Efficient and Selective Aerobic Alcohol Oxidation Catalyzed by Copper(II)/2,2,6,6,-Tetramethylpiperidine-1-oxyl at Room Temperature

A simple and efficient copper(II)/2,2,6,6,-tetramethylpiperidine-1-oxyl (TEMPO)-catalyzed aerobic oxidation of both primary and secondary benzylic, allylic, and aliphatic alcohols to their corresponding aldehydes and ketones at room temperature using the copper(II) complex [Cu(mu-Cl)(Cl)-(phen)](2) as the Cu(II) source is reported. The conversion of both electron-rich and electron-neutral benzyl alcohols is smooth and faster than those of electron-deficient ones. The chemoselectivity of a primary benzyl alcohol over the secondary alcohol is also observed. Alcohols regarded as difficult substrates for oxidation due to their coordinating ability with transition metal catalyst such as 4-(methylthio)-benzyl alcohol and 3-pyridinemethanol are also oxidized easily. In addition, a lignin model alcohol is oxidized to the corresponding aldehyde in excellent yield. Conversions of benzylic and allylic alcohols are faster as compared to those of aliphatic alcohols in accordance with their C-alpha-H bond strengths. A plausible mechanism of the TEMPO-based catalytic cycle is proposed.