Metal catalysis in oxidation by peroxides. Part II. Kinetics and mechanism of molybdenum-catalyzed oxidation of sulphides and alkenes with hydrogen peroxide

The oxidation of p-chlorophenylmethyl sulphide and of a series of phenylsubstituted arylmethyl sulphides, of p-chlorophenylmethylsulphoxide and 1-methylcyclohexene with hydrogen peroxide in the presence of catalytic amounts of three molybdenum derivatives, MoO2(acac)2, Mo(CO)6 and MoO5·HMPT·H2O has been studied in ethanol. The reactions afford the corresponding sulphoxides, the sulphone and the epoxide, respectively, in quantitative yields. Kinetic studies indicate that the oxidation rate is first-order in the substrate and catalyst but zero-order in hydrogen peroxide. The reactivity order observed for the three different substrate parallels their nucleophilicity, i.e. sulphide > sulphoside > alkene. The rates of sulphide oxidation, on the other hand, are very little affected by substitution in the phenyl ring. On the basis of the identity of the catalytic efficiency of the three Mo species employed, the oxidizing agent formed in solution is suggested to be the peroxocomplex MoO5(EtOH)2. The results will be discussed in the light of the two alternative mechanisms proposed for the oxygen transfer process, i.e. the normal 'electrophilic' mechanism and the two-step one involving prior coordination of the substrate. © 1981.