Tuning the Reactivity of TEMPO during Electrocatalytic Alcohol Oxidations in Room-Temperature Ionic Liquids
Tuning the Reactivity of TEMPO during Electrocatalytic Alcohol Oxidations in Room-Temperature Ionic Liquids
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DOI：
10.1021/acssuschemeng.9b01823
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发表时间：
2019-07-01
影响因子：
8.4
通讯作者：
Walsh, Darren A.
中科院分区：
文献类型：
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作者：
Delorme, Astrid E.;Sans, Victor;Walsh, Darren A.
2,2,6,6-四甲基哌啶-1-氧（TEMPO）是一种很有前途的、可持续的、无金属的醇氧化介质。在这篇贡献中，我们描述了如何通过溶剂介质的设计来改变TEMPO在室温离子液体（RTILs）中电催化醇氧化的选择性。在一系列铵基、磷基和咪唑基RTILs中，TEMPO的循环伏安法表明，随着RTIL阴离子氢键碱度的降低，TEMPO被氧化的电位从677 mV（相对于十甲基二茂铁/十甲基铁、dmFc/dmFc（+）、氧化还原对的电位）增加到788 mV。电位的增加伴随着苯甲醇氧化速率常数的增加，从约0.1 dm(3) mol(-1) s（-1）增加到约0.7 dm(3) mol(-1) s(-1)，表明通过明智地选择RTIL阴离子来操纵TEMPO的反应性的能力。在一系列RTIL中，醇的氧化速率依次为2-丁醇< 1-苯乙醇<辛醇<苄基醇，RTIL的1-辛基-3-甲基咪达唑双（三氟甲磺酰基）亚胺（[NTf2](-)）对伯醇的氧化表现出明显的选择性。此外，在[NTf2]（-）基RTILs中，反应动力学和选择性比在乙腈中更好，乙腈通常是间接醇电氧化的首选溶剂。最后，我们证明了在流动电解系统中使用RTILs可以进行电解tempo介导的醇氧化，具有优异的产率和反应选择性，证明了这种系统提供的机会。
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 dm(3) mol(-1) s(-1) to about 0.7 dm(3) 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.