Stoichiometric oxidations of σ-bonds: Radical and possible non-radical pathways

Many transition metal complexes accomplish or catalyze the oxidation of CH, OH, and other σ-bonds. Under aerobic conditions, metal complexes typically modulate an autoxidation radical chain. In anaerobic reactions, a metal complex can be the reactive species that attacks the σ-bond, in many cases by abstracting a hydrogen atom from the substrate. Examples described here include the oxidation of alkylaromatic compounds by ruthenium oxo complexes and reactions of deprotonated iron(III) complexes. In general, these reactions occur with addition of H+ to a ligand and e− to the metal center. Rate constants for such hydrogen-atom transfer reactions can, in many cases, be predicted by the Marcus cross relation. Autoxidation and metal-mediated radical mechanisms are so prevalent that proposals of non-radical oxidations of CH bonds carry a higher burden of proof. It is argued here that the oxidation of H2 by OsO4 occurs by a non-radical, [3 + 2] mechanism. OsO4 oxidizes alkanes under similar aqueous conditions. For example, isobutane is oxidized to tert-butanol, and cyclohexane to adipate and succinate. The alkane oxidations do not have the hallmarks of a radical mechanism but sufficient questions remain that a radical pathway cannot be excluded at this time.

Oxidizing transition metal complexes can abstract hydrogen atoms from CH and OH bonds. Examples reviewed here include oxidations of alkylaromatics and hydroxylamines by ruthenium-oxo and iron(III) complexes. Rate constants for such reactions can often be predicted from the reaction driving force and intrinsic barriers. Novel OsO4 oxidations of H2 and alkanes, in contrast, may not occur by a radical mechanism. Figure optionsDownload as PowerPoint slide