Arene and Phenol oxidation with hydrogen peroxide using ‘sandwich’ type substituted polyoxometalates as catalysts

The fused polycyclic systems (phenantrene, anthracene, naphthalene), naphthols (1 and 2), phenol, polyalkyl-substituted phenols (hydroquinone, catechol, m-methoxyphenol, o-cresol) and toluene were catalytically oxidized with H2O2 using a lacunary phosphotungstic acid derivative whose formula is Na8H[ΔPW9O34] · 19H2O (Δ-PW9). The reaction was carried out under homogeneous conditions of catalysis at 55 °C, using acetone as solvent. Under such conditions, the naphthols were selectively oxidized in the presence of phenols. It was observed that the conversion to quinones from naphthols was higher than that from polycyclic fused systems.Considering the performance of the above mentioned reactions, the 2-naphthol oxidation was selected to test the complex heteropolytungstate catalysts. These catalysts are derived from the coupling of two fragments [PW9O34] (lacunar, Keggin-like, Δ-PW9) of general formula K10[(PW9O34)2M4(H2O)2] · 20H2O in which M stands for Co, Zn and Mn divalent ions. The reaction was evaluated by using acetone and/or acetonitrile solvents at their reflux temperatures. Under such conditions, the β-naphthol was selectively oxidized to 1,2-naphthoquinone. The catalytic selectivity and the influence of the heteropolytungstate metallic clusters on the primary lacunar phase were also comparatively analyzed.