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

Alkanes (cyclooctane, n-octane, adamantane, ethane) can be efficiently oxid ized by hydrogen peroxide in acetonitrile using tetra-n-butylammonium salts of the vanadium-containing polyphosphomolybdates [PMo11 VO40](4-) and [PMo 6V5O39](12-) as catalysts. The oxidation of alkanes gives rise to the corre sponding alkyl hydroperoxides as the main products, which slowly decompose in the course of the reaction to produce the corresponding ketones (aldehyd es) and alcohols. The reaction in acetic acid and water is much less effici ent. The oxidation of cyclooctane at 60 degreesC in acetonitrile gives with in 9 h oxygenates with turnover numbers > 1000 and yields > 30% based on th e alkane. Pyrazine-2-carboxylic acid added as co-catalyst accelerates the r eaction but does not enhance the product yield. The oxidation of the cis- a nd trans-isomers of decalin proceeds without retention of configuration. Th e mechanism assumed involves the reduction of V(V) to V(IV) by a first mole cule 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 re act with aerobic oxygen. The alkyl peroxy radicals thus formed are then con verted into alkyl hydroperoxides. (C) 2001 Elsevier Science B.V. All rights reserved.