Benzophenone-Sensitized Photolysis of N,N-Dibenzyl-O-acylhydroxylamines. Mechanism of the Imine-Forming Radical Elimination Reaction

Article abstract of DOI:10.1246/bcsj.64.2140

Title hydroxylamines (1) undergo benzophenone-sensitized photolysis to give N-benzylidenebenzylamine (2), p-substituted benzoic acids (3), and monosubstituted benzenes (4) derived from decarboxylation of p-substituted benzoyloxyl radicals.Analysis of the linear Stern-Volmer plots, both for the sensitized photolysis of 1 and for the phosphorescence quenching of benzophenone by 1, demonstrates the participation of triplet 1 in this photolysis.The logarithm of the ratio (k_r/k_d), where k_r is the rate constant for homolytic cleavage of the N-O bond in triplet 1 and k_d is that for its deactivation, used as a measure of the triplet-state reactivities of 1, exhibited a good Hammett-type relationship against the substituent constant (?) with a slope (ρ) of -0.75.This indicates the exclusive occurence of homolysis of the N-O bond in the first excited triplet state of 1.Thus the decomposition of 1 activated by triplet benzophenone is classified as an imine-forming radical elimination.The results of deuterium isotope and micellar effects on the sensitized photolysis suggest that the "out-of-cage" products 4 are obtained by decarboxylation and subsequent hydrogen abstraction of p-substituted benzoyloxyl radicals which escaped from the triplet cage in competition with spin inversion to the singlet cage.In this singlet cage, hydrogen abstraction giving 2 and 3 and recombination affording the starting 1 were found to take place exclusively.