Mechanism of Sulfoxide Formation through Reaction of Sulfur Radical Cation Complexes with Superoxide or Hydroxide Ion in Oxygenated Aqueous Solution

We have characterized and quantified several pathways which transform aliphatic sulfur radical cations into sulfoxides in aqueous solution. Sulfur radical cations were produced photochemically via one-electron photooxidation through triplet 4-carboxybenzophenone. Sulfur radical cations and superoxide yield sulfoxide, confirmed by oxygen product isotope effects and an inhibitory role of superoxide dismutase. On the basis of competition experiments with superoxide dismutase the rate constant for the reaction between dimethylsulfide radical cations and superoxide was derived as (2.3 ± 1.2) × 1011 M-1 s-1. A demetalated variant of superoxide dismutase did not inhibit superoxide mediated sulfoxide formation, confirming the importance of an active site of the enzyme for inhibition. The stoichiometry of 2 equiv of sulfoxide per reaction of superoxide with a sulfur radical cation suggests a pathway like the singlet oxygen mediated sulfoxide formation, i.e., via a persulfoxide intermediate formed via (i) direct coupling of superoxide with the sulfur radical cation or (ii) electron transfer followed by addition of the product singlet oxygen to a nonoxidized sulfide. In aqueous solution the persulfoxide may add water to yield a hydroperoxy sulfurane prior to its reaction with a second nonoxidized sulfide. At pH values larger than 9, hydroxide ion starts to compete with superoxide for sulfur radical cations and reacts with the persulfoxide or hydroperoxy sulfurane intermediates, initiating less effective pathways of sulfoxide formation. One pathway involves the formation of hydroxysulfuranyl radicals and their reaction with oxygen, supported by product and solvent isotope effects. Besides superoxide and hydroxide-mediated sulfoxide formation there is an additional route involving methylthiomethylperoxyl radicals. Based on oxygen product isotope effects, the latter appear to transfer oxygen onto the sulfide rather than reacting via electron transfer.