Formation of intramolecular three-electron-bonded 2 sigma/1 sigma* radicalcations upon reduction of dialkylsulfinyl sulfides by H-atoms

The reaction of H-atoms with sulfinyl sulfides RS(CH2)(n)SOR' (R, R' = alky l) in strongly acidic aqueous medium leads, among other reactions, to the r eduction of the sulfoxide moiety. This results in the formation of a S.+ ra dical cation entity which coordinates with the original sulfide moiety to y ield the (>S thereforeS<)(+) three-electron-bonded 2<sigma>/1 sigma* radica l cation. The optical properties of this reductively generated species are identical to those for the transients obtained previously upon one-electron oxidation of dithiaalkanes. At sulfinyl sulfide concentrations in the 10(- 4)-10(-2) M range the H-.-atom-induced process occurs intramolecularly. The most stable three-electron-bonded radical cation is formed when the two in teracting sulfurs are linked via a -(CH2)(3)- chain and a five-membered rin g structure can be established. The yields of the (>S thereforeS<)(+)-type transients range from G = 0.2-0.7 (species per 10 J absorbed energy) with t he high value pertaining to the reduction of MeS(CH3)(3)SOMe (0.01 M) in aq ueous, N-2-purged, 3 M HClO4 solutions. These yields are significantly belo w the yield of H-<bullet>-atoms, indicating competing processes such as H-a tom abstraction and reaction of H-. with 2-methyl-2-propanol (which was add ed for removal of (OH)-O-. radicals). The rate constant estimate of (1.8 +/ - 0.2) x 10(7) M-1 s(-1) for the reaction of H-. + MeS(CH2)(3)SOMe appears to be typical with respect to the order of magnitude for the H-.-induced re duction of the sulfoxide moiety. Electrochemical cyclic voltammetry experim ents on the oxidation of RS(CH2)(n)SR', RS(CH2)(n)SOR', RSO(CH2)(n)SOR', an d RS(CH2)(n)SO2R' showed relatively little influence of the sulfur-linking alkyl chain length and the nature of the terminal alkyl substituents except for the dithiaalkanes. This may indicate the lack of any significant elect ronic interaction between the S and SO moieties in the alkylsulfinyl sulfid es, SO and SO, in the bis(alkylsulfinyl)alkanes, and S and SO2 in the alkyl sulfonyl sulfides. The electrochemical data further suggest that, concernin g RS(CH2)(n)SOR', the lower oxidation potential pertains to the oxidation o f the sulfide moiety (yielding RSO(CH2)(n)SOR'). The SO --> SO2 oxidation, on the other hand, requires potentials which are more positive by about 0.2 V. It appears that the RS(CH2)(3)SOR' --> RS(CH2)(3)SO2R' oxidation prevai ls when R' is electron density releasing, while an electron density releasi ng R favors the RS(CH2)(n)SOR' --> RSO(CH2)(3)SOR' oxidation.