Aerobic Alcohol Oxidations Catalyzed by Oxorhenium Complexes Containing Redox-Active Ligands



The capacity of five-coordinate oxorhenium(V) anions with redox-active catecholate ligands to homolyze O2 and afford dioxorhenium(VII) products is utilized for the development of new aerobic alcohol oxidation catalysts. The reaction of [ReVII(O)2(cat)2] with benzyl alcohol (BnOH) affords the expected products of net H2 transfer: [ReV(O)(cat)2], benzaldehyde, and presumably H2O. However, mechanistic studies reveal that the formation of the active oxidant requires both the dioxo and monooxo species, so BnOH oxidation by[ReVII(O)2(cat)2] exhibits an unexpected catalytic dependence on [ReV(O)(cat)2]. Attempts to oxidize more thermodynamically challenging primary alcohols, which include CH3OH, using the [ReVII(O)2(cat)2] + [ReV(O)(cat)2] system did not yield aldehyde products. However, experiments performed in CH3OH allowed the observation of a catalytically active intermediate species, which provides an insight into the mechanism of catalytic action and catalyst degradation. Based on these observations, complexes that contain a more oxidatively robust [Br4cat]2– ligand were shown to exhibit higher catalytic activity as measured by total turnover number. The requirement for a redox-active ligand for catalyst function has both benefits and limitations that are discussed in the context of aerobic alcohol oxidation catalysis.