PRODUCTS AND KINETICS OF PHOTOREDUCTION OF ACETOPHENONE BY AMINES AND ALCOHOLS

Photoreduction of acetophenone by 2-butylamine leads to acetophenone pinacol and to N-2-butylidene-2-butylamine. Photoreduction of acetophenone and of p-methylacetophenone by α-methylbenzylamine leads to the three products of coupling of the radicals formed by the initial abstraction of hydrogen, pinacols, amino alcohols, and diamines. The radicals derived from the purely aliphatic alcohols and amines reduce a second molecule of ketone. The aryl alkyl ketone- and amine-derived radicals are of similar stability and both survive and couple. Acetophenone is not photoreduced by α-methylbenzyl acetate, and is photoreduced with low efficiency by N-acetyl-α-methylbenzylamine. Quantum yield for photoreduction of acetophenone by 1 M α-methylbenzyl alcohol in benzene is 0.37, by 0.5 M α-methylbenzylamine is 0.49, and by 0.5 M 2-propanol is 0.75. Light absorbing transients are formed to a greater extent and photoreduction is less efficient at high concentrations of each of these reducing agents. Efficiency of photoreduction first increases with dilution with benzene, and then decreases at high dilution. Extrapolation of the dilute solution values leads to hypothetical limiting quantum yields for photoreduction of acetophenone by α-methylbenzyl alcohol in benzene, ϕ 0.54; by 2-propanol in benzene, ϕ 1.2, with kd/kt = 0.28 M; by α-methylbenzylamine in benzene, ϕ 0.61, with kd/kir = 0.049 M. For photoreduction by this amine a Stern-Volmer treatment of quenching by naphthalene leads to kqkir = 84, kir = 7.1 × 107 M−1 sec−1, and kd = 3.5 × 106 sec−1. Photoreduction of acetophenone by 1 M 2-butylamine in benzene, ϕ ∼1.1, is 1.40 as efficient as in neat 2-butylamine. A light-absorbing transient is not formed in this amine, and the lower efficiency in this neat amine may be due to π,π* character in ketone triplet in this medium, or to deactivating solvation of ketone and triplet by the amine. © 1969, American Chemical Society. All rights reserved.