Kinetics and mechanism of oxidation of aryl methyl sulfoxides with (salen)Mn III/H 2O 2 catalytic system

Abstract

The kinetics of (salen)Mn III 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)Mn III, 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 H 2O 2 and sulfoxide has been found to be 1:1 and the product analysis confirms the formation of sulfone and the regeneration of (salen)Mn III 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, k 2 have been evaluated. The proposed mechanism has been well supported by electronic-oxidant and electronic-substrate effect studies.