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

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journal contribution
posted on 2019-05-22, 00:00 authored by Astrid E. Delorme, Victor Sans, Peter Licence, Darren A. Walsh
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.