Aerobic Oxidation of Alcohols Catalyzed by Iron or Vanadium Compounds
Both iron and vanadium compounds are generally inexpensive and play important roles in living systems (especially in redox reaction). Although they also have great potential in organic syntheses as catalysts for the aerobic oxidation of alcohols, they were seldom used for this purpose until the end of the twentieth century. A few examples of the aerobic oxidation of alcohols catalyzed by iron compounds have been reported since 2002. Several alcohols can be oxidized to the corresponding carbonyl compound in high yield using these methods. Effective aerobic oxidations of alcohols catalyzed by vanadium compounds began to be reported in 1999. In the first studies, the substrates were limited to α-hydroxycarbonyls and propargylic alcohols. Several groups developed kinetic resolutions of α-hydroxycarbonyls using asymmetric aerobic oxidation catalyzed by a chiral vanadium complex. Recently, the aerobic oxidation of a variety of alcohols, including unactivated aliphatic alcohols, catalyzed by vanadium compounds was accomplished. Furthermore, vanadium compounds supported by solids or ionic liquids have been developed, and they can be used repeatedly for aerobic oxidation of alcohols without a decrease in activity. Oxidation of hydroquinones to quinines and cleavage of the carbon–carbon bonds of glycols and α-hydroxycarbonyls can be achieved by using vanadium-catalyzed aerobic oxidations.