Photolysis of alkyl aryl sulfoxides: alpha-cleavage, hydrogen abstraction, and racemization

The photochemistry of a series of alkyl aryl sulfoxides is described. The initial event of the photolysis process is homolytic cleavage to form sulfinyl/alkyl radical pairs. The radical pair partitions between recombination to starting material, formation of sulfenic esters, disproportionation to an olefin and benzenesulfenic acid, and formation of typical radical escape products. The quantum yield for conversion depends on the structure or the reactivity of the alkyl radical, with the sequence benzyl > tertiary alkyl > secondary alkyl > primary alkyl > (di)aryl. The high racemization efficiency of some aryl primary-alkyl sulfoxides suggests that another nonradical pathway for the photoracemization process may exist. Product analysis does not support any hydrogen abstraction pathways.