Azirines, and Iminocarbenes

The formation of 2//-azirines by thermolysis and photolysis of vinyl azides has been reviewed previously.19-20 

Smolinsky21 found that the gas-phase pyrolysis or liquid-phase photolysis of a-azidostyrenes (1) produced 50-60% yields of 3-aryl-2//-azirines (2) together with small amounts (5-10%) of ketenimines (3) (Eq. 1). 

Boyer22 reported the formation of phenylacetonitrile by photolysis or pyrolysis of / -styryl azide (4), photolysis of /S-styryl isocyanate, and triethyl phosphite deoxygenation of / -nitrostyrene. A nitrene mechanism was proposed (Eq. 2). 

However, Isomura et al.23 showed that an unstable azirine (5) is formed initially by photolysis or thermolysis of 4. The subsequent thermolysis (300°C, gas phase) or photolysis of the azirine gave a 1 1 mixture of phenylacetonitrile and indole (Eq. 3). Further examples of indole formation have been reported.23,24 

The generality of azirine formation from vinyl azides was demonstrated by Hassner and co-workers,25 who also isolated the bicyclic azirine 7 from 

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Vapor phase pyrolysis of two unsymmetrically substituted 1,2,3-triazole isomers, like (75) and (76), involves the antiaromatic l//-azirine derivative (77) which rearranges to two isomeric 2//-azirines (78) and (79) (71CC1519, 73JCS(Pi)555> and other products. Iminocar-benes (80) are most likely involved as intermediates. Furthermore, nitrogen has been extruded from several IH- 1,2,3-triazoles by flash vacuum pyrolysis. 1-Alkyl-1,2,3-triazoles (81) give nitriles (82) and (83) (via Wolff rearrangement) and (hydroxy)isoquinolines (84) and (85) (by [1,4-H] transfer in the iminocarbene) (75JCS(P1)1). 

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... 

The dimethylamino-2//-azirine 39 undergoes exclusive C—C bond breaking by flash vacuum pyrolysis (350 C, 0.1 torr).50 In this case, the intermediate iminocarbene (or nitrile ylide) 40 is stabilized by the amino group. Only the carbene product 42 and not the nitrene product 41 was isolated (Eq. 13). 

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