Reduction of permanganate by thioanisole: lewis acid catalysis
The oxidation of sulfides and sulfoxides by permanganate in anhydrous acetone solutions is catalyzed by Lewis acids such as iron(III) chloride, zinc chloride, and mercury(II) chloride. The reaction kinetics unequivocally confirm that the function of these catalysts is to activate the oxidant by forming permanganate/Lewis acid complexes analogous to the protonation of Mn by Bronsted acids. A Hammett analysis of the rate constants for the oxidation of a series of substituted thioanisoles gives a negative rho value (-1. 11) indicative of an electron deficient transition state. No secondary kinetic isotope effect is observed when the hydrogens of the methyl group are replaced by deuterium. Despite previous observations that sulfoxides are preferentially 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 oxidant via initial ligation.