Carbenes nitrile ylide structure

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

Studies of the reaction of photochemically generated carbenes with nitriles to give nitrile ylides have again attracted attention. Flash photolysis of 1-naphthyldiEizomethane (51) in acetonitrile or of the azirine (52) gave the same transient species to which the nitrile ylide structure (53) was assigned. The first stable nitrile ylide was also obtained in this way irradiation of diazotetrakis(trifluoromethyl)cyclopentadiene (54) in the presence of 1-cyanoadamantane (55) gave the crystalline ylide (56).An X-ray structure determination showed that the 1,3-dipole is essentially linear. 

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

The first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

The direct assembly of nitrile ylides by the reaction of carbenes (e.g., 84) with nitriles has, in principle, great potential for structural variation as both components are easily accessible. The first reports of such reactions appeared in the early 1980s and were concerned with the reaction of nitriles with stabilized carbenes such as 84 and 86 (41 3). 

The iminocarbenes 33 (Scheme 7) may also be regarded as nitrile ylides (35). Ab initio calculations45 indicate that nitrilium betaines are best described as hybrids of the bent dipolar and carbenic structures (e.g., 33 and 35). Consequently, the generation of the nitrile ylide 35 in the gas phase should give the same product as obtained from the azirine 31. This was confirmed by flash vacuum pyrolysis of the appropriate oxazol-5(2//)-one (36), which gave a quantitative yield of the azadiene 34.46... 

Chromium (and tungsten) imino carbene complexes 113 are found to be synthetic equivalents for nitrile ylides on the basis of the resonance structure (Scheme 5.23) [34]. The reaction of the imino carbene complex 113 with alkyne 114 affords pyrrole 115. Imidazoles 116 and 120, pyrrolidines 118, and oxazoline 119 are also obtained by the [3+2] cycloaddition of the imino carbene complexes 113 with benzonitrile, alkenes 117, and benzaldehyde, respectively. Although the regiochemical outcome of these reactions is unpredictable, the regioselectivity can be correlated with the resonance structure in the case of the reaction with 3-hexyn-2-one. 

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