Practical Aerobic Oxidation of Alcohols: A Ligand-Enhanced 2,2,6,6-Tetramethylpiperidine-1-oxy/Manganese Nitrate Catalyst System

A highly efficient, ligand-enhanced 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO)/Mn(NO3)(2) catalyst system for the aerobic oxidation of alcohols is described. From the series of coordinating ligands studied herein, 2-picolinic acid (PyCOOH) improves the catalytic activity of TEMPO/Mn(NO3)(2) remarkably. Under ambient air at room temperature in acetic acid, the ligand-enhanced catalyst converts aliphatic and benzylic primary alcohols that bear various functional groups into their respective aldehydes with near quantitative conversions. The applicability of the catalyst for convenient preparative synthesis was demonstrated by conducting oxidations on a gram scale. A change of TEMPO to the sterically less demanding 9-azabicyclo[3.3.1]nonane N-oxyl results in a Mn catalyst that is also able to oxidize secondary alcohols to ketones. Mechanistic studies showed that alcohols are oxidized by the oxoammonium cation derived from the nitroxyl radical. The active oxidant is regenerated by Mn(NO3)(2), and this process is greatly promoted by the coordination of PyCOOH to Mn.