MECHANISM OF OXIDATION OF ALIPHATIC THIOETHERS TO SULFOXIDES BY HYDROXYL RADICALS - THE IMPORTANCE OF MOLECULAR-OXYGEN

The sulfoxide yield in the .OH radical-induced oxidation of dimethylsu lfide in aqueous solution is dramatically enhanced by admission of mol ecular oxygen. Various possible mechanistic pathways leading to sulfox ide can now be eliminated from speculation as a result of a detailed s tudy on the influence of sulfide concentration, pH, isotope effects, r adical cation stability and deprotonation kinetics, and electron-trans fer reactions from transient radicals to special additives. The major route to sulfoxide is demonstrated to involve a 2sigma/1sigma three-e lectron-bonded radical cation, (>S:S<)+, which, after reaction with OH - (H2O), generates a transient neutral sulfuranyl-type radical, >S(OH) S.<. The latter is prone to O2-addition, and the thioperoxyl radical, >S(OH)S(OO.)<, generated thereby decays intramolecularly into one mole cule each of sulfoxide, sulfide, and superoxide. The mechanism may be generalized to the free radical-induced oxidation of any sulfide (at l east aliphatic ones) and would thus potentially be of interest, for ex ample, for the stability of sulfide-based drugs or biological material such as methionine-containing peptides and proteins.