Kinetics and mechanism of thioether oxidation with H 2O 2 in the presence of Ti(IV)-substituted heteropolytungstates

Abstract

Kinetics of thioether oxidation with concentrated (86%) and diluted (35%) aqueous H 2O 2 in the presence of catalytic amounts of (Bu 4N) 7{[PW 11O 39Ti] 2OH} dimeric heteropolytungstate ( 1) has been studied in acetonitrile. With 86% H 2O 2 the reaction was found to be first order in the oxidant, 0.5 order in the catalyst and practically independent on the sulfide concentration and its nature, whereas with 35% H 2O 2 the reaction was first order in both the catalyst and substrate, the order in the oxidant being changed from first to zero with increasing H 2O 2 concentration. Both 31P NMR and kinetic data are consistent with mechanism, including fast dissociation of 1 to the monomer ( 2), the equilibrium formation of active hydroperoxo complex PW 11TiOOH ( B) from 2, and interaction of B with thioether. The rate-limiting steps are the formation of B and the interaction of B with sulfide at low and high H 2O concentrations, respectively. The kinetic study of stoichiometric reaction between B, generated in situ from inactive side-on peroxo complex PW 11TiO 2 ( A), and thioethers showed first order dependence on both B and sulfide concentration, thus indicating that no binding of thioether to Ti(IV) occurs. The lack of Hammett-type correlation and specific products obtained in oxidation of benzyl phenyl sulfide allowed to rule out electrophilic oxygen transfer and suggest a mechanism that involves the formation of a thioether cation radical intermediate.