Efficient and selective aerobic oxidation of alcohols into aldehydes and ketones using ruthenium/TEMPO as the catalytic system

The combination of RuC2(PPh3)(3) and TEMPO affords an efficient catalytic system for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding aldehydes and ketones, in > 99% selectivity in all cases. The Ru/TEMPO system displayed a preference for primary vs secondary alcohols. Results from Hammett correlation studies (p = -0.58) and the primary kinetic isotope effect (k(H)/k(D) = 5.1) for the catalytic aerobic benzyl alcohol oxidations are inconsistent with either an oxoruthenium (O=Ru) or an oxoammonium based mechanism. We postulate a hydridometal mechanism, involving a RuH2(PPh3)(3) species as the active catalyst. TEMPO acts as a hydrogen transfer mediator and is either regenerated by oxygen, under catalytic aerobic conditions, or converted to TEMPH under stoichiometric anaerobic conditions.