Physical aspects of N-benzoylaziridine photochemistry: characterization of azomethine ylides and related photointermediates

Abstract

Results of a time-resolved study based on nanosecond laser flash photolysis are presented for phototransients derived from three N-benzoylaziridines in fluid solutions. The major transients, best assigned as azomethine ylides, are characterized by two maxima (380-440 and 560-610 nm) in the absorption spectrum in the visible region and by microsecond lifetimes at room temperature ( Τ^Y = 0.2-31 µs); these are sluggish in their reactivity toward dimethyl acetylenedicarboxylate (k_q^Y ≤ X l0⁴ M^-1 s^-1 in benzene). Relatively short-lived triplets (Τ_T = 1-90 ns in benzene), quenchable by biphenyl and 2,5-dimethyl-2,4-hexadienea,c t as precursors for the ylides. Between trawl -benzoyl-2,3-diphenylaziridin(el a) and its cis isomer (lb), T_T is considerably shorter for the former (1 ns for la vs 40 ns for lb, on the basis of the assumption that the rate constant for triplet quenching by 2,5-dimethyl-2,4-hexadieneis 2 X l0⁹ M^-1 s^-1 in benzene); this is explainable in terms of efficient intramolecular quenching of the benzamide-type triplet by a β-phenyl group in la while this interaction is less favorable in the most stable conformer of lb (wherein the benzoyl moiety protrudes away from the cis phenyl groups). Steady-state photolysis and product analysis studies have provided evidence for photocleavage of both C-N and C-C bonds of the aziridine ring.