Metal catalysis in oxidation by peroxides. Part 5. On the mechanism of vanadium(V) catalysed oxidation by hydrogen peroxide

The oxidation of p-chlorophenyl methyl sulphide and of a series of phenyl substituted aryl methyl sulphides with hydrogen peroxide in the presence of catalytic amounts of bisacetylacetonato-oxovanadium(IV) [VaO(acac)2] in ethanol, at 25°C, has been studied. The reaction affords the parent sulphoxide in quantitative yield. Oxidation rates are little affected by substitution in the phenyl ring of the sulphide. Comparison with vanadium catalysed oxidation of sulphides by tert-butyl hydroperoxide shows that the Va(V)-H2O2 system is much more effective than Va(V)-ButO2H system. The initial rates of the reaction, which is first-order in sulphide and in vanadium catalyst, show a rather complicated dependence on the initial concentration of the peroxide. Initial rates are found to increase with increasing [H2O2]0 at low peroxide initial concentration, then decrease at higher [H2O2]0; both the positive and the negative dependence show a fractionary kinetic order. Also on the ground of a kinetic investigation of the Va(V) catalysed autodecomposition of H2O2, which proceeds at measurable rate only at much higher Va(V) concentrations than those employed in the oxidation experiments, the involvement of two peroxyvanadium(V) species in the oxidation mechanism, corresponding to the equilibrium addition of one and two molecules of H2O2 to Va(V), respectively, is suggested. Whereas the mono-adduct is believed to be a strong oxidant, the di-adduct appears to behave, in our system, as a much weaker oxidizing agent. This is likely related to the fact that, in the conditions adopted, the di-adduct may be largely ionized to its monoanionic form.