Highly Selective Oxidation of Alkylphenols to <i>p</i>‑Benzoquinones with Aqueous Hydrogen Peroxide Catalyzed
by Divanadium-Substituted Polyoxotungstates
- Publication date
- Publisher
Abstract
The
catalytic performance of divanadium- and dititanium-substituted
γ-Keggin polyoxotungstates, TBA<sub>4</sub>H[γ-PW<sub>10</sub>V<sub>2</sub>O<sub>40</sub>] (<b>I</b>, TBA = tetra-<i>n</i>-butylammonium), TBA<sub>4</sub>H<sub>2</sub>[γ-SiW<sub>10</sub>V<sub>2</sub>O<sub>40</sub>] (<b>II</b>), and TBA<sub>8</sub>[{γ-SiW<sub>10</sub>Ti<sub>2</sub>O<sub>36</sub>(OH)<sub>2</sub>}<sub>2</sub>(μ-O)<sub>2</sub>] (<b>III</b>) has
been assessed in the selective oxidation of industrially important
alkylphenols/naphthols with the green oxidant 35% aqueous H<sub>2</sub>O<sub>2</sub>. 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 H<sub>2</sub>O<sub>2</sub> 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 <sup>51</sup>V and <sup>31</sup>P NMR spectroscopy