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

Delorme, Astrid E.; Sans, Victor; Licence, Peter; Walsh, Darren A.

Abstract

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 < 1phenylethanol < octanol < benzyl alcohol, and the RTIL 1-octyl-3-methylmidazolium bis(trifluoromethanesulfonyl)imide ([NTf2]–) shows clear selectivity towards 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.

Citation

Delorme, A. E., Sans, V., Licence, P., & Walsh, D. A. (2019). Tuning the Reactivity of TEMPO during Electrocatalytic Alcohol Oxidations in Room-Temperature Ionic Liquids. ACS Sustainable Chemistry and Engineering, 7(13), 11691-11699. https://doi.org/10.1021/acssuschemeng.9b01823

• Journal Article Type: Article
• Acceptance Date: May 22, 2019
• Online Publication Date: May 22, 2019
• Publication Date: Jul 1, 2019
• Deposit Date: Jun 14, 2019
• Publicly Available Date: May 23, 2020
• Journal: ACS Sustainable Chemistry & Engineering
• Electronic ISSN: 2168-0485
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 7
• Issue: 13
• Pages: 11691-11699
• DOI: https://doi.org/10.1021/acssuschemeng.9b01823
• Public URL: https://nottingham-repository.worktribe.com/output/2188265
• Publisher URL: https://pubs.acs.org/doi/10.1021/acssuschemeng.9b01823