Photochemical ring-opening in 2,3-diphenyl aziridines. Transient-spectral and kinetic behavior of azomethine ylides and related photointermediates.

By employing laser pulses at various wavelengths for nanosecond flash photolysis, a comprehensive time-resolved study has been performed on transient azomethine ylides photogenerated from several 2,3-diphenyl aziridines in fluid solutions under three different conditions, namely, by direct 266 nm excitation, under reversible electron-transfer sensitization by 1,4-dicyanonaphthalene singlet excited state, and via energy transfer from acetone triplet. Under each of the three conditions of photoexcitation, azomethine ylides are readily formed as transient species, characterized by broad, structureless absorption spectra with maxima at 470-500 nm and mostly complex decay kinetics in mu s-ms time domain. Under acetone triplet sensitization, a second, shorter-lived transient species with absorption maximum at similar to 360 nm is observed to grow and decay in the same time range as that of the growth of ylides. This species has been identified as the ring-opened precursor ylide triplet. The azomethine ylides are practically nonquenchable by oxygen, except that under acetone triplet sensitization in air-saturated acetonitrile, their decay is significantly enhanced. The latter is explained in terms of quenching through dipolarophilic reaction with singlet oxygen. A value of 1.6 X 10(9) M-1 s(-1) has been estimated for the rate constant for reaction between singlet oxygen and ylide from trans-2,3-diphenylaziridine. We also report rate constants, in the range 2 x 10(3) to 4 x 10(9) M-1 s(-1), for the quenching of azomethine ylides by two dipolarophiles, namely, maleic anhydride and dimethyl acetylene dicarboxylate. The dipolarophilic reactivity of ylides carrying bulky substituents on the N atom is relatively subdued. Acetic acid proved to be a modest quencher of ylides with rate constants close to 10(6) M-1 s(-1).