Selective Aerobic Oxidation of Alcohols with NO3- Activated Nitroxyl Radical/Manganese Catalyst System

A homogeneous Mn(NO3)(2)/2,2,6,6-tetramethylpiperidin-1-yl)oxyl/2-picolinic acid catalyst system is highly active and versatile for the selective aerobic oxidation of alcohols (2,2,6,6-tet-ramethylpiperidin-1-yl)oxyl = TEMPO, 2-picolinic acid = PyCOOH). The catalytic method enables near quantitative conversion of various primary alcohols to the respective aldehydes using a very simple reaction setup and workup. This study presents findings on the catalyst stability and mechanisms of deactivation. The results show that NO3- plays a crucial catalytic role in the reaction as a source of oxygen activating NOx species. Yet, disproportionation of NO3- to the volatile NO2 during the reaction leads to catalyst deactivation under open air conditions. Catalyst deactivation through this route can be overcome by adding a catalytic amount of nitrate salt, for example NaNO3 into the reaction. This stabilizes the Mn(NO3)(2)/TEMPO/PyCOOH catalyst and enables oxidation of various primary alcohols to the respective aldehydes using low catalyst loadings under ambient conditions. Secondary alcohols can be oxidized with a modified catalyst utilizing sterically accessible nitroxyl radical 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) instead of TEMPO. At the end of the alcohol oxidation, pure carbonyl products and the reusable catalyst can be recovered simply by extracting with organic solvent and dilute aqueous acid, followed by evaporation of both phases.