Tuning the Reactivity of TEMPO during Electrocatalytic Alcohol Oxidations in Room-Temperature Ionic Liquids

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) is a promising, sustainable, metal-free mediator for oxidation of alcohols. In this contribution, we describe how the selectivity of TEMPO for electrocatalytic alcohol oxidations in room-temperature ionic liquids (RTILs) can be changed by design of the solvent medium. Cyclic voltammetry of TEMPO in a series of ammonium-, phosphonium-, and imidazolium-based RTILs reveals that the potential at which TEMPO is oxidized increases from 677 mV (vs the potential of the decamethylferrocene/decamethylferrocinium, dmFc/dmFc⁺, redox couple) to 788 mV as the H-bond basicity of the RTIL anions decreases. The increase in potential is accompanied by an increase in the rate constant for oxidation of benzyl alcohol from about 0.1 dm³ mol^–1 s^–1 to about 0.7 dm³ mol^–1 s^–1, demonstrating the ability to manipulate the reactivity of TEMPO by judicious choice of the RTIL anions. The rate of alcohol oxidation in a series of RTILs increases in the order 2-butanol < 1-phenylethanol < octanol < benzyl alcohol, and the RTIL 1-octyl-3-methylmidazolium bis­(trifluoromethanesulfonyl)­imide ([NTf₂]⁻) shows clear selectivity toward the oxidation of primary alcohols. In addition, the reaction kinetics and selectivity are better in [NTf₂]⁻-based RTILs than in acetonitrile, often the solvent-of-choice in indirect alcohol electrooxidations. Finally, we demonstrate that electrolytic TEMPO-mediated alcohol oxidations can be performed using RTILs in a flow-electrolysis system, with excellent yields and reaction selectivity, demonstrating the opportunities offered by such systems.