A Novel Halogenation Protocol and Non-heme Iron Hydroxylase Mimics as Catalysts for C-H Oxidation and O2 Activation

Abstract

Hydrocarbons react with molar concentrations of peracetic acid and halide salts to yield predominantly monohalogenated products under optimum conditions, with chlorination being more oxidatively efficient than bromination. The alkane halogenation proceeds at ambient temperature and does not require a heavy-metal catalyst. The observed reactivity is consistent with a radical mechanism, in which the peracid initially reacts with the halide ions to yield halogen-atom radicals, which ultimately oxidize the hydrocarbon. Although the reactivity proceeds slightly more efficiently in acetonitrile, the halogenation protocol works well in water. Iron complexes with the tetradentate N-donor ligand N,N′-di(phenylmethyl)-N,N′-bis(2-pyridinylmethyl)-1,2-cyclohexanediamine (bbpc) are reported. Despite the benzyl groups present on the amines, the iron compounds catalyze the oxygenation of cyclohexane by peroxides to an extent similar to those employing less sterically encumbered ligands. The catalytic activity is strongly dependent on the counterion, with the highest activity and the strongest preference for alkane hydroxylation correlating to the most weakly coordinating anion, SbF6−. The selectivity for the alcohol product over the ketone is amplified when acetic acid is present as an additive. When hydrocarbon substrates with both secondary and tertiary carbons are oxidized by H2O2, the catalyst directs oxidation toward the secondary carbons to a greater degree than other previously reported iron-containing homogeneous catalysts. [Fe(bbpc)(MeCN)2](SbF6)2 is also found to react with O2 in the presence of hydrocarbons. During these reactions, a ferric hydroperoxide species is observed and identified by mass spectrometry and electron paramagnetic resonance. The rate of formation of the Fe(III)-OOH species scales with the concentration of hydrocarbon and the strength of the C-H bond, suggesting that it may be formed via a hydrogen atom abstraction by a ferric superoxo species.