Oxidation of Alcohols by [Cp*Rh(ppy)(OH)]+

Rh(III) polypyridine complexes ([Cp*Rh(ppy)(H2O)(2+); ppy = 2,2'-bipyridine, 2,2'bipyridine-4,4'-dicarboxylate, o-phenanthroline, tetrahydro-4,4'-dialkyl-bis-oxazole) oxidize in organic or aqueous alkaline solution primary and secondary alcohols to aldehydes or ketones and are thereby reduced to the Rh(I) complexes Cp*Rh(ppy). The Rh(III) form can be regenerated by oxidants like pyruvate or oxygen, making the reaction quasi-catalytic. The reaction follows an autocatalytic pathway; hydrogen transfer from the alpha -CH2 group of an alcoholate complex [Cp*Rh(ppy)(OR)](+) to Cp*Rh(I)(ppy) is suggested to yield the Rh(III) intermediate Cp*Rh(ppy)H as the key and rate determining step. The knowledge of Rh(III)/Rh(I) redox potentials allows to estimate the thermodynamic driving force of the reaction which is not more than about 300 mV.