Metal catalysis in oxidation by peroxides. Part 7. Ligand exchange and catalytic activity of molybdenum(VI) species in the oxidation of organic sulphides by t-butyl hydroperoxide

Abstract

The oxidation of di-n-butyl sulphide by t-butyl hydroperoxide catalysed by bis(acetylacetonato)dioxmolybdenum(VI)[MoO₂(acac)₂] has been studied in MeOH, EtOH, and PrⁱOH at 25°. The oxidation rates depend on the nature of the alcoholic solvent with the reactivity order PrⁱOH > EtOH > MeOH. Evidence, based on ¹H n.m.r. and kinetic analysis, is given to show that the acetylacetone ligands in MoO₂(acac)₂ are displaced by solvent alcohol and that two molybdenum(VI) species, with zero and one diketonate ligands bonded to the metal, respectively, are formed. The original bis(diketone) complex appears to be devoid of any catalytic activity, whereas the two intermediate Mo^VI species have similar efficiency as catalysts. In agreement with this observation it was found that added acetylacetone inhibits the oxidation. The precursor complex bis(dipivaloylmethanato)dioxomolybdenum(VI) was observed to have lower reactivity than MoO₂(acac)₂. A change of the hydroperoxide to BuⁿOOH in the MoO₂(acac)₂ catalysed oxidation has a negligible effect on the rate. The nature of the catalytic complexes and the mechanism of the oxygen-transfer process are discussed.