Photochemistry of triarylsulfonium salts

Article abstract of DOI:10.1021/ja00172a015

The photolysis of triphenylsulfonium, tris(4-methylphenyl)sulfonium, tris(4-chlorophenyl)sulfonium, several monosubstituted (4-F, 4-Cl, 4-Me, 4-MeO, 4-PhS, and 4-PhCO), and disubstituted (4,4′-Me₂ and 4,4′-(MeO)₂) triphenylsulfonium salts was examined in solution. It was found that direct irradiation of triphenylsulfonium salts produced new rearrangement products, phenylthiobiphenyls, along with diphenyl sulfide, which had been previously reported. Similarly, the triarylsulfonium salts, with the exception of the [4-(phenylthio)phenyl]diphenylsulfonium salts, gave the new rearrangement products. The mechanism for direct photolysis is proposed to occur from the singlet excited state to give a predominant heterolytic cleavage along with some homolytic cleavage. The heterolytic cleavage gives phenyl cation and diphenyl sulfide, whereas homolytic cleavage gives the singlet phenyl radical and diphenylsulfinyl radical cation pair. These pairs of intermediates then produce the observed photoproducts by an in-cage recombination mechanism and also by reactions with the solvent. The effect of solvent viscosity, solvent polarity, anion, and aryl substituent was examined. The triplet sensitization of the sulfonium salts was also investigated. In contrast to previous reports, the triplet state of the sulfonium salt is labile, leading to a triplet geminate radical pair of phenyl radical and diphenylsulfinyl radical cation. These species ultimately form benzene and diphenyl sulfide as products. Direct photolysis of the [4-(phenylthio)phenyl]diphenylsulfonium salt, gave exclusively diphenyl sulfide, benzene, and acid and decomposes via the triplet excited state.