Regioselectivity in the Reductive Bond Cleavage of Diarylalkylsulfonium Salts: Variation with Driving Force andStructure of Sulfuranyl Radical Intermediates

This investigation was stimulated by reports that one-electron reductions of monoaryldialkylsulfonium
salts never give aryl bond cleavage whereas reductions of diarylmonoalkylsulfonium salts
preferentially give aryl bond cleavage. We studied the product ratios from the reductive cleavage of di-4-
tolylethylsulfonium and di-4-tolyl-2-phenylethylsulfonium salts by a variety of one-electron reducing agents
ranging in potential from -0.77 to +2.5 eV (vs SCE) and including thermal reductants, indirect electrolyses
mediated by a series of cyanoaromatics, and excited singlet states. We report that the cleavage products
vary from regiospecific alkyl cleavage to predominant aryl cleavage as a function of the potential of the
reducing agent. We conclude that differences between the reductive cleavages of mono- and diarylsulfonium
salts are direct consequences of the structures of the sulfuranyl radical intermediates and the bond
dissociation energies of the alkyl and aryl bonds. Competitions between the rates of cleavage and oxidation
of the intermediate sulfuranyl radicals and between concerted and stepwise mechanisms are discussed to
explain the variations in bond cleavage products as a function of the driving forces for the reductions.
Density functional theory investigations of the nature of the antibonding S-alkyl and S-aryl orbitals of the
starting sulfonium salts provide additional insight.