High-valent iron in chemical and biological oxidations

Various aspects of the reactivity of iron(IV) in chemical and biological systems are reviewed. Accumulated evidence shows that the ferryl species [Fe(IV){double bond, long}O]²⁺ can be formed under a variety of conditions including those related to the ferrous ion-hydrogen peroxide system known as Fenton's reagent. Early evidence that such a species could hydroxylate typical aliphatic C-H bonds included regioselectivities and stereospecificities for cyclohexanol hydroxylation that could not be accounted for by a freely diffusing hydroxyl radical. Iron(IV) porphyrin complexes are also found in the catalytic cycles of cytochrome P450 and chloroperoxidase. Model oxo-iron(IV) porphyrin complexes have shown reactivity similar to the proposed enzymatic intermediates. Mechanistic studies using mechanistically diagnostic substrates have implicated a radical rebound scenario for aliphatic hydroxylation by cytochrome P450. Likewise, several non-heme diiron hydroxylases, AlkB (Ω-hydroxylase), sMMO (soluble methane monooxygenase), XylM (xylene monooxygenase) and T4moH (toluene monooxygenase) all show clear indications of radical rearranged products indicating that the oxygen rebound pathway is a ubiquitous mechanism for hydrocarbon oxygenation by both heme and non-heme iron enzymes.