title: Regioselectivity in the reductive bond cleavage of diarylalkylsulfonium salts: variation with driving force and structure of sulfuranyl radical intermediates.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Rochester, Rochester, New York 14627, USA. kamp@chem.rochester.edu
authors.forename: Jack A
authors.initials: JA
authors.lastname: Kampmeier
authors.forename: A K M Mansurul
authors.initials: AK
authors.lastname: Hoque
authors.forename: Franklin D
authors.initials: FD
authors.lastname: Saeva
authors.forename: Donald K
authors.initials: DK
authors.lastname: Wedegaertner
authors.forename: Pia
authors.initials: P
authors.lastname: Thomsen
authors.forename: Saif
authors.initials: S
authors.lastname: Ullah
authors.forename: Jacob
authors.initials: J
authors.lastname: Krake
authors.forename: Torben
authors.initials: T
authors.lastname: Lund
mesh_terms: D018360:Crystallography, X-Ray; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D012492:Salts / Q000737:chemistry; D013237:Stereoisomerism; D013440:Sulfides / Q000737:chemistry*; D013452:Sulfonium Compounds / Q000737:chemistry*
chemical_list: D005609:Free Radicals; D012492:Salts; D013440:Sulfides; D013452:Sulfonium Compounds