Oxidation of thioethers and sulfoxides with hydrogen peroxide using TS-1 as catalyst

A combined experimental and molecular simulation study of the oxidation of thioethers with hydrogen peroxide using Ti-containing zeolites as catalysts is described and discussed. Two aspects of reaction selectivity were explored. First, regioselectivity was studied for the oxidation of allyl methyl thioether with TS-1 as catalyst, and only products for the oxidation at sulfur, i.e., the sulfoxide and sulfone, were observed. Second, shape-selective oxidation was studied using four isomeric butyl methyl thioethers. For n-, iso- and sec-butyl methyl thioethers the dominant product in the TS-1 catalysed reaction was the sulfone, but for tert-butyl methyl thioether, the dominant product was from partial oxidation to the sulfoxide. Molecular simulations were used to investigate the origin of this effect. For all substrates used in this study, the oxidation of the thioether to the sulfoxide was found to occur readily by a non-catalysed solution reaction and this was studied in detail. However, the oxidation of the sulfoxide to the sulfone was only observed in the catalysed reactions. It was observed that the non-catalysed reaction can be suppressed by carrying out the catalysed reaction in the presence of a base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), that is too large to diffuse into the intracrystalline pore structure of TS-1. In the presence of DBU, the rate of reaction with TS-1 as catalyst was much lower and the relative ratios of the sulfoxide and sulfone products can be explained through a combination of intramolecular steric hindrance and the shape selectivity of the zeolite.