Oxygen activation by metal complexes and alkyl hydroperoxides. Applications of mechanistic probes to explore the role of alkoxyl radicals in alkane functionalization

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Abstract
The mechanism of the oxidation of cycloalkanes by tertiary alkyl hydroperoxides catalysed by iron(III) dichlorotris(2-pyridylmethyl)amine [FeIIICl2(TPA)]+ and by the acetate bridged (µ-oxo) di-iron complex [Fe2III(TPA)2O(OAc)]3+ has been investigated. Product studies do not support oxidation via a high valent iron–oxo intermediate (formally FeVO), but are consistent with a mechanism involving hydrogen atom abstraction from the alkane by alkoxyl radicals derived from the hydroperoxide. In the presence of a large excess of tert-butyl hydroperoxide, the oxidation of cyclohexane yields cyclohexanone, cyclohexanol and tert-butylcyclohexyl peroxide in more than stoichiometric amounts and, in the case of the mono-iron catalyst, one equivalent of cyclohexyl chloride. Replacement of Me3COOH by hydroperoxides, which could yield tert-alkoxyl radicals having much shorter lifetimes than the tert-butoxyl radical prevents oxidation of the cycloalkane. The products obtained with these hydroperoxide mechanistic probes are those derived from the fast unimolecular reactions (generally β-scissions) of the corresponding alkoxyl radicals. The inapplicability of dimethyl sulfide as a mechanistically diagnostic trap for the putative FeVO intermediate and the value of di-tert-butyl hyponitrite as a non-iron-based source of tert-butoxyl radicals are discussed.

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