Kinetics and mechanism of thioether oxidation with H2O2 in the presence ofTi(IV)-substituted heteropolytungstates

Kinetics of thioether oxidation with concentrated (86%) and diluted (35%) a queous H2O2 in the presence of catalytic amounts of (Bu4N)(7){[PW11O39Ti](2 )OH} dimeric heteropolytungstate (1) has been studied in acetonitrile. With 86% H2O2 the reaction was found to be first order in the oxidant, 0.5 orde r in the catalyst and practically independent on the sulfide concentration and its nature, whereas with 35% H2O2 the reaction was first order in both the catalyst and substrate, the order in the oxidant being changed from fir st to zero with increasing H2O2 concentration. Both P-31 NMR and kinetic da ta are consistent with mechanism, including fast dissociation of 1 to the m onomer (2), the equilibrium formation of active hydroperoxo complex PW11TiO OH (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 hi gh H2O concentrations, respectively. The kinetic study of stoichiometric re action between B, generated in situ from inactive side-on peroxo complex PW 11TiO2 (A), and thioethers showed first order dependence on both B and sulf ide concentration, thus indicating that no binding of thioether to Ti(IV) o ccurs. The lack of Hammett-type correlation and specific products obtained in oxidation of benzyl phenyl sulfide allowed to rule out electrophilic oxy gen transfer and suggest a mechanism that involves the formation of a thioe ther cation radical intermediate. (C) 2000 Elsevier Science B.V. All rights reserved.