Mechanism of 2,2′6,6′-Tetramethylpiperidin-<i>N</i>-oxyl-Mediated Oxidation of Alcohols in Ionic Liquids

Abstract

The mechanism of 2,2'6,6'-tetramethylpiperidin- N-oxyl (TEMPO)-mediated oxidation of alcohols to aldehydes and ketones in ionic liquids has been investigated using cyclic voltammetry and rotating disk electrode voltammetry. It is shown that the presence of bases (B) and their conjugate acids (BH (+)), as well as their p K as, strongly influences the rate of reaction. Data indicated that the first step in the oxidation is the formation of the alcoholate species via acid-base equlibrium with B. The alcoholate subsequently reacts with the active form of TEMPO (T (+), i.e., the one-electron oxidized form) forming an intermediate that further reacts with T (+) and B returning TEMPO catalytically, BH (+), and the carbonyl product. A kinetic model incorporating this pre-equilibrium step has been derived, which accounts for the experimentally observed reaction kinetics. Overall, the rate of reaction is controlled by the equilibrium constant for the pre-equilibrium step; as such, strong bases are required for more kinetically efficient transformations using this redox catalyst.