Counter Electrode Reactions—Important Stumbling Blocks on the Way to a Working Electro‐organic Synthesis

Abstract

Over the past two decades, electro‐organic synthesis has gained significant interest, both in technical and academic research as well as in terms of applications. The omission of stoichiometric oxidizers or reducing agents enables a more sustainable route for redox reactions in organic chemistry. Even if it is well‐known that every electrochemical oxidation is only viable with an associated reduction reaction and vice versa, the relevance of the counter reaction is often less addressed. In this Review, the importance of the corresponding counter reaction in electro‐organic synthesis is highlighted and how it can affect the performance and selectivity of the electrolytic conversion. A selection of common strategies and unique concepts to tackle this issue are surveyed to provide a guide to select appropriate counter reactions for electro‐organic synthesis.

Discovery of Anodic Thiourea Oxidation as a Sustainable Counter Reaction to Boost Electro-Reductive Organic Transformations

“… a Reaction conditions of undivided cell: 1a (1.0 mmol), anodic feedstock (1.0 mmol), n Bu 4 NBF 4 (1.0 mmol), and THF/H 2 O (4.5:0.5 mL) in an undivided cell with a platinum plate anode (10 mm × 10 mm × 0.1 mm) and a carbon plate cathode (10 mm × 10 mm × 1 mm), constant current = 20 mA ( j anode = 20 mA cm –2 ), r.t., 2.5 h (2.0 F/mol). Reaction conditions of divided cell: 1a (0.25 mmol), anodic feedstock (0.25 mmol), n Bu 4 NBF 4 (4.0 mmol), and THF/H 2 O (18:2 mL) in a divided cell with a FAB-PK-130 anion exchange membrane, constant current = 5 mA ( j anode = 5 mA cm –2 ), r.t., and 2.5 h (2.0 F/mol). b Collected from Figures and S1. c Conversion (%) determined by 1 H NMR using 1,3,5-trimethoxybenzene as an internal standard. d Isolated yield (%). e Collected from ref (vs SHE at 25 °C). See the Supporting Information for details. …”

“…10−15 Nevertheless, the counter reaction for electro-reductive organic transformations continues to pose a challenge, which usually involves either the oxygen evolution reaction (OER) or the sacrifice of the metal anode (Scheme 1B,1C). 7 The OER process requires relatively high overpotentials and restricts the overall efficiency of electrolysis due to its sluggish kinetics and thermodynamics. The use of sacrificial metal anode leads to excess generation of metal waste, complicated post-processing, and infeasible industrial scale-up.…”

“…Organic electrosynthesis has been revived as a powerful tool for chemical transformations and a sustainable alternative to conventional redox reactions over the past decade. − In a typical electrochemical cell, the power supply always pushes electrons into the cathode from the anode, leading to the coupled anodic oxidation and cathodic reduction. As such, the consideration of an appropriate counter reaction is relevant to ensure the high efficiency of the target electro-transformation. − For instance, the proton reduction with low overpotentials and favourable kinetics is often employed to satiate the cathodic process when an electro-oxidative organic transformation is of interest (Scheme A). − Nevertheless, the counter reaction for electro-reductive organic transformations continues to pose a challenge, which usually involves either the oxygen evolution reaction (OER) or the sacrifice of the metal anode (Scheme B,1C) . The OER process requires relatively high overpotentials and restricts the overall efficiency of electrolysis due to its sluggish kinetics and thermodynamics.…”

Electrochemical CO2 Utilization for the Synthesis of α-Hydroxy Acids

“…When stable anode materials, such as platinum, glassy carbon, or boron-doped diamond, are used, reactions are usually carried out in divided cells to avoid unwanted oxidation of the starting material or product of the cathodic process . A typical anodic reaction occurring in systems with inert anodes is the oxidation of the solvent or supporting electrolyte anions . Several approaches have been pursued to avoid sacrificial anodes in electrocarboxylation reactions.…”

“…36 A typical anodic reaction occurring in systems with inert anodes is the oxidation of the solvent or supporting electrolyte anions. 37 Several approaches have been pursued to avoid sacrificial anodes in electrocarboxylation reactions. Muchez et al presented an undivided cell setup with a stable anode where the TEMPO (2,2,6,6-tetramethylpiperidinyloxyl)-mediated oxidation of benzyl alcohol to benzaldehyde is the anodic process.…”