully genetically encoded, assemble and function inside of cells, and can be optimized for different substrates, activities, and selectivities. We are learning how to use nature's innovation mechanisms to marry some of the synthetic chemists' favorite transformations with the exquisite selectivity and tunability of enzymes.

α-Unsubstituted Pyrroles by NHC-Catalyzed Three-Component Coupling: Direct Synthesis of a Versatile Atorvastatin Derivative.

PubMed

Fleige, Mirco; Glorius, Frank

2017-08-10

A practical one-pot cascade reaction protocol provides direct access to valuable 1,2,4-trisubstituted pyrroles. The process involves an N-heterocyclic carbene (NHC)-catalyzed Stetter-type hydroformylation using glycolaldehyde dimer as a novel C1 building-block, followed by a Paal-Knorr condensation with primary amines. The reaction makes use of simple and commercially available starting-materials and catalyst, an important feature regarding applicability and utility. Low catalyst loading under mild reaction conditions afforded a variety of 1,2,4-substituted pyrroles in a transition-metal-free reaction with high step economy and good yields. This methodology is applied in the synthesis of a versatile Atorvastatin precursor, in which a variety of modifications at the pyrrole core structure are possible. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Adhesive curing through low-voltage activation

PubMed Central

Ping, Jianfeng; Gao, Feng; Chen, Jian Lin; Webster, Richard D.; Steele, Terry W. J.

2015-01-01

Instant curing adhesives typically fall within three categories, being activated by either light (photocuring), heat (thermocuring) or chemical means. These curing strategies limit applications to specific substrates and can only be activated under certain conditions. Here we present the development of an instant curing adhesive through low-voltage activation. The electrocuring adhesive is synthesized by grafting carbene precursors on polyamidoamine dendrimers and dissolving in aqueous solvents to form viscous gels. The electrocuring adhesives are activated at −2 V versus Ag/AgCl, allowing tunable crosslinking within the dendrimer matrix and on both electrode surfaces. As the applied voltage discontinued, crosslinking immediately terminated. Thus, crosslinking initiation and propagation are observed to be voltage and time dependent, enabling tuning of both material properties and adhesive strength. The electrocuring adhesive has immediate implications in manufacturing and development of implantable bioadhesives. PMID:26282730

Z-Selective iridium-catalyzed cross-coupling of allylic carbonates and α-diazo esters.

PubMed

Thomas, Bryce N; Moon, Patrick J; Yin, Shengkang; Brown, Alex; Lundgren, Rylan J

2018-01-07

A well-defined Ir-allyl complex catalyzes the Z -selective cross-coupling of allyl carbonates with α-aryl diazo esters. The process overrides the large thermodynamic preference for E -products typically observed in metal-mediated coupling reactions to enable the synthesis of Z , E -dieneoates in good yield with selectivities consistently approaching or greater than 90 : 10. This transformation represents the first productive merger of Ir-carbene and Ir-allyl species, which are commonly encountered intermediates in allylation and cyclopropanation/E-H insertion catalysis. Potentially reactive functional groups (aryl halides, ketones, nitriles, olefins, amines) are tolerated owing to the mildness of reaction conditions. Kinetic analysis of the reaction suggests oxidative addition of the allyl carbonate to an Ir-species is rate-determining. Mechanistic studies uncovered a pathway for catalyst activation mediated by NEt 3 .

Determination of βS haplotypes in patients with sickle-cell anemia in the state of Rio Grande do Norte, Brazil

PubMed Central

Cabral, Cynthia Hatsue Kitayama; Serafim, Édvis Santos Soares; de Medeiros, Waleska Rayane Dantas Bezerra; de Medeiros Fernandes, Thales Allyrio Araújo; Kimura, Elza Miyuki; Costa, Fernando Ferreira; de Fátima Sonati, Maria; Rebecchi, Ivanise Marina Moretti; de Medeiros, Tereza Maria Dantas

2011-01-01

βS haplotypes were studied in 47 non-related patients with sickle-cell anemia from the state of Rio Grande do Norte, Brazil. Molecular analysis was conducted by PCR/RFLP using restriction endonucleases XmnI, HindIII, HincII and HinfI to analyze six polymorphic sites from the beta cluster. Twenty-seven patients (57.5%) were identified with genotype CAR/CAR, 9 (19.1%) CAR/BEN, 6 (12.8%) CAR/CAM, 1 (2.1%) BEN/BEN, 2 (4.3%) CAR/Atp, 1 (2.1%) BEN/Atp and 1 (2.1%) with genotype Atp/Atp. The greater frequency of Cameroon haplotypes compared to other Brazilian states suggests the existence of a peculiarity of African origin in the state of Rio Grande do Norte. PMID:21931513

Studies of a Diazo Cyclopropanation Strategy for the Total Synthesis of (-)-Lundurine A.

PubMed

Huang, Hong-Xiu; Jin, Shuai-Jiang; Gong, Jin; Zhang, Dan; Song, Hao; Qin, Yong

2015-09-14

The bioactive Kopsia alkaloids lundurines A-D are the only natural products known to contain indolylcyclopropane. Achieving their syntheses can provide important insights into their biogenesis, as well as novel synthetic routes for complex natural products. Asymmetric total synthesis of (-)-lundurine A has previously been achieved through a Simmons-Smith cyclopropanation strategy. Here, the total synthesis of (-)-lundurine A was carried out using a metal-catalyzed diazo cyclopropanation strategy. In order to avoid a carbene CH insertion side reaction during cyclopropanation of α-diazo- carboxylates or cyanides, a one-pot, copper-catalyzed Bamford-Stevens diazotization/diazo decomposition/cyclopropanation cascade was developed, involving hydrazone. This approach simultaneously generates the C/D/E ring system and the two chiral quaternary centers at C2 and C7. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Palladium coupling catalysts for pharmaceutical applications.

PubMed

Doucet, Henri; Hierso, Jean-Cyrille

2007-11-01

This review discusses recent advances made in the area of palladium-catalyzed coupling reactions and describes a selection of the catalytic systems that are useful in the preparation of valuable compounds for the pharmaceutical industry. Most of these types of syntheses have used either simple palladium salts or palladium precursors associated with electron-rich mono- or bidentate phosphine ligands as catalysts. For some reactions, ligands such as triphenyl phosphine, 1,1'-bis(diphenylphosphino)ferrocene, a carbene or a bipyridine have also been employed. Several new procedures for the Suzuki cross-coupling reaction, the activation of aryl chlorides, the functionalization of aromatics and the synthesis of heteroaromatics are discussed. The C-H activation/ functionalization reactions of aryl and heteroaryl derivatives have emerged as powerful tools for the preparation of biaryl compounds, and the recent procedures and catalysts employed in this promising field are also highlighted herein.

2-Ferrocenyl-2-thiazoline as a building block of novel phosphine-free ligands.

PubMed

Corona-Sánchez, Ricardo; Toscano, Rubén A; Ortega-Alfaro, M Carmen; Sandoval-Chávez, César; López-Cortés, José G

2013-09-07

New 1,2-disubstituted ferrocenes [5(b-j), in which R = -SMe, -SPh, -SiPr, -SiMe3, -SePh, -SnBu3, -B(OH)2, -Me, -I] with a thiazoline ring in the ferrocene backbone using as key intermediate a ferrocenyl Fischer carbene complex were synthesized. The capability of the 2-thiazoline moiety as an ortho-directed metalation group was demonstrated by subsequent quenching of lithium intermediate with several electrophiles, proving to be an excellent method for synthesizing bidentate ligands. The catalytic scope of the [N,S] ligand 5b as the corresponding palladium complex 5b-PdCl(2) in a microwave-promoted Heck reaction was also tested. Results obtained showed better catalytic activity of 5b-PdCl(2) compared to other catalytic systems based on a [N,S] ferrocenyl ligand.

Catalytic Enantioselective Olefin Metathesis in Natural Product Synthesis. Chiral Metal-Based Complexes that Deliver High Enantioselectivity and More

PubMed Central

Malcolmson, Steven J.; Meek, Simon J.; Zhugralin, Adil R.

2012-01-01

Chiral olefin metathesis catalysts enable chemists to access enantiomerically enriched small molecules with high efficiency; synthesis schemes involving such complexes can be substantially more concise than those that would involve enantiomerically pure substrates and achiral Mo alkylidenes or Ru-based carbenes. The scope of research towards design and development of chiral catalysts is not limited to discovery of complexes that are merely the chiral versions of the related achiral variants. A chiral olefin metathesis catalyst, in addition to furnishing products of high enantiomeric purity, can offer levels of efficiency, product selectivity and/or olefin stereoselectivity that are unavailable through the achiral variants. Such positive attributes of chiral catalysts (whether utilized in racemic or enantiomerically enriched form) should be considered as general, applicable to other classes of transformations. PMID:19967680

Tetanus toxin is labeled with photoactivatable phospholipids at low pH

DOE Office of Scientific and Technical Information (OSTI.GOV)

Montecucco, C.; Schiavo, G.; Brunner, J.

1986-02-25

The mechanism of cell penetration by tetanus toxin is unknown; it has been suggested that the toxin may penetrate into the lipid bilayer from a low-pH vesicular compartment. In this work, the interaction of tetanus toxin with liposomal model membranes has been studied by following its photoinduced cross-linking with either a nitrene or a carbene photolytically generated from corresponding light-sensitive phosphatidylcholine analogues. The toxin was labeled only at pHs lower than 5.5. The low pH acquired hydrophobicity of tetanus toxin appears to be confined to its light chain and to the 45-kDa NH2-terminal fragment of the heavy chain. Negatively chargedmore » lipids promote the interaction of this toxin with the hydrocarbon chain of phospholipids. The relevance of the present findings to the possible mechanism of nerve cell penetration by tetanus toxin is discussed.« less

Heptagraphene: Tunable dirac cones in a graphitic structure

DOE PAGES

Lopez-Bezanilla, Alejandro; Martin, Ivar; Littlewood, Peter B.

2016-09-13

Here, we predict the existence and dynamical stability of heptagraphene, a new graphitic structure formed of rings of 10 carbon atoms bridged by carbene groups yielding seven-membered rings. Despite the rectangular unit cell, the band structure is topologically equivalent to that of strongly distorted graphene. Density-functional-theory calculations demonstrate that heptagraphene has Dirac cones on symmetry lines that are robust against biaxial strain but which open a gap under shear. At high deformation values bond reconstructions lead to different electronic band arrangements in dynamically stable configurations. Within a tight-binding framework this richness of the electronic behavior is identified as a directmore » consequence of the symmetry breaking within the cell which, unlike other graphitic structures, leads to band gap opening. A combined approach of chemical and physical modification of graphene unit cell unfurls the opportunity to design carbon-based systems in which one aims to tune an electronic band gap.« less

Photochemically Switching Diamidocarbene Spin States Leads to Reversible Büchner Ring Expansions.

PubMed

Perera, Tharushi A; Reinheimer, Eric W; Hudnall, Todd W

2017-10-18

The discovery of thermal and photochemical control by Woodward and Hoffmann revolutionized how we understand chemical reactivity. Similarly, we now describe the first example of a carbene that exhibits differing thermal and photochemical reactivity. When a singlet ground-state N,N'-diamidocarbene 1 was photolyzed at 380 nm, excitation to a triplet state was observed. The triplet-state electronic structure was characteristic of the expected biradical σ 1 p π 1 spin configuration according to a combination of spectroscopic and computational methods. Surprisingly, the triplet state of 1 was found to engage a series of arenes in thermally reversible Büchner ring expansion reactions, marking the first examples where both cyclopropanation and ring expansion of arenes were rendered reversible. Not only are these photochemical reactions different from the known thermal chemistry of 1, but the reversibility enabled us to perform the first examples of photochemically induced arene exchange/expansion reactions at a single carbon center.

One-electron-mediated rearrangements of 2,3-disiladicarbene.

PubMed

Mondal, Kartik Chandra; Samuel, Prinson P; Roesky, Herbert W; Aysin, Rinat R; Leites, Larissa A; Neudeck, Sven; Lübben, Jens; Dittrich, Birger; Holzmann, Nicole; Hermann, Markus; Frenking, Gernot

2014-06-25

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si═Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2(•-), as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2(•-).

Azido, triazolyl, and alkynyl complexes of gold(I): syntheses, structures, and ligand effects.

PubMed

Robilotto, Thomas J; Deligonul, Nihal; Updegraff, James B; Gray, Thomas G

2013-08-19

Gold(I) triazolyl complexes are prepared in [3 + 2] cycloaddition reactions of (tertiary phosphine)gold(I) azides with terminal alkynes. Seven such triazolyl complexes, not previously prepared, are described. Reducible functional groups are accommodated. In addition, two new (N-heterocyclic carbene)gold(I) azides and two new gold(I) alkynyls are described. Eight complexes are crystallographically authenticated; aurophilic interactions appear in one structure only. The packing diagrams of gold(I) triazolyls all show intermolecular hydrogen bonding between N-1 of one molecule and N-3 of a neighbor. This hydrogen bonding permeates the crystal lattice. Density-functional theory calculations of (triphenylphosphine)gold(I) triazolyls and the corresponding alkynyls indicate that the triazolyl is a stronger trans-influencer than is the alkynyl, but the alkynyl is more electron-releasing. These results suggest that trans-influences in two-coordinate gold(I) complexes can be more than a simple matter of ligand donicity.

A mechanistical study on non-equilibrium reaction pathways in solid nitromethane (CH3NO2) and D3-nitromethane (CD3NO2) upon interaction with ionizing radiation

NASA Astrophysics Data System (ADS)

Kaiser, Ralf I.; Maksyutenko, Pavlo

2015-07-01

Thin films of homogeneously mixed nitromethane (CH3NO2) and D3-nitromethane (CD3NO2) ices were exposed to energetic electrons and Lyman α photons. The isotopically mixed reaction products were probed in situ via reflectron time-of-flight mass spectrometry coupled with pulsed photoionization during temperature programmed desorption. This study provides compelling evidence on three high energy pathways, which have not been observed under collisionless conditions in the gas phase. These are decomposition of nitromethane (CH3NO2) via atomic oxygen loss to nitrosomethane (CH3NO), fragmentation of nitromethane (CH3NO2) and/or methylnitrite (CH3ONO) via carbene (CH2) loss, and carbonsbnd hydrogen bond rupture processes in nitromethane (CH3NO2) and/or methylnitrite (CH3ONO).

Benzene construction via organocatalytic formal [3+3] cycloaddition reaction.

PubMed

Zhu, Tingshun; Zheng, Pengcheng; Mou, Chengli; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2014-09-25

The benzene unit, in its substituted forms, is a most common scaffold in natural products, bioactive molecules and polymer materials. Nearly 80% of the 200 best selling small molecule drugs contain at least one benzene moiety. Not surprisingly, the synthesis of substituted benzenes receives constant attentions. At present, the dominant methods use pre-existing benzene framework to install substituents by using conventional functional group manipulations or transition metal-catalyzed carbon-hydrogen bond activations. These otherwise impressive approaches require multiple synthetic steps and are ineffective from both economic and environmental perspectives. Here we report an efficient method for the synthesis of substituted benzene molecules. Instead of relying on pre-existing aromatic rings, here we construct the benzene core through a carbene-catalyzed formal [3+3] reaction. Given the simplicity and high efficiency, we expect this strategy to be of wide use especially for large scale preparation of biomedicals and functional materials.

ADMET Polymerization Activities of Electrochemically Reduced W-Based Active Species for Ge- and Sn-Containing Dienes

NASA Astrophysics Data System (ADS)

Imamoglu, Yavuz; Aydogdu, Cemil; Karabulut, Solmaz; Düz, Bülent

In the last 20 years metal atom-containing polymers have become important classes of polymers [1]. Properties like high thermic stability, electric, and photo conductometry make them very interesting for producing films, fibers, and coating [2]. Many of these compounds can be synthesized by conventional methods [3]. For producing metal-containing polymers anionic, cationic, and radicalic polymerizations were used [4-6]. Metal-containing polymers were also synthesized via acyclic diene metathesis (ADMET) polymerization that is facilitated by Schrock’s molybdenum alkylidene, or Grubbs’ ruthenium carbene catalyst [7-9]. In 1979, Gilet and coworkers succeeded in synthesizing metathetically active species from electrochemical reduction of WCl6 and MoCl5 [10,11]. In the light of these works, we have showed that electrochemically generated tungsten-based active species (WCl6-e--Al-CH2Cl2) catalyzes various metathesis-related reactions [12-16].

Activating catalysts with mechanical force.

PubMed

Piermattei, Alessio; Karthikeyan, S; Sijbesma, Rint P

2009-05-01

Homogeneously catalysed reactions can be 'switched on' by activating latent catalysts. Usually, activation is brought about by heat or an external chemical agent. However, activation of homogeneous catalysts with a mechanical trigger has not been demonstrated. Here, we introduce a general method to activate latent catalysts by mechanically breaking bonds between a metal and one of its ligands. We have found that silver(I) complexes of polymer-functionalized N-heterocyclic carbenes, which are latent organocatalysts, catalyse a transesterification reaction when exposed to ultrasound in solution. Furthermore, ultrasonic activation of a ruthenium biscarbene complex with appended polymer chains results in catalysis of olefin metathesis reactions. In each case, the catalytic activity results from ligand dissociation, brought about by transfer of mechanical forces from the polymeric substituents to the coordination bond. Mechanochemical catalyst activation has potential applications in transduction and amplification of mechanical signals, and mechanically initiated polymerizations hold promise as a novel repair mechanism in self-healing materials.

NHC-catalysed benzoin condensation - is it all down to the Breslow intermediate?

PubMed

Rehbein, Julia; Ruser, Stephanie-M; Phan, Jenny

2015-10-01

The Breslow catalytic cycle describing the benzoin condensation promoted by N-heterocyclic carbenes (NHC) as proposed in the late 1950s has since then been tried by generations of physical organic chemists. Emphasis has been laid on proofing the existence of an enaminol like structure (Breslow intermediate) that explains the observed umpolung of an otherwise electrophilic aldehyde. The present study is not focusing on spectroscopic elucidation of a thiazolydene based Breslow intermediate but rather tries to clarify if this key-intermediate is indeed directly linked with the product side of the overall reaction. The here presented EPR-spectroscopic and computational data provide a fundamentally different view on how the benzoin condensation may proceed: a radical pair could be identified as a second key-intermediate that is derived from the Breslow-intermediate via an SET process. These results highlight the close relationship to the Cannizarro reaction and oxidative transformations of aldehydes under NHC catalysis.

Origin of chemoselectivity in N-heterocyclic carbene catalyzed cross-benzoin reactions: DFT and experimental insights.

PubMed

Langdon, Steven M; Legault, Claude Y; Gravel, Michel

2015-04-03

An exploration into the origin of chemoselectivity in the NHC-catalyzed cross-benzoin reaction reveals several key factors governing the preferred pathway. In the first computational study to explore the cross-benzoin reaction, a piperidinone-derived triazolium catalyst produces kinetically controlled chemoselectivity. This is supported by (1)H NMR studies as well as a series of crossover experiments. Major contributors include the rapid and preferential formation of an NHC adduct with alkyl aldehydes, a rate-limiting carbon-carbon bond formation step benefiting from a stabilizing π-stacking/π-cation interaction, and steric penalties paid by competing pathways. The energy profile for the analogous pyrrolidinone-derived catalyst was found to be remarkably similar, despite experimental data showing that it is less chemoselective. The chemoselectivity could not be improved through kinetic control; however, equilibrating conditions show substantial preference for the same cross-benzoin product kinetically favored by the piperidinone-derived catalyst.

Asymmetric catalytic cascade reactions for constructing diverse scaffolds and complex molecules.

PubMed

Wang, Yao; Lu, Hong; Xu, Peng-Fei

2015-07-21

With the increasing concerns about chemical pollution and sustainability of resources, among the significant challenges facing synthetic chemists are the development and application of elegant and efficient methods that enable the concise synthesis of natural products, drugs, and related compounds in a step-, atom- and redox-economic manner. One of the most effective ways to reach this goal is to implement reaction cascades that allow multiple bond-forming events to occur in a single vessel. This Account documents our progress on the rational design and strategic application of asymmetric catalytic cascade reactions in constructing diverse scaffolds and synthesizing complex chiral molecules. Our research is aimed at developing robust cascade reactions for the systematic synthesis of a range of interesting molecules that contain structural motifs prevalent in natural products, pharmaceuticals, and biological probes. The strategies employed to achieve this goal can be classified into three categories: bifunctional base/Brønsted acid catalysis, covalent aminocatalysis/N-heterocyclic carbene catalysis, and asymmetric organocatalytic relay cascades. By the use of rationally designed substrates with properly reactive sites, chiral oxindole, chroman, tetrahydroquinoline, tetrahydrothiophene, and cyclohexane scaffolds were successfully assembled under bifunctional base/Brønsted acid catalysis from simple and readily available substances such as imines and nitroolefins. We found that some of these reactions are highly efficient since catalyst loadings as low as 1 mol % can promote the multistep sequences affording complex architectures with high stereoselectivities and yields. Furthermore, one of the bifunctional base/Brønsted acid-catalyzed cascade reactions for the synthesis of chiral cyclohexanes has been used as a key step in the construction of the tetracyclic core of lycorine-type alkaloids and the formal synthesis of α-lycorane. Guided by the principles of covalent aminocatalysis and N-heterocyclic carbene catalysis, we synthesized chiral piperidine, indole, and cyclobutane derivatives. The synthesis of chiral cyclobutanes and pyrroloindolones showed unprecedented reactivity of substrates and catalysts. The development of the strategy of asymmetric organocatalytic relay cascades has provided a useful tool for the controlled synthesis of specific diastereomers in complex molecules. This Account gives a panoramic view and the logic of our research on the design, development, and applications of asymmetric catalytic cascade reactions that will potentially provide useful insights into exploring new reactions.

Reactions of a Ruthenium Complex with Substituted N-Propargyl Pyrroles.

PubMed

Chia, Pi-Yeh; Huang, Shou-Ling; Liu, Yi-Hong; Lin, Ying-Chih

2016-04-05

In an investigation into the chemical reactions of N-propargyl pyrroles 1 a-c, containing aldehyde, keto, and ester groups on the pyrrole ring, with [Ru]-Cl ([Ru]=Cp(PPh3 )2 Ru; Cp=C5 H5 ), an aldehyde group in the pyrrole ring is found to play a crucial role in stimulating the cyclization reaction. The reaction of 1 a, containing an aldehyde group, with [Ru]-Cl in the presence of NH4 PF6 yields the vinylidene complex 2 a, which further reacts with allyl amine to give the carbene complex 6 a with a pyrrolizine group. However, if 1 a is first reacted with allyl amine to yield the iminenyne 8 a, then the reaction of 8 a with [Ru]-Cl in the presence of NH4 PF6 yields the ruthenium complex 9 a, containing a cationic pyrrolopyrazinium group, which has been fully characterized by XRD analysis. These results can be adequately explained by coordination of the triple bond of the propargyl group to the ruthenium metal center first, followed by two processes, that is, formation of a vinylidene intermediate or direct nucleophilic attack. Additionally, the deprotonation of 2 a by R4 NOH yields the neutral acetylide complex 3 a. In the presence of NH4 PF6 , the attempted alkylation of 3 a resulted in the formation the Fischer-type amino-carbene complex 5 a as a result of the presence of NH3, which served as a nucleophile. With KPF6, the alkylation of 3 a with ethyl and benzyl bromoacetates afforded the disubstituted vinylidene complexes 10 a and 11 a, containing ester groups, which underwent deprotonation reactions to give the furyl complexes 12 a and 13 a, respectively. For 13 a, containing an O-benzyl group, subsequent 1,3-migration of the benzyl group was observed to yield product 14 a with a lactone unit. Similar reactivity was not observed for the corresponding N-propargyl pyrroles 1 b and 1 c, which contained keto and ester groups, respectively, on the pyrrole ring. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Highly selective biaryl cross-coupling reactions between aryl halides and aryl Grignard reagents: a new catalyst combination of N-heterocyclic carbenes and iron, cobalt, and nickel fluorides.

PubMed

Hatakeyama, Takuji; Hashimoto, Sigma; Ishizuka, Kentaro; Nakamura, Masaharu

2009-08-26

Combinations of N-heterocyclic carbenes (NHCs) and fluoride salts of the iron-group metals (Fe, Co, and Ni) have been shown to be excellent catalysts for the cross-coupling reactions of aryl Grignard reagents (Ar(1)MgBr) with aryl and heteroaryl halides (Ar(2)X) to give unsymmetrical biaryls (Ar(1)-Ar(2)). Iron fluorides in combination with SIPr, a saturated NHC ligand, catalyze the biaryl cross-coupling between various aryl chlorides and aryl Grignard reagents in high yield and high selectivity. On the other hand, cobalt and nickel fluorides in combination with IPr, an unsaturated NHC ligand, exhibit interesting complementary reactivity in the coupling of aryl bromides or iodides; in contrast, with these substrates the iron catalysts show a lower selectivity. The formation of homocoupling byproducts is suppressed markedly to less than 5% in most cases by choosing the appropriate metal fluoride/NHC combination. The present catalyst combinations offer several synthetic advantages over existing methods: practical synthesis of a broad range of unsymmetrical biaryls without the use of palladium catalysts and phosphine ligands. On the basis of stoichiometric control experiments and theoretical studies, the origin of the unique catalytic effect of the fluoride counterion can be ascribed to the formation of a higher-valent heteroleptic metalate [Ar(1)MF(2)]MgBr as the key intermediate in our proposed catalytic cycle. First, stoichiometric control experiments revealed the stark differences in chemical reactivity between the metal fluorides and metal chlorides. Second, DFT calculations indicate that the initial reduction of di- or trivalent metal fluoride in the wake of transmetalation with PhMgCl is energetically unfavorable and that formation of a divalent heteroleptic metalate complex, [PhMF(2)]MgCl (M = Fe, Co, Ni), is dominant in the metal fluoride system. The heteroleptic ate-complex serves as a key reactive intermediate, which undergoes oxidative addition with PhCl and releases the biaryl cross-coupling product Ph-Ph with reasonable energy barriers. The present cross-coupling reaction catalyzed by iron-group metal fluorides and an NHC ligand provides a highly selective and practical method for the synthesis of unsymmetrical biaryls as well as the opportunity to gain new mechanistic insights into the metal-catalyzed cross-coupling reactions.

N-heterocycle carbene (NHC)-ligated cyclopalladated N,N-dimethylbenzylamine: a highly active, practical and versatile catalyst for the Heck-Mizoroki reaction.

PubMed

Peh, Guang-Rong; Kantchev, Eric Assen B; Zhang, Chi; Ying, Jackie Y

2009-05-21

The wide dissemination of catalytic protocols in academic and industrial laboratories is facilitated by the development of catalysts that are not only highly active but also user-friendly, stable to moisture, air and long term storage and easy to prepare on a large scale. Herein we describe a protocol for the Heck-Mizoroki reaction mediated by cyclopalladated N,N-dimethylbenzylamine (dmba) ligated with a N-heterocyclic carbene, 1,3-bis(mesityl)imidazol-2-ylidene (IMes), that fulfils these criteria. The precatalyst can be synthesized on approximately 100 g scale by a tri-component, sequential, one-pot reaction of N,N-dimethylbenzylamine, PdCl2 and IMes.HCl in refluxing acetonitrile in air in the presence of K2CO3. This single component catalyst is stable to air, moisture and long term storage and can be conveniently dispensed as a stock solution in NMP. It mediates the Heck-Mizoroki reaction of a range of aryl- and heteroaryl bromides in reagent grade NMP at the 0.1-2 mol% range without the need for rigorous anhydrous techniques or a glovebox, and is active even in air. The catalyst is capable of achieving very high levels of catalytic activity (TON of up to 5.22 x 10(5)) for the coupling of a deactivated arylbromide, p-bromoanisole, with tBu acrylate as a benchmark substrate pair. A wide range of aryl bromides, iodides and, for the first time with a NHC-Pd catalyst, a triflate was coupled with diverse acrylate derivatives (nitrile, tert-butyl ester and amides) and styrene derivatives. The use of excess (>2 equiv.) of the aryl bromide and tert-butyl acrylate leads to mixture of tert-butyl beta,beta-diarylacrylate and tert-butyl cinnamate derivatives depending on the substitution pattern of the aryl bromide. Electron rich m- and p-substituted arylbromides give the diarylated products exclusively, whereas electron-poor aryl bromides give predominantly mono-arylated products. For o-substituted aryl bromides, no doubly arylated products could be obtained under any conditions. Overall, the active catalyst (IMes-Pd) shows higher activity with electron-rich aryl halides, a marked difference compared with the more commonly used phosphane-Pd or non-ligated Pd catalysts.

The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules.

PubMed

Smith, Caleb A; Gillespie, Blanton R; Heard, George L; Setser, D W; Holmes, Bert E

2017-11-22

The recombination of CF 3 and CHF 2 radicals in a room-temperature bath gas was used to prepare vibrationally excited CF 3 CHF 2 * molecules with 101 kcal mol -1 of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF 3 , CF 3 (F)C: and C 2 F 4 product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF 3 (F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 10 4 , 4.7 × 10 4 , and 1.1 × 10 4 s -1 for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E 0 ) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol -1 to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF 3 (F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol -1 . Hydrogen-bonded complexes between HF and the H atom migration transition state of CH 3 (F)C: and the F atom migration transition state of CF 3 (F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.

2-Hydroxyethyl substituted NHC precursors: Synthesis, characterization, crystal structure and carbonic anhydrase, α-glycosidase, butyrylcholinesterase, and acetylcholinesterase inhibitory properties

NASA Astrophysics Data System (ADS)

Erdemir, Fatoş; Barut Celepci, Duygu; Aktaş, Aydın; Taslimi, Parham; Gök, Yetkin; Karabıyık, Hasan; Gülçin, İlhami

2018-03-01

This study contains novel a serie synthesis of N-heterocyclic carbene (NHC) precursors that 2-hydroxyethyl substituted. The NHC precursors have been prepared from 1-(2- hydroxyethyl)benzimidazole and alkyl halides. The novel NHC precursors have been characterized by using 1H NMR, 13C NMR, FTIR spectroscopy and elemental analysis techniques. Molecular and crystal structures of 2a, 2d, 2e, 2f and 2g were obtained with single-crystal X-ray diffraction studies. These novel NHC precursor's derivatives effectively inhibited the α-glycosidase, cytosolic carbonic anhydrase I and II isoforms (hCA I and II), butyrylcholinesterase (BChE) and acetylcholinesterase (AChE). Inhibition constant (Ki) were found in the range of 0.30-9.22 nM for α-glycosidase, 13.90-41.46 nM for hCA I, 12.82-49.95 nM for hCA II, 145.82-882.01 nM for BChE, and 280.92-1370.01 nM for AChE, respectively.

Nanostructured Thin Films Obtained from Fischer Aminocarbene Complexes

PubMed Central

Lazo-Jiménez, Rosa E.; Ortega-Alfaro, M. Carmen; López-Cortés, José G.; Alvarez-Toledano, Cecilio; Chávez-Carvayar, José Á.; Ignés-Mullol, Jordi; González-Torres, Maykel; Carreón-Castro, Pilar

2016-01-01

The synthesis of four amphiphilic organometallic complexes with the general formula RC = M(CO)5NH(CH2)15CH3, where R is a ferrocenyl 2(a-b) or a phenyl 4(a-b) group as a donor moiety and a Fischer carbene of chromium (0) or tungsten (0) as an acceptor group, are reported. These four push-pull systems formed Langmuir (L) monolayers at the air-water interface, which were characterized by isotherms of surface pressure versus molecular area and compression/expansion cycles (hysteresis curves); Brewster angle microscopic images were also obtained. By using the Langmuir–Blodgett (LB) method, molecular monolayers were transferred onto glass substrates forming Z-type multilayers. LB films were characterized through ultraviolet-visible spectroscopy, atomic force microscopy and X-ray diffraction techniques. Results indicated that films obtained from 2b complex [(Ferrocenyl)(hexadecylamine)methylidene] pentacarbonyl tungsten (0) are the most stable and homogeneous; due to their properties, these materials may be incorporated into organic electronic devices. PMID:28773289

Kinetic and thermodynamic acidities of substituted 1-benzyl-1-methoxy-2-nitroethylenes. Strong reduction of the transition state imbalance compared to other nitroalkanes.

PubMed

Bernasconi, Claude F; Ali, Mahammad; Gunter, Janette C

2003-01-08

Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z = m-NO(2), m-CF(3), m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50% water (v/v) at 20 degrees C. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH(-) and piperidine and on the reversible deprotonation of 2-NO(2)() by piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine in the same solvent. These data allowed a determination of the Brønsted coefficients alpha (dependence on acidity of 2-Z) and beta (dependence on amine basicity). The fact that alpha > beta indicates the presence of a transition state imbalance which, however, is much smaller than that for the deprotonation of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study of the deprotonation of Fischer carbene complexes are discussed.

Diaryl-1,2,3-Triazolylidene Platinum(II) Complexes.

PubMed

Soellner, Johannes; Strassner, Thomas

2018-04-11

Control of the excited state geometry by rational ligand design leads to a new class of phosphorescent emitters with extraordinary photophysical properties. Extension of the π-system in the triplet state leading to a significant bathochromic shift of the emission was avoided by introduction of additional steric demand. We report the synthesis, characterization and photophysical properties of novel platinum(II) complexes bearing C^C* cyclometalated mesoionic carbene (MIC) with different β-diketonate ligands. The MIC ligand precursors were prepared from 1-phenyl-1,2,3-triazole using arylation protocols, introducing phenyl or mesityl functionalities. A solid state structure confirming the NMR assignments is presented. The emission properties were investigated in detail at room temperature and 77 K and are supported by DFT calculations and cyclic voltammetry. All complexes, with emission maxima between 502-534 nm, emit with quantum efficiencies ranging from 70-84 % in PMMA films. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Final Technical Report: Vibrational Spectroscopy of Transient Combustion Intermediates Trapped in Helium Nanodroplets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Douberly, Gary Elliott

The objective of our experimental research program is to isolate and stabilize transient intermediates and products of prototype combustion reactions. This will be accomplished by Helium Nanodroplet Isolation, a novel technique where liquid helium droplets freeze out high energy metastable configurations of a reacting system, permitting infrared spectroscopic characterizations of products and intermediates that result from hydrocarbon radical reactions with molecular oxygen and other small molecules relevant to combustion environments. The low temperature (0.4 K) and rapid cooling associated with He droplets provides a perfectly suited medium to isolate and probe a broad range of molecular radical and carbene systemsmore » important to combustion chemistry. The sequential addition of molecular species to He droplets often leads to the stabilization of high-energy, metastable cluster configurations that represent regions of the potential energy surface far from the global minimum. Single and double resonance IR laser spectroscopy techniques, along with Stark and Zeeman capabilities, are being used to probe the structural and dynamical properties of these systems.« less

Catalytic asymmetric trifluoromethylthiolation via enantioselective [2,3]-sigmatropic rearrangement of sulfonium ylides

NASA Astrophysics Data System (ADS)

Zhang, Zhikun; Sheng, Zhe; Yu, Weizhi; Wu, Guojiao; Zhang, Rui; Chu, Wen-Dao; Zhang, Yan; Wang, Jianbo

2017-10-01

The trifluoromethylthio (SCF3) functional group has been of increasing importance in drug design and development as a consequence of its unique electronic properties and high stability coupled with its high lipophilicity. As a result, methods to introduce this highly electronegative functional group have attracted considerable attention in recent years. Although significant progress has been made in the introduction of SCF3 functionality into a variety of molecules, there remain significant challenges regarding the enantioselective synthesis of SCF3-containing compounds. Here, an asymmetric trifluoromethylthiolation that proceeds through the enantioselective [2,3]-sigmatropic rearrangement of a sulfonium ylide generated from a metal carbene and sulfide (Doyle-Kirmse reaction) has been developed using chiral Rh(II) and Cu(I) catalysts. This transformation features mild reaction conditions and excellent enantioselectivities (up to 98% yield and 98% e.e.), thus providing a unique, highly efficient and enantioselective method for the construction of C(sp3)-SCF3 bonds bearing chiral centres.

METHOD FOR PRODUCING ISOTOPIC METHANES FROM LITHIUM CARBONATE AND LITHIUM HYDRIDE

DOEpatents

Frazer, J.W.

1959-10-27

A process is descrlbed for the production of methane and for the production of methane containing isotopes of hydrogen and/or carbon. Finely divided lithium hydrlde and litldum carbonate reactants are mixed in intimate contact and subsequently compacted under pressures of from 5000 to 60,000 psl. The compacted lithium hydride and lithium carbenate reactunts are dispised in a gas collecting apparatus. Subsequently, the compact is heated to a temperature in the range 350 to 400 deg C whereupon a solid-solid reaction takes place and gaseous methane is evolved. The evolved methane is contaminated with gaseous hydrogen and a very small amount of CO/sub 2/; however, the desired methane product is separated from sald impurities by well known chemical processes, e.g., condensation in a cold trap. The product methane contalns isotopes of carbon and hydrogen, the Isotopic composition being determined by the carbon isotopes originally present In the lithium carbonate and the hydrogen isotopes originally present in the lithium hydride.

Pyrene-based bisazolium salts: from luminescence properties to janus-type bis-N-heterocyclic carbenes.

PubMed

Gonell, Sergio; Poyatos, Macarena; Peris, Eduardo

2014-07-28

A series of pyrene-based bisazolium salts have been obtained starting from 4,5,9,10-tetrabromo-2,7-di-tert-butylpyrene. The synthetic procedure to the pyrene-bisazoliums (PBIs) reveals an unexpected behavior, as a consequence of the presence of the alkyl groups (alkyl=Me, Et, n-Pr, and n-Bu) coming from the trisalkoxyformate in the final products, instead of the expected tBu of tAmyl groups from the starting tetra-aminated pyrenes. All bisazoliums show fluorescence properties, with emissions in the range of 370-420 nm, and quantum yields ranging from 0.29 to 0.41. The PBIs were used as bis-NHC precursors in the preparation of a series of dirhodium and diiridium complexes, which have been fully characterized. The electrochemical studies on selected dimetallic complexes reveal that the electronic communication between the metals through the polyaromatic linker is negligible. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Aqueous NMR Signal Enhancement by Reversible Exchange in a Single Step Using Water-Soluble Catalysts

PubMed Central

2016-01-01

Two synthetic strategies are investigated for the preparation of water-soluble iridium-based catalysts for NMR signal amplification by reversible exchange (SABRE). In one approach, PEGylation of a variant N-heterocyclic carbene provided a novel catalyst with excellent water solubility. However, while SABRE-active in ethanol solutions, the catalyst lost activity in >50% water. In a second approach, synthesis of a novel di-iridium complex precursor where the cyclooctadiene (COD) rings have been replaced by CODDA (1,2-dihydroxy-3,7-cyclooctadiene) leads to the creation of a catalyst [IrCl(CODDA)IMes] that can be dissolved and activated in water—enabling aqueous SABRE in a single step, without need for either an organic cosolvent or solvent removal followed by aqueous reconstitution. The potential utility of the CODDA catalyst for aqueous SABRE is demonstrated with the ∼(−)32-fold enhancement of 1H signals of pyridine in water with only 1 atm of parahydrogen. PMID:27350846

Asymmetric total synthesis of onoseriolide, bolivianine, and isobolivianine.

PubMed

Du, Biao; Yuan, Changchun; Yu, Tianzi; Yang, Li; Yang, Yang; Liu, Bo; Qin, Song

2014-02-24

In this article, we describe our efforts on the total synthesis of bolivianine (1) and isobolivianine (2), involving the synthesis of onoseriolide (3). The first generation synthesis of bolivianine was completed in 21 steps by following a chiral resolution strategy. Based on the potential biogenetic relationship between bolivianine (1), onoseriolide (3), and β-(E)-ocimene (8), the second generation synthesis of bolivianine was biomimetically achieved from commercially available (+)-verbenone in 14 steps. The improved total synthesis features an unprecedented palladium-catalyzed intramolecular cyclopropanation through an allylic metal carbene, for the construction of the ABC tricyclic system, and a Diels-Alder/intramolecular hetero-Diels-Alder (DA/IMHDA) cascade for installation of the EFG tricyclic skeleton with the correct stereochemistry. Transformation from bolivianine to isobolivianine was facilitated in the presence of acid. The biosynthetic mechanism and the excellent regio- and endo selectivities in the cascade are well supported by theoretical chemistry based on the DFT calculations. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A ligand-directed divergent catalytic approach to establish structural and functional scaffold diversity

NASA Astrophysics Data System (ADS)

Lee, Yen-Chun; Patil, Sumersing; Golz, Christopher; Strohmann, Carsten; Ziegler, Slava; Kumar, Kamal; Waldmann, Herbert

2017-02-01

The selective transformation of different starting materials by different metal catalysts under individually optimized reaction conditions to structurally different intermediates and products is a powerful approach to generate diverse molecular scaffolds. In a more unified albeit synthetically challenging strategy, common starting materials would be exposed to a common metal catalysis, leading to a common intermediate and giving rise to different scaffolds by tuning the reactivity of the metal catalyst through different ligands. Herein we present a ligand-directed synthesis approach for the gold(I)-catalysed cycloisomerization of oxindole-derived 1,6-enynes that affords distinct molecular scaffolds following different catalytic reaction pathways. Varying electronic properties and the steric demand of the gold(I) ligands steers the fate of a common intermediary gold carbene to selectively form spirooxindoles, quinolones or df-oxindoles. Investigation of a synthesized compound collection in cell-based assays delivers structurally novel, selective modulators of the Hedgehog and Wnt signalling pathways, autophagy and of cellular proliferation.

Divergent Pathways Involving 1,3-Dipolar Addition and N-N Bond Splitting of an Organic Azide across a Zirconium Methylidene.

PubMed

Kurogi, Takashi; Mane, Manoj V; Zheng, Shuai; Carroll, Patrick J; Baik, Mu-Hyun; Mindiola, Daniel J

2018-02-12

The zirconium methylidene (PNP)Zr=CH 2 (OAr) (1) reacts with N 3 Ad to give two products (PNP)Zr=NAd(OAr) (2) and (PNP)Zr(η 2 -N=NAd)(N=CH 2 )(OAr) (3), both resulting from a common cycloaddition intermediate (PNP)Zr(CH 2 N 3 Ad)(OAr) (A). Using a series of control experiments in combination with DFT calculations, it was found that 2 results from a nitrene by a carbene metathesis reaction in which N 2 acts as a delivery vehicle and forms N 2 CH 2 as a side product. In the case of 3, N-N bond splitting of the azide at the α-position allowed the isolation of a rare example of a parent ketimide complex of zirconium. Isotopic labeling studies and solid-state X-ray analysis are presented for 2 and 3, in addition to an independent synthesis for the former. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Vitamin B1-catalyzed acetoin formation from acetaldehyde: a key step for upgrading bioethanol to bulk C₄ chemicals.

PubMed

Lu, Ting; Li, Xiukai; Gu, Liuqun; Zhang, Yugen

2014-09-01

The production of bulk chemicals and fuels from renewable biobased feedstocks is of significant importance for the sustainability of human society. The production of ethanol from biomass has dramatically increased and bioethanol also holds considerable potential as a versatile building block for the chemical industry. Herein, we report a highly selective process for the conversion of ethanol to C4 bulk chemicals, such as 2,3-butanediol and butene, via a vitamin B1 (thiamine)-derived N-heterocyclic carbene (NHC)-catalyzed acetoin condensation as the key step to assemble two C2 acetaldehydes into a C4 product. The environmentally benign and cheap natural catalyst vitamin B1 demonstrates high selectivity (99%), high efficiency (97% yield), and high tolerance toward ethanol and water impurities in the acetoin reaction. The results enable a novel and efficient process for ethanol upgrading. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Z-Selective iridium-catalyzed cross-coupling of allylic carbonates and α-diazo esters† †Electronic supplementary information (ESI) available: Full procedures, computational details and characterization data. See DOI: 10.1039/c7sc04283c

PubMed Central

Thomas, Bryce N.; Moon, Patrick J.; Yin, Shengkang; Brown, Alex

2017-01-01

A well-defined Ir–allyl complex catalyzes the Z-selective cross-coupling of allyl carbonates with α-aryl diazo esters. The process overrides the large thermodynamic preference for E-products typically observed in metal-mediated coupling reactions to enable the synthesis of Z,E-dieneoates in good yield with selectivities consistently approaching or greater than 90 : 10. This transformation represents the first productive merger of Ir–carbene and Ir–allyl species, which are commonly encountered intermediates in allylation and cyclopropanation/E–H insertion catalysis. Potentially reactive functional groups (aryl halides, ketones, nitriles, olefins, amines) are tolerated owing to the mildness of reaction conditions. Kinetic analysis of the reaction suggests oxidative addition of the allyl carbonate to an Ir-species is rate-determining. Mechanistic studies uncovered a pathway for catalyst activation mediated by NEt3. PMID:29629093

Blue phosphorescent nitrile containing C^C* cyclometalated NHC platinum(II) complexes.

PubMed

Tronnier, Alexander; Metz, Stefan; Wagenblast, Gerhard; Muenster, Ingo; Strassner, Thomas

2014-02-28

Since C^C* cyclometalated Pt(II) complexes with N-heterocyclic carbene (NHC) ligands have been identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results regarding quantum yields, colour and stability have been presented. Herein, we report on four nitrile substituted complexes with a chelating NHC ligand (1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazole or 4-(tert-butyl)-1-(4-cyanophenyl)-3-methyl-1H-imidazole) and a bidentate monoanionic auxiliary ligand (acetylacetone or dimesitoylmethane). The complexes have been fully characterized including extensive 2D NMR studies (COSY, HSQC, HMBC, NOESY, (195)Pt NMR), three of them also by solid-state structures. Photophysical measurements in amorphous PMMA films and pure emitter films at room temperature reveal the impact of the mesityl groups in the auxiliary ligand, which led to a significant increase of the quantum yield, while the decay lifetimes decreased. The electron withdrawing nitrile groups shift the emission towards blue colour coordinates.

Porous polycarbene-bearing membrane actuator for ultrasensitive weak-acid detection and real-time chemical reaction monitoring.

PubMed

Sun, Jian-Ke; Zhang, Weiyi; Guterman, Ryan; Lin, Hui-Juan; Yuan, Jiayin

2018-04-30

Soft actuators with integration of ultrasensitivity and capability of simultaneous interaction with multiple stimuli through an entire event ask for a high level of structure complexity, adaptability, and/or multi-responsiveness, which is a great challenge. Here, we develop a porous polycarbene-bearing membrane actuator built up from ionic complexation between a poly(ionic liquid) and trimesic acid (TA). The actuator features two concurrent structure gradients, i.e., an electrostatic complexation (EC) degree and a density distribution of a carbene-NH 3 adduct (CNA) along the membrane cross-section. The membrane actuator performs the highest sensitivity among the state-of-the-art soft proton actuators toward acetic acid at 10 -6  mol L -1 (M) level in aqueous media. Through competing actuation of the two gradients, it is capable of monitoring an entire process of proton-involved chemical reactions that comprise multiple stimuli and operational steps. The present achievement constitutes a significant step toward real-life application of soft actuators in chemical sensing and reaction technology.

Local structure of Iridium organometallic catalysts covalently bonded to carbon nanotubes.

NASA Astrophysics Data System (ADS)

Blasco, J.; Cuartero, V.; Subías, G.; Jiménez, M. V.; Pérez-Torrente, J. J.; Oro, L. A.; Blanco, M.; Álvarez, P.; Blanco, C.; Menéndez, R.

2016-05-01

Hybrid catalysts based on Iridium N-heterocyclic carbenes anchored to carbon nanotubes (CNT) have been studied by XAFS spectroscopy. Oxidation of CNT yields a large amount of functional groups, mainly hydroxyl groups at the walls and carboxylic groups at the tips, defects and edges. Different kinds of esterification reactions were performed to functionalize oxidized CNT with imidazolium salts. Then, the resulting products were reacted with an Ir organometallic compound to form hybrid catalysts efficient in hydrogen transfer processes. XANES spectroscopy agree with the presence of Ir(I) in these catalysts and the EXAFS spectra detected differences in the local structure of Ir atoms between the initial Ir organometallic compound and the Ir complexes anchored to the CNT. Our results confirm that the halide atom, present in the Ir precursor, was replaced by oxygen from -OH groups at the CNT wall in the first coordination shell of Ir. The lability of this group accounts for the good recyclability and the good efficiency shown by these hybrid catalysts.

Heteroleptic Cycloplatinated N-Heterocyclic Carbene Complexes: A New Approach to Highly Efficient Blue-Light Emitters.

PubMed

Fuertes, Sara; Chueca, Andrés J; Arnal, Lorenzo; Martín, Antonio; Giovanella, Umberto; Botta, Chiara; Sicilia, Violeta

2017-05-01

New heteroleptic compounds of platinum(II)-containing cyclometalated N-heterocyclic carbenes, [PtCl(R-C^C*)(PPh 3 )] [R-CH^C*-κC* = 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene (R-C = Naph; 1A), 1-[4-(ethoxycarbonyl)phenyl]-3-methyl-1H-imidazol-2-ylidene (R = CO 2 Et; 1B), and [Pt(R-C^C*)(py)(PPh 3 )]PF 6 (py = pyridine; R-C = Naph, 2A; R = CO 2 Et, 2B], have been prepared and fully characterized. All of them were obtained as the trans-(C*,PPh 3 ) isomer in high yields. The selectivity of their synthesis has been explained in terms of the degree of transphobia (T) of pairs of ligands in trans positions. X-ray diffraction studies on both 2A and 2B revealed that only in 2A, containing a C^C* with a more extended π system, do the molecules assemble themselves into head-to-tail pairs through intermolecular π···π contacts. The photophysical properties of 2A and 2B and those of the related compounds [Pt(NC-C^C*)(PPh 3 )L]PF 6 [NC-CH^C*-κC* = 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene; L = pyridine (py; 2C), 2,6-dimethylphenylisocyanide (CNXyl; 3C), and 2-mercapto-1-methylimidazole (MMI; 4C)] have been examined to analyze the influence of the R substituent on R-C^C* (R-C = Naph; R = CO 2 Et, CN) and that of the ancillary ligands (L) on them. Experimental data and time-dependent density functional theory calculations showed the similarity of the electronic features associated with R-C^C* (R = CN, CO 2 Et) and their difference with respect to R-C^C* (R-C = Naph). All of the compounds are very efficient blue emitters in poly(methyl methacrylate) films under an argon atmosphere, with QY values ranging from 68% (2B) to 93% (2C). In the solid state, the color of the emission changes to yellowish-orange for compounds 2A (λ max = 600 nm) and 3C (λ max = 590 nm) because of the formation of aggregates through intermolecular π···π interactions. 2C and 3C were chosen to fabricate fully solution-processed electroluminescent devices with blue-light (2C), yellow-orange-light (3C), and white-light (mixtures of 2C and 3C) emission from neat films of the compounds as emitting layers.

Diverse reactivity of a tricoordinate organoboron L2PhB: (L = oxazol-2-ylidene) towards alkali metal, group 9 metal, and coinage metal precursors† †Electronic supplementary information (ESI) available: Experimental and calculation details, and crystallographic information for 2, 3, 4, 6, 8. CCDC 1038665, 1038666, 1038667, 1011534, and 1011533. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc00404g Click here for additional data file. Click here for additional data file.

PubMed Central

Kong, Lingbing; Ganguly, Rakesh; Li, Yongxin

2015-01-01

The reactivity of a tricoordinate organoboron L2PhB: (L = oxazol-2-ylidene) 1 towards metal precursors and its coordination chemistry were comprehensively studied. While the boron center in 1 is reluctant to coordinate to the alkali metals in their trifluoromethanesulfonate salts (MOTf) (M = Li, Na, K), the unprecedented compound 2 containing two L2PhB: units linked by a cyclic Li(OTf)2Li spacer was obtained from the reaction of 1 with LiOTf. Treatment of 1 with group 9 metal complexes [MCl(COD)]2 (M = Rh, Ir) afforded the first zwitterionic rhodium(i)–boronium complex 3 and the iridium(iii)–borane complex 4, respectively. The reaction pathway may involve C–H activation followed by proton migration from the metals to the boron center, demonstrating the first example of the deprotonation of metal hydrides by a basic boron. In the reactions with coinage metals, 1 could act as a two-electron reducing agent towards the metal chlorides MCl (M = Cu, Ag, Au). Meanwhile, the reaction of 1 with gold chloride supported by a N-heterocyclic carbene (NHC) produced a heteroleptic cationic gold complex [(L2PhB)Au(NHC)]Cl (6) featuring both carbene and L2PhB: ligands on the gold atom. In contrast, an isolable gold chloride complex (L2PhB)AuCl (8) was obtained by direct complexation between 1 and triphenylphosphine-gold chloride via ligand exchange. X-ray diffraction analysis and computational studies revealed the nature of the B:→Au bonding interaction in complexes 6 and 8. Natural Population Analysis (NPA) and Natural Bond Orbital (NBO) analysis support the strong σ-donating property of the L2PhB: ligand. Moreover, preliminary studies showed that complex 8 can serve as an efficient precatalyst for the addition of X–H (X = N, O, C) to alkynes under ambient conditions, demonstrating the first application of a metal complex featuring a neutral boron-based ligand in catalysis. PMID:29308167

Understanding chemical reactions of CO2 and its isoelectronic molecules with 1-butyl-3-methylimidazolium acetate by changing the nature of the cation: the case of CS2 in 1-butyl-1-methylpyrrolidinium acetate studied by NMR spectroscopy and density functional theory calculations.

PubMed

Cabaço, M Isabel; Besnard, Marcel; Chávez, Fabián Vaca; Pinaud, Noël; Sebastião, Pedro J; Coutinho, João A P; Danten, Yann

2014-06-28

NMR spectroscopy ((1)H, (13)C, (15)N) shows that carbon disulfide reacts spontaneously with 1-butyl-1-methylpyrrolidinium acetate ([BmPyrro][Ac]) in the liquid phase. It is found that the acetate anions play an important role in conditioning chemical reactions with CS2 leading, via coupled complex reactions, to the degradation of this molecule to form thioacetate anion (CH3COS(-)), CO2, OCS, and trithiocarbonate (CS3 (2-)). In marked contrast, the cation does not lead to the formation of any adducts allowing to conclude that, at most, its role consists in assisting indirectly these reactions. The choice of the [BmPyrro](+) cation in the present study allows disentangling the role of the anion and the cation in the reactions. As a consequence, the ensemble of results already reported on CS2-[Bmim][Ac] (1), OCS-[Bmim][Ac] (2), and CO2-[Bmim][Ac] (3) systems can be consistently rationalized. It is argued that in system (1) both anion and cation play a role. The CS2 reacts with the acetate anion leading to the formation of CH3COS(-), CO2, and OCS. After these reactions have proceeded the nascent CO2 and OCS interact with the cation to form imidazolium-carboxylate ([Bmim] CO2) and imidazolium-thiocarboxylate ([Bmim] COS). The same scenario also applies to system (2). In contrast, in the CO2-[Bmim] [Ac] system a concerted cooperative process between the cation, the anion, and the CO2 molecule takes place. A carbene issued from the cation reacts to form the [Bmim] CO2, whereas the proton released by the ring interacts with the anion to produce acetic acid. In all these systems, the formation of adduct resulting from the reaction between the solute molecule and the carbene species originating from the cation is expected. However, this species was only observed in systems (2) and (3). The absence of such an adduct in system (1) has been theoretically investigated using DFT calculations. The values of the energetic barrier of the reactions show that the formation of [Bmim] CS2 is unfavoured and that the anion offers a competitive reactive channel via an oxygen-sulphur exchange mechanism with the solute in systems (1) and (2).

Development of Catalysts and Ligands for Enantioselective Gold Catalysis

PubMed Central

Wang, Yi-Ming; Lackner, Aaron D.; Toste, F. Dean

2014-01-01

CONSPECTUS The use of Au(I) complexes for the catalytic activation of C-C π-bonds has been the subject of intense investigation in the last decade or so. The facile formation of carbon-carbon and carbon-heteroatom bonds facilitated by gold naturally led to efforts to render these transformations enantioselective. Early examples of enantioselective gold-catalyzed transformations have focused on bis(phosphinegold) complexes derived from axially chiral scaffolds. Although these complexes were highly successful in some reactions like cyclopropanation, careful choice of the weakly coordinating ligand (or counterion) was needed to obtain high levels of enantioselectivity for the case of allene hydroamination. These counterion effects led us to use the anion itself as a source of chirality, which was successful in the case of allene hydroalkoxylation. In order to expand the scope of reactions amenable to enantioselective gold catalysis to cycloadditions and other carbocyclization processes, a new class of mononuclear phosphite and phosphoramidite ligands was developed to supplement the previously widely utilized phosphines. Finally carbene ligands, in particular, the acyclic diaminocarbenes, have also been successfully applied to enantioselective transformations. PMID:24228794

Gold(I) NHC-based homo- and heterobimetallic complexes: synthesis, characterization and evaluation as potential anticancer agents.

PubMed

Bertrand, Benoît; Citta, Anna; Franken, Inge L; Picquet, Michel; Folda, Alessandra; Scalcon, Valeria; Rigobello, Maria Pia; Le Gendre, Pierre; Casini, Angela; Bodio, Ewen

2015-09-01

While N-heterocyclic carbenes (NHC) are ubiquitous ligands in catalysis for organic or industrial syntheses, their potential to form transition metal complexes for medicinal applications has still to be exploited. Within this frame, we synthesized new homo- and heterobimetallic complexes based on the Au(I)-NHC scaffold. The compounds were synthesized via a microwave-assisted method developed in our laboratories using Au(I)-NHC complexes carrying a pentafluorophenol ester moiety and another Au(I) phosphane complex or a bipyridine ligand bearing a pendant amine function. Thus, we developed two different methods to prepare homo- and heterobimetallic complexes (Au(I)/Au(I) or Au(I)/Cu(II), Au(I)/Ru(II), respectively). All the compounds were fully characterized by several spectroscopic techniques including far infrared, and were tested for their antiproliferative effects in a series of human cancer cells. They showed moderate anticancer properties. Their toxic effects were also studied ex vivo using the precision-cut tissue slices (PCTS) technique and initial results concerning their reactivity with the seleno-enzyme thioredoxin reductase were obtained.

Carbon Dioxide Utilization by the Five-Membered Ring Products of Cyclometalation Reactions

PubMed Central

Omae, Iwao

2016-01-01

In carbon dioxide utilization by cyclometalated five-membered ring products, the following compounds are used in four types of applications: 1. 2-Phenylpyrazole iridium compounds, pincer phosphine iridium compounds and 2-phenylimidazoline iridium compounds are used as catalysts for both formic acid production from CO2 and H2, and hydrogen production from the formic acid. This formic acid can be a useful agent for H2 production and storage for fuel cell electric vehicles. 2. Other chemicals, e.g., dimethyl carbonate, methane, methanol and CO, are produced with dimethylaminomethylphenyltin compounds, pincer phosphine iridium compounds, pincer phosphine nickel compound and ruthenium carbene compound or 2-phenylpyridine iridium compounds, and phenylbenzothiazole iridium compounds as the catalysts for the reactions with CO2. 3. The five-membered ring intermediates of cyclometalation reactions with the conventional substrates react with carbon dioxide to afford their many types of carboxylic acid derivatives. 4. Carbon dioxide is easily immobilized at room temperature with immobilizing agents such as pincer phosphine nickel compounds, pincer phosphine palladium compounds, pincer N,N-dimethylaminomethyltin compounds and tris(2-pyridylthio)methane zinc compounds. PMID:28503084

Amination with Pd-NHC complexes: rate and computational studies involving substituted aniline substrates.

PubMed

Hoi, Ka Hou; Çalimsiz, Selçuk; Froese, Robert D J; Hopkinson, Alan C; Organ, Michael G

2012-01-02

The amination of aryl chlorides with various aniline derivatives using the N-heterocyclic carbene-based Pd complexes Pd-PEPPSI-IPr and Pd-PEPPSI-IPent (PEPPSI=pyridine, enhanced precatalyst, preparation, stabilization, and initiation; IPr=diisopropylphenylimidazolium derivative; IPent= diisopentylphenylimidazolium derivative) has been studied. Rate studies have shown a reliance on the aryl chloride to be electron poor, although oxidative addition is not rate limiting. Anilines couple best when they are electron rich, which would seem to discount deprotonation of the intermediate metal ammonium complex as being rate limiting in favour of reductive elimination. In previous studies with secondary amines using PEPPSI complexes, deprotonation was proposed to be the slow step in the cycle. These experimental findings relating to mechanism were corroborated by computation. Pd-PEPPSI-IPr and the more hindered Pd-PEPPSI-IPent catalysts were used to couple deactivated aryl chlorides with electron poor anilines; while the IPr catalysis was sluggish, the IPent catalyst performed extremely well, again showing the high reactivity of this broadly useful catalyst. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Traceless Immobilization of Analytes for High-Throughput Experiments with SAMDI Mass Spectrometry.

PubMed

Helal, Kazi Y; Alamgir, Azmain; Berns, Eric J; Mrksich, Milan

2018-06-21

Label-free assays, and particularly those based on the combination of mass spectroscopy with surface chemistries, enable high-throughput experiments of a broad range of reactions. However, these methods can still require the incorporation of functional groups that allow immobilization of reactants and products to surfaces prior to analysis. In this paper, we report a traceless method for attaching molecules to a self-assembled monolayer for matrix-assisted laser desorption and ionization (SAMDI) mass spectrometry. This method uses monolayers that are functionalized with a 3-trifluoromethyl-3-phenyl-diazirine group that liberates nitrogen when irradiated and gives a carbene that inserts into a wide range of bonds to covalently immobilize molecules. Analysis of the monolayer with SAMDI then reveals peaks for each of the adducts formed from molecules in the sample. This method is applied to characterize a P450 drug metabolizing enzyme and to monitor a Suzuki-Miyaura coupling chemical reaction and is important because modification of the substrates with a functional group would alter their activities. This method will be important for high-throughput experiments in many areas, including reaction discovery and optimization.

Functionalization of Carbon-Hydrogen Bonds Through Transition Metal Carbenoid Insertion

NASA Astrophysics Data System (ADS)

Davies, Huw M. L.; Dick, Allison R.

The functionalization of carbon-hydrogen bonds through transition metal carbenoid insertion is becoming a powerful method for the construction of new carbon-carbon bonds in organic synthesis. This chapter will highlight recent developments in this field, while placing it within its historical context. Intramolecular carbenoid C-H insertion will be covered first, focusing on formation of three- and six-membered rings, as well as the use of nontraditional substrates. Additionally, the most recent progress in asymmetric catalysis will be discussed. The bulk of the chapter will concentrate on intermolecular transformations, emphasizing both the effect of substrate structure and the influence of carbene substituent electronics on the regioselectivity of the reactions. Vinyldiazoacetates will be covered as a distinct class of carbenoid precursor, as they have been shown to initiate a variety of unique transformations, such as the combined C-H activation/Cope rearrangement. Finally, the synthetic utility of carbenoid C-H insertion reactions, both intra- and intermolecular, will be displayed through their use in the total syntheses of a number of natural products and pharmaceuticals.

Functionalization of carbon-hydrogen bonds through transition metal carbenoid insertion.

PubMed

Davies, Huw M L; Dick, Allison R

2010-01-01

The functionalization of carbon-hydrogen bonds through transition metal carbenoid insertion is becoming a powerful method for the construction of new carbon-carbon bonds in organic synthesis. This chapter will highlight recent developments in this field, while placing it within its historical context. Intramolecular carbenoid C-H insertion will be covered first, focusing on formation of three- and six-membered rings, as well as the use of nontraditional substrates. Additionally, the most recent progress in asymmetric catalysis will be discussed. The bulk of the chapter will concentrate on intermolecular transformations, emphasizing both the effect of substrate structure and the influence of carbene substituent electronics on the regioselectivity of the reactions. Vinyldiazoacetates will be covered as a distinct class of carbenoid precursors, as they have been shown to initiate a variety of unique transformations, such as the combined C-H activation/Cope rearrangement. Finally, the synthetic utility of carbenoid C-H insertion reactions, both intra- and intermolecular, will be displayed through their use in the total syntheses of a number of natural products and pharmaceuticals.

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

PubMed Central

Koh, Ming Joo; Nguyen, Thach T.; Zhang, Hanmo; Schrock, Richard R.; Hoveyda, Amir H.

2016-01-01

Olefin metathesis has made a significant impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is very limited. In this manuscript, we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of an in situ-generated catalyst with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents and proceed to high conversion at ambient temperature within four hours. Many alkenyl chlorides, bromides and fluorides can be obtained in up to 91 percent yield and complete Z selectivity. This method can be used to easily synthesize biologically active compounds and to perform the site- and stereoselective fluorination of other organic compounds. PMID:27008965

Cu-catalyzed formal methylative and hydrogenative carboxylation of alkynes with carbon dioxide: efficient synthesis of α,β-unsaturated carboxylic acids.

PubMed

Takimoto, Masanori; Hou, Zhaomin

2013-08-19

The sequential hydroalumination or methylalumination of various alkynes catalyzed by different catalyst systems, such those based on Sc, Zr, and Ni complexes, and the subsequent carboxylation of the resulting alkenylaluminum species with CO2 catalyzed by an N-heterocyclic carbene (NHC)-copper catalyst have been examined in detail. The regio- and stereoselectivity of the overall reaction relied largely on the hydroalumination or methylalumination reactions, which significantly depended on the catalyst and alkyne substrates. The subsequent Cu-catalyzed carboxylation proceeded with retention of the stereoconfiguration of the alkenylaluminum species. All the reactions could be carried out in one-pot to afford efficiently a variety of α,β-unsaturated carboxylic acids with well-controlled configurations, which are difficult to construct by previously reported methods. This protocol could be practically useful and attractive because of its high regio- and stereoselectivity, simple one-pot reaction operation, and the use of CO2 as a starting material. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Water oxidation catalyzed by mononuclear ruthenium complexes with a 2,2'-bipyridine-6,6'-dicarboxylate (bda) ligand: how ligand environment influences the catalytic behavior.

PubMed

Staehle, Robert; Tong, Lianpeng; Wang, Lei; Duan, Lele; Fischer, Andreas; Ahlquist, Mårten S G; Sun, Licheng; Rau, Sven

2014-02-03

A new water oxidation catalyst [Ru(III)(bda)(mmi)(OH2)](CF3SO3) (2, H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; mmi = 1,3-dimethylimidazolium-2-ylidene) containing an axial N-heterocyclic carbene ligand and one aqua ligand was synthesized and fully characterized. The kinetics of catalytic water oxidation by 2 were measured using stopped-flow technique, and key intermediates in the catalytic cycle were probed by density functional theory calculations. While analogous Ru-bda water oxidation catalysts [Ru(bda)L2] (L = pyridyl ligands) are supposed to catalyze water oxidation through a bimolecular coupling pathway, our study points out that 2, surprisingly, undergoes a single-site water nucleophilic attack (acid-base) pathway. The diversion of catalytic mechanisms is mainly ascribed to the different ligand environments, from nonaqua ligands to an aqua ligand. Findings in this work provide some critical proof for our previous hypothesis about how alternation of ancillary ligands of water oxidation catalysts influences their catalytic efficiency.

Total synthesis of the thiopeptide antibiotic amythiamicin D.

PubMed

Hughes, Rachael A; Thompson, Stewart P; Alcaraz, Lilian; Moody, Christopher J

2005-11-09

The thiopeptide (or thiostrepton) antibiotics are a class of sulfur containing highly modified cyclic peptides with interesting biological properties, including reported activity against MRSA and malaria. Described herein is the total synthesis of the thiopeptide natural product amythiamicin D, which utilizes a biosynthesis-inspired hetero-Diels-Alder route to the pyridine core of the antibiotic as a key step. Preliminary studies using a range of serine-derived 1-ethoxy-2-azadienes established that hetero-Diels-Alder reaction with N-acetylenamines proceeded efficiently under microwave irradiation to give 2,3,6-trisubstituted pyridines. The thiazole building blocks of the antibiotic were obtained by either classical Hantzsch reactions or by dirhodium(II)-catalyzed chemoselective carbene N-H insertion followed by thionation, and were combined to give the bis-thiazole that forms the left-hand fragment of the antibiotic. The key Diels-Alder reaction of a tris-thiazolyl azadiene with benzyl 2-(1-acetylaminoethenyl)thiazole-4-carboxylate gave the core tetrathiazolyl pyridine, which was elaborated into the natural product by successive incorporation of glycine and bis-thiazole fragments followed by macrocyclization.

Silylene-Nickel Promoted Cleavage of B-O Bonds: From Catechol Borane to the Hydroborylene Ligand.

PubMed

Hadlington, Terrance J; Szilvási, Tibor; Driess, Matthias

2017-06-19

The first 16 valence electron [bis(NHC)](silylene)Ni 0 complex 1, [( TMS L)ClSi:→Ni(NHC) 2 ], bearing the acyclic amido-chlorosilylene ( TMS L)ClSi: ( TMS L=N(SiMe 3 )Dipp; Dipp=2,6-Pr i 2 C 6 H 4 ) and two NHC ligands (N-heterocyclic carbene=:C[(Pr i )NC(Me)] 2 ) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi-NiH complex 2. Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene-coordinated (chloro)(silyl)nickel(II) complex 3, {[cat( TMS L)Si](Cl)Ni←:BH(NHC) 2 }, via the cleavage of two B-O bonds and simultaneous formation of two Si-O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:→Ni moiety in the breaking of incredibly strong B-O bonds. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chiral Nickel(II) Complex Catalyzed Enantioselective Doyle-Kirmse Reaction of α-Diazo Pyrazoleamides.

PubMed

Lin, Xiaobin; Tang, Yu; Yang, Wei; Tan, Fei; Lin, Lili; Liu, Xiaohua; Feng, Xiaoming

2018-03-07

Although high enantioselectivity of [2,3]-sigmatropic rearrangement of sulfonium ylides (Doyle-Kirmse reaction) has proven surprisingly elusive using classic chiral Rh(II) and Cu(I) catalysts, in principle it is due to the difficulty in fine discrimination of the heterotopic lone pairs of sulfur and chirality inversion at sulfur of sulfonium ylides. Here, we show that the synergistic merger of new α-diazo pyrazoleamides and a chiral N, N'-dioxide-nickel(II) complex catalyst enables a highly enantioselective Doyle-Kirmse reaction. The pyrazoleamide substituent serves as both an activating and a directing group for the ready formation of a metal-carbene- and Lewis-acid-bonded ylide intermediate in the assistance of a dual-tasking nickel(II) complex. An alternative chiral Lewis-acid-bonded ylide pathway greatly improves the product enantiopurity even for the reaction of a symmetric diallylsulfane. The majority of transformations over a series of aryl- or vinyl-substituted α-diazo pyrazoleamindes and sulfides proceed rapidly (within 5-20 min in most cases) with excellent results (up to 99% yield and 96% ee), providing a breakthrough in enantioselective Doyle-Kirmse reaction.

Dynamic Kinetic Resolution Enabled by Intramolecular Benzoin Reaction: Synthetic Applications and Mechanistic Insights.

PubMed

Zhang, Guoxiang; Yang, Shuang; Zhang, Xiaoyan; Lin, Qiqiao; Das, Deb K; Liu, Jian; Fang, Xinqiang

2016-06-29

The highly enantio-, diastereo-, and regioselective dynamic kinetic resolution of β-ketoesters and 1,3-diketones was achieved via a chiral N-heterocyclic carbene catalyzed intramolecular cross-benzoin reaction. A variety of tetralone derivatives bearing two contiguous stereocenters and multiple functionalities were liberated in moderate to excellent yields and with high levels of stereoselectivity (>95% ee and >20:1 dr in most cases). In addition, the excellent regioselectivity control for aryl/alkyl 1,3-diketones, and the superior electronic differentiation of 1,3-diarylketones were highlighted. Moreover, a set of new mechanistic rationale that differs with the currently widely accepted understanding of intramolecular benzoin reactions was established to demonstrate the superior preference of benzoin over aldol transformation: (1) A coexistence of competitive aldol and benzoin reactions was detected, but a retro-aldol-irreversible benzoin process performs a vital role in the generation of predominant benzoin products. (2) The most essential role of an N-electron-withdrawing substituent in triazolium catalysts was revealed to be accelerating the rate of the benzoin transformation, rather than suppressing the aldol process through reducing the inherent basicity of the catalyst.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Galan, Brandon R.; Wiedner, Eric S.; Helm, Monte L.

Nickel(II) complexes containing chelating N-heterocyclic carbene-phosphine ligands ([NiL2](BPh4)2, for which L = [MeIm(CH2)2PR2]) have been synthesized for the purpose of studying how this class of ligand effects the electrochemical properties compared to the nickel bis- diphosphine analogues. The nickel complexes were synthesized and characterized by x-ray crystallography and electrochemical methods. Based on the half wave potentials (E1/2), substitution of an NHC for one of the phosphines in a diphoshine ligand results in shifts in potential to 0.6 V to 1.2 V more negative than the corresponding nickel bis-diphosphine complexes. These quantitative results highlight the substantial effect that NHC ligands canmore » have upon the electronic properties of the metal complexes. BRG, JCL, and AMA acknowledge the support by the US Department of Energy Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. MLH acknoledges the support of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less

Carbon Dioxide Utilization by the Five-Membered Ring Products of Cyclometalation Reactions.

PubMed

Omae, Iwao

2016-04-01

In carbon dioxide utilization by cyclometalated five-membered ring products, the following compounds are used in four types of applications: 1. 2-Phenylpyrazole iridium compounds, pincer phosphine iridium compounds and 2-phenylimidazoline iridium compounds are used as catalysts for both formic acid production from CO 2 and H 2 , and hydrogen production from the formic acid. This formic acid can be a useful agent for H 2 production and storage for fuel cell electric vehicles. 2. Other chemicals, e.g. , dimethyl carbonate, methane, methanol and CO, are produced with dimethylaminomethylphenyltin compounds, pincer phosphine iridium compounds, pincer phosphine nickel compound and ruthenium carbene compound or 2-phenylpyridine iridium compounds, and phenylbenzothiazole iridium compounds as the catalysts for the reactions with CO 2 . 3. The five-membered ring intermediates of cyclometalation reactions with the conventional substrates react with carbon dioxide to afford their many types of carboxylic acid derivatives. 4. Carbon dioxide is easily immobilized at room temperature with immobilizing agents such as pincer phosphine nickel compounds, pincer phosphine palladium compounds, pincer N , N -dimethylaminomethyltin compounds and tris(2-pyridylthio)methane zinc compounds.

Explicitly correlated coupled cluster calculations for propadienylidene (H(2)CCC).

PubMed

Botschwina, Peter; Oswald, Rainer

2010-09-16

Propadienylidene (H(2)CCC), a reactive carbene of interest to combustion processes and astrochemistry, has been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x (x = a, b) level. Vibrational configuration interaction (VCI) has been employed to calculate accurate wavenumbers for the fundamental vibrations of H(2)CCC, D(2)CCC, and HDCCC. The symmetric CH stretching vibration of H(2)CCC is predicted to occur at ν(1) = 2984 cm(-1). Absorptions observed by argon matrix infrared spectroscopy at 3049.5 and 3059.6 cm(-1) are reassigned to the combination tone ν(2) + ν(4), which interacts with ν(1) and is predicted to have a higher intensity than the latter. Furthermore, IR bands detected at 865.4 and 868.8 cm(-1) are assigned to ν(6)(HDCCC), and those observed at 904.0 and 909.8 cm(-1) are assigned to the out-of-plane bending vibration ν(8)(HDCCC). An accurate value of 79.8 +/- 0.2 kJ mol(-1) is recommended for the zero-point vibrational energy of H(2)CCC.

Rational synthesis of normal, abnormal and anionic NHC–gallium alkyl complexes: structural, stability and isomerization insights† †Dedicated to the memory of Paul von R. Schleyer. ‡ ‡Electronic supplementary information (ESI) available: CIF files giving crystallographic results, experimental details and copies of the NMR spectra. CCDC 1405459–1405464. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc02086g

PubMed Central

Uzelac, Marina; Hernán-Gómez, Alberto; Armstrong, David R.; Kennedy, Alan R.

2015-01-01

Advancing the rational design of main-group N-heterocyclic carbene complexes, this study reports the synthesis, X-ray crystallographic and NMR spectroscopic characterisation of a novel series of Ga complexes containing neutral or anionic NHC ligands using the unsaturated carbene IPr (IPr = 1,3-bis-(2,6-di-isopropylphenyl)imidazol-2-ylidene). Starting from normal adduct GaR3·IPr (1) (R = CH2SiMe3), the addition of polar LiR led to the formation of NHC-stabilised gallate species IPr·LiGaR4 (2), resulting from co-complexation of the single-metal species. Contrastingly, reversing the order of addition of these organometallic reagents, by treating unsaturated free IPr, first with LiR followed by GaR3, furnished novel heteroleptic gallate (THF)2Li[:C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (3), which contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Ga and through its normal C2 position to Li. Electrophilic interception studies of 3 using methyl triflate (MeOTf), methanol and imidazolium salt (IMes·HCl) led to the isolation and structural elucidation of the two novel neutral abnormal NHC (aNHC) complexes [CH3C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (4) and aIPr·GaR3 (5) (aIPr = HC{[N(2,6-iPr2C6H3)]2CHC}). These studies disclose the preference of the anionic IPr ligand present in 3 to react with electrophiles via its C2 position, leaving its Ga–C4 bond intact. Abnormal complex 5 can also be accessed by a thermally induced rearrangement of its normal isomer 1. Combining NMR spectroscopic and kinetic studies with DFT calculations, new light has been shed on this intriguing transformation, which suggests that it occurs via a dissociative mechanism, highlighting the importance of the donor ability of the solvent used in these thermal isomerizations as well as the steric bulk of the substituents on the NHC and the Ga reagent. These findings intimate that relief of the steric hindrance around Ga by forming an abnormal complex is a key driving force behind these rearrangements. PMID:29910864

Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal

PubMed Central

2017-01-01

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6′-dihydroxybipyridine (6,6′-dhbp)) for both their proton-responsive features and for metal–ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = tBu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ir) or methoxy (5Ir); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)Ru(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ru) or methoxy (5Ru); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir, and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = tBu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1–5. In general, NHC-pyOR complexes 1–3 were modest precatalysts for both reactions. NHC complexes 1–3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP, which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir) is 5–8 times more active than x = methoxy (5Ir). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir. Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6′-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation. PMID:29540958

Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal.

PubMed

Siek, Sopheavy; Burks, Dalton B; Gerlach, Deidra L; Liang, Guangchao; Tesh, Jamie M; Thompson, Courtney R; Qu, Fengrui; Shankwitz, Jennifer E; Vasquez, Robert M; Chambers, Nicole; Szulczewski, Gregory J; Grotjahn, Douglas B; Webster, Charles Edwin; Papish, Elizabeth T

2017-03-27

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO 2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6'-dihydroxybipyridine (6,6'-dhbp)) for both their proton-responsive features and for metal-ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N -heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-py OR )Cl]OTf complexes where R = t Bu ( 1 ), H ( 2 ), or Me ( 3 ). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6'-dxbp)Cl]OTf, where x = hydroxy ( 4 Ir ) or methoxy ( 5 Ir ); 4 Ir was reported previously, but 5 Ir is new. The analogous ruthenium complexes were also tested using [(η 6 -cymene)Ru(6,6'-dxbp)Cl]OTf, where x = hydroxy ( 4 Ru ) or methoxy ( 5 Ru ); 4 Ru and 5 Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1 , 2 , 3 , 5 Ir , and for two [Ag(NHC-py OR ) 2 ]OTf complexes 6 (R = t Bu) and 7 (R = Me). The aqueous catalytic studies of both CO 2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1 - 5 . In general, NHC-py OR complexes 1 - 3 were modest precatalysts for both reactions. NHC complexes 1 - 3 all underwent transformations under basic CO 2 hydrogenation conditions, and for 3 , we trapped a product of its transformation, 3 SP , which we characterized crystallographically. For CO 2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy ( 4 Ir ) is 5-8 times more active than x = methoxy ( 5 Ir ). Notably, ruthenium complex 4 Ru showed 95% of the activity of 4 Ir . For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4 Ir ≫ 4 Ru and 4 Ir ≈ 5 Ir . Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6'-dhbp are deprotonated and alkali metals can bind and help to activate CO 2 . Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO 2 hydrogenation and formic acid dehydrogenation.

Are Nonadiabatic Reaction Dynamics the Key to Novel Organosilicon Molecules? The Silicon (Si(3P))-Dimethylacetylene (C4H6(X1A1g)) System as a Case Study.

PubMed

Thomas, Aaron M; Dangi, Beni B; Yang, Tao; Kaiser, Ralf I; Lin, Lin; Chou, Tzu-Jung; Chang, Agnes H H

2018-06-06

The bimolecular gas phase reaction of ground-state silicon (Si; 3 P) with dimethylacetylene (C 4 H 6 ; X 1 A 1g ) was investigated under single collision conditions in a crossed molecular beams machine. Merged with electronic structure calculations, the data propose nonadiabatic reaction dynamics leading to the formation of singlet SiC 4 H 4 isomer(s) and molecular hydrogen (H 2 ) via indirect scattering dynamics along with intersystem crossing (ISC) from the triplet to the singlet surface. The reaction may lead to distinct energetically accessible singlet SiC 4 H 4 isomers ( 1 p8- 1 p24) in overall exoergic reaction(s) (-107 -20 +12 kJ mol -1 ). All feasible reaction products are either cyclic, carry carbene analogous silylene moieties, or carry C-Si-H or C-Si-C bonds that would require extensive isomerization from the initial collision complex(es) to the fragmenting singlet intermediate(s). The present study demonstrates the first successful crossed beams study of an exoergic reaction channel arising from bimolecular collisions of silicon, Si( 3 P), with a hydrocarbon molecule.

Synthesis of alkyl-substituted six-membered lactones through ring-closing metathesis of homoallyl acrylates. An easy route to pyran-2-ones, constituents of tobacco flavor.

PubMed

D'Annibale, Andrea; Ciaralli, Laura; Bassetti, Mauro; Pasquini, Chiara

2007-08-03

The ring-closing metathesis (RCM) reactions of homoallylic acrylates bearing alkyl substituents on various positions of their skeleton afford the corresponding pentenolides in the presence of carbene ruthenium catalysts. For R3 = R4 = H, or R3 = Me, R4 = H, the reactions are catalyzed by complex [RuCl2(PCy3)2(=CHPh)], while a second-generation Grubbs catalyst is required when R3 = H and R4 = Me, R3 = R4 = Me, or R3 = i-Pr and R4 = H. Alkyl substitution at the homoallylic carbon (R1, R2) increases the yield of the reaction when both the acrylic and/or homoallylic double bonds are methyl-substituted. The interaction of the catalyst with the substrate in the initiation stage involves the homoallylic double bond rather than the acrylic moiety, and the resulting alkylidene species from the first-generation Grubbs catalyst can be observed by 1H and 31P NMR. The racemic tobacco constituents 4-isopropyl-5,6-dihydropyran-2-one and 4-isopropyltetrahydropyran-2-one are prepared via a short reaction sequence, involving the RCM reaction as the key transformation.

Pincer-CNC mononuclear, dinuclear and heterodinuclear Au(III) and Pt(II) complexes supported by mono- and poly-N-heterocyclic carbenes: synthesis and photophysical properties.

PubMed

Gonell, S; Poyatos, M; Peris, E

2016-04-07

A family of cyclometallated Au(iii) and Pt(ii) complexes containing a CNC-pincer ligand (CNC = 2,6-diphenylpyridine) supported by pyrene-based mono- or bis-NHC ligands have been synthesized and characterized, together with the preparation of a Pt-Au hetero-dimetallic complex based on a Y-shaped tris-NHC ligand. The photophysical properties of all the new species and of two related Ru(ii)-arene complexes were studied and compared. Whereas the pyrene-based complexes only exhibit emission in solution, those containing the Y-shaped tris-NHC ligand are only luminescent when dispersed in poly(methyl methacrylate) (PMMA). In particular, the pyrene-based complexes were found to be emissive in the range of 373-440 nm, with quantum yields ranging from 3.1 to 6.3%, and their emission spectra were found to be almost superimposable, pointing to the fluorescent pyrene-centered nature of the emission. This observation suggests that the emission properties of the pyrene fragment may be combined with some of the numerous applications of NHCs as supporting ligands allowing, for instance, the design of biological luminescent agents.

Dual descriptors within the framework of spin-polarized density functional theory.

PubMed

Chamorro, E; Pérez, P; Duque, M; De Proft, F; Geerlings, P

2008-08-14

Spin-polarized density functional theory (SP-DFT) allows both the analysis of charge-transfer (e.g., electrophilic and nucleophilic reactivity) and of spin-polarization processes (e.g., photophysical changes arising from electron transitions). In analogy with the dual descriptor introduced by Morell et al. [J. Phys. Chem. A 109, 205 (2005)], we introduce new dual descriptors intended to simultaneously give information of the molecular regions where the spin-polarization process linking states of different multiplicity will drive electron density and spin density changes. The electronic charge and spin rearrangement in the spin forbidden radiative transitions S(0)-->T(n,pi(*)) and S(0)-->T(pi,pi(*)) in formaldehyde and ethylene, respectively, have been used as benchmark examples illustrating the usefulness of the new spin-polarization dual descriptors. These quantities indicate those regions where spin-orbit coupling effects are at work in such processes. Additionally, the qualitative relationship between the topology of the spin-polarization dual descriptors and the vertical singlet triplet energy gap in simple substituted carbene series has been also discussed. It is shown that the electron density and spin density rearrangements arise in agreement with spectroscopic experimental evidence and other theoretical results on the selected target systems.

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents

PubMed Central

2016-01-01

Alkene metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular alkene metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C (0.6 active sites/W). This high activity at low temperature broadens the scope of these catalysts to functionalized substrates. This activation process can also be applied to classical industrial catalysts. We provide evidence for the formation of a metallacyclopentane intermediate and propose how the active species are formed. PMID:27610418

Bonding properties and bond activation of ylides: recent findings and outlook.

PubMed

Urriolabeitia, Esteban P

2008-11-14

The interaction of phosphorus and nitrogen ylides with metallic precursors has been examined from different points of view. The first one is related to the bonding properties of the ylides. Ylides with a unique stabilizing group bond through different atoms (the Calpha or the heteroatoms); while ylides with two stabilizing groups never coordinate through the Calpha atom. In the second section we examine the cause of the stereoselective coordination of bisylides of phosphorus, nitrogen and arsenic, and of mixed bisylides. We describe here the very interesting conformational preferences found in these systems, which have been determined and characterized. The DFT study of these bisylides has allowed for the characterization of strong intramolecular PO and AsO interactions, as well as moderate CHO[double bond, length as m-dash]C hydrogen bonds as the source of these conformational preferences. The third topic is related to the amazing reactivity of phosphorus ylides in bond activation processes. Depending on the nature of the metallic precursors, ylides can behave as sources of carbenes, of phosphine derivatives, of other ylides or of orthometallated complexes through P[double bond, length as m-dash]C, P-C or C-H bond activation reactions.

Chemistry and structure of coal-derived asphaltenes, Phase III. Quarterly progress report, January--March 1978

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yen, T. F.

1978-01-01

The solubility limits of Synthoil and PAMCO asphaltenes have been measured as a function of Hildebrand solubility parameters and hydrogen bonding. Solvents with moderate hydrogen bonding capacity such as dioxane, ethyl benzoate and dibutyl phthalate were found to be most effective in dissolving asphaltenes over the widest range of solubility parameters. VPO molecular weight studies of coal liquid derived carbenes, as a function of concentration in the solvent THF, indicate that these fractions are more strongly self-associated than the corresponding asphaltenes, and generally afford high infinite dilution number average molecular weights: Synthoil, 861; HRI H-Coal, 1156; Cat. Inc. SRC, 1228;more » PAMCO SRC, 1054. The variable ESR temperature dependence of the spin intensity for a Synthoil asphaltene-I/sub 2/ charge transfer followed a 1/T (Curie--Weiss) dependence over the temperature range from 25/sup 0/ to -114/sup 0/C suggesting that independent, non-interacting donor and acceptor doublets were formed. Weight percent OH values, determined from 'H NMR analysis of silylated asphaltenes, were found to provide a reasonably linear correlation with the absorbance of the monomeric OH infrared stretching bands of the asphaltenes.« less

Fischer carbene mediated covalent grafting of a peptide nucleic acid on gold surfaces and IR optical detection of DNA hybridization with a transition metalcarbonyl label

NASA Astrophysics Data System (ADS)

Srivastava, Pratima; Ghasemi, Mahsa; Ray, Namrata; Sarkar, Amitabha; Kocabova, Jana; Lachmanova, Stepanka; Hromadova, Magdalena; Boujday, Souhir; Cauteruccio, Silvia; Thakare, Pramod; Licandro, Emanuela; Fosse, Céline; Salmain, Michèle

2016-11-01

Amine-reactive surfaces comprising N-hydroxysuccinimide ester groups as well as much more unusual Fischer alkoxymetallocarbene groups were generated on gold-coated surfaces via self-assembled monolayers of carboxy- and azido-terminated thiolates, respectively. These functions were further used to immobilize homothymine peptide nucleic acid (PNA) decamer in a covalent fashion involving the primary amine located at its N-terminus. These stepwise processes were monitored by polarization modulation reflection - absorption infrared spectroscopy (PM-RAIRS) that gave useful information on the molecular composition of the organic layers. PNA grafting and hybridization with complementary DNA strand were successfully transduced by quartz crystal microbalance (QCM) measurements. Unfortunately, attempts to transduce the hybridization optically by IR in a label-free fashion were inconclusive. Therefore we undertook to introduce an IR reporter group, namely a transition metalcarbonyl (TMC) entity at the 5‧ terminus of complementary DNA. Evidence for the formation of PNA-DNA heteroduplex was brought by the presence of ν(Ctbnd O) bands in the 2000 cm-1 region of the IR spectrum of the gold surface owing to the metalcarbonyl label.

Preclinical anti-cancer activity and multiple mechanisms of action of a cationic silver complex bearing N-heterocyclic carbene ligands.

PubMed

Allison, Simon J; Sadiq, Maria; Baronou, Efstathia; Cooper, Patricia A; Dunnill, Chris; Georgopoulos, Nikolaos T; Latif, Ayşe; Shepherd, Samantha; Shnyder, Steve D; Stratford, Ian J; Wheelhouse, Richard T; Willans, Charlotte E; Phillips, Roger M

2017-09-10

Organometallic complexes offer the prospect of targeting multiple pathways that are important in cancer biology. Here, the preclinical activity and mechanism(s) of action of a silver-bis(N-heterocyclic carbine) complex (Ag8) were evaluated. Ag8 induced DNA damage via several mechanisms including topoisomerase I/II and thioredoxin reductase inhibition and induction of reactive oxygen species. DNA damage induction was consistent with cytotoxicity observed against proliferating cells and Ag8 induced cell death by apoptosis. Ag8 also inhibited DNA repair enzyme PARP1, showed preferential activity against cisplatin resistant A2780 cells and potentiated the activity of temozolomide. Ag8 was substantially less active against non-proliferating non-cancer cells and selectively inhibited glycolysis in cancer cells. Ag8 also induced significant anti-tumour effects against cells implanted intraperitoneally in hollow fibres but lacked activity against hollow fibres implanted subcutaneously. Thus, Ag8 targets multiple pathways of importance in cancer biology, is less active against non-cancer cells and shows activity in vivo in a loco-regional setting. Crown Copyright © 2017. Published by Elsevier B.V. All rights reserved.

Spin Contamination Error in Optimized Geometry of Singlet Carbene (1A1) by Broken-Symmetry Method

NASA Astrophysics Data System (ADS)

Kitagawa, Yasutaka; Saito, Toru; Nakanishi, Yasuyuki; Kataoka, Yusuke; Matsui, Toru; Kawakami, Takashi; Okumura, Mitsutaka; Yamaguchi, Kizashi

2009-10-01

Spin contamination errors of a broken-symmetry (BS) method in optimized structural parameters of the singlet methylene (1A1) molecule are quantitatively estimated for the Hartree-Fock (HF) method, post-HF methods (CID, CCD, MP2, MP3, MP4(SDQ)), and a hybrid DFT (B3LYP) method. For the purpose, the optimized geometry by the BS method is compared with that of an approximate spin projection (AP) method. The difference between the BS and the AP methods is about 10-20° in the HCH angle. In order to examine the basis set dependency of the spin contamination error, calculated results by STO-3G, 6-31G*, and 6-311++G** are compared. The error depends on the basis sets, but the tendencies of each method are classified into two types. Calculated energy splitting values between the triplet and the singlet states (ST gap) indicate that the contamination of the stable triplet state makes the BS singlet solution stable and the ST gap becomes small. The energy order of the spin contamination error in the ST gap is estimated to be 10-1 eV.

Iridium N-Heterocyclic Carbene Complexes as Efficient Catalysts for Magnetization Transfer from para-Hydrogen

PubMed Central

2011-01-01

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)2(IMes)(py)3]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H2. These reversible processes bring para-H2 and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in 1H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)2(η2-H2)(IMes)(py)2]+, an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach. PMID:21469642

Iridium N-heterocyclic carbene complexes as efficient catalysts for magnetization transfer from para-hydrogen.

PubMed

Cowley, Michael J; Adams, Ralph W; Atkinson, Kevin D; Cockett, Martin C R; Duckett, Simon B; Green, Gary G R; Lohman, Joost A B; Kerssebaum, Rainer; Kilgour, David; Mewis, Ryan E

2011-04-27

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)(2)(IMes)(py)(3)]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H(2). These reversible processes bring para-H(2) and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in (1)H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)(2)(η(2)-H(2))(IMes)(py)(2)](+), an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach.

Sequence change in the HS2-LCR and Ggamma-globin gene promoter region of sickle cell anemia patients.

PubMed

Adorno, E V; Moura-Neto, J P; Lyra, I; Zanette, A; Santos, L F O; Seixas, M O; Reis, M G; Goncalves, M S

2008-02-01

The fetal hemoglobin (HbF) levels and betaS-globin gene haplotypes of 125 sickle cell anemia patients from Brazil were investigated. We sequenced the Ggamma- and Agamma-globin gene promoters and the DNase I-2 hypersensitive sites in the locus control regions (HS2-LCR) of patients with HbF level disparities as compared to their betaS haplotypes. Sixty-four (51.2%) patients had CAR/Ben genotype; 36 (28.8%) Ben/Ben; 18 (14.4%) CAR/CAR; 2 (1.6%) CAR/Atypical; 2 (1.6%) Ben/Cam; 1 (0.8%) CAR/Cam; 1 (0.8%) CAR/Arab-Indian, and 1 (0.8%) Sen/Atypical. The HS2-LCR sequence analyses demonstrated a c.-10.677G>A change in patients with the Ben haplotype and high HbF levels. The Gg gene promoter sequence analyses showed a c.-157T>C substitution shared by all patients, and a c.-222_-225del related to the Cam haplotype. These results identify new polymorphisms in the HS2-LCR and Gg-globin gene promoter. Further studies are required to determine the correlation between HbF synthesis and the clinical profile of sickle cell anemia patients.

Homo-Roche Ester Derivatives By Asymmetric Hydrogenation and Organocatalysis

PubMed Central

Khumsubdee, Sakunchai; Zhou, Hua; Burgess, Kevin

2013-01-01

Asymmetric hydrogenation routes to homologs of The Roche ester tend to be restricted to hydrogenations of itaconic acid derivatives, ie substrates that contain a relatively unhindered, 1,1-disubstituted, alkene. This is because in hydrogenations mediated by RhP2 complexes, the typical catalysts, it is difficult to obtain high conversions using the alternative substrate for the same product, the isomeric trisubstituted alkenes (D in the text). However, chemoselective modification of the identical functional groups in itaconic acid derivatives are difficult, hence it would be favorable to use the trisubstituted alkene. Trisubstituted alkene substrates can be hydrogenated with high conversions using chiral analogs of Crabtree’s catalyst of the type IrN(carbene). This paper demonstrates such reactions are scalable (tens of grams) and can be manipulated to give optically pure homo-Roche ester chirons. Organocatalytic fluorination, chlorination, and amination of the homo-Roche building blocks was performed to demonstrate that they could be easily transformed into functionalized materials with two chiral centers and α,ω-groups that provide extensive scope for modifications. A synthesis of (S,S)- and (R,S)-γ-hydroxyvaline was performed to illustrate one application of the amination product. PMID:24219839

NHC-catalysed benzoin condensation – is it all down to the Breslow intermediate?† †Electronic supplementary information (ESI) available: Characterisation data of products, substrates and catalysts, EPR and NMR spectra and progress curves as well as computational details are found. See DOI: 10.1039/c5sc02186c Click here for additional data file.

PubMed Central

Ruser, Stephanie-M.; Phan, Jenny

2015-01-01

The Breslow catalytic cycle describing the benzoin condensation promoted by N-heterocyclic carbenes (NHC) as proposed in the late 1950s has since then been tried by generations of physical organic chemists. Emphasis has been laid on proofing the existence of an enaminol like structure (Breslow intermediate) that explains the observed umpolung of an otherwise electrophilic aldehyde. The present study is not focusing on spectroscopic elucidation of a thiazolydene based Breslow intermediate but rather tries to clarify if this key-intermediate is indeed directly linked with the product side of the overall reaction. The here presented EPR-spectroscopic and computational data provide a fundamentally different view on how the benzoin condensation may proceed: a radical pair could be identified as a second key-intermediate that is derived from the Breslow-intermediate via an SET process. These results highlight the close relationship to the Cannizarro reaction and oxidative transformations of aldehydes under NHC catalysis. PMID:29449915

Harnessing polarisation transfer to indazole and imidazole through signal amplification by reversible exchange to improve their NMR detectability

PubMed Central

Fekete, Marianna; Rayner, Peter J.; Green, Gary G. R.

2017-01-01

The signal amplification by reversible exchange (SABRE) approach has been used to hyperpolarise the substrates indazole and imidazole in the presence of the co‐ligand acetonitrile through the action of the precataysts [IrCl(COD)(IMes)] and [IrCl(COD)(SIMes)]. 2H‐labelled forms of these catalysts were also examined. Our comparison of the two precatalysts [IrCl(COD)(IMes)] and [IrCl(COD)(SIMes)], coupled with 2H labelling of the N‐heterocyclic carbene and associated relaxation and polarisation field variation studies, demonstrates the critical and collective role these parameters play in controlling the efficiency of signal amplification by reversible exchange. Ultimately, with imidazole, a 700‐fold1H signal gain per proton is produced at 400 MHz, whilst for indazole, a 90‐fold increase per proton is achieved. The co‐ligand acetonitrile proved to optimally exhibit a 190‐fold signal gain per proton in these measurements, with the associated studies revealing the importance the substrate plays in controlling this value. Copyright © 2017 The Authors. Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd. PMID:28497481

Origins of Protons in C-H Bond Insertion Products of Phenols: Proton-Self-Sufficient Function via Water Molecules.

PubMed

Luo, Zhoujie; Gao, Ya; Zhu, Tong; Zhang, John Zenghui; Xia, Fei

2017-08-31

Water molecules can serve as proton shuttles for proton transfer in the C-H bond insertion reactions catalyzed by transition metal complexes. Recently, the control experiments performed for C-H bond insertion of phenol and anisol by gold carbenes show that large discrepancy exists in the yields of hydrogenated and deuterated products. Thus, we conducted a detailed theoretical analysis on the function of water molecules in the C-H bond insertion reactions. The comparison of calculated results and control experiments indicates that the solution water molecules play a crucial role of proton shuttle in C-H bond insertion. In particular, it was found that the hydroxyl groups in phenols were capable of donating protons via water shuttles for the production of C-H products, which had a substantial influence on the yields of inserted products. The hydroxyl groups instead of C-H bonds in phenols function like "proton reservoirs" in the C-H bond insertion, which we call the "proton self-sufficient" (PSS) function of phenol. The PSS function of phenol indicates that the substrates with and without proton reservoirs will lead to different C-H bond insertion products.

Matrix Isolation Spectroscopy and Photochemistry of Triplet 1,3-DIMETHYLPROPYNYLIDENE (MeC3Me)

NASA Astrophysics Data System (ADS)

Knezz, Stephanie N.; Waltz, Terese A.; Haenni, Benjamin C.; Burrmann, Nicola J.; McMahon, Robert J.

2015-06-01

Acetylenic carbenes and conjugated carbon chain molecules of the HCnH family are relevant to the study of combustion and chemistry in the interstellar medium (ISM). Propynylidene (HC3H) has been thoroughly studied and its structure and photochemistry determined. Here, we produce triplet diradical 1,3-dimethylpropynylidene (MeC3Me) photochemically from a precursor diazo compound in a cryogenic matrix (N2 or Ar) at 10 K, and spectroscopic analysis is carried out. The infrared, electronic absorption, and electron paramagnetic resonance spectra were examined in light of the parent (HC3H) system to ascertain the effect of alkyl substituents on delocalized carbon chains of this type. Computational analysis, EPR, and infrared analysis indicate a triplet ground state with a quasilinear structure. Infrared experiments reveal photochemical reaction to penten-3-yne upon UV irradiation. Further experimental and computational results pertaining to the structure and photochemistry will be presented. Seburg, R. A.; Patterson, E. V.; McMahon, R. J., Structure of Triplet Propynylidene (HCCCH) as Probed by IR, UV/vis, and EPR Spectroscopy of Isotopomers. Journal of the American Chemical Society 2009, 131 (26), 9442-9455.

Computation provides chemical insight into the diverse hydride NMR chemical shifts of [Ru(NHC)4(L)H]0/+ species (NHC = N-heterocyclic carbene; L = vacant, H2, N2, CO, MeCN, O2, P4, SO2, H-, F- and Cl-) and their [Ru(R2PCH2CH2PR2)2(L)H]+ congeners.

PubMed

Häller, L Jonas L; Mas-Marzá, Elena; Cybulski, Mateusz K; Sanguramath, Rajashekharayya A; Macgregor, Stuart A; Mahon, Mary F; Raynaud, Christophe; Russell, Christopher A; Whittlesey, Michael K

2017-02-28

Relativistic density functional theory calculations, both with and without the effects of spin-orbit coupling, have been employed to model hydride NMR chemical shifts for a series of [Ru(NHC) 4 (L)H] 0/+ species (NHC = N-heterocyclic carbene; L = vacant, H 2 , N 2 , CO, MeCN, O 2 , P 4 , SO 2 , H - , F - and Cl - ), as well as selected phosphine analogues [Ru(R 2 PCH 2 CH 2 PR 2 ) 2 (L)H] + (R = i Pr, Cy; L = vacant, O 2 ). Inclusion of spin-orbit coupling provides good agreement with the experimental data. For the NHC systems large variations in hydride chemical shift are shown to arise from the paramagnetic term, with high net shielding (L = vacant, Cl - , F - ) being reinforced by the contribution from spin-orbit coupling. Natural chemical shift analysis highlights the major orbital contributions to the paramagnetic term and rationalizes trends via changes in the energies of the occupied Ru d π orbitals and the unoccupied σ* Ru-H orbital. In [Ru(NHC) 4 (η 2 -O 2 )H] + a δ-interaction with the O 2 ligand results in a low-lying LUMO of d π character. As a result this orbital can no longer contribute to the paramagnetic shielding, but instead provides additional deshielding via overlap with the remaining (occupied) d π orbital under the L z angular momentum operator. These two effects account for the unusual hydride chemical shift of +4.8 ppm observed experimentally for this species. Calculations reproduce hydride chemical shift data observed for [Ru( i Pr 2 PCH 2 CH 2 P i Pr 2 ) 2 (η 2 -O 2 )H] + (δ = -6.2 ppm) and [Ru(R 2 PCH 2 CH 2 PR 2 ) 2 H] + (ca. -32 ppm, R = i Pr, Cy). For the latter, the presence of a weak agostic interaction trans to the hydride ligand is significant, as in its absence (R = Me) calculations predict a chemical shift of -41 ppm, similar to the [Ru(NHC) 4 H] + analogues. Depending on the strength of the agostic interaction a variation of up to 18 ppm in hydride chemical shift is possible and this factor (that is not necessarily readily detected experimentally) can aid in the interpretation of hydride chemical shift data for nominally unsaturated hydride-containing species. The synthesis and crystallographic characterization of the BAr F 4 - salts of [Ru(IMe 4 ) 4 (L)H] + (IMe 4 = 1,3,4,5-tetramethylimidazol-2-ylidene; L = P 4 , SO 2 ; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ) and [Ru(IMe 4 ) 4 (Cl)H] are also reported.

Rh-Catalyzed Intermolecular Reactions of α-Alkyl-α-Diazo Carbonyl Compounds with Selectivity over β-Hydride Migration.

PubMed

DeAngelis, Andrew; Panish, Robert; Fox, Joseph M

2016-01-19

Rh-carbenes derived from α-diazocarbonyl compounds have found broad utility across a remarkable range of reactivity, including cyclopropanation, cyclopropenation, C-H insertions, heteroatom-hydrogen insertions, and ylide forming reactions. However, in contrast to α-aryl or α-vinyl-α-diazocarbonyl compounds, the utility of α-alkyl-α-diazocarbonyl compounds had been moderated by the propensity of such compounds to undergo intramolecular β-hydride migration to give alkene products. Especially challenging had been intermolecular reactions involving α-alkyl-α-diazocarbonyl compounds. This Account discusses the historical context and prior limitations of Rh-catalyzed reactions involving α-alkyl-α-diazocarbonyl compounds. Early studies demonstrated that ligand and temperature effects could influence chemoselectivity over β-hydride migration. However, effects were modest and conflicting conclusions had been drawn about the influence of sterically demanding ligands on β-hydride migration. More recent advances have led to a more detailed understanding of the reaction conditions that can promote intermolecular reactivity in preference to β-hydride migration. In particular, the use of bulky carboxylate ligands and low reaction temperatures have been key to enabling intermolecular cyclopropenation, cyclopropanation, carbonyl ylide formation/dipolar cycloaddition, indole C-H functionalization, and intramolecular bicyclobutanation with high chemoselectivity over β-hydride migration. Cyclic α-diazocarbonyl compounds have been shown to be particularly resilient toward β-hydride migration and are the first class of compounds that can engage in intermolecular reactivity in the presence of tertiary β-hydrogens. DFT calculations were used to propose that for cyclic α-diazocarbonyl compounds, ring constraints relieve steric interaction for intermolecular reactions and thereby accelerate the rate of intermolecular reactivity relative to intramolecular β-hydride migration. Enantioselective reactions of α-alkyl-α-diazocarbonyl compounds have been developed using bimetallic N-imido-tert-leucinate-derived complexes. The most effective complexes were found by computation and X-ray crystallography to adopt a "chiral crown" conformation in which all of the imido groups are presented on one face of the paddlewheel complex in a chiral arrangement. Insight from computational studies guided the design and synthesis of a mixed ligand paddlewheel complex, Rh2(S-PTTL)3TPA, the structure of which bears similarity to the chiral crown complex Rh2(S-PTTL)4. Rh2(S-PTTL)3TPA engages substrate classes (aliphatic alkynes, silylacetylenes, α-olefins) that are especially challenging in intermolecular reactions of α-alkyl-α-diazoesters and catalyzes enantioselective cyclopropanation, cyclopropenation, and indole C-H functionalization with yields and enantioselectivities that are comparable or superior to Rh2(S-PTTL)4. The work detailed in this Account describes progress toward enabling a more general utility for α-alkyl-α-diazo compounds in Rh-catalyzed carbene reactions. Further studies on ligand design and synthesis will continue to broaden the scope of their selective reactions.

Ammonia and hydrazine. Transition-metal-catalyzed hydroamination and metal-free catalyzed functionalization

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bertrand, Guy

2012-06-29

The efficient and selective preparation of organic molecules is critical for mankind. For the future, it is of paramount importance to find catalysts able to transform abundant and cheap feedstocks into useful compounds. Acyclic and heterocyclic nitrogen-containing derivatives are common components of naturally occurring compounds, agrochemicals, cosmetics, and pharmaceuticals; they are also useful intermediates in a number of industrial processes. One of the most widely used synthetic strategies, allowing the formation of an N-C bond, is the addition of an N-H bond across a carbon-carbon multiple bond, the so-called hydroamination reaction. This chemical transformation fulfills the principle of “green chemistry”more » since it ideally occurs with 100% atom economy. Various catalysts have been found to promote this reaction, although many limitations remain; one of the most prominent is the lack of methods that permit the use of NH 3 and NH 2NH 2 as the amine partners. In fact, ammonia and hydrazine have rarely succumbed to homogeneous catalytic transformations. Considering the low cost and abundance of ammonia (136 million metric tons produced in 2011) and hydrazine, catalysts able to improve the reactivity and selectivity of the NH 3- and NH 2NH 2-hydroamination reaction, and more broadly speaking the functionalization of these chemicals, are highly desirable. In the last funded period, we discovered the first homogeneous catalysts able to promote the hydroamination of alkynes and allenes with ammonia and the parent hydrazine. The key feature of our catalytic systems is that the formation of catalytically inactive Werner complexes is reversible, in marked contrast to most of the known ammonia and hydrazine transition metal complexes. This is due to the peculiar electronic properties of our neutral ancillary ligands, especially their strong donating capabilities. However, our catalysts currently require high temperatures and long reaction times. To address this issue, we have developed several new families of carbon- and boron-based ligands, which are even better donors. The corresponding metal complexes (particularly gold, rhodium, iridium, and ruthenium) of all these species will be tested in the Markovnikov and anti-Markovnikov hydroamination of alkynes, allenes, and also alkenes with ammonia and hydrazine. We will also develop metal-free catalytic processes for the functionalization of ammonia and hydrazine. By possessing both a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble and can mimic the chemical behavior of transition metals. Our preliminary results demonstrate that specially designed carbenes can split the N–H bond of ammonia by an initial nucleophilic activation that prevents the formation of Lewis acid-base adducts, which is the major hurdle for the transition metal catalyzed functionalization of NH 3. The use of purely organic compounds as catalysts will eliminate the major drawbacks of transition-metal-catalysis technology, which are the excessive cost of metal complexes (metal + ligands) and in many cases the toxicity of the metal.« less

Methylene migration and coupling on a non-reducible metal oxide: The reaction of dichloromethane on stoichiometric α-Cr 2O 3(0001)

DOE PAGES

Dong, Yujung; Brooks, John D.; Chen, Tsung-Liang; ...

2014-09-17

The reaction of CH 2Cl 2 over the nearly-stoichiometric α-Cr 2O 3(0001) surface produces gas phase ethylene, methane and surface chlorine adatoms. The reaction is initiated by the decomposition of CH 2Cl 2 into surface methylene and chlorine. Photoemission indicates that surface cations are the preferred binding sites for both methylene and chlorine adatoms. Two reaction channels are observed for methylene coupling to ethylene in temperature-programmed desorption (TPD). A desorption-limited, low-temperature route is attributed to two methylenes bound at a single site. The majority of ethylene is produced by a reaction-limited process involving surface migration (diffusion) of methylene as themore » rate-limiting step. DFT calculations indicate the surface diffusion mechanism is mediated by surface oxygen anions. The source of hydrogen for methane formation is adsorbed background water. Chlorine adatoms produced by the dissociation of CH 2Cl 2 deactivate the surface by simple site-blocking of surface Cr 3+ sites. Finally, a comparison of experiment and theory shows that DFT provides a better description of the surface chemistry of the carbene intermediate than DFT+U using reported parameters for a best representation of the bulk electronic properties of α-Cr 2O 3.« less

A Mechanistical Study on the Formation of Dimethyl Ether (CH3OCH3) and Ethanol (CH3CH2OH) in Methanol-containing Ices and Implications for the Chemistry of Star-forming Regions

NASA Astrophysics Data System (ADS)

Bergantini, Alexandre; Góbi, Sándor; Abplanalp, Matthew J.; Kaiser, Ralf I.

2018-01-01

The underlying formation mechanisms of complex organic molecules (COMs)—in particular, structural isomers—in the interstellar medium (ISM) are largely elusive. Here, we report new experimental findings on the role of methanol (CH3OH) and methane (CH4) ices in the synthesis of two C2H6O isomers upon interaction with ionizing radiation: ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3). The present study reproduces the interstellar abundance ratios of both species with ethanol to dimethyl ether branching ratios of (2.33 ± 0.14):1 suggesting that methanol and methane represents the key precursor to both isomers within interstellar ices. Exploiting isotopic labeling combined with reflectron time-of-flight mass spectrometry (Re-TOF-MS) after isomer selective vacuum ultra-violet (VUV) photoionization of the neutral molecules, we also determine the formation mechanisms of both isomers via radical–radical recombination versus carbene (CH2) insertion with the former pathway being predominant. Formation routes to higher molecular weight reaction products such as ethylene glycol (HOCH2CH2OH), dimethyl peroxide (CH3OOCH3), and methoxymethanol (CH3OCH2OH) are discussed briefly as well.

Controlling the hydrogenolysis of silica-supported tungsten pentamethyl leads to a class of highly electron deficient partially alkylated metal hydrides

DOE PAGES

Maity, Niladri; Barman, Samir; Callens, Emmanuel; ...

2015-11-30

The well-defined single-site silica-supported tungsten complex [(Si–O–)W(Me) 5], 1, is an excellent precatalyst for alkane metathesis. The unique structure of 1 allows the synthesis of unprecedented tungsten hydrido methyl surface complexes via a controlled hydrogenolysis. Specifically, in the presence of molecular hydrogen, 1 is quickly transformed at -78 °C into a partially alkylated tungsten hydride, 4, as characterized by 1H solid-state NMR and IR spectroscopies. Species 4, upon warming to 150 °C, displays the highest catalytic activity for propane metathesis yet reported. DFT calculations using model systems support the formation of [(Si–O–)WH 3(Me) 2], as the predominant species at -78more » °C following several elementary steps of hydrogen addition (by σ-bond metathesis or α-hydrogen transfer). Rearrangement of 4 occuring between -78 °C and room temperature leads to the formation of an unique methylidene tungsten hydride [(Si–O–)WH 3(CH 2)], as determined by solid-state 1H and 13C NMR spectroscopies and supported by DFT. Thus for the first time, a coordination sphere that incorporates both carbene and hydride functionalities has been observed.« less

Amino Acid-Assisted Dehalogenation of Carbon Tetrachloride by Green Rust: Inhibition of Chloroform Production.

PubMed

Yin, Weizhao; Strobel, Bjarne W; B Hansen, Hans Christian

2017-03-21

Layered Fe II -Fe III hydroxides (green rusts, GRs) are promising reactants for reductive dechlorination of chlorinated solvents due to high reaction rates and the opportunity to inject reactive slurries of the compounds into contaminant plumes. However, it is necessary to develop strategies that reduce the formation of toxic byproducts such as chloroform (CF). In this study, carbon tetrachloride (CT) dehalogenation by the chloride form of GR (GR Cl ) was tested in the presence of glycine (GLY) and other selected amino acids. GLY, alanine (ALA), and serine (SER) all resulted in remarkable suppression of CF formation with only ∼10% of CF recovery while sarcosine (SAR) showed insignificant effects. For two nonamino acid buffers, TRIS had little effect while HEPES resulted in a 40 times lower rate constant compared to experiments in which no buffer was added. The Fe II complexing properties of the amino acids and buffers caused variable extents of GR Cl dissolution which was linearly correlated with CF suppression and dehalogenation rate. We hypothesize that the CF suppression seen for amino acids is caused by stabilization of carbene intermediates via the carbonyl group. Different effects on CF suppression and CT dehalogenation rate were expected because of the different structural and chemical properties of the amino acids.

Ligand effects on the hydrogenation of biomass-inspired substrates with bifunctional Ru, Ir, and Rh complexes.

PubMed

Jansen, Eveline; Jongbloed, Linda S; Tromp, Dorette S; Lutz, Martin; de Bruin, Bas; Elsevier, Cornelis J

2013-09-01

We herein report on the application and structural investigation of a new set of complexes that contain bidentate N-heterocyclic carbenes (NHCs) and primary amine moieties of the type [M(arene)Cl(L)] [M=Ru, Ir, or Rh; arene=p-cymene or pentamethylcyclopentadienyl; L=1-(2-aminophenyl)-3-(n-alkyl)imidazol-2-ylidine]. These complexes were tested and compared in the hydrogenation of acetophenone with hydrogen. Structural variations in the chelate ring size of the heteroditopic ligand revealed that smaller chelate ring sizes in combination with ring conjugation in the ligand are beneficial for the activity of this type of catalyst, favoring an inner-sphere coordination pathway. Additionally, increasing the steric bulk of the alkyl substituent on the NHC aided the reaction, showing almost no induction period and formation of a more active catalyst for the n-butyl complex relative to complexes with smaller Me and Et substituents. As is common in hydrogenation reactions, the activity of the complexes decreases in the order Ru>Ir>Rh. The application of [Ru(p-cym)Cl(L)]PF6 , which outperforms its reported analogues, has been successfully extended to the hydrogenation of more challenging biomass-inspired substrates. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Toward organometallic (99m)Tc imaging agents: synthesis of water-stable (99)Tc-NHC complexes.

PubMed

Benz, Michael; Spingler, Bernhard; Alberto, Roger; Braband, Henrik

2013-11-20

(99)Tc(V)O2-NHC complexes containing monodentate and bidentate N-heterocyclic carbenes (NHCs) have been prepared by the reactions of [TcO(glyc)2](-) (glyc = ethyleneglycolato) with 1,3-dimethylimidazoline-2-ylidene (L1), 1,1'-methylene-3,3'-dimethyl-4,4'-diimidazoline-2,2'-diylidene (L2), and 1,1'-methylene-3,3'-diethyl-4,4'-diimidazoline-2,2'-diylidene (L3) in THF. The resulting complexes were fully characterized and their stabilities investigated. While complexes with monodentate NHCs only are hydrolytically unstable, complexes containing bidentate NHCs are water-stable over a broad pH range. The high water stability allows interconversion of the {(99)Tc(V)O2}(+) core into {(99)Tc(V)OCl}(2+) with HCl as the H(+) and Cl(-) source. An alternative procedure to obtain (99)Tc(V)O2-NHC complexes is the in situ deprotonation of imidazolium salts, enabling the preparation of (99)Tc(V)O2-NHC compounds without free NHCs, thus increasing the scope of NHC ligands drastically. The remarkable stability and pH-controllable reactivity of the new complexes underlines the potential of NHCs as stabilizing ligands for (99)Tc complexes and paves the way for the first (99m)Tc-NHC complexes in the future.

Electrochemistry of 1,1'-bis(2,4-dialkylphosphetanyl)ferrocene and 1,1'-bis(2,5-dialkylphospholanyl)ferrocene ligands: free phosphines, metal complexes, and chalcogenides.

PubMed

Mandell, Chelsea L; Kleinbach, Shannon S; Dougherty, William G; Kassel, W Scott; Nataro, Chip

2010-10-18

The oxidative electrochemistries of a series of chiral bisphosphinoferrocene ligands, 1,1'-bis(2,4-dialkylphosphetanyl)ferrocene (FerroTANE) and 1,1'-bis(2,5-dialkylphospholanyl)ferrocene (FerroLANE), were examined. The reversibility of the oxidation is sensitive to the steric bulk of the alkyl groups. New transition metal compounds and phosphine chalcogenides of these ligands were prepared and characterized. X-ray crystal structures of 10 of these compounds are reported. The percent buried volume (%V(bur)) is a recently developed measurement based on crystallographic data that examines the steric bulk of N-heterocyclic carbene and phosphine ligands. The %V(bur) for the FerroTANE and FerroLANE structures with methyl or ethyl substituents suggests these ligands are similar in steric properties to 1,1'-bis(diphenylphosphino)ferrocene (dppf). In addition the %V(bur) has been found to correlate well with the Tolman cone angle for phosphine chalcogenides. The oxidative electrochemistries of the transition metal complexes occur at more positive potentials than the free ligands. While a similar positive shift is seen for the oxidative electrochemistries of the phosphine chalcogenides, the oxidation of the phosphine selenides does not occur at the iron center, but rather oxidation occurs at the selenium atoms.

Heterobimetallic complexes containing an N-heterocyclic carbene based multidentate ligand and catalyzed tandem click/Sonogashira reactions.

PubMed

Gu, Shaojin; Xu, Daichao; Chen, Wanzhi

2011-02-21

Mono- and polynuclear complexes containing 3-(1,10-phenanthrolin-2-yl)-1-(pyridin-2-ylmethyl)imidazolylidene (L), [NiL(2)](PF(6))(2) (2), [CoL(2)](PF(6))(3) (3), [PtLCl](PF(6)) (4), [PdAgL(2)](PF(6))(3) (5), [PdCuL(2)](PF(6))(3) (6), [Pd(2)L(2)Cl(2)](PF(6))(2) (7), and [Pd(3)L(2)Cl(4)](PF(6))(2) (8) have been prepared and fully characterized by NMR, ESI-MS spectroscopy, and X-ray crystallography. In complexes 2-4, the ligand binds to metals in a pincer NNC fashion with the pyridine group uncoordinated. Complexes 5 and 6 are isostructural to each other in which the palladium ions are surrounded by two pyridines and two imidazolylidenes and Ag(I) or Cu(I) is coordinated by two 1,10-phenanthroline moieties. In the trinuclear palladium complex 8, one palladium ion has an identical coordination mode as in 5 and 6, and the other two palladium ions are bonded to the 1,10-phenanthroline. Complex 6 exhibits excellent catalytic activity for the tandem click/Sonogashira reaction of 1-(bromomethyl)-4-iodobenzene, NaN(3), and ethynylbenzene in which three C-N bonds and one C-C bond are formed in a single flask.

Hydrophobic photolabeling identifies BHA2 as the subunit mediating the interaction of bromelain-solubilized influenza virus hemagglutinin with liposomes at low pH

DOE Office of Scientific and Technical Information (OSTI.GOV)

Harter, C.; Baechi, T.S.; Semenza, G.

1988-03-22

To investigate the molecular basis of the low-pH-mediated interaction of the bromelain-solubilized ectodomain of influenza virus hemagglutinin (BHA) with membranes, we have photolabeled BHA in the presence of liposomes with the two carbene-generating, membrane-directed reagents 3-(trifluoromethyl)-3-(m-(/sup 125/I)iodophenyl)diazirine ((/sup 125/I)TID) and a new analogue of a phospholipid, 1-palmitoyl-2-(11-(4-(3-(trifluoromethyl)diazirinyl)phenyl)(2-/sup 3/H) undecanoyl)-sn-glycero-3-phosphocholine ((/sup 3/H)-PTPC/11). With the latter reagent, BHA was labeled in a strictly pH-dependent manner, i.e., at pH 5 only, whereas with (/sup 125/I)TID, labeling was seen also at pH 7. In all experiments, the label was selectively incorporated into the BHA2 polypeptide, demonstrating that the interaction of BHA with membranes ismore » mediated through this subunit, possibly via its hydrophobic N-terminal segment. Similar experiments with a number of other water-soluble proteins (ovalbumin, carbonic anhydrase, alpha-lactalbumin, trypsin, and soybean trypsin inhibitor) indicate that the ability to interact with liposomes at low pH is not a property specific for BHA but is observed with other, perhaps most, proteins.« less

Organocatalytic upgrading of furfural and 5-hydroxymethyl furfural to C10 and C12 furoins with quantitative yield and atom-efficiency.

PubMed

Zang, Hongjun; Chen, Eugene Y X

2015-03-30

There is increasing interest in the upgrading of C5 furfural (FF) and C6 5-hydroxymethyl furfural (HMF) into C10 and C12 furoins as higher energy-density intermediates for renewable chemicals, materials, and biofuels. This work utilizes the organocatalytic approach, using the in situ generated N,S-heterocyclic carbene catalyst derived from thiazolium ionic liquids (ILs), to achieve highly efficient self-coupling reactions of FF and HMF. Specifically, variations of the thiazolium IL structure have led to the most active and efficient catalyst system of the current series, which is derived from a new thiazolium IL carrying the electron-donating acetate group at the 5-ring position. For FF coupling by this IL (0.1 mol %, 60 °C, 1 h), when combined with Et3N, furoin was obtained in >99% yield. A 97% yield of the C12 furoin was also achieved from the HMF coupling by this catalyst system (10 mol % loading, 120 °C, 3 h). On the other hand, the thiazolium IL bearing the electron-withdrawing group at the 5-ring position is the least active and efficient catalyst. The mechanistic aspects of the coupling reaction by the thiazolium catalyst system have also been examined and a mechanism has been proposed.

The development and catalytic uses of N-heterocyclic carbene gold complexes.

PubMed

Nolan, Steven P

2011-02-15

Gold has emerged as a powerful synthetic tool in the chemist's arsenal. From the early use of inorganic salts such as AuCl and AuCl(3) as catalysts, the field has evolved to explore ligands that fine-tune reactivity, stability, and, more recently, selectivity in gold-mediated processes. Substrates generally contain alkenes or alkynes, and they usually involve straightforward protocols in air with solvents that can often times be of technical grade. The actual catalytic species is the putative cationic gold(I) complex [Au(L)](+) (where L is a phosphorus-based species or N-heterocyclic carbene, NHC). The early gold systems bearing phosphine and phosphite ligands provided important transformations and served as useful mechanistic probes. More recently, the use of NHCs as ligands for gold has rapidly gained in popularity. These two-electron donor ligands combine strong σ-donating properties with a steric profile that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the gold-NHC complexes have been used as well-defined precatalysts and have permitted the isolation of reactive single-component systems that are now used instead of the initial [Au(L)Cl]/silver salt method. Because some are now commercially available, NHC-containing gold(I) complexes are gathering increasing interest. In this Account, we describe the chronological development of this chemistry in our laboratories, highlighting the advantages of this family of gold complexes and reviewing their synthesis and applications in catalysis. We first outline the syntheses, which are straightforward. The complexes generally exhibit high stability, allowing for indefinite storage and easy handling. We next consider catalysis, particularly examining efficacy in cycloisomerization, other skeletal rearrangements, addition of water to alkynes and nitriles, and C-H bond activation. These processes are quite atom-economical, and in the most recent C-H reactions the only byproduct is water. State-of-the-art methodology now involves single-component catalysts, precluding the need for costly silver co-catalysts. Remarkably, the use of an NHC as a supporting ligand has permitted the isolation of [Au(L)(S)](+) species (where S is a solvent molecule such as a nitrile), which can act as single-component catalysts. Some improvements are still needed, as the single components are most often synthesized with a silver reagent. Owing to the stabilizing effect of NHC coordination, some NHC-containing systems can catalyze extremely challenging reactions (at temperatures as high as 140 °C) and react at very low loadings of gold (ppm levels). Our latest developments deal with C-H bond functionalization and hold great promise. We close with a selection of important developments by the community with gold-NHC complexes. As demonstrated by the turns and twists encountered during our own journey in the gold-NHC venture, the chemistry described here, combining fundamental organometallic, catalytic, and organic methodology, remains rich in opportunities, especially considering that only a handful of gold(I) architectures has been studied. We hope this Account will encourage young researchers to explore this emerging area, as the adage "the more you do, the more you have to do" surely holds true in gold-mediated catalysis.

The divergent effects of strong NHC donation in catalysis† †Electronic supplementary information (ESI) available: Rate profiles for decomposition of u-GIIm and s-GIIm; X-ray crystallographic details; NOESY spectra, and derivation of the [PCy3]-independence of decomposition. CCDC 1400077. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc02592c

PubMed Central

Lummiss, Justin A. M.; Higman, Carolyn S.; Fyson, Devon L.; McDonald, Robert

2015-01-01

Strong σ-donation from NHC ligands (NHC = N-heterocyclic carbene) is shown to have profoundly conflicting consequences for the reactivity of transition-metal catalysts. Such donation is regarded as central to high catalyst activity in many contexts, of which the second-generation Grubbs metathesis catalysts (RuCl2(NHC)(PCy3)( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CHPh), GII) offer an early, prominent example. Less widely recognized is the dramatically inhibiting impact of NHC ligation on initiation of GII, and on re-entry into the catalytic cycle from the resting-state methylidene species RuCl2(NHC)(PCy3)( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CH2), GIIm. Both GII and the methylidene complexes are activated by dissociation of PCy3. The impact of NHC donicity on the rate of PCy3 loss is explored in a comparison of s-GIIm, vs.u-GIIm, in which the NHC ligand is saturated H2IMes or unsaturated IMes, respectively. PCy3 loss is nearly an order of magnitude slower for the IMes derivative (a difference that is replicated, albeit smaller, for the benzylidene precatalysts GII). Proposed as an overlooked contributor to these rate differences is an increase in the Ru–PCy3 bond strength arising from π-back-donation onto the phosphine ligand. Strong σ-donation from the IMes ligand, coupled with the inability of this unsaturated NHC to participate in significant π-backbonding, amplifies Ru → PCy3 π-back-donation. The resulting increase in Ru–P bond strength greatly inhibits entry into the active cycle. For s-GII, in contrast, the greater π-acceptor capacity of the NHC ligand enables competing Ru → H2IMes back-donation (as confirmed by NOE experiments, which reveal restricted rotation about the Ru–NHC bond for H2IMes, but not IMes). Ru → PCy3 back-donation is thus attenuated in the H2IMes complexes, accounting for the greater lability of the PCy3 ligand in s-GIIm and s-GII. Similarly inhibited initiation is predicted for other metal–NHC catalysts in which a π-acceptor ligand L must be dissociated to permit substrate binding. Conversely, enhanced reactivity can be expected where such L ligands are pure σ-donors. These effects are expected to be particularly dramatic where the NHC ligand has minimal π-acceptor capacity (as in the unsaturated Arduengo carbenes), and in geometries that maximize NHC–M–L orbital interactions. PMID:29861923

Studies of Metal-Metal Bonded Compounds in Catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berry, John F.

The overall goals of this research are (1) to define the fundamental coordination chemistry underlying successful catalytic transformations promoted by metal-metal bonded compounds, and (2) to explore new chemical transformations that occur at metal-metal bonded sites that could lead to the discovery of new catalytic processes. Transformations of interest include metal-promoted reactions of carbene, nitrene, or nitrido species to yield products with new C–C and C–N bonds, respectively. The most promising suite of transition metal catalysts for these transformations is the set of metal-metal bonded coordination compounds of Ru and Rh of the general formula M 2(ligand) 4, where Mmore » = Ru or Rh and ligand = a monoanionic, bridging ligand such as acetate. Development of new catalysts and improvement of catalytic conditions have been stymied by a general lack of knowledge about the nature of highly reactive intermediates in these reactions, the knowledge that is to be supplied by this work. Our three specific objectives for this year have been (A) to trap, isolate, and characterize new reactive intermediates of general relevance to catalysis, (B) to explore the electronic structure and reactivity of these unusual species, and how these two properties are interrelated, and (C) to use our obtained mechanistic knowledge to design new catalysts with a focus on Earth-abundant first-row transition metal compounds.« less

PAH chemistry at eV internal energies. 1. H-shifted isomers

NASA Astrophysics Data System (ADS)

Trinquier, Georges; Simon, Aude; Rapacioli, Mathias; Gadéa, Florent Xavier

2017-06-01

The PAH family of organic compounds (polycyclic aromatic hydrocarbons), involved in several fields of chemistry, has received particular attention in astrochemistry, where their vibrational spectroscopy, thermodynamics, dynamics, and fragmentation properties are now abundantly documented. This survey aims at drawing trends for low spin-multiplicity surfaces of PAHs bearing internal energies in the range 1-10 eV. It addresses some typical alternatives to the ground-state regular structures of PAHs, making explicit possible intramolecular rearrangements leading to high-lying minima. These isomerisations should be taken into consideration when addressing PAH processing in astrophysical conditions. The first part of this double-entry study focuses on the hydrogen-shifted forms, which bear both a carbene center and a saturated carbon. It rests upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states. Systematically searched for in neutral species, these H-shifted minima are lying 4-5 eV above the regular all-conjugated forms, and are separated by barriers of about 1 eV. General hydrogen-shifting is found to be easier for cationic species as the relative energies of their H-shifted minima are 1-1.5 eV lower than those for neutral species. As much as possible, classical knowledge and concepts of organic chemistry such as aromaticity and Clar's rules are invoked for result interpretation.

Ancillary ligand effects upon dithiolene redox noninnocence in tungsten bis(dithiolene) complexes.

PubMed

Yan, Yong; Keating, Christopher; Chandrasekaran, Perumalreddy; Jayarathne, Upul; Mague, Joel T; DeBeer, Serena; Lancaster, Kyle M; Sproules, Stephen; Rubtsov, Igor V; Donahue, James P

2013-06-03

An expanded set of compounds of the type [W(S2C2Me2)2L1L2](n) (n = 0: L1 = L2 = CO, 1; L1 = L2 = CN(t)Bu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2-: L1 = L2 = CN(-), [6](2-)) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, υ(CCchelate), and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, 1 and [6](2-), are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO π* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via π-backbonding. The CN(-) π* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene π* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s).

Insights on the mechanism of thioredoxin reductase inhibition by gold N-heterocyclic carbene compounds using the synthetic linear selenocysteine containing C-terminal peptide hTrxR(488-499): an ESI-MS investigation.

PubMed

Pratesi, Alessandro; Gabbiani, Chiara; Michelucci, Elena; Ginanneschi, Mauro; Papini, Anna Maria; Rubbiani, Riccardo; Ott, Ingo; Messori, Luigi

2014-07-01

Gold-based drugs typically behave as strong inhibitors of the enzyme thioredoxin reductase (hTrxR), possibly as the consequence of direct Gold(I) coordination to its active site selenocysteine. To gain a deeper insight into the molecular basis of enzyme inhibition and prove gold-selenocysteine coordination, the reactions of three parent Gold(I) NHC compounds with the synthetic C-terminal dodecapeptide of hTrxR containing Selenocysteine at position 498, were investigated by electrospray ionization mass spectrometry (ESI-MS). Formation of 1:1 Gold-peptide adducts, though in highly different amounts, was demonstrated in all cases. In these adducts the same [Au-NHC](+) moiety is always associated to the intact peptide. Afterward, tandem MS experiments, conducted on a specific Gold-peptide complex, pointed out that Gold is coordinated to the selenolate group. The relatively large strength of the Gold-selenolate coordinative bond well accounts for potent enzyme inhibition typically afforded by these Gold(I) compounds. In a selected case, the time course of enzyme inhibition was explored. Interestingly, enzyme inhibition turned out to show up very quickly and reached its maximum just few minutes after mixing. Overall, the present results offer some clear insight into the process of thioredoxin reductase inhibition by Gold-based compounds. Copyright © 2014 Elsevier Inc. All rights reserved.

Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles.

PubMed

Wentrup, Curt

2017-03-08

Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N 2 elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5-aryltetrazoles can result in double N 2 elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2,5-disubstituted tetrazoles are excellent precursors of nitrile imines (propargylic, allenic, or carbenic), which are isolable at low temperatures in some cases (e.g., aryl- and silylnitrile imines) or rearrange to carbodiimides. 1,5-Disubstituted tetrazoles are precursors of imidoylnitrenes, which also rearrange to carbodiimides or add intramolecularly to aryl substituents to yield indazoles and related compounds. Where relevant for the mechanistic understanding, pyrolysis under flow conditions or in solution or the solid state will be mentioned. Results of photolysis reactions and computational chemistry complementing the FVP results will also be mentioned in several places.

Expanding the Enzyme Universe: Accessing Non-Natural Reactions by Mechanism-Guided Directed Evolution

PubMed Central

Renata, Hans; Wang, Z. Jane

2015-01-01

High selectivities and exquisite control over reaction outcomes entice chemists to use biocatalysts in organic synthesis. However, many useful reactions are not accessible because they are not in nature’s known repertoire. We will use this review to outline an evolutionary approach to engineering enzymes to catalyze reactions not found in nature. We begin with examples of how nature has discovered new catalytic functions and how such evolutionary progressions have been recapitulated in the laboratory starting from extant enzymes. We then examine non-native enzyme activities that have been discovered and exploited for chemical synthesis, emphasizing reactions that do not have natural counterparts. The new functions have mechanistic parallels to the native reaction mechanisms that often manifest as catalytic promiscuity and the ability to convert from one function to the other with minimal mutation. We present examples of how non-natural activities have been improved by directed evolution, mimicking the process used by nature to create new catalysts. Examples of new enzyme functions include epoxide opening reactions with non-natural nucleophiles catalyzed by a laboratory-evolved halohydrin dehalogenase, cyclopropanation and other carbene transfer reactions catalyzed by cytochrome P450 variants, and non-natural modes of cyclization by a modified terpene synthase. Lastly, we describe discoveries of non-native catalytic functions that may provide future opportunities for expanding the enzyme universe. PMID:25649694

Organocatalytic Upgrading of Furfural and 5-Hydroxymethyl Furfural to C10 and C12 Furoins with Quantitative Yield and Atom-Efficiency

PubMed Central

Zang, Hongjun; Chen, Eugene Y. X.

2015-01-01

There is increasing interest in the upgrading of C5 furfural (FF) and C6 5-hydroxymethyl furfural (HMF) into C10 and C12 furoins as higher energy-density intermediates for renewable chemicals, materials, and biofuels. This work utilizes the organocatalytic approach, using the in situ generated N,S-heterocyclic carbene catalyst derived from thiazolium ionic liquids (ILs), to achieve highly efficient self-coupling reactions of FF and HMF. Specifically, variations of the thiazolium IL structure have led to the most active and efficient catalyst system of the current series, which is derived from a new thiazolium IL carrying the electron-donating acetate group at the 5-ring position. For FF coupling by this IL (0.1 mol %, 60 °C, 1 h), when combined with Et3N, furoin was obtained in >99% yield. A 97% yield of the C12 furoin was also achieved from the HMF coupling by this catalyst system (10 mol % loading, 120 °C, 3 h). On the other hand, the thiazolium IL bearing the electron-withdrawing group at the 5-ring position is the least active and efficient catalyst. The mechanistic aspects of the coupling reaction by the thiazolium catalyst system have also been examined and a mechanism has been proposed. PMID:25830482

Catalysts For Hydrogenation And Hydrosilylation Methods Of Making And Using The Same

DOEpatents

Dioumaev, Vladimir K.; Bullock, R. Morris

2004-05-18

A compound is provided including an organometallic complex represented by the formula I: wherein M is an atom of molybdenum or tangsten, Cp is substituted or unsubstituted cyclopentadienyl radical represented by the formula [C.sub.5 Q.sup.1 Q.sup.2 Q.sup.3 Q.sup.4 Q.sup.5 ], wherein Q.sup.1 to Q.sup.5 are independently selected from the group consisting of H radical, C.sub.1-20 hydrocarbyl radical, substituted hydrocarbyl radical, halogen radical, halogen-substituted hydrocarbyl radical, --OR, --C(O)R', --CO.sub.2 R', --SiR'.sub.3 and --NR'R", wherein R' and R" are independently selected from the group consisting of H radical, C.sub.1-20 hydrocarbyl radical, halogen radical, and halogen-substituted hydrocarbyl radical, wherein said Q.sup.1 to Q.sup.5 radicals are optionally linked to each other to form a stable bridging group, NHC is any N-heterocyclic carbene ligand, L is either any neutral electron donor ligand, wherein k is a number from 0 to 1 or L is an anionic ligand wherein k is 2, and A.sup.- is an anion. Processes using the organometallic complex as catalyst for hydrogenation of aldehydes and ketones are provided. Processes using the organometallic complex as catalyst for the hydrosilylation of aldehydes, ketones and esters are also provided.

Mild partial deoxygenation of esters catalyzed by an oxazolinylborate-coordinated rhodium silylene

DOE PAGES

Xu, Songchen; Boschen, Jeffery S.; Biswas, Abhranil; ...

2015-08-17

An electrophilic, coordinatively unsaturated rhodium complex supported by borate-linked oxazoline, oxazoline-coordinated silylene, and N-heterocyclic carbene donors [{κ³-N,Si,C-PhB(Ox Me²)(Ox Me²SiHPh)Im Mes}Rh(H)CO][HB(C₆F₅)₃] (2, Ox Me² = 4,4-dimethyl-2-oxazoline; Im Mes = 1-mesitylimidazole) is synthesized from the neutral rhodium silyl {PhB(Ox Me²)₂Im Mes}RhH(SiH 2Ph)CO (1) and B(C 6F 5) 3. The unusual oxazoline-coordinated silylene structure in 2 is proposed to form by rearrangement of an unobserved isomeric cationic rhodium silylene species [{PhB(Ox Me²)₂Im Mes}RhH(SiHPh)CO][HB(C₆F₅)₃] generated by H abstraction. Complex 2 catalyzes reductions of organic carbonyl compounds with silanes to give hydrosilylation products or deoxygenation products. The pathway to these reactions is primarily influenced bymore » the degree of substitution of the organosilane. Reactions with primary silanes give deoxygenation of esters to ethers, amides to amines, and ketones and aldehydes to hydrocarbons, whereas tertiary silanes react to give 1,2-hydrosilylation of the carbonyl functionality. In contrast, the strong Lewis acid B(C₆F₅)₃ catalyzes the complete deoxygenation of carbonyl compounds to hydrocarbons with PhSiH₃ as the reducing agent.« less

Mild partial deoxygenation of esters catalyzed by an oxazolinylborate-coordinated rhodium silylene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Songchen; Boschen, Jeffery S.; Biswas, Abhranil

An electrophilic, coordinatively unsaturated rhodium complex supported by borate-linked oxazoline, oxazoline-coordinated silylene, and N-heterocyclic carbene donors [{κ³-N,Si,C-PhB(Ox Me²)(Ox Me²SiHPh)Im Mes}Rh(H)CO][HB(C₆F₅)₃] (2, Ox Me² = 4,4-dimethyl-2-oxazoline; Im Mes = 1-mesitylimidazole) is synthesized from the neutral rhodium silyl {PhB(Ox Me²)₂Im Mes}RhH(SiH 2Ph)CO (1) and B(C 6F 5) 3. The unusual oxazoline-coordinated silylene structure in 2 is proposed to form by rearrangement of an unobserved isomeric cationic rhodium silylene species [{PhB(Ox Me²)₂Im Mes}RhH(SiHPh)CO][HB(C₆F₅)₃] generated by H abstraction. Complex 2 catalyzes reductions of organic carbonyl compounds with silanes to give hydrosilylation products or deoxygenation products. The pathway to these reactions is primarily influenced bymore » the degree of substitution of the organosilane. Reactions with primary silanes give deoxygenation of esters to ethers, amides to amines, and ketones and aldehydes to hydrocarbons, whereas tertiary silanes react to give 1,2-hydrosilylation of the carbonyl functionality. In contrast, the strong Lewis acid B(C₆F₅)₃ catalyzes the complete deoxygenation of carbonyl compounds to hydrocarbons with PhSiH₃ as the reducing agent.« less

Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers.

PubMed

Liu, Yizhu; Persson, Petter; Sundström, Villy; Wärnmark, Kenneth

2016-08-16

The photophysics and photochemistry of transition metal complexes (TMCs) has long been a hot field of interdisciplinary research. Rich metal-based redox processes, together with a high variety in electronic configurations and excited-state dynamics, have rendered TMCs excellent candidates for interconversion between light, chemical, and electrical energies in intramolecular, supramolecular, and interfacial arrangements. In specific applications such as photocatalytic organic synthesis, photoelectrochemical cells, and light-driven supramolecular motors, light absorption by a TMC-based photosensitizer and subsequent excited-state energy or electron transfer constitute essential steps. In this context, TMCs based on rare and expensive metals, such as ruthenium and iridium, are frequently employed as photosensitizers, which is obviously not ideal for large-scale implementation. In the search for abundant and environmentally benign solutions, six-coordinate Fe(II) complexes (Fe(II)L6) have been widely considered as highly desirable alternatives. However, not much success has been achieved due to the extremely short-lived triplet metal-to-ligand charge transfer ((3)MLCT) excited state that is deactivated by low-lying metal-centered (MC) states on a 100 fs time scale. A fundamental strategy to design useful Fe-based photosensitizers is thus to destabilize the MC states relative to the (3)MLCT state by increasing the ligand field strength, with special focus on making eg σ* orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from Ru(II)L6 complexes unfortunately met with limited success in this regard, despite their close chemical kinship. In this Account, we summarize recent promising results from our and other groups in utilizing strongly σ-donating N-heterocyclic carbene (NHC) ligands to make strong-field Fe(II)L6 complexes with significantly extended (3)MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the (3)MLCT lifetime by more than 2 orders of magnitude compared to the parental [Fe(tpy)2](2+) (tpy = 2,2':6',2″-terpyridine) complex. Quantum chemical (QC) studies also revealed that the (3)MC instead of the (5)MC state likely dictates the deactivation of the (3)MLCT state, a behavior distinct from traditional Fe(II)L6 complexes but rather resembling Ru analogues. A heteroleptic Fe(II) NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the (3)MLCT lifetime relative to its parental [Fe(bpy)3](2+) (bpy = 2,2'-bipyridine) complex. Again, a Ru-like deactivation mechanism of the (3)MLCT state was indicated by QC studies. With a COOH-functionalized homoleptic complex, a record (3)MLCT lifetime of 37 ps was recently observed on an Al2O3 nanofilm. As a proof of concept, it was further demonstrated that the significant improvement in the (3)MLCT lifetime indeed benefits efficient light harvesting with Fe(II) NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy (3)MLCT state to a TiO2 nanofilm was achieved by a stable Fe(II) complex. This is in complete contrast to conventional Fe(II)L6-derived photosensitizers that could only make use of high-energy photons. These exciting results significantly broaden the understanding of the fundamental photophysics and photochemistry of d(6) Fe(II) complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element.

Metallosupramolecular Architectures Obtained from Poly-N-heterocyclic Carbene Ligands.

PubMed

Sinha, Narayan; Hahn, F Ekkehardt

2017-09-19

Over the past two decades, self-assembly of supramolecular architectures has become a field of intensive research due to the wide range of applications for the resulting assemblies in various fields such as molecular encapsulation, supramolecular catalysis, drug delivery, metallopharmaceuticals, chemical and photochemical sensing, and light-emitting materials. For these purposes, a large number of coordination-driven metallacycles and metallacages featuring different sizes and shapes have been prepared and investigated. Almost all of these are Werner-type coordination compounds where metal centers are coordinated by nitrogen and/or oxygen donors of polydentate ligands. With the evolving interest in the coordination chemistry of N-heterocyclic carbenes (NHCs), discrete supramolecular complexes held together by M-C NHC bonds have recently become of interest. The construction of such metallosupramolecular assemblies requires the synthesis of suitable poly-NHC ligands where the NHC donors form labile bonds with metal centers thus enabling the formation of the thermodynamically most stable reaction product. In organometallic chemistry, these conditions are uniquely met by the combination of poly-NHCs and silver(I) ions where the resulting assemblies also offer the possibility to generate new structures by transmetalation of the poly-NHC ligands to additional metal centers forming more stable C NHC -M bonds. Stable metallosupramolecular assemblies obtained from poly-NHC ligands feature special properties such as good solubility in many less polar organic solvents and the presence of the often catalyticlly active {M(NHC) n } moiety as building block. In this Account, we review recent developments in organometallic supramolecular architectures derived from poly-NHC ligands. We describe dinuclear (M = Ag I , Au I , Cu I ) tetracarbene complexes obtained from bis-NHC ligands with an internal olefin or two external coumarin pendants and their postsynthetic modification via a photochemically induced single or double [2 + 2] cycloaddition to form dinuclear tetracarbene complexes featuring cyclobutane units. Even three-dimensional cage-like structures can be prepared by this postsynthetic strategy. Cylinder-like trinuclear, tetranuclear, and hexanuclear (M = Ag I , Au I , Cu I , Hg II , Pd II ) complexes have been obtained from benzene-bridged tris-, tetrakis-, or hexakis-NHC ligands. These complexes resemble polynuclear assemblies obtained from related polydentate Werner-type ligands. Contrary to the Werner-type complexes, cylinder-like assemblies with three, four, or six silver(I) ions sandwiched in between two tris-, tetrakis-, or hexakis-NHC ligands undergo a facile transmetalation reaction to give the complexes featuring more stable M-C NHC bonds, normally with retention of the metallosupramolecular structure. This unique behavior of NHC-Ag + complexes allows the prepration of assemblies containing various metals from the poly-NHC silver(I) assemblies. Narcissistic self-sorting phenomena have also been observed for mixtures of selected poly-NHC ligands and silver(I) ions. Even a very early type of metallosupramolecular assembly, the tetranuclear molecular square, can be prepared from four bridging dicarbene ligands and four transition metal ions either by a stepwise assembly or by a single-step protocol. At this point, it appears that procedures for the synthesis of metallosupramolecular assemblies using polydentate Werner-type ligands and metal ions can be transferred to organometallic chemistry by using suitable poly-NHC ligands. The resulting structures feature stable M-C NHC bonds (with the exception of the labile C NHC -Ag + bond) when compared to M-N/M-O bonds in classical Werner-type complexes. The generally good solubility of the compounds and the presence of the often catalytically active {M(NHC) n } moiety make organometallic supramolecular complexes a promising new class of molecular hosts for catalytic transformations and encapsulation of selected substrates.

Summary of Research/Publications

NASA Technical Reports Server (NTRS)

1997-01-01

Summary of research/publications include:(1) Comment on broadening of water microwave lines by collisions with helium atoms; (2) Calculations of ion-molecule deuterium fractionation reactions involving HD; (3) Ab initio predictions on the rotational spectra of carbon-chain carbene molecules; (4) Theoretical IR spectra of ionized naphthalene; (5) Improved collisional excitation rates for interstellar water; (6) Calculations on the competition between association and reaction for C3H+ + H2; (7) Theoretical infrared spectra of some model polycyclic aromatic hydrocarbons: effect of ionization; (8) Calculations concerning interstellar isomeric abundance ratios for C3H and C3H2; (9) New calculations on the ion-molecule processes C2H2+ + H2 C2H3+ + H and C2H2+ + H2 C2H4+; (10) Anisotropic rigid rotor potential energy function for H2O-H2; (11) A correlated ab initio study of linear carbon-chain radicals CnH (n=2-7); (12) Ab initio characterization of MgCCH, MgCCH+, and MgC2 and pathways to their formation in the interstellar medium; (13) Why HOC+ is detectable in interstellar clouds: The rate of the reaction between HOC+ and H2; (14) A correlated ab initio study of the X 2A 1 and A 2E states of MgCH3; (15) On the stability of interstellar carbon clusters: The rate of the reaction between C3 and O; and (16) The rate of the reaction between CN and C2H2 at interstellar temperatures.

Heterobimetallic N-Heterocyclic Carbene Complexes: A Synthetic, Spectroscopic, and Theoretical Study.

PubMed

Pell, Thomas P; Wilson, David J D; Skelton, Brian W; Dutton, Jason L; Barnard, Peter J

2016-07-18

A new synthetic methodology has been developed for the preparation of heterobimetallic group 11 and group 12 complexes of a symmetrical bis-NHC "pincer" ligand. The synthetic route involved the initial preparation of a mononuclear [Au(NHC)2](+) complex with pendent imidazole moieties on the NHC ligands. Subsequent alkylation of the imidazole groups with Et3OBF4 and metalation with a second metal ion (Ag(I) or Hg(II)) provided two heterobimetallic complexes. Four homobimetallic (Cu(I)2, Ag(I)2, Au(I)2, and Hg(II)2) complexes of the same bis-NHC "pincer" ligand were also prepared. The homobimetallic Cu(I)2, Au(I)2, and Hg(II)2 complexes and heterobimetallic Au(I)-Ag(I) and Au(I)-Hg(II) complexes and the synthetic intermediates for the heterobimetallic complexes were characterized by X-ray crystallography. These X-ray structures show that the bimetallic complexes adopt "twisted" conformations in the solid state, supporting short M···M interactions. Crystalline samples of the homobimetallic Ag(I)2 and Au(I)2 and heterobimetallic Au(I)-Ag(I) and Au(I)-Hg(II) complexes were emissive at room temperature and at 77 K. The geometries of the synthesized complexes were optimized at the M06-L/def2-SVP level of theory, and the electronic nature of the M···M interactions for all synthesized complexes was investigated using natural bond orbital (NBO) calculations.

Heterobimetallic Activation of Dioxygen

PubMed Central

York, John T.; Young, Victor G.; Tolman, William B.

2008-01-01

Reaction of the known germylene Ge[N(SiMe3)2]2 and a new heterocyclic variant Ge[(NMes)2(CH)2] with [LMe2 Cu]2 (LMe2 = the β -diketiminate derived from 2-(2,6-dimethylphenyl)amino-4-(2,6-dimethylphenyl)imino-2-pentene) yielded novel Cu(I)-Ge(II) complexes LMe2Cu-Ge[(NMes)2(CH)2] (1a) and LMe2Cu-Ge[N(SiMe3)2]2 (1b), which were characterized by spectroscopy and X-ray crystallography. The lability of the Cu(I)-Ge(II) bond in 1a and b was probed by studies of their reactivity with benzil, PPh3, and an N-heterocyclic carbene (NHC). Notably, both complexes are cleaved rapidly by PPh3 and the NHC to yield stable Cu(I) adducts (characterized by X-ray diffraction) and the free germylene. In addition, the complexes are highly reactive with O2 and exhibit chemistry which depends on the bound germylene. Thus, oxygenation of 1a results in scission and formation of thermally unstable LMe2CuO2, which subsequently decays to [(LMe2Cu)2(μ-O)2], while 1b yields LMe2Cu(μ-O)2Ge[N(SiMe3)2]2, a novel heterobimetallic intermediate having [CuIII(μ-O)2GeIV]3+ core. The isolation of the latter species by direct oxygenation of a Cu(I)-Ge(II) precursor represents a new route to heterobimetallic oxidants comprising copper. PMID:16676981

Exploring the Transphobia Effect on Heteroleptic NHC Cycloplatinated Complexes.

PubMed

Fuertes, Sara; Chueca, Andrés J; Sicilia, Violeta

2015-10-19

The synthesis of 1-(4-cyanophenyl)-1H-imidazol (1) has been carried out by an improved method. Then its corresponding imidazolium iodide salt, 2, has been used to prepare the N-heterocyclic carbene (NHC) cycloplatinated compound [{Pt(μ-Cl)(C^C*)}2] (4) (HC^C*-κC* = 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene) following a step-by-step protocol. The intermediate complex [PtCl(η(3)-2-Me-C3H4) (HC^C*-κC*)] (3) has also been isolated and characterized. Using 4 as precursor, several heteroleptic complexes of stoicheometry [PtCl(C^C*)L] (L = PPh3 (5), pyridine (py, 6), 2,6-dimethylphenyl isocyanide (CNXyl, 7), and 2-mercapto-1-methylimidazole (MMI, 8)) and [Pt(C^C*)LL']PF6 (L = PPh3, L' = py (9), CNXyl (10), and MMI (11)) have been synthesized. Complexes 6-8 were obtained as a mixture of cis- and trans-(C*,L) isomers, while trans-(C*,L) isomer was the only one observed for complexes 5 and 9-11. Their geometries have been discussed in terms of the degree of transphobia (T) of pairs of trans ligands and supported by theoretical calculations. The trans influence of the two σ Pt-C bonds present in these molecules, Pt-C(Ar) and Pt-C*(NHC), has been compared from the J(Pt-P) values observed in the new complex [Pt(C^C*)(dppe)]PF6 (dppe = 1, 2-bis(diphenylphosphino)ethane, 12).

Generating carbyne equivalents with photoredox catalysis

NASA Astrophysics Data System (ADS)

Wang, Zhaofeng; Herraiz, Ana G.; Del Hoyo, Ana M.; Suero, Marcos G.

2018-02-01

Carbon has the unique ability to bind four atoms and form stable tetravalent structures that are prevalent in nature. The lack of one or two valences leads to a set of species—carbocations, carbanions, radicals and carbenes—that is fundamental to our understanding of chemical reactivity. In contrast, the carbyne—a monovalent carbon with three non-bonded electrons—is a relatively unexplored reactive intermediate; the design of reactions involving a carbyne is limited by challenges associated with controlling its extreme reactivity and the lack of efficient sources. Given the innate ability of carbynes to form three new covalent bonds sequentially, we anticipated that a catalytic method of generating carbynes or related stabilized species would allow what we term an ‘assembly point’ disconnection approach for the construction of chiral centres. Here we describe a catalytic strategy that generates diazomethyl radicals as direct equivalents of carbyne species using visible-light photoredox catalysis. The ability of these carbyne equivalents to induce site-selective carbon-hydrogen bond cleavage in aromatic rings enables a useful diazomethylation reaction, which underpins sequencing control for the late-stage assembly-point functionalization of medically relevant agents. Our strategy provides an efficient route to libraries of potentially bioactive molecules through the installation of tailored chiral centres at carbon-hydrogen bonds, while complementing current translational late-stage functionalization processes. Furthermore, we exploit the dual radical and carbene character of the generated carbyne equivalent in the direct transformation of abundant chemical feedstocks into valuable chiral molecules.

Halogen bonding from a hard and soft acids and bases perspective: investigation by using density functional theory reactivity indices.

PubMed

Pinter, Balazs; Nagels, Nick; Herrebout, Wouter A; De Proft, Frank

2013-01-07

Halogen bonds between the trifluoromethyl halides CF(3)Cl, CF(3)Br and CF(3)I, and dimethyl ether, dimethyl sulfide, trimethylamine and trimethyl phosphine were investigated using Pearson's hard and soft acids and bases (HSAB) concept with conceptual DFT reactivity indices, the Ziegler-Rauk-type energy-decomposition analysis, the natural orbital for chemical valence (NOCV) framework and the non-covalent interaction (NCI) index. It is found that the relative importance of electrostatic and orbital (charge transfer) interactions varies as a function of both the donor and acceptor molecules. Hard and soft interactions were distinguished and characterised by atomic charges, electrophilicity and local softness indices. Dual-descriptor plots indicate an orbital σ hole on the halogen similar to the electrostatic σ hole manifested in the molecular electrostatic potential. The predicted high halogen-bond-acceptor affinity of N-heterocyclic carbenes was evidenced in the highest complexation energy for the hitherto unknown CF(3) I·NHC complex. The dominant NOCV orbital represents an electron-density deformation according to a n→σ*-type interaction. The characteristic signal found in the reduced density gradient versus electron-density diagram corresponds to the non-covalent interaction between contact atoms in the NCI plots, which is the manifestation of halogen bonding within the NCI theory. The unexpected C-X bond strengthening observed in several cases was rationalised within the molecular orbital framework. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Diastereoselective Radical‐Type Cyclopropanation of Electron‐Deficient Alkenes Mediated by the Highly Active Cobalt(II) Tetramethyltetraaza[14]annulene Catalyst

PubMed Central

Chirila, Andrei; Gopal Das, Braja; Paul, Nanda D.

2017-01-01

Abstract A new protocol for the catalytic synthesis of cyclopropanes using electron‐deficient alkenes is presented, which is catalysed by a series of affordable, easy to synthesise and highly active substituted cobalt(II) tetraaza[14]annulenes. These catalysts are compatible with the use of sodium tosylhydrazone salts as precursors to diazo compounds in one‐pot catalytic transformations to afford the desired cyclopropanes in almost quantitative yields. The reaction takes advantage of the metalloradical character of the Co complexes to activate the diazo compounds. The reaction is practical and fast, and proceeds from readily available starting materials. It does not require the slow addition of diazo reagents or tosylhydrazone salts or heating and tolerates many solvents, which include protic ones such as MeOH. The CoII complexes derived from the tetramethyltetraaza[14]annulene ligand are easier to prepare than cobalt(II) porphyrins and present a similar catalytic carbene radical reactivity but are more active. The reaction proceeds at 20 °C in a matter of minutes and even at −78 °C in a few hours. The catalytic system is robust and can operate with either the alkene or the diazo reagent as the limiting reagent, which inhibits the dimerisation of diazo compounds totally. The protocol has been applied to synthesise a variety of substituted cyclopropanes. High yields and selectivities were achieved for various substrates with an intrinsic preference for trans cyclopropanes. PMID:28529668

Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution.

PubMed

Liu, Yuesheng; Luo, Lun; Xiao, Jie; Wang, Lei; Song, You; Qu, Jingping; Luo, Yi; Deng, Liang

2015-05-18

The salt elimination reactions of (IPr2Me2)2FeCl2 (IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with the corresponding aryl Grignard reagents afford [(IPr2Me2)2FeAr2] (Ar = Ph, 3; C6H4-p-Me, 4; C6H4-p-(t)Bu, 5; C6H3-3,5-(CF3)2, 6) in good yields. X-ray crystallographic studies revealed the presence of both tetrahedral and trans square planar isomers for 3 and 6 and the tetrahedral structures for 4 and 5. Magnetic susceptibility and (57)Fe Mössbauer spectrum measurements on the solid samples indicated the high-spin (S = 2) and intermediate-spin (S = 1) nature of the tetrahedral and square planar structures, respectively. Solution property studies, including solution magnetic susceptibility measurement, variable-temperature (1)H and (19)F NMR, and absorption spectroscopy, on 3-6, as well as an (57)Fe Mössbauer spectrum study on a frozen tetrahydrofuran solution of tetrahedral [(IPr2Me2)2(57)FePh2] suggest the coexistence of tetrahedral and trans square planar structures in solution phase. Density functional theory calculations on (IPr2Me2)2FePh2 disclosed that the tetrahedral and trans square planar isomers are close in energy and that the geometry isomerization can occur by spin-change-coupled geometric transformation on four-coordinate iron(II) center.

Synthesis of Quaternary Carbon Stereogenic Centers through Enantioselective Cu-Catalyzed Allylic Substitutions with Vinylaluminum Reagents

PubMed Central

Gao, Fang; McGrath, Kevin P.; Lee, Yunmi; Hoveyda, Amir H.

2010-01-01

Catalytic enantioselective allylic substitution (EAS) reactions, which involve the use of alkyl- or aryl-substituted vinylaluminum reagents and afford 1,4-dienes containing a quaternary carbon stereogenic center at their C-3 site, are disclosed. The C–C bond forming transformations are promoted by 0.5–2.5 mol % of sulfonate bearing chiral bidentate N-heterocyclic carbene (NHC) complexes, furnishing the desired products efficiently (66–97% yield of isolated products) and in high site- (>98% SN2′) and enantioselectivity [up to 99:1 enantiomer ratio (er)]. To the best of our knowledge, the present report puts forward the first cases of allylic substitution reactions that result in the generation of all-carbon quaternary stereogenic centers through the addition of a vinyl unit. The aryl- and vinyl-substituted vinylaluminum reagents, which cannot be prepared in high efficiency through direct reaction with diisobutylaluminum hydride, are accessed through a recently introduced Ni-catalyzed reaction of the corresponding terminal alkynes with the same inexpensive metal-hydride agent. Sequential Ni-catalyzed hydrometallations and Cu-catalyzed C–C bond forming reactions allow for efficient and selective synthesis of a range of enantiomerically enriched EAS products, which cannot cannot be accessed by previously disclosed strategies (due to inefficient vinylmetal synthesis or low reactivity and/or selectivity with Si-substituted derivatives). The utility of the protocols developed is demonstrated through a concise enantioselective synthesis of natural product bakuchiol. PMID:20860365

Organic reactions catalyzed by methylrhenium trioxide: Reactions of ethyl diazoacetate and organic azides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Z.; Espenson, J.H.

1996-10-16

Methylrhenium trioxide (CH{sub 3}ReO{sub 3} or MTO) catalyzes several classes of reactions of ethyl diazoacetate, EDA. It is the first high valent oxo complex for carbene transfer. Under mild conditions and in the absence of other substrates, EDA was converted to a 9:1 mixture of diethyl maleate and diethyl fumarate. In the presence of alcohols, {alpha}-alkoxy ethyl acetates were obtained in good yield. The yields dropped for the larger and more branched alcohols, the balance of material being diethyl maleate and fumarate. An electron-donating group in the para position of phenols favors the formation of {alpha}-phenoxy ethyl acetates. The usemore » of EDA to form {alpha}-thio ethyl acetates and N-substituted glycine ethyl esters, on the other hand, is hardly affected by the size or structure of the parent thiol or amine, with all of these reactions proceeding in high yield. MTO-catalyzed cycloaddition reactions occur between EDA and aromatic imines, olefins, and carbonyl compounds. Three-membered ring products are formed: aziridines, cyclopropanes, and epoxides, respectively. The reactions favor the formation of trans products, and provide a convenient route for the preparation of aziridines. Intermediate carbenoid and nitrenoid species have been proposed. In the presence of an oxygen source such as an epoxide, ethyl diazoacetate and azibenzil are converted to an oxalic acid monoethyl ester and to benzil; at the same time the epoxide was converted to an olefin. 75 refs., 1 fig., 7 tabs.« less

Gold(I)-assisted catalysis - a comprehensive view on the [3,3]-sigmatropic rearrangement of allyl acetate

NASA Astrophysics Data System (ADS)

Freindorf, Marek; Cremer, Dieter; Kraka, Elfi

2018-03-01

The unified reaction valley approach (URVA) combined with the local mode, ring puckering and electron density analysis is applied to elucidate the mechanistic differences of the non-catalysed and the Au[I]-N-heterocyclic carbene (NHC)-catalysed [3,3]-sigmatropic rearrangement of allyl acetate. Using a dual-level approach (DFT and DLPNO-CCSD(T)), the influence of solvation, counter-ions, bulky and electron withdrawing/donating substituents as well as the exchange of the Au[I]-NHC with a Au[I]-phosphine catalyst is investigated. The catalyst breaks up the rearrangement into two steps by switching between Au[I]-π and Au[I]-σ complexation, thus avoiding the energy-consuming CO cleavage in the first step. Based on local stretching force constants ka(C=C), we derive for the first time a quantitative measure of the π-acidity of the Au[I] catalyst; in all catalysed reactions, the bond order n(C=C) drops from 2 to 1.65. The ring puckering analysis clarifies that all reactions start and end via a six-membered ring with a boat form. All Au[I]-σ-complex intermediates show a considerable admixture of the chair form. The non-catalysed [3,3]-sigmatropic rearrangement goes through a maximum of charge separation between the allyl and acetate units at the transition state, while all catalysed reactions proceed via a minimum of charge separation reached in the region of the Au[I]-σ-complex.

Engineered Protein Coatings to Improve the Osseointegration of Dental and Orthopaedic Implants

PubMed Central

Raphel, Jordan; Karlsson, Johan; Galli, Silvia; Wennerberg, Ann; Lindsay, Christopher; Haugh, Matthew; Pajarinen, Jukka; Goodman, Stuart B.; Jimbo, Ryo; Andersson, Martin; Heilshorn, Sarah C.

2016-01-01

Here we present the design of an engineered, elastin-like protein (ELP) that is chemically modified to enable stable coatings on the surfaces of titanium-based dental and orthopaedic implants by novel photocrosslinking and solution processing steps. The ELP includes an extended RGD sequence to confer bio-signaling and an elastin-like sequence for mechanical stability. ELP thin films were fabricated on cp-Ti and Ti6Al4V surfaces using scalable spin and dip coating processes with photoactive covalent crosslinking through a carbene insertion mechanism. The coatings withstood procedures mimicking dental screw and hip replacement stem implantations, a key metric for clinical translation. They promoted rapid adhesion of MG63 osteoblast-like cells, with over 80% adhesion after 24 hours, compared to 38% adhesion on uncoated Ti6Al4V. MG63 cells produced significantly more mineralization on ELP coatings compared to uncoated Ti6Al4V. Human bone marrow mesenchymal stem cells (hMSCs) had an earlier increase in alkaline phosphatase activity, indicating more rapid osteogenic differentiation and mineral deposition on adhesive ELP coatings. Rat tibia and femur in vivo studies demonstrated that cell-adhesive ELP-coated implants increased bone-implant contact area and interfacial strength after one week. These results suggest that ELP coatings withstand surgical implantation and promote rapid osseointegration, enabling earlier implant loading and potentially preventing micromotion that leads to aseptic loosening and premature implant failure. PMID:26790146

Multifunctional Fischer Aminocarbene Complexes as Hole or Electron Transporting Layers in Organic Solar Cells.

PubMed

Vidal-García, Pablo; Sánchez-Vergara, María Elena; Corona-Sánchez, Ricardo; Jiménez-Sandoval, Omar; Mercado, Efraín Gutiérrez-Rivas; Toscano, Rubén A; Álvarez-Toledano, Cecilio

2018-03-24

A new series of Fischer carbenes have been synthetized and examined as hole-transporting or electron-transporting layers (HTLs or ETLs) in the fabrication of organic solar cells (OSCs). The synthesis of three Fischer aminocarbene complexes with the general formula [Cr(CO)₅{C(NHCH₂)Ar}] (Ar = 2-pyridyl ( 3a ), 3-pyridyl ( 3b ) and 4-pyridyl ( 3c )) is reported. The molecular structure of complex 3b has been confirmed by X-ray analysis. In order to study the possible applications of the three Fischer aminocarbenes in OSCs, thin films of these complexes were prepared using a vacuum deposition process. These organometallic films were chemically and morphologically characterized by IR spectroscopy, SEM, AFM and XRD. According to the IR and Tauc analysis, the vacuum deposition process generates thin films free of impurities with an activation energy of 4.0, 2.7 and 2.1 eV for 3a , 3b y 3c, respectively. The UV-vis spectra of the amorphous aminocarbene films show that they are practically transparent to the visible radiation of the electromagnetic spectrum. This is due to the fact that their absorption is located mainly in the ultraviolet range. Two OSCs with bulk-heterojunction configuration were manufactured in order to prove the use of the aminocarbenes as ETL o HTL. The aminocarbene [Cr(CO)₅{C(NHCH₂) 4-pyridyl}] ( 3c ) proved to be suitable as ETL with a fill factor (FF) of 0.23 and a short circuit current density ( J SC ) of 1.037 mA/cm².

Ionic liquids as an electrolyte for the electro synthesis of organic compounds.

PubMed

Kathiresan, Murugavel; Velayutham, David

2015-12-25

The use of ionic liquids (ILs) as a solvent and an electrolyte for electro organic synthesis has been reviewed. To date several ILs exist, however the ILs based on tetraalkylammonium, pyrrolidinium, piperidinium and imidazolium cations with BF4(-), PF6(-), and TFSI anions have been widely used and explored the most. Electro organic synthesis in ionic liquid media leading to the synthesis of a wide range of organic compounds has been discussed. Anodic oxidation or cathodic reduction will generate radical cation or anion intermediates, respectively. These radicals can undergo self coupling or coupling with other molecules yielding organic compounds of interest. The cation of the IL is known to stabilize the radical anion extensively. This stabilization effect has a specific impact on the electrochemical CO2 reduction and coupling to various organics. The relative stability of the intermediates in IL leads to the formation of specific products in higher yields. Electrochemical reduction of imidazolium or thiazolium based ILs generates N-heterocyclic carbenes that have been shown to catalyze a wide range of base or nucleophile catalyzed organic reactions in IL media, an aspect that falls into the category of organocatalysis. Electrochemical fluorination or selective electrochemical fluorination is another fascinating area that delivers selectively fluorinated organic products in Et3N·nHF or Et4NF·nHF adducts (IL) via anodic oxidation. Oxidative polymerization in ILs has been explored the most; although morphological changes were observed compared to the conventional methods, polymers were obtained in good yields and in some cases ILs were used as dopants to improve the desired properties.

Reactive carbon-chain molecules: synthesis of 1-diazo-2,4-pentadiyne and spectroscopic characterization of triplet pentadiynylidene (H-C[triple bond]C-:C-C[triple bond]C-H).

PubMed

Bowling, Nathan P; Halter, Robert J; Hodges, Jonathan A; Seburg, Randal A; Thomas, Phillip S; Simmons, Christopher S; Stanton, John F; McMahon, Robert J

2006-03-15

1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.

A heterobimetallic complex featuring a Ti–Co multiple bond and its application to the reductive coupling of ketones to alkenes† †Electronic supplementary information (ESI) available: Experimental procedures, additional spectroscopic data for 1–4, and computational details of 2 and 3. CCDC 1037714–1037716. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc03772c Click here for additional data file. Click here for additional data file.

PubMed Central

Wu, Bing; Bezpalko, Mark W.; Foxman, Bruce M.

2015-01-01

To explore metal–metal multiple bonds between first row transition metals, Ti/Co complexes supported by two phosphinoamide ligands have been synthesized and characterized. The Ti metalloligand Cl2Ti(XylNPiPr2)2 (1) was treated with CoI2 under reducing conditions, permitting isolation of the Ti/Co complex [(μ-Cl)Ti(XylNPiPr2)2CoI]2 (2). One electron reduction of complex 2 affords ClTi(XylNPiPr2)2CoPMe3 (3), which features a metal–metal triple bond and an unprecedentedly short Ti–Co distance of 2.0236(9) Å. This complex is shown to promote the McMurry coupling reaction of aryl ketones into alkenes, with concomitant formation of the tetranuclear complex [Ti(μ3-O)(NXylPiPr2)2CoI]2 (4). A cooperative mechanism involving bimetallic C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O bond activation and a cobalt carbene intermediate is proposed. PMID:29142672

Gold(I) NHC Complexes: Antiproliferative Activity, Cellular Uptake, Inhibition of Mammalian and Bacterial Thioredoxin Reductases, and Gram-Positive Directed Antibacterial Effects.

PubMed

Schmidt, Claudia; Karge, Bianka; Misgeld, Rainer; Prokop, Aram; Franke, Raimo; Brönstrup, Mark; Ott, Ingo

2017-02-03

Gold complexes with N-heterocyclic carbene (NHC) ligands represent a promising class of metallodrugs for the treatment of cancer or infectious diseases. In this report, the synthesis and the biological evaluation of halogen-containing NHC-Au I -Cl complexes are described. The complexes 1 and 5 a-5 f displayed good cytotoxic activity against tumor cells, and cellular uptake studies suggested that an intact Au-NHC fragment is essential for the accumulation of high amounts of both the metal and the NHC ligand. However, the bioavailability was negatively affected by serum components of the cell culture media and was influenced by likely transformations of the complex. One example (5 d) efficiently induced apoptosis in vincristine- and daunorubicin-resistant P-glycoprotein overexpressing Nalm-6 leukemia cells. Cellular uptake studies with this compound showed that both the wild-type and resistant Nalm-6 cells accumulated comparable amounts of gold, indicating that the gold drug was not excreted by P-glycoprotein or other efflux transporters. The effective inhibition of mammalian and bacterial thioredoxin reductases (TrxR) was confirmed for all of the gold complexes. Antibacterial screening of the gold complexes showed a particularly high activity against Gram-positive strains, reflecting their high dependence on an intact Trx/TrxR system. This result is of particular interest as the inhibition of bacterial TrxR represents a relatively little explored mechanism of new anti-infectives. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Catalytic activation of carbon-carbon bonds in cyclopentanones.

PubMed

Xia, Ying; Lu, Gang; Liu, Peng; Dong, Guangbin

2016-11-24

In the chemical industry, molecules of interest are based primarily on carbon skeletons. When synthesizing such molecules, the activation of carbon-carbon single bonds (C-C bonds) in simple substrates is strategically important: it offers a way of disconnecting such inert bonds, forming more active linkages (for example, between carbon and a transition metal) and eventually producing more versatile scaffolds. The challenge in achieving such activation is the kinetic inertness of C-C bonds and the relative weakness of newly formed carbon-metal bonds. The most common tactic starts with a three- or four-membered carbon-ring system, in which strain release provides a crucial thermodynamic driving force. However, broadly useful methods that are based on catalytic activation of unstrained C-C bonds have proven elusive, because the cleavage process is much less energetically favourable. Here we report a general approach to the catalytic activation of C-C bonds in simple cyclopentanones and some cyclohexanones. The key to our success is the combination of a rhodium pre-catalyst, an N-heterocyclic carbene ligand and an amino-pyridine co-catalyst. When an aryl group is present in the C3 position of cyclopentanone, the less strained C-C bond can be activated; this is followed by activation of a carbon-hydrogen bond in the aryl group, leading to efficient synthesis of functionalized α-tetralones-a common structural motif and versatile building block in organic synthesis. Furthermore, this method can substantially enhance the efficiency of the enantioselective synthesis of some natural products of terpenoids. Density functional theory calculations reveal a mechanism involving an intriguing rhodium-bridged bicyclic intermediate.

Long Carbon Chains in the Warm Carbon-chain-chemistry Source L1527: First Detection of C7H in Molecular Clouds

NASA Astrophysics Data System (ADS)

Araki, Mitsunori; Takano, Shuro; Sakai, Nami; Yamamoto, Satoshi; Oyama, Takahiro; Kuze, Nobuhiko; Tsukiyama, Koichi

2017-09-01

Long carbon-chain molecules were searched for toward the low-mass star-forming region L1527, which is a prototypical source of warm carbon-chain chemistry (WCCC), using the 100 m Green Bank Telescope. Long carbon-chain molecules, C7H (2Π1/2), C6H (2Π3/2 and 2Π1/2), CH3C4H, and C6H2 (cumulene carbene, CCCCCCH2), and cyclic species of C3H and C3H2O were detected. In particular, C7H was detected for the first time in molecular clouds. The column density of C7H is determined to be 6 × 1010 cm-2. The column densities of the carbon-chain molecules including CH3C4H and C6H in L1527 relative to those in the starless dark cloud Taurus Molecular Cloud-1 Cyanopolyyne Peak (TMC-1 CP) tend to be systematically lower for long carbon-chain lengths. However, the column densities of C7H and C6H2 do not follow this trend and are found to be relatively abundant in L1527. This result implies that these long carbon-chain molecules are remnants of the cold starless phase. The results—that both the remnants and WCCC products are observed toward L1527—are consistent with the suggestion that the protostar can also be born in the parent core at a relatively early stage in the chemical evolution.

A Broadly Applicable NHC–Cu-Catalyzed Approach for Efficient, Site-, and Enantioselective Coupling of Readily Accessible (Pinacolato)alkenylboron Compounds to Allylic Phosphates and Applications to Natural Product Synthesis

PubMed Central

2015-01-01

A set of protocols for catalytic enantioselective allylic substitution (EAS) reactions that allow for additions of alkenyl units to readily accessible allylic electrophiles is disclosed. Transformations afford 1,4-dienes that contain a tertiary carbon stereogenic site and are promoted by 1.0–5.0 mol % of a copper complex of an N-heterocyclic carbene (NHC). Aryl- as well as alkyl-substituted electrophiles bearing a di- or trisubstituted alkene may be employed. Reactions can involve a variety of robust alkenyl–(pinacolatoboron) [alkenyl–B(pin)] compounds that can be either purchased or prepared by various efficient, site-, and/or stereoselective catalytic reactions, such as cross-metathesis or proto-boryl additions to terminal alkynes. Vinyl-, E-, or Z-disubstituted alkenyl-, 1,1-disubstituted alkenyl-, acyclic, or heterocyclic trisubstituted alkenyl groups may be added in up to >98% yield, >98:2 SN2′:SN2, and 99:1 enantiomeric ratio (er). NHC–Cu-catalyzed EAS with alkenyl–B(pin) reagents containing a conjugated carboxylic ester or aldehyde group proceed to provide the desired 1,4-diene products in good yield and with high enantioselectivity despite the presence of a sensitive stereogenic tertiary carbon center that could be considered prone to epimerization. In most instances, the alternative approach of utilizing an alkenylmetal reagent (e.g., an Al-based species) represents an incompatible option. The utility of the approach is illustrated through applications to enantioselective synthesis of natural products such as santolina alcohol, semburin, nyasol, heliespirone A, and heliannuol E. PMID:24467274

A broadly applicable NHC-Cu-catalyzed approach for efficient, site-, and enantioselective coupling of readily accessible (pinacolato)alkenylboron compounds to allylic phosphates and applications to natural product synthesis.

PubMed

Gao, Fang; Carr, James L; Hoveyda, Amir H

2014-02-05

A set of protocols for catalytic enantioselective allylic substitution (EAS) reactions that allow for additions of alkenyl units to readily accessible allylic electrophiles is disclosed. Transformations afford 1,4-dienes that contain a tertiary carbon stereogenic site and are promoted by 1.0-5.0 mol % of a copper complex of an N-heterocyclic carbene (NHC). Aryl- as well as alkyl-substituted electrophiles bearing a di- or trisubstituted alkene may be employed. Reactions can involve a variety of robust alkenyl-(pinacolatoboron) [alkenyl-B(pin)] compounds that can be either purchased or prepared by various efficient, site-, and/or stereoselective catalytic reactions, such as cross-metathesis or proto-boryl additions to terminal alkynes. Vinyl-, E-, or Z-disubstituted alkenyl-, 1,1-disubstituted alkenyl-, acyclic, or heterocyclic trisubstituted alkenyl groups may be added in up to >98% yield, >98:2 SN2':SN2, and 99:1 enantiomeric ratio (er). NHC-Cu-catalyzed EAS with alkenyl-B(pin) reagents containing a conjugated carboxylic ester or aldehyde group proceed to provide the desired 1,4-diene products in good yield and with high enantioselectivity despite the presence of a sensitive stereogenic tertiary carbon center that could be considered prone to epimerization. In most instances, the alternative approach of utilizing an alkenylmetal reagent (e.g., an Al-based species) represents an incompatible option. The utility of the approach is illustrated through applications to enantioselective synthesis of natural products such as santolina alcohol, semburin, nyasol, heliespirone A, and heliannuol E.

Trans-Metal-Trapping Meets Frustrated-Lewis-Pair Chemistry: Ga(CH2SiMe3)3-Induced C–H Functionalizations

PubMed Central

2017-01-01

Merging two topical themes in main-group chemistry, namely, cooperative bimetallics and frustrated-Lewis-pair (FLP) activity, this Forum Article focuses on the cooperativity-induced outcomes observed when the tris(alkyl)gallium compound GaR3 (R = CH2SiMe3) is paired with the lithium amide LiTMP (TMP = 2,2,6,6-tetramethylpiperidide) or the sterically hindered N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu). When some previously published work are drawn together with new results, unique tandem reactivities are presented that are driven by the steric mismatch between the individual reagents of these multicomponent reagents. Thus, the LiTMP/GaR3 combination, which on its own fails to form a cocomplex, functions as a highly regioselective base (LiTMP)/trap (GaR3) partnership for the metalation of N-heterocycles such as diazines, 1,3-benzoazoles, and 2-picolines in a trans-metal-trapping (TMT) process that stabilizes the emerging sensitive carbanions. Taking advantage of related steric incompatibility, a novel monometallic FLP system pairing GaR3 with ItBu has been developed for the activation of carbonyl compounds (via C=O insertion) and other molecules with acidic hydrogen atoms such as phenol and phenylacetylene. Shedding new light on how these non-cocomplexing partnerships operate and showcasing the potential of gallium reagents to engage in metalation reactions or FLP activations, areas where the use of this group 13 metal is scant, this Forum Article aims to stimulate more interest and activity toward the advancement of organogallium chemistry. PMID:28485929

Real-time sub-Ångstrom imaging of reversible and irreversible conformations in rhodium catalysts and graphene

NASA Astrophysics Data System (ADS)

Kisielowski, Christian; Wang, Lin-Wang; Specht, Petra; Calderon, Hector A.; Barton, Bastian; Jiang, Bin; Kang, Joo H.; Cieslinski, Robert

2013-07-01

The dynamic responses of a rhodium catalyst and a graphene sheet are investigated upon random excitation with 80 kV electrons. An extraordinary electron microscope stability and resolution allow studying temporary atom displacements from their equilibrium lattice sites into metastable sites across projected distances as short as 60 pm. In the rhodium catalyst, directed and reversible atom displacements emerge from excitations into metastable interstitial sites and surface states that can be explained by single atom trajectories. Calculated energy barriers of 0.13 eV and 1.05 eV allow capturing single atom trapping events at video rates that are stabilized by the Rh [110] surface corrugation. Molecular dynamics simulations reveal that randomly delivered electrons can also reversibly enhance the sp3 and the sp1 characters of the sp2-bonded carbon atoms in graphene. The underlying collective atom motion can dynamically stabilize characteristic atom displacements that are unpredictable by single atom trajectories. We detect three specific displacements and use two of them to propose a path for the irreversible phase transformation of a graphene nanoribbon into carbene. Collectively stabilized atom displacements greatly exceed the thermal vibration amplitudes described by Debye-Waller factors and their measured dose rate dependence is attributed to tunable phonon contributions to the internal energy of the systems. Our experiments suggest operating electron microscopes with beam currents as small as zepto-amperes/nm2 in a weak-excitation approach to improve on sample integrity and allow for time-resolved studies of conformational object changes that probe for functional behavior of catalytic surfaces or molecules.

Radical-molecule reaction C3H+H2O: a mechanistic study.

PubMed

Dong, Hao; Ding, Yi-Hong; Sun, Chia-Chung

2005-02-08

Despite the importance of the C(3)H radical in both combustion and interstellar space, the reactions of C(3)H toward stable molecules have never been studied. In this paper, we report our detailed mechanistic study on the radical-molecule reaction C(3)H+H(2)O at the Becke's three parameter Lee-Yang-Parr-B3LYP6-311G(d,p) and coupled cluster with single, double, and triple excitations-CCSD(T)6-311G(2d,p) (single-point) levels. It is shown that the C(3)H+H(2)O reaction initially favors formation of the carbene-insertion intermediates HCCCHOH (1a,1b) rather than the direct H- or OH-abstraction process. Subsequently, the isomers (1a,1b) can undergo a direct H- extrusion to form the well-known product propynal HCCCHO (P(5)). Highly competitively, (1a,1b) can take the successive 1,4- and 1,2-H-shift interconversion to isomer H(2)CCCHO(2a,2b) and then to isomer H(2)CCHCO(3a,3b), which can finally take a direct C-C bond cleavage to give product C(2)H(3) and CO (P(1)). The other products are kinetically much less feasible. With the overall entrance barrier 10.6 kcal/mol, the title reaction can be important in postburning processes. Particularly, our calculations suggest that the title reaction may play a role in the formation of the intriguing interstellar molecule, propynal HCCCHO. The calculated results will also be useful for the analogous C(3)H reactions such as with ammonia and alkanes.

Automated Transition State Search and Its Application to Diverse Types of Organic Reactions.

PubMed

Jacobson, Leif D; Bochevarov, Art D; Watson, Mark A; Hughes, Thomas F; Rinaldo, David; Ehrlich, Stephan; Steinbrecher, Thomas B; Vaitheeswaran, S; Philipp, Dean M; Halls, Mathew D; Friesner, Richard A

2017-11-14

Transition state search is at the center of multiple types of computational chemical predictions related to mechanistic investigations, reactivity and regioselectivity predictions, and catalyst design. The process of finding transition states in practice is, however, a laborious multistep operation that requires significant user involvement. Here, we report a highly automated workflow designed to locate transition states for a given elementary reaction with minimal setup overhead. The only essential inputs required from the user are the structures of the separated reactants and products. The seamless workflow combining computational technologies from the fields of cheminformatics, molecular mechanics, and quantum chemistry automatically finds the most probable correspondence between the atoms in the reactants and the products, generates a transition state guess, launches a transition state search through a combined approach involving the relaxing string method and the quadratic synchronous transit, and finally validates the transition state via the analysis of the reactive chemical bonds and imaginary vibrational frequencies as well as by the intrinsic reaction coordinate method. Our approach does not target any specific reaction type, nor does it depend on training data; instead, it is meant to be of general applicability for a wide variety of reaction types. The workflow is highly flexible, permitting modifications such as a choice of accuracy, level of theory, basis set, or solvation treatment. Successfully located transition states can be used for setting up transition state guesses in related reactions, saving computational time and increasing the probability of success. The utility and performance of the method are demonstrated in applications to transition state searches in reactions typical for organic chemistry, medicinal chemistry, and homogeneous catalysis research. In particular, applications of our code to Michael additions, hydrogen abstractions, Diels-Alder cycloadditions, carbene insertions, and an enzyme reaction model involving a molybdenum complex are shown and discussed.

Photoelectron Spectroscopy of Substituted Phenylnitrenes

NASA Astrophysics Data System (ADS)

Wijeratne, Neloni R.; Da Fonte, Maria; Wenthold, Paul G.

2009-06-01

Nitrenes are unusual molecular structures with unfilled electronic valences that are isoelectronic with carbenes. Although, both can be generated by either thermal or photochemical decomposition of appropriate precursors they usually exhibit different reactivities. In this work, we carry out spectroscopic studies of substituted phenylnitrene to determine how the introduction of substituents will affect the reactivity and its thermochemical properties. All studies were carried out by using the newly constructed time-of-flight negative ion photoelectron spectrometer (NIPES) at Purdue University. The 355 nm photoelectron spectra of the o-, m-, and p-chlorophenyl nitrene anions are fairly similar to that measured for phenylnitrene anion. All spectra show low energy triplet state and a high energy singlet state. The singlet state for the meta isomer is well-resolved, with a well defined origin and observable vibrational structure. Whereas the singlet states for the ortho and para isomers have lower energy onsets and no resolved structure. The isomeric dependence suggests that the geometry differences result from the resonance interaction between the nitrogen and the substituent. Quinoidal resonance structures are possible for the open-shell singlet states of the o- and p-chlorinated phenyl nitrenes. The advantages of this type of electronic structures for the open-shell singlet states is that the unpaired electrons can be more localized on separate atoms in the molecules, minimizing the repulsion between. Because the meta position is not in resonance with the nitrenes, substitution at that position should not affect the structure of the open-shell singlet state. The measured electron affinities (EA) of the triplet phenylnitrenes are in excellent agreement with the values predicted by electronic structure calculations. The largest EA, 1.82 eV is found for the meta isomer, with para being the smallest, 1.70 eV.

Catalytic Transformation of Aldehydes with Nickel Complexes through η(2) Coordination and Oxidative Cyclization.

PubMed

Hoshimoto, Yoichi; Ohashi, Masato; Ogoshi, Sensuke

2015-06-16

Chemists no longer doubt the importance of a methodology that could activate and utilize aldehydes in organic syntheses since many products prepared from them support our daily life. Tremendous effort has been devoted to the development of these methods using main-group elements and transition metals. Thus, many organic chemists have used an activator-(aldehyde oxygen) interaction, namely, η(1) coordination, whereby a Lewis or Brønsted acid activates an aldehyde. In the field of coordination chemistry, η(2) coordination of aldehydes to transition metals by coordination of a carbon-oxygen double bond has been well-studied; this activation mode, however, is rarely found in transition-metal catalysis. In view of the distinctive reactivity of an η(2)-aldehyde complex, unprecedented reactions via this intermediate are a distinct possibility. In this Account, we summarize our recent results dealing with nickel(0)-catalyzed transformations of aldehydes via η(2)-aldehyde nickel and oxanickelacycle intermediates. The combination of electron-rich nickel(0) and strong electron-donating N-heterocyclic carbene (NHC) ligands adequately form η(2)-aldehyde complexes in which the aldehyde is highly activated by back-bonding. With Ni(0)/NHC catalysts, processes involving intramolecular hydroacylation of alkenes and homo/cross-dimerization of aldehydes (the Tishchenko reaction) have been developed, and both proceed via the simultaneous η(2) coordination of aldehydes and other π components (alkenes or aldehydes). The results of the mechanistic studies are consistent with a reaction pathway that proceeds via an oxanickelacycle intermediate generated by the oxidative cyclization with a nickel(0) complex. In addition, we have used the η(2)-aldehyde nickel complex as an effective activator for an organosilane in order to generate a silicate reactant. These reactions show 100% atom efficiency, generate no wastes, and are conducted under mild conditions.

Globin haplotypes of human T-cell lymphotropic virus type I-infected individuals in Salvador, Bahia, Brazil, suggest a post-Columbian African origin of this virus.

PubMed

Alcantara, Luiz Carlos; Van Dooren, Sonia; Gonçalves, Marilda Souza; Kashima, Simone; Costa, Maria Cristina Ramos; Santos, Fred Luciano Neves; Bittencourt, Achilea Lisboa; Dourado, Inês; Filho, Antonio Andrade; Covas, Dimas Tadeu; Vandamme, Anne-Mieke; Galvão-Castro, Bernardo

2003-08-01

The city of Salvador, Bahia, Brazil, has sociodemographic characteristics similar to some African cities. Up to now, it has had the highest prevalence of human T-cell lymphotropic virus type I (HTLV-I) infection (1.74%) in the country. To investigate which strains of HTLV-I are circulating in Salvador, we studied isolates from 82 patients infected with HTLV-I: 19 from the general population, 21 from pregnant women, 16 from intravenous drug users, and 26 from patients and their family attending a neurologic clinic. Phylogenetic analysis from part of the LTR fragments showed that most of these isolates belonged to the Transcontinental subgroup of the Cosmopolitan subtype (HTLV-Ia). Only one sample from a pregnant woman was closely related to the Japanese subgroup, suggesting recent introduction of a Japanese HTLV-I lineage into Salvador. betaA-Globin haplotypes were examined in 34 infected individuals and found to be atypical, confirming the racial heterogeneity of this population. A total of 20 chromosomes were characterized as Central African Republic (CAR) haplotype (29.4%), 31 (45.6%) were characterized as Benin (BEN) haplotype, and 17 (25%) were characterized as Senegal (SEN) haplotype. Five patients' genotypes (14.7%) were CAR/CAR; 10 (29,4%), BEN/BEN; 9 (26.5%), CAR/BEN; 2 (5.9%), BEN/SEN; and 7 (20.6%), SEN/SEN. One patient's genotype (2.9%) was CAR/SEN. The betaA-globin haplotype distribution in Salvador is unusual compared with other Brazilian states. Our data support the hypothesis of multiple post-Columbian introductions of African HTLV-Ia strains in Salvador, Bahia, Brazil.

White Light Emission from Planar Remote Phosphor Based on NHC Cycloplatinated Complexes.

PubMed

Fuertes, Sara; Chueca, Andrés J; Perálvarez, Mariano; Borja, Pilar; Torrell, Marc; Carreras, Josep; Sicilia, Violeta

2016-06-29

We report on the generation of bright white luminescence through solid-state illumination of remote phosphors based on novel cycloplatinated N-heterocyclic carbene (NHC) compounds. Following a stepwise protocol we got the new NHC compound [{Pt(μ-Cl)(C(∧)C*)}2] (4) (HC(∧)C*-κC* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene), which was used together with the related ones 4a (HC(∧)C*-κC*= 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene) and 4b (HC(∧)C*-κC*= 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene) as starting materials for the synthesis of the new ionic derivatives [Pt(R-C(∧)C*) (CNR')2]PF6 (R = -COOEt, R' = t-Bu (5), Xyl (6); R = -CN, R' = t-Bu (7), Xyl (8); R(∧)C = Naph, R' = t-Bu (9), Xyl (10)). The X-ray structures of 6 and 8-10 have been determined. The photophysical properties of these cationic compounds have been studied and supported by the time-dependent-density functional theory (TD-DFT) calculations. The compounds 5, 8, and 9 have been revealed as the most efficient emitters in the solid state with quantum yields of 41%, 21%, and 40%, respectively. White-light remote-phosphors have been prepared just by stacking different combinations of these compounds and [Pt(bzq) (CN) (CN(t)Bu)] (R1) as blue (5, 8), yellow (9), and red (R1) components onto the same substrate. The CCT (correlated color temperature) and the CRI (color rendering index) of the emitted white-light have been tuned by accurately controlling the individual contributions.

The Use of Cyclometalated NHCs and Pyrazoles for the Development of Fully Efficient Blue PtII Emitters and Pt/Ag Clusters.

PubMed

Arnal, Lorenzo; Fuertes, Sara; Martín, Antonio; Sicilia, Violeta

2018-05-15

New bis-pyrazole complexes [Pt(C^C*)(RpzH) 2 ]X, containing a cyclometalated N-heterocyclic carbene ligand (HC^C*=1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene) were prepared as chloride (X=Cl - , RpzH: 3,5-Me 2 pzH 1 a, 4-MepzH 2 a, pzH 3 a), perchlorate (X=ClO 4 - , 1 b-3 b), or hexafluorophosphate (X=PF 6 - , RpzH: 3,5-Me 2 pzH 1 c) salts. The X-ray structure of 1 a showed that the Cl - anion is trapped by the cation through two N-H⋅⋅⋅Cl bonds. In solution of methanol, acetone and THF at RT, 1 a-3 a coexist in equilibrium with the corresponding [PtCl(C^C*)(RpzH)] (B) and RpzH species. In CH 2 Cl 2 , this equilibrium takes place just for 2 a and 3 a, but it is completely shifted to the left at 243 and 223 K for 2 a and 3 a, respectively. The low-lying absorption and emission bands were assigned to intraligand (ILCT) charge transfer on the NHC group. Quantum yield measurements in PMMA films revealed that 1 b, 2 b and 1 c are amongst the most efficient blue-light emitters, with values up to 100 %. Proton abstraction from the coordinated 3,5-Me 2 pzH in 1 b by NEt 3 and replacement by Ag + afforded a neutral [Pt 2 Ag 2 ] cluster containing Pt→Ag dative bonds. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Genetically programmed chiral organoborane synthesis

NASA Astrophysics Data System (ADS)

Kan, S. B. Jennifer; Huang, Xiongyi; Gumulya, Yosephine; Chen, Kai; Arnold, Frances H.

2017-12-01

Recent advances in enzyme engineering and design have expanded nature’s catalytic repertoire to functions that are new to biology. However, only a subset of these engineered enzymes can function in living systems. Finding enzymatic pathways that form chemical bonds that are not found in biology is particularly difficult in the cellular environment, as this depends on the discovery not only of new enzyme activities, but also of reagents that are both sufficiently reactive for the desired transformation and stable in vivo. Here we report the discovery, evolution and generalization of a fully genetically encoded platform for producing chiral organoboranes in bacteria. Escherichia coli cells harbouring wild-type cytochrome c from Rhodothermus marinus (Rma cyt c) were found to form carbon-boron bonds in the presence of borane-Lewis base complexes, through carbene insertion into boron-hydrogen bonds. Directed evolution of Rma cyt c in the bacterial catalyst provided access to 16 novel chiral organoboranes. The catalyst is suitable for gram-scale biosynthesis, providing up to 15,300 turnovers, a turnover frequency of 6,100 h-1, a 99:1 enantiomeric ratio and 100% chemoselectivity. The enantiopreference of the biocatalyst could also be tuned to provide either enantiomer of the organoborane products. Evolved in the context of whole-cell catalysts, the proteins were more active in the whole-cell system than in purified forms. This study establishes a DNA-encoded and readily engineered bacterial platform for borylation; engineering can be accomplished at a pace that rivals the development of chemical synthetic methods, with the ability to achieve turnovers that are two orders of magnitude (over 400-fold) greater than those of known chiral catalysts for the same class of transformation. This tunable method for manipulating boron in cells could expand the scope of boron chemistry in living systems.

Theoretical mechanistic study on the ion-molecule reaction of SiCN+/SiNC+ with H2O.

PubMed

Wang, Jian; Ding, Yi-hong; Sun, Chia-chung

2005-02-15

The gas-phase ion-molecule reactions play very important roles in interstellar and in plasma chemistry. Motivated by recent astrophysical detection of the SiCN/SiNC radicals and laboratory characterization of some SiCN-containing species, we carried out a detailed potential energy survey on the SiCN+/SiNC(+) + H2O reaction at the Becke's three-parameter Lee-Yang-Parr-B3LYP/6-311G(d,p) and coupled cluster with single, double, and triple excitations-CCSD(T)/6-311 + G(2df,p) (single-point) levels as an attempt towards understanding the SiCN+/SiNC+ reaction mechanisms. In contrast to the carbene-featured analogous CCN+/CNC(+) + H2X (X=O,S) reactions, the title reaction SiCN+/SiNC(+) + H2O are not associated with any competitive silylene-insertion characters. Moreover, the -CN <--> -NC interconversion has a low barrier and plays an important role in determining the final product distributions. This is also in marked difference from the CCN+/CNC+ reaction. It is shown that the isomeric sila-cations SiCN+ and SiNC+ can both react with H2O to barrierlessly generate the major product P1 HOSi(+) + HCN and the minor one P3 HOSi(+) + HNC, whereas other low-lying products such as P2 SiNCO(+) + H2, and P(0) H2NSi(+) + CO are kinetically unfeasible. The high efficiency of the SiCN+/SiNC+ reaction towards H2O and the potential importance of SiCN+/SiNC+ ion chemistry in interstellar and SiCN-based microelectric and photoelectric processes strongly appeals for future laboratory investigations on the SiCN+/SiNC+ chemical reactivity.

Catalytic activation of carbon–carbon bonds in cyclopentanones

PubMed Central

Xia, Ying; Lu, Gang; Liu, Peng; Dong, Guangbin

2017-01-01

In the chemical industry, molecules of interest are based primarily on carbon skeletons. When synthesizing such molecules, the activation of carbon–carbon single bonds (C–C bonds) in simple substrates is strategically important: it offers a way of disconnecting such inert bonds, forming more active linkages (for example, between carbon and a transition metal) and eventually producing more versatile scaffolds1–13. The challenge in achieving such activation is the kinetic inertness of C–C bonds and the relative weakness of newly formed carbon–metal bonds6,14. The most common tactic starts with a three- or four-membered carbon-ring system9–13, in which strain release provides a crucial thermodynamic driving force. However, broadly useful methods that are based on catalytic activation of unstrained C–C bonds have proven elusive, because the cleavage process is much less energetically favourable. Here we report a general approach to the catalytic activation of C–C bonds in simple cyclopentanones and some cyclohexanones. The key to our success is the combination of a rhodium pre-catalyst, an N-heterocyclic carbene ligand and an amino-pyridine co-catalyst. When an aryl group is present in the C3 position of cyclopentanone, the less strained C–C bond can be activated; this is followed by activation of a carbon–hydrogen bond in the aryl group, leading to efficient synthesis of functionalized α-tetralones—a common structural motif and versatile building block in organic synthesis. Furthermore, this method can substantially enhance the efficiency of the enantioselective synthesis of some natural products of terpenoids. Density functional theory calculations reveal a mechanism involving an intriguing rhodium-bridged bicyclic intermediate. PMID:27806379

Tuning the Electronic Structure of Fe(II) Polypyridines via Donor Atom and Ligand Scaffold Modifications: A Computational Study.

PubMed

Bowman, David N; Bondarev, Alexey; Mukherjee, Sriparna; Jakubikova, Elena

2015-09-08

Fe(II) polypyridines are an important class of pseudo-octahedral metal complexes known for their potential applications in molecular electronic switches, data storage and display devices, sensors, and dye-sensitized solar cells. Fe(II) polypyridines have a d(6) electronic configuration and pseudo-octahedral geometry and can therefore possess either a high-spin (quintet) or a low-spin (singlet) ground state. In this study, we investigate a series of complexes based on [Fe(tpy)2](2+) (tpy = 2,2';6',2″-terpyridine) and [Fe(dcpp)2](2+) (dcpp = 2,6-bis(2-carboxypyridyl)pyridine). The ligand field strength in these complexes is systematically tuned by replacing the central pyridine with five-membered (N-heterocyclic carbene, pyrrole, furan) or six-membered (aryl, thiazine-1,1-dioxide, 4-pyrone) moieties. To determine the impact of ligand substitutions on the relative energies of metal-centered states, the singlet, triplet, and quintet states of the Fe(II) complexes were optimized in water (PCM) using density functional theory at the B3LYP+D2 level with 6-311G* (nonmetals) and SDD (Fe) basis sets. It was found that the dcpp ligand scaffold allows for a more ideal octahedral coordination environment in comparison to the tpy ligand scaffold. The presence of six-membered central rings also allows for a more ideally octahedral coordination environment relative to five-membered central rings, regardless of the ligand scaffold. We find that the ligand field strength in the Fe(II) polypyridines can be tuned by altering the donor atom identity, with C donor atoms providing the strongest ligand field.

Lyman α photolysis of solid nitromethane (CH3NO2) and D3-nitromethane (CD3NO2)--untangling the reaction mechanisms involved in the decomposition of model energetic materials.

PubMed

Maksyutenko, Pavlo; Muzangwa, Lloyd G; Jones, Brant M; Kaiser, Ralf I

2015-03-21

Solid nitromethane (CH3NO2) along with its isotopically labelled counterpart D3-nitromethane (CD3NO2) ices were exposed to Lyman α photons to investigate the mechanism involved in the decomposition of energetic materials in the condensed phase. The chemical processes in the ices were monitored online and in situ via infrared spectroscopy complimented by temperature programmed desorption studies utilizing highly sensitive reflectron time-of-flight mass spectrometry coupled with pulsed photoionization (ReTOF-PI) at 10.49 eV. The infrared data revealed the formation of cis-methylnitrite (CH3ONO), formaldehyde (H2CO), water (H2O), carbon monoxide (CO), and carbon dioxide (CO2). Upon sublimation of the irradiated samples, three classes of higher molecular weight products, which are uniquely formed in the condensed phase, were identified via ReTOF-PI: (i) nitroso compounds [nitrosomethane (CH3NO), nitrosoethane (C2H5NO), nitrosopropane (C3H7NO)], (ii) nitrite compounds [methylnitrite (CH3ONO), ethylnitrite (C2H5ONO), propylnitrite (C3H7ONO)], and (iii) higher molecular weight molecules [CH3NONOCH3, CH3NONO2CH3, CH3OCH2NO2, ONCH2CH2NO2]. The mechanistical information obtained in the present study suggest that the decomposition of nitromethane in the condensed phase is more complex compared to the gas phase under collision-free conditions opening up not only hitherto unobserved decomposition pathways of nitromethane (hydrogen atom loss, oxygen atom loss, retro carbene insertion), but also the blocking of several initial decomposition steps due to the 'matrix cage effect'.

Direct synthesis of cis-dihalido-bis(NHC) complex of nickel(II) and catalytic application in olefin addition polymerization: effect of halogen co-ligands and density functional theory study.

PubMed

Zhang, Dao; Zhou, Sen; Li, Zhiming; Wang, Quanrui; Weng, Linhong

2013-09-07

Two novel amine-containing N-heterocyclic carbene ligand precursors [H(1a-b)]Br have been prepared in good yield and fully characterized. Direct syntheses of cis- and trans-dihalido-bis(NHC) nickel complexes [Ni(NHC)2X2] (X = Cl, Br) are reported. The solid structures of trans-[Ni(1a-b)2Br2] (2a-b) and cis-[Ni(1a)2Cl2] (3) were determined by single-crystal X-ray analysis and 3 was found to be the first example of cis-configuration coordination of monodentate NHC ligands to a metal center for dihalido-bis(NHC) nickel complexes. DFT calculations were conducted to determine the energy difference between cis- and trans-isomers of complexes 2a and 3 bearing bromide and chloride co-ligands. The cis-[Ni(1a)2Cl2] (cis-3) is 1.77-1.55 kcal mol(-1) lower in energy than its trans-isomer in polar solvents including CH2Cl2 and THF, while the trans-[Ni(1a)2Br2] (trans-2a) is more stable than the cis-isomer similarly in the gas phase. The cis nickel complex 3 with two coordinated monodentate NHCs was tested for olefin addition polymerization at standard conditions. It was found that cis-3 was inactive in ethylene polymerization but showed moderate catalytic activities (0.5-3.0 × 10(6) g of PNB (mol of Ni)(-1) h(-1)) in the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst.

Molecules in Laboratory and in Interstellar Space?

NASA Astrophysics Data System (ADS)

Thimmakondu, Venkatesan S.

2016-06-01

In this talk, the quantum chemistry of astronomically relevant molecules will be outlined with an emphasis on the structures and energetics of C_7H_2 isomers, which are yet to be identified in space. Although more than 100's of isomers are possible for C_7H_2, to date only 6 isomers had been identified in the laboratory. The equilibrium geometries of heptatriynylidene (1), cyclohepta-1,2,3,4-tetraen-6-yne (2), and heptahexaenylidene (3), which we had investigated theoretically will be discussed briefly. While 1 and 3 are observed in the laboratory, 2 is a hypothetical molecule. The theoretical data may be useful for the laboratory detection of 2 and astronomical detection of 2 and 3. THIS WORK IS SUPPORTED BY A RESEARCH GRANT (YSS/2015/00099) FROM SERB, DST, GOVERNMENT OF INDIA. Apponi, A. P.; McCarthy, M. C.; Gottlieb, C. A.; Thaddeus, P. Laboratory Detection of Four New Cumulene Carbenes: H_2C_7, H_2C_8, H_2C_9, and D_2C10, Astrophys. J. 2000, 530, 357-361 Ball, C. D; McCarthy, M. C.; Thaddeus, P. Cavity Ringdown Spectroscopy of the Linear Carbon Chains HC_7H, HC_9H, HC11H, and HC13H. J. Chem. Phys. 2000, 112, 10149-10155 Dua, S.; Blanksby, S. J.; Bowie, J. H. Formation of Neutral C_7H_2 Isomers from Four Isomeric C_7H_2 Radical Anion Precursors in the Gas Phase. J. Phys. Chem. A, 2000, 104, 77-85. Thimmakondu, V. S. The equilibrium geometries of heptatriynylidene, cyclohepta-1,2,3,4-tetraen-6-yne, and heptahexaenylidene, Comput. Theoret. Chem. 2016, 1079, 1-10

A DFT study on NHC-catalyzed intramolecular aldehyde-ketone crossed-benzoin reaction: mechanism, regioselectivity, stereoselectivity, and role of NHC.

PubMed

Zhang, Wei; Wang, Yang; Wei, Donghui; Tang, Mingsheng; Zhu, Xinju

2016-07-06

A systematic theoretical study has been carried out to understand the mechanism and stereoselectivity of N-heterocyclic carbene (NHC)-catalyzed intramolecular crossed-benzoin reaction of enolizable keto-aldehyde using density functional theory (DFT) calculations. The calculated results reveal that the most favorable pathway contains four steps, i.e., the nucleophilic attack of NHC on the carbonyl carbon atom of a formyl group, the formation of a Breslow intermediate, a ring-closure process coupled with proton transfer, and regeneration of the catalyst. For the formation of the Breslow intermediate via the [1,2]-proton transfer process, apart from the direct proton transfer mechanism, the base Et3N and the in situ generated Brønsted acid Et3N·H(+) mediated proton transfer mechanisms have also been investigated; the free energy barriers for the crucial proton transfer steps are found to be significantly lowered by explicit inclusion of the Brønsted acid Et3N·H(+). The computational results show that the ring-closure process is the stereoselectivity-determining step, in which two chirality centers assigned on the coupling carbon atoms are formed, and the S-configured diastereomer is the predominant product, which is in good agreement with the experimental observations. NCI and NBO analyses are employed to disclose the origin of stereoselectivity and regioselectivity. Moreover, a global reaction index (GRI) analysis has been performed to confirm that NHC mainly plays the role of a Lewis base. The mechanistic insights obtained in the present study should be valuable for the rational design of an effective organocatalyst for this kind of reaction with high stereoselectivity and regioselectivity.

Ruthenium bidentate phosphine complexes for the coordination and catalytic dehydrogenation of amine- and phosphine-boranes.

PubMed

Ledger, Araminta E W; Ellul, Charles E; Mahon, Mary F; Williams, Jonathan M J; Whittlesey, Michael K

2011-07-25

Addition of the amine-boranes H(3)B⋅NH(2)tBu, H(3)B⋅NHMe(2) and H(3)B⋅NH(3) to the cationic ruthenium fragment [Ru(xantphos)(PPh(3))(OH(2))H][BAr(F)(4)] (2; xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; BAr(F)(4)=[B{3,5-(CF(3))(2)C(6)H(3)}(4)](-)) affords the η(1)-B-H bound amine-borane complexes [Ru(xantphos)(PPh(3))(H(3)B⋅NH(2)tBu)H][BAr(F)(4)] (5), [Ru(xantphos)(PPh(3))(H(3) B⋅NHMe(2))H][BAr(F)(4)] (6) and [Ru(xantphos)(PPh(3))(H(3)B⋅NH(3))H][BAr(F)(4)] (7). The X-ray crystal structures of 5 and 7 have been determined with [BAr(F)(4)] and [BPh(4)] anions, respectively. Treatment of 2 with H(3)B⋅PHPh(2) resulted in quite different behaviour, with cleavage of the B-P interaction taking place to generate the structurally characterised bis-secondary phosphine complex [Ru(xantphos)(PHPh(2))(2)H][BPh(4)] (9). The xantphos complexes 2, 5 and 9 proved to be poor precursors for the catalytic dehydrogenation of H(3)B⋅NHMe(2). While the dppf species (dppf=1,1'-bis(diphenylphosphino)ferrocene) [Ru(dppf)(PPh(3))HCl] (3) and [Ru(dppf)(η(6)-C(6)H(5)PPh(2))H][BAr(F)(4)] (4) showed better, but still moderate activity, the agostic-stabilised N-heterocyclic carbene derivative [Ru(dppf)(ICy)HCl] (12; ICy=1,3-dicyclohexylimidazol-2-ylidene) proved to be the most efficient catalyst with a turnover number of 76 h(-1) at room temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis, Characterization, and In Vivo Efficacy of Shell Cross-Linked Nanoparticle Formulations Carrying Silver Antimicrobials as Aerosolized Therapeutics

PubMed Central

2014-01-01

The use of nebulizable, nanoparticle-based antimicrobial delivery systems can improve efficacy and reduce toxicity for treatment of multi-drug-resistant bacteria in the chronically infected lungs of cystic fibrosis patients. Nanoparticle vehicles are particularly useful for applying broad-spectrum silver-based antimicrobials, for instance, to improve the residence time of small-molecule silver carbene complexes (SCCs) within the lung. Therefore, we have synthesized multifunctional, shell cross-linked knedel-like polymeric nanoparticles (SCK NPs) and capitalized on the ability to independently load the shell and core with silver-based antimicrobial agents. We formulated three silver-loaded variants of SCK NPs: shell-loaded with silver cations, core-loaded with SCC10, and combined loading of shell silver cations and core SCC10. All three formulations provided a sustained delivery of silver over the course of at least 2–4 days. The two SCK NP formulations with SCC10 loaded in the core each exhibited excellent antimicrobial activity and efficacy in vivo in a mouse model of Pseudomonas aeruginosa pneumonia. SCK NPs with shell silver cation-load only, while efficacious in vitro, failed to demonstrate efficacy in vivo. However, a single dose of core SCC10-loaded SCK NPs (0.74 ± 0.16 mg Ag) provided a 28% survival advantage over sham treatment, and administration of two doses (0.88 mg Ag) improved survival to 60%. In contrast, a total of 14.5 mg of Ag+ delivered over 5 doses at 12 h intervals was necessary to achieve a 60% survival advantage with a free-drug (SCC1) formulation. Thus, SCK NPs show promise for clinical impact by greatly reducing antimicrobial dosage and dosing frequency, which could minimize toxicity and improve patient adherence. PMID:23718195