Highly Selective Oxidation of Alkylphenols to p-Benzoquinones with Aqueous Hydrogen Peroxide Catalyzed by Divanadium-Substituted Polyoxotungstates

The catalytic performance of divanadium- and dititanium- substituted gamma-Keggin polyoxotungstates, TBA(4)H[gamma-PW10V2O40] (I, TBA = tetra-n-butylammonium), TBA(4)H(2)[gamma-SiW10V2O40] (II), and TBA(8)[{gamma-SiW10Ti2O36(OH)(2)}(2)(mu-O)(2)] (III) has been assessed in the selective oxidation of industrially important alkylphenols/naphthols with the green oxidant 35% aqueous H2O2. Phosphotungstate I revealed a superior catalytic performance in terms of activity and selectivity and produced alkylsubstituted p-benzo- and naphthoquinones with good to excellent yields. By applying the optimized reaction conditions, 2,3,5-trimethyl-p-benzoquinone (TMBQ vitamin E key intermediate) was obtained in a nearly quantitative yield via oxidation of 2,3,6-trimethylphenol (TMP). The efficiency of H2O2 utilization reached 90%. The catalyst retained its structure under turnover conditions and could be recycled and reused. An active peroxo vanadium complex responsible for the oxidation of TMP to TMBQ has been identified using V-51 and P-31 NMR spectroscopy.