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 ions

Gaebert C, Siegner C, Mattay J, Toubartz M, Steenken S. 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 ions. JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2. 1998;(12):2735-2740.The reactive intermediates generated by laser flash photolysis (lambda(exc) = 248 or 193 nm) of the aziridines (E)-1-butyl-2,3-diphenylaziridine 1, (E)-2,3-diphenylaziridine 2, 1,2-dliphenylaziridine 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 10(6) and 10(9) M-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