Mechanism of Selective C-H Hydroxylation Mediated by Manganese Aminopyridine Enzyme Models

The mechanism of selective oxidation of aliphatic C-H groups with H2O2 in the presence of aminopyridine Mn complexes, modeling the reactivities of natural oxygenases of the cytochrome P450 superfamily, has been examined. The oxygenation of C-H groups proceeds via hydrogen atom abstraction by the electrophilic metal site; the logarithm of C-H oxidation rates correlates linearly with bond dissociation energies for homolytic C-H bond cleavage. Hammett correlations and stereospecificity studies reflect the formation of a short-lived electron-deficient radical intermediate. Isotopic labeling studies confirm the incorporation of O-18 from added (H2O)-O-18, thus providing so far lacking evidence for the oxomanganese(V)-mediated oxidation mechanism.