Reduction of permanganate by thioanisole: Lewis acid catalysis

The oxidation of sulfides and sulfoxides by permanganate in anhydrous aceto ne solutions is catalyzed by Lewis acids such as iron(III) chloride, zinc c hloride, and mercury(II) chloride. The reaction kinetics unequivocally conf irm that the function of these catalysts is to activate the oxidant by form ing permanganate/Lewis acid complexes analogous to the protonation of MnO4- by Bronsted acids. A Hammett analysis of the rate constants for the oxidat ion of a series of substituted thioanisoles gives a negative rho value (-1. 11) indicative of an electron deficient transition state. No secondary kine tic isotope effect is observed when the hydrogens of the methyl group are r eplaced by deuterium. Despite previous observations that sulfoxides are pre ferentially oxidized in competitive experiments, sulfides are oxidized more rapidly when individual rates are measured. All of these observations are most consistent with a mechanism in which the reductant reacts with the oxi dant via initial ligation.