Oxidations of cycloalkanes and benzene by hydrogen peroxide catalyzed by an {FeIIIN2S2} centre

The iron(III) complex [Fe(gma)(PBu3)] [H-2(gma)=glyoxal-bis(2-mercaptoanil)], bearing an (FeN2S2) centre, was found to act as a selective catalyst or catalyst precursor towards the oxidation, under mild conditions, of (i) cycloalkanes (cyclopentane and cyclohexane) to the corresponding cyclic alcohols and cyclic ketones, and (ii) benzene to phenol, by aqueous H2O2 in CH3CN and in the presence of pyrazinecarboxylic acid (Hpca) or nitric acid as additives. The system exhibits a good selectivity towards the alcohols (typically ca. 90% upon deoxygenation of the formed cycloalkyl hydroperoxides by PPh3). The influence of various factors, such as temperature, amounts of reagents and reaction time, were investigated in a systematic way, and overall yields of alcohols and ketones up to 17% and of phenol up to 20% were achieved. Catalyst turnover numbers (TONs) of 490, 415 and 110 were obtained in the oxidation of cyclopentane, cyclohexane and phenol, respectively. The turnover frequency (TOF) is comparable with those of alkane monooxygenases. The reactions proceed via both oxygen- and carbon-radicals as shown by experiments with radical traps. (C) 2008 Elsevier B.V. All rights reserved.