Alkane oxidation with hydrogen peroxide catalyzed homogeneously by vanadium-containing polyphosphomolybdates

Alkanes (cyclooctane, n-octane, adamantane, ethane) can be efficiently oxidized by hydrogen peroxide in acetonitrile using tetra-n-butylammonium salts of the vanadium-containing polyphosphomolybdates [PMo11 VO40](4-) and [PMo6V5O39](12-) as catalysts. The oxidation of alkanes gives rise to the corresponding alkyl hydroperoxides as the main products, which slowly decompose in the course of the reaction to produce the corresponding ketones (aldehydes) and alcohols. The reaction in acetic acid and water is much less efficient. The oxidation of cyclooctane at 60 degreesC in acetonitrile gives within 9 h oxygenates with turnover numbers > 1000 and yields > 30% based on the alkane. Pyrazine-2-carboxylic acid added as co-catalyst accelerates the reaction but does not enhance the product yield. The oxidation of the cis- and trans-isomers of decalin proceeds without retention of configuration. The mechanism assumed involves the reduction of V(V) to V(IV) by a first molecule of hydrogen peroxide, followed by the reaction of V(IV) with a second H2O2 Molecule to generate hydroxyl radicals. The latter abstract a hydrogen atom from the alkane, RH, leading to alkyl radicals, R-., which rapidly react with aerobic oxygen. The alkyl peroxy radicals thus formed are then converted into alkyl hydroperoxides. (C) 2001 Elsevier Science B.V. All rights reserved.