Metal-ion promotion of the oxidative dehydrogenation of coordinated amines and alcohols

The oxidative dehydrogenations of amines and alcohols are promoted by their coordination to transition metal centers, with ruthenium and osmium being particularly effective. The dehydrogenation reaction is well known in macrocyclic chemistry, and has been reported for a variety of monodentate and bidentate ligands as well. The initial step in the process is the one-electron oxidation of the metal center. However, the mechanism of the subsequent intramolecular redox reaction (in which the ligand is oxidized and the metal reduced) is ambiguous—it may take place either by one-electron steps through a ligand-radical intermediate, or involve higher oxidation states of the metal so that alternative two-electron pathways are possible. This review investigates studies of the mechanistic features of the reaction. The particular efficacy of ruthenium and osmium in promotion of ligand oxidation is related to their ability to attain an oxidation state two units greater than the final state, stabilized by deprotonation, and allowing a low-energy pathway for the even-electron processes required in the dehydrogenations of the amine and alcohol substrates