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

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 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 < 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.

Synthesis of Cu(II) complexes by N,O‐donor ligand transformation and their catalytic role in visible‐light‐driven alcohol oxidation

“…As oxidation of alcohol to aldehyde and carboxylic acid is a very important reaction in organic as well as biochemistry, many catalytic systems have been developed with different reaction conditions and yields. [ 31–61 ] Despite several reports, the existing methodologies are often plagued by disadvantages like harsh reaction conditions, use of toxic metals, and poor recyclability. Photocatalysis emerged as an important tool to carry out such reaction in a more environmentally friendly condition without application of external heat.…”

“…Alcohol oxidation by copper catalyst is a well-known process and extensively studied as reported in literature. [31][32][33][34][35][36][37][38][39] The oxidation process has been explored in various methodologies including traditional heating, [40][41][42][43][44] microwave irradiation, [39,[45][46][47] electrochemical method, [48][49][50][51][52] and ultrasonic irradiation. [53][54][55] Interestingly, though alcohol oxidation is reported by photocatalytic systems like Ag 2 S-CdS junction [56][57][58] and platinum nanocluster/graphitic carbon nitride, [59,60] however, use of copper complexes as photocatalyst remains rare and largely unexplored.…”

Significantly Enhanced Photoelectrocatalytic Alcohol Oxidation Performance of CdS Nanowire-Supported Pt via the “Bridge” Role of Nitrogen-Doped Graphene Quantum Dots

“…It is commonly accepted that the long lifetime stability is another essential parameter for assessing the performance of catalysts and also has instructional meaning for practical applications . Therefore, the scan cycling experiments of two electrodes with and without visible light were conducted.…”

“…It is commonly accepted that the long lifetime stability is another essential parameter for assessing the performance of catalysts and also has instructional meaning for practical applications. 44 Therefore, the scan cycling experiments of two electrodes with and without visible light were conducted. As shown in Figure 7A, the i p of all electrodes exhibits a similar trend, first increases with scanning cycles, attains the highest i p , and later gradually decreases with the further increment of cycles.…”

Membrane-Supported Recovery of Homogeneous Organocatalysts: A Review

“…Biomass-derived 5-hydroxymethylfurfural (HMF) was converted into 2,5-diformylfuran (DFF) in a galvanostatic setup using the compact ElectraSyn reactor in an environmentally friendly organic electrosynthesis (Figure 7). In comparison to the previous methods [64][65][66][67], which used precious metals (like Pt) as the electrode material, graphite (anode) and stainless steel (cathode) were chosen to achieve a cost-effective process. As catalysts, native TEMPO and recyclable homogeneous and heterogeneous derivatives were applied.…”