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

Abstract

Abstract The capacity of five‐coordinate oxorhenium(V) anions with redox‐active catecholate ligands to homolyze O 2 and afford dioxorhenium(VII) products is utilized for the development of new aerobic alcohol oxidation catalysts. The reaction of [Re VII (O) 2 (cat) 2 ] – with benzyl alcohol (BnOH) affords the expected products of net H 2 transfer: [Re V (O)(cat) 2 ] – , benzaldehyde, and presumably H 2 O. However, mechanistic studies reveal that the formation of the active oxidant requires both the dioxo and monooxo species, so BnOH oxidation by[Re VII (O) 2 (cat) 2 ] – exhibits an unexpected catalytic dependence on [Re V (O)(cat) 2 ] – . Attempts to oxidize more thermodynamically challenging primary alcohols, which include CH 3 OH, using the [Re VII (O) 2 (cat) 2 ] – + [Re V (O)(cat) 2 ] – system did not yield aldehyde products. However, experiments performed in CH 3 OH 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 [Br 4 cat] 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.