A Water-Soluble and “Self-Assembled” Polyoxometalate as a Recyclable Catalyst for Oxidation of Alcohols in Water with Hydrogen Peroxide

Several catalytic methods for the oxidation of alcohols to carbonyl compounds have been published in recent years and another has just been reported.  Na₁₂[WZnZn₂-(H₂O)₂(ZnW₉)₃₄)₂] which can be prepared by drop-wise addition of zinc nitrate to a nitric acid solution of sodium tungstate, is an effective catalyst for which oxidises secondary alcohols to ketones (yields ~ 95%) and activated primary alcohols to carboxylic acids (benzyl alcohol gives 100% benzoic acid).

Unactivated primary alcohols such as pentanol do not give such good results (66% conversion to a mixture of pentanal (9%) and pentanoic acid (91%) , and 61% conversion to a 41:59 mixture of pentanal and pentanoic acid in the presence of TEMPO).   1,3-Diols, such as 2-ethyl-1,3-hexanediol, are oxidised to b –ketoacids in good yield.

All the examples cited were carried out in a two phase system with the aqueous polyoxometalate oxidant and the neat (liquid) alcohol.

Other examples include the successful oxidation of 2-butyl-4-chloro-5-hydroxymethylimidazole to the corresponding carboxylic acid (> 95% yield), but the reaction failed with 1-cyclohexyl-3,3,3-trifluoro-2-propanol.   ( J. Am. Chem. Soc. , 2003, 125, 5280-5281).