Effect of the hydrogen peroxide concentration in stereospecific oxidation of alkanes by models of non-heme oxygenases

Abstract

The biomimetic oxidation of alkanes (cyclohexane, adamantane, cis-1,2-dimethylcyclohexane) with hydrogen peroxide catalyzed by Fe(II) complexes containing tetradentate nitrogen ligands (M = [Fe(bpmen)(MeCN)2](ClO4)2 (bispicolyl-1,2-dimethylethylenediamine), [Fe(bpen)(MeCN)2](ClO4)2 (bispicolylethylenediamine), and [Fe(tpcaH)(MeCN)2]2(ClO4)4 (tripyridylcarboxamide) is studied. The effects of the hydrogen peroxide concentration on the alcohol/ketone (A/K) ratio and on the regioselectivity of oxidation (3/2) are discovered. Rather high stereospecificity (RC = 96–99%) persisting at high hydrogen peroxide concentrations is hardly consistent with the participation of the HO. radical, inferred from the rather low regioselectivity and low A/K ratio observed under these conditions. The molecular mechanism of oxygen transfer from hydrogen peroxide, which was earlier proved reliably for low concentrations of hydrogen peroxide ([H2O2]/[M] ≪ 10), can be applied to high peroxide concentrations ([H2O2]/[M] > 10) if a new ferryl species containing two equivalents of the oxidant is assumed to be involved in the process. This assumption is confirmed by the direct stereospecific formation of alkyl hydroperoxide from alkane at a high concentration of hydrogen peroxide.