Oxalic acid-improved mild cyclohexane oxidation catalyzed by VO(acac)2: non-radical versus radical mechanism

Abstract

The efficiency of the vanadyl(IV)acetylacetonate (1)–catalyzed oxidation of cyclohexane by hydrogen peroxide in acetonitrile increased in the presence of oxalic acid (2). The addition of 2 leads to the substitution of acac ligands by oxalate resulting in formation of vanadyl(IV)oxalate (3). That promotes the formation of cyclohexanol, cyclohexanone and cyclohexylhydroperoxide principal products. The observed alteration in the product ratio was subjected to the mixed mechanism of substrate transformation: mainly the free radical in 2–free process and radical/non-radical one in the presence of oxalic acid. In the last case, the putative VO(η2-O2)(oxalate) intermediates can directly transfer the bounded oxygen onto the substrate without the involvement of free radicals. The obviousness of two mechanisms has been shown by comparison of 2 and the influence of free radical scavenger N-benzylidene-tert-butylamine (InH) on the process kinetics.