Azirines, nitrile ylide structure photolysis

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. 

It was found that the azirine-nitrile ylide isomerization was a completely reversible process. The unlabeled nitrile ylide showed a prominent band at 1926 cm that underwent a 66-cm shift with N substitution. This shift was interpreted as being consistent with an allene-like skeleton (8) rather than the alternative pwpargyl-like stmcture (9). This conclusion was supported by the spectra from the C- and H-labeled variants. Warming the nitrile ylide in a xenon matrix from 12 to 82 K provided no new absorptions suggesting that the allene-like structure may also be adopted in solution. Some absorption spectra for benzonitriho benzylide (DPNY) and some substituted benzonitrilio methylides obtained via pulsed-laser photolysis of azirines are given in Table 7.1 (5). 

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. 

Work on the molecular structure of benzonitrilio methylide 8/9 has been carried out via Fourier transform infrared (FTIR) smdies on it and five isotope-labeled variants. The nitrile ylides were generated in nitrogen matrices at 12 K either directly, by photolysis of the azirine 7, or indirectly from the azidostyrenes 6 (4). 

---