Sulfoxidation with hydrogen peroxide catalyzed by [SeO4{WO(O2)2}2]2−

Abstract

The selenium-containing dinuclear peroxotungstate, [(n-C₄H₉)₄N]₂[SeO₄{WO(O₂)₂}₂] (I), acts as a homogeneous catalyst for the selective oxidation of various kinds of cyclic mono- and disulfides with 30% aqueous H₂O₂. The cyclic disulfides were selectively oxidized to the corresponding monosulfoxides with one equivalent of H₂O₂ with respect to the sulfides. In the presence of two equivalents of H₂O₂, the oxidation of dibenzothiophene gave the corresponding sulfone in 98% yield under the mild conditions. The negative Hammett ρ value (−0.62) for the competitive oxidation of p-substituted thioanisoles and the low X_SO (X_SO = (nucleophilic oxidation)/(total oxidation)) value of 0.14 for the I-catalyzed oxidation of thianthrene 5-oxide (SSO) revealed that I is a strong electrophilic oxidant. The reactivities of the di- and tetranuclear peroxotungstates with XO₄ⁿ⁻ ligands (X = Se(VI), As(V), P(V), S(VI), and Si(IV)) were strongly dependent on the kinds of hetero atoms. The reaction rates for the sulfoxidation decreased with an increase in the X_SO values and a peroxotungstate with a stronger electrophilicity was more active for the sulfoxidation. The kinetic and mechanistic investigations showed that the electrophilic attack of the peroxo oxygen at the sulfur atom is a key step in the sulfoxidation. The computational investigation supported the high chemoselectivitiy for the sulfoxidation of diallyl sulfide.