An unusual non-symmetrical nitrile ylide dimerization catalyzed by bis(diisopropylamino)carbene

Carbene-nitrile ylide 2 undergoes an unusual non-symmetrical dimerization catalyzed by stable bis(diisopropylamino)carbene.     

Unprecedented 1,4-stannatropy: effective generation of azomethine ylides as nitrile ylide equivalents from N-(stannylmethyl)thioamides

Generation and cycloaddition of less- or non-stabilized azomethine ylides, or nitrile ylide equivalents, via unprecedented 1,4-stannatropy of N-(tributylstannylmethyl)thioamides were achieved. The reactions with dipolarophiles, such as electron-deficient alkenes and alkynes, afforded corresponding pyrrolidine and pyrrole derivatives effectively.     

A novel class of nitrile ylide

Isothiocarbamoyl chlorides IV on treatment with NEt₃ afforded nitrile ylides V which reacted with dimethyl acetylenedicarboxylate to give 2H-pyrroles VI and with ethyl cyanoformate to give 4H-imidazoles VII.     

Formation and reactivity of gem-difluoro-substituted pyridinium ylides: Experimental and DFT investigation

According to DFT B3LYP/6-31G* calculations the reaction of difluorocarbene with pyridines proceeds reversibly with the formation of thermodynamically unstable intermediates, difluoro-substituted pyridinium ylides, which dissociate to carbene and pyridine with low activation barrier. The equilibrium constant of the reaction increases with increasing electron-withdrawing ability of substituents in the pyridine ring. Difluoroylides were generated from 4-cyano, 4-benzoyl- and 4-ethoxycarbonyl-substituted pyridines under difluorocarbene generation conditions (CF₂Br₂/Pb/Bu₄NBr/CH₂Cl₂/ultrasound) and trapped with dimethyl maleate or fumaronitrile. 3-Fluoroindolizines were isolated as final products of the reaction which involves dehydrofluorination of the primary cycloadducts followed by dehydrogenation by active MnO₂.     

Generation of functionalized azomethine ylides and their application to stereoselective heterocycle synthesis: an equivalent process of C-unsubstituted nitrile ylide cycloaddition reaction

The synthetic process equivalent to C-unsubstituted (CH) nitrile ylides cycloaddition reaction is achieved via cycloaddition of NH-azomethine ylide and the following fission reaction of the cycloadducts under acidic conditions. Cycloaddition of NH-azomethine ylide generated by a thermal 1,2-prototropy in 4-oxo-4H-pyrido[1,2-a]pyrimidine-3-carbaldehyde system with maleimides provides proline derivatives under extremely mild conditions. Heating their adducts in AcOH at 85 °C causes a cleavage of C-C bond between the proline and heterocyclic moiety to give the parent heterocyclic system and dehydroproline derivatives, which is regarded as a cycloadduct of C-unsubstituted (CH) nitrile ylide. This cycloaddition-fission reaction sequences can be applied to one-pot three-component reaction.     

S,C-sulfonium ylides from thiophenes: potential carbene precursors

Photolysis of S,C-sulfonium ylides derived from thioanisol, thiophene, benzothiophene, or dibenzothiophene provides products deriving from dicarbomethoxycarbene. In methanol, no rearrangement of the carbene to the ketene derivative is observed. Formation of dibenzothiophene and benzothiophene is quantitative. For the thiophene-based ylide, insertion of the carbene into the alpha-CH bond of thiophene is observed. Evidence is presented that supports formation of both singlet and triplet carbene.     

3-pyridylcarbene and 3-pyridylnitrene: ring opening to nitrile ylides

Photolysis of 3-pyridyldiazomethane in an Ar matrix at 7-10 K gives 3-pyridylcarbene. Further photolysis causes ring opening to nitrile ylide 26 (formonitrile pent-2-en-4-ynylide) as the major reaction together with a minor amount of ring expansion to 1-azacyclohepta-1,3,4,6-tetraene, 27. Matrix photolysis of 3-azidopyridine leads to ring opening to formonitrile N-cyanovinylmethylide, 33.     

Density functional study of the relative reactivity in the concerted 1,3‐dipolar cycloaddition of nitrile ylide to disubstituted ethylenes

Density functional theory was used to perform a theoretical evaluation of (E)‐1,2‐disubstituted ethylenes as dipolarophiles for the 1,3‐dipolar cycloaddition reaction. The reactivities of electron‐withdrawing and ‐donating substituted ethylenes were examined by estimating their activation energies. The calculated activation energies predicted that the most reactive species is (E)‐1,2‐C₂H₂(NO)₂, whereas the least reactive is (E)‐2‐butene. Namely, it was demonstrated that 16‐electron 1,3‐dipole reactants with more electropositive substituents in terminal positions and ethylenes that possess more strongly electron‐withdrawing substituents facilitate 1,3‐dipolar cycloaddition reactions. All of the theoretical results can be rationalized using the configuration mixing model. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 318–323, 2001     

The carbene reactivity surface: a classification

A new two-dimensional classification of singlet carbenes based on the difference in reactivity of their insertion reactions into the C-H bonds of acetonitrile and isobutane is presented. This classification combines the stability and the philicity of divalent species. Until now all of the experimentally based philicity scales are based on the addition to alkenes. Moreover, a new terminology for describing the reactivity of carbenes is introduced. Among the alkyl carbenes, acetyl carbene (2) and cyclopentadienylidene are shown as highly reactive electrophilic carbenes, whereas the other alkylidenes and alkenylidenes investigated are all less active than 2 and more nucleophilic. The stabilized-nucleophilic bicyclo[2.1.1]hex-2-en-5-ylidene (13) possesses a stability similar to that of cyclic alkyl amino carbene (CAAC) 18 and aminophosphoniocarbene 7. Strong hydrogen bridging is found between a C-H bond of acetonitrile and the nucleophilic carbenes 13 and 14.     

Intramolecular 1,3-cycloadditions of nitrile ylides bearing an acetylenic function

Treatment of amides 1a,b with thionyl chloride followed by reaction with triethylamine gave 1,4-dihydro[1]benzopyrano[4,3-b]pyrrole derivatives ( 4a,b ) arising from intramolecular 1,3-cycloaddition of intermediate nitrile ylides ( 3a,b ).     

Neutral carbene analogues of group 13 elements: the dimerization reaction to a biradicaloid

The dimerization reactions of the neutral carbene analogues with the group 13 elements boron, aluminum, gallium, and indium are studied. Besides boron, all monomeric species possess singlet ground states. For Al, bulky substituted cases were investigated; they reveal no essential changes in the singlet-triplet energy separations compared with the parent species. The dimerization energies increase with an increase in the bulk of the substituents; this is a consequence of an enhancement of van der Waals forces for association. The latter is opposed by entropic forces, which facilitate dissociation. An equilibrium between monomeric and dimeric structures is predicted because of enthalpy versus entropy control. The low-temperature domain association should prevail in the formation of a dimer with Al (Ga) within the formal oxidation state I+. The Al-Al bond refers to a chelated biradicaloid species with an energetically low-lying triplet state. It emerges from the metal-metal contacts in the dimer. The biradical character of the dimer decreases in the order E = Al ? Ga > In. The carbene analogue of In forms upon dimerization of only weak coordinative metal-metal interactions.     

Catalytic [2,3]-sigmatropic rearrangement of sulfur ylide derived from metal carbene

This paper reviews the most recent development of [2,3]-sigmatropic rearrangement of sulfur ylide generated from the reaction of sulfide with metal carbene. The metal carbene was formed from decomposition of diazo compounds catalyzed by transition metal complexes such as Cu(I) and Rh(II).     

Frontiers in catalytic nitrile hydration: Nitrile and cyanohydrin hydration catalyzed by homogeneous organometallic complexes

Acrylic monomers are a significant part of the global economy, contributing to the manufacture of over a billion tons of diverse polymeric consumer products every year. The development of more efficient, greener methods to manufacture this highly demanded class of compounds is an important goal in the realization of a sustainable chemical industry. The pursuit of environmentally benign production processes has inspired a rich body of industrial and academic research on methods for the catalytic hydration of nitriles, and this review surveys both established and newer methods of generating acrylic amides, acids, and esters from nitrile and cyanohydrin substrates. The review also examines synthetic and mechanistic studies of homogeneously catalyzed nitrile hydration reactions with an emphasis on explicating the parameters that impact catalyst performance. The final section is a discussion of catalyst properties, gleaned from the mechanistic studies, that will be useful in designing the next generation of nitrile hydration catalysts.     

Organocatalytic tail-to-tail dimerization of olefin: umpolung of methyl methacrylate mediated by N-heterocyclic carbene

Highly selective tail-to-tail dimerization of methyl methacrylate has been realized by an N-heterocyclic carbene catalyst, giving dimethyl 2,5-dimethyl-2-hexenedioate with an E/Z ratio of 95:5 in 86% isolated yield. The umpolung mechanism is proposed on the basis of interception of the intermediates using ESI-MS analyses and deuterium-labeling experiments.