Highly Selective Oxidation of Alkylphenols to <i>p</i>‑Benzoquinones with Aqueous Hydrogen Peroxide Catalyzed by Divanadium-Substituted Polyoxotungstates

The catalytic performance of divanadium- and dititanium-substituted γ-Keggin polyoxotungstates, TBA₄H­[γ-PW₁₀V₂O₄₀] (<b>I</b>, TBA = tetra-<i>n</i>-butylammonium), TBA₄H₂[γ-SiW₁₀V₂O₄₀] (<b>II</b>), and TBA₈[{γ-SiW₁₀Ti₂O₃₆(OH)₂}₂(μ-O)₂] (<b>III</b>) has been assessed in the selective oxidation of industrially important alkylphenols/naphthols with the green oxidant 35% aqueous H₂O₂. Phosphotungstate <b>I</b> revealed a superior catalytic performance in terms of activity and selectivity and produced alkylsubstituted <i>p</i>-benzo- and naphthoquinones with good to excellent yields. By applying the optimized reaction conditions, 2,3,5-trimethyl-<i>p</i>-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 ⁵¹V and ³¹P NMR spectroscopy