Metal catalysis in oxidation by peroxides. Part 2. Molybdenum catalysed oxidation of organosulphur compounds by t-butyl hydroperoxide

Abstract

The reaction of di-n-butyl sulphide with t-butyl hydroperoxide in dry ethanol at 25°, catalysed by dioxomolybdenum(VI) acetylacetonate, affords di-n-butyl sulphoxide in quantitative yield. The kinetics conform to a rate law which is first order in sulphide and hydroperoxide; in the range of catalyst concentrations from 1.0 × 10^–7 to ca. 200 × 10^–7M, it is first order in the molydbdenum chelate, allowing a catalytic third-order rate constant (k₃) of 162 l² mol^–2 s^–1 to be estimated. This is ca. 2 × 10⁴ higher than the k₃ value determined for catalysis by methanesulphonic acid, a typical strong protic acid, in the same solvent. At a given sulphide and metal catalyst concentration, dependence of rates on the initial concentration of Bu^tO₂H suggests that a Michaelis–Mententype rate law holds. A Lineweaver–Burk type treatment of data allows values of 0.56 mol l^–1 for K′(a constant related to the dissociation constant of a postulated intermediate hydroperoxide–metal ion complex) and 103 l mol^–1 s^–1 for k_II(the limiting specific rate constant for rate-determining nucleophilic attack by the sulphide on the metal ion-activated peroxide molecule) to be estimated. The relative rates of oxidation of some representative substrates were found to decrease in the order Buⁿ₂S (100) > Buⁿ₂SO (0.15) cyclohexene (0.01), indicating that the molybdenum-catalysed reaction is quite sensitive to substrate nucleophilicity.