Kinetics and mechanism of the tungsten-catalyzed oxidation of organic sulphides and alkenes by hydrogen peroxide

The oxidation of a series of phenyl-substituted arylmethyl sulphides and of cyclohexene and 1-methylcyclohexene by hydrogen peroxide in the presence of catalytic amounts of WO5· HMPT · H2O and W(CO)6 has been studied in ethanol (HMPT = hexamethyl phosphoric acid triamide). The reactions afford the corresponding sulphoxides and epoxides in quantitative yield. A subsequent ethanolysis of the oxirane ring occurs in solution. Kinetic studies indicate that the reaction is first-order in the substrate and in the catalyst, whereas a zero-order in hydrogen peroxide is found. The identity of the catalytic efficiency of the two W(VI) species employed suggests that HMPT ligand is displaced in solution. The substituent effect on the oxidation rates of arylmethyl sulphides follows the Hammett free-energy relationship (ρ{variant} ≈ -1). Sulphides are much more reactive than cyclohexene which, in turn, is 2.5-fold less reactive than 1-methylcyclohexene. Reaction rates are remarkably enhanced by addition of CH3SO3H, whereas added HMPT at relatively high concentration inhibits the oxidation. The data are discussed in the light of the results of previous studies on metal-hydrogen peroxide systems, pointing out similarities and differences. The conclusion is reached that W(VI)-H2O2 is a particularly efficient oxidant.