Aqueous biphasic oxidation: A water-soluble polyoxometalate catalyst for selective oxidation of various functional groups with hydrogen peroxide

A "sandwich" type polyoxometalate, Na₁₂[(WZn ₃(H₂O)₂][(ZnW₉O₃4) ₂], was used as an oxidation catalyst in aqueous biphasic reaction media to effect oxidation of alcohols, diols, pyridine derivatives, amines and aniline derivatives with hydrogen peroxide. The catalyst was shown by ¹⁸³W NMR to be stable in aqueous solutions in the presence of H ₂O₂ and showed only minimal non-productive decomposition of the oxidant. Secondary alcohols were selectively oxidized to ketones, while primary alcohols tended to be oxidized to the corresponding carboxylic acids, although secondary alcohols were selectively oxidized in the presence of primary alcohols. Vicinal diols yielded carbon-carbon bond cleavage products in very high yields. Pyridine derivatives were oxidized to the respective TV-oxides, but strongly electron-withdrawing moieties inhibited the oxidation reaction. Primary amines were oxidized to the oximes, but significantly hydrolyzed in situ. Aniline derivatives were oxidized to the corresponding azoxy or nitro products depending on the substitution pattern in the aromatic ring. Catalyst recovery and recycle was demonstrated.