Radical Cleavage and Competing Photoreactions of Phenacyl Sulfides

Article abstract of DOI:10.1021/ja00244a032

The photochemistry of ketones with the stuctures PhCOCH2SR, PhCOCH2S(O)R, PhCOCH2SO2R, and p-X-PhCOCH2SPh 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 β-cleavage.The sulfur-centered radicals give coupling products.The maximum quantum yield for β-cleavage is 0.40; apparently 60percent of the initially formed radical pairs undergo in-cage reaction.When R = methyl or butyl, some acetophenone is formed by γ-hydrogen abstraction as well.Alkyl subtituents on the α-carbon enhance the disproportionation reactions of the phenacyl radicals.Measurements of quantum yields and triplet life times (by Stern-Volmer quenching of acetophenone formation) allowed determination of rate constants for β-cleavage as follows: PhS, 1E10-1E11; MeS(O), 6 * 1E9; BuS, 1.5 * 1E8; BuSO2, 1 * 1E7 s^-1.Ring substituents increase triplet lifetimes.Absorption and phosphorescence spectra indicate that the n,?^* and ?,?^* transitions both involve some population of the C-S ?^* orbital.This mixing, together with 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.