Laser flash photolysis of aziridines. Spectroscopic and kinetic characterization of azomethine ylides. Their [3 + 2] cyclization with alkenes and protonation by water–alcohols to yield iminium ions1

Abstract

The reactive intermediates generated by laser flash photolysis (λ_exc = 248 or 193 nm) of the aziridines (E)-1-butyl-2,3-diphenylaziridine 1, (E)-2,3-diphenylaziridine 2, 1,2-diphenylaziridine 3, 1-(p-methoxyphenyl)-2-phenylaziridine 4 and 1-butyl-2-phenylaziridine 5 in acetonitrile and alcohols as solvents were identified as the corresponding azomethine ylides and spectroscopically and kinetically characterized. The rate constants for the [3 + 2] reaction of the ylides with electron-deficient alkenes to give pyrrolidines are between 10⁶ and 10⁹M^–1 s^–1. In the case of 1 and 2 iminium ions are produced by protonation (by H₂O or ROH) of the ylides (also studied by time-resolved conductance). For this reaction, with MeOH as the proton donor, the kinetic isotope effect is k(MeO–H)/k(MeO–D) = 5–7, from which it is concluded that the transition state for protonation of the carbanionic site of the ylide is linear.