Radical cleavage and competing photoreactions of phenacyl sulfides

The photochemistry of ketones with the structures PhCOCH/sub 2/SR, PhCOCH/sub 2/S(O)R, PhCOCH/sub 2/SO/sub 2/R, and p-X-PhCOCH/sub 2/SPh has been studied. They all give primarily acetophenone as product when irradiated in the presence of benzenethiol, which traps free phenacyl radicals formed by excited state ..beta..-cleavage. The sulfur-centered radicals give coupling products. The maximum quantum yield for this ..beta..-cleavage is 0.40; apparently 60% of the initially formed radical pairs undergo in-cage reaction. When R = methyl or butyl, some acetophenone is formed by ..gamma..-hydrogen abstraction as well. Alkyl substituents on the ..cap alpha..-carbon enhance the disproportionation reactions of the phenacyl radicals. Measurements of quantum yields and triplet lifetimes (by Stern-Volmer quenching of acetophenone formation) allowed determination of rate constants for ..beta..-cleavage as follows: PhS, 10/sup 10/-10/sup 11/; MeS(O), 6 x 10/sup 9/; BuS, 1.5 x 10/sup 8/; BuSO/sub 2/, 1 x 10/sup 7/ s/sup -1/. Ring substituents increase triplet lifetimes. Absorption and phosphorescence spectra and indicate that the n,..pi..* and ..pi..,..pi.. transitions both involve some population of the C-S sigma* orbital. This mixing, together with the free spin density on the excited carbonyl carbon, appears to determine the rate constant for cleavage. Radical cleavage is also very fast and efficient for p-((phenylthio)methyl)acetophenone.