Thioether Oxidation with H2O2 Catalyzed by Nb‐Substituted Polyoxotungstates: Mechanistic Insights

Nb‐monosubstituted polyoxotungstates of the Lindqvist and Keggin structures, (Bu4N)3[Nb(O)W5O18] (1) and (Bu4N)4[PW11NbO40] (2), respectively, catalyze oxidation of organic sulfides with aqueous H2O2 in acetonitrile. The corresponding peroxo complexes (Bu4N)3[Nb(O2)W5O18] (3) and (Bu4N)4[PW11Nb(O2)O39] (4), formed upon interaction of 1 and 2 with H2O2, are able to oxidize sulfides to sulfoxides and sulfones under stoichiometric conditions. The product analysis of the oxidation of thianthrene 5‐oxide, competitive sulfide–sulfoxide oxidation and Hammett plot implicated electrophilic oxidative properties of 4 and a complex oxidative nature of 3. Protonation of peroxo complex 3 leading to the formation of the peroxo species (Bu4N)2[HNb(O2)W5O18] (5) strongly increases its electrophilicity, which has a great impact on its reactivity and sulfoxidation selectivity. Nb‐peroxopolyoxotungstates of the Lindqvist and Keggin structures, (Bu4N)3[Nb(O2)W5O18] and (Bu4N)4[PW11Nb(O2)O39], catalyze oxidation of thioethers with H2O2 and are able to oxidize them under stoichiometric conditions to produce sulfoxides and sulfones. Protonation of the Lindqvist peroxo complex strongly increases its reactivity and sulfoxidation selectivity.