Photochemical α Cleavage and Free-Radical Reactions of Some Deoxybenzoins

The effect of α-methyl and α-phenyl substituents on the photochemical α-cleavage reactions of alkyl phenyl ketones has been investigated, α cleavage is the only primary process observed upon irradiation of the deoxybenzoins 1-4 in degassed benzene solution. Photoreduction competes with a cleavage of deoxybenzoin in 2-propanol-benzene solution. The reactions of the benzoyl and benzyl free radicals formed upon α cleavage have been studied in some detail. Spectroscopic and energy transfer data indicate that α cleavage occurs exclusively from the lowest ³n,π^* excited state. Triplet lifetimes were measured by Stern-Volmer analysis of product quenching by naphthalene and, for deoxybenzoin, by quenching of room temperature phosphorescence. The effects of substituents upon triplet lifetimes indicate that the rate constants do not depend on the stability of the resulting radicals. From a comparison of the rate constants for photochemical α cleavage of the deoxybenzoins studied and the rate constants for thermolysis of corresponding peresters it is concluded that the transition state for α cleavage resembles the excited ketone rather than the radical pair.