Flash photolysis studies of nitrile imines and related intermediates photogenerated from sydnones and tetrazoles in fluid solutions

The spectral and kinetic behavior of nitrile imines photogenerated from sydnones and tetrazoles in fluid solutions has been studied by laser and lamp flash photolysis. The nitrile imines are characterized by lifetimes of milliseconds and are quenchable by the dipolarophile dimethyl acetylenedicarboxylate (kq = (5 - 9) × 103 M-1 s-1 for N,C-diarylnitrile imines in benzene) and by carboxylic acids (kq = 108 - (2 × 109) M-1 s-1 with trifluoroacetic acid in benzene and acetonitrile). The phototransformation of 3,4-diaryl-sydnones to the corresponding N,C-diarylnitrile imines occurs rapidly (within nanoseconds of laser excitation); this suggests that bicyclic diaziridine, diazirine or 1,2,3-oxadiazolin-5-one intermediates, postulated in the literature as precursors for nitrile imines, are either very short lived or not involved at all. The laser flash photolysis of the sydnones unsubstituted at the 4-position or bearing a methyl group at this position gives rise to additional, fast-decaying, transient species (t = 0.3 - 4 µs in benzene) which become progressively longer lived upon interaction with hydroxylic reagents (water, alcohols and carboxylic acids). Possible assignments of these transient species in terms of ylide structures are discussed in the light of the results of steady state photolysis at low temperatures