Kinetics and mechanism of oxidation of aryl methyl sulfoxides with (salen)MnIII/H2O2 catalytic system

The kinetics of (salen)MnIII complexes-catalysed oxidation of aryl methyl sulfoxides with hydrogen peroxide in 80% acetonitrile–20% water solvent mixture at 25 °C has been followed spectrophotometrically. The reaction is first-order in (salen)MnIII, zero-order in hydrogen peroxide and fractional-order in sulfoxide. Also, it has been found that nitrogenous bases affect the oxidation, while free-radical inhibitor does not. However, an increase in the water content of the solvent mixture causes an increase in the rate of reaction. Stoichiometry between H2O2 and sulfoxide has been found to be 1:1 and the product analysis confirms the formation of sulfone and the regeneration of (salen)MnIII complex. These observations have been well analyzed in favor of a Michaelis–Menten type mechanism, involving a manganese(III)–hydroperoxide complex as the reactive species. Using the derived rate law, the oxidant–substrate complex formation constant, K and the oxidant–substrate complex decomposition rate constant, k2 have been evaluated. The proposed mechanism has been well supported by electronic-oxidant and electronic-substrate effect studies.

The oxidation of aryl methyl sulfoxides with H2O2 in the presence of (salen)MnIII complex is first-order in complex, zero-order in H2O2 and fractional-order in sulfoxide. A Michaelis–Menten type mechanism involving MnIII–hydroperoxide complex as reactive species has been proposed.Figure optionsDownload high-quality image (22 K)Download as PowerPoint slide