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.

Keywords: Electrochemistry; Hydrogen; Oxygen; Paired Electrolyses; Sacrificial Anodes.

Scheme 1

Basic concepts of an electrochemical redox reaction.

Figure 1

Schematic presentation of a two‐ and three‐electrode cell used in electro‐organic synthesis. The electrode potential is also shown.

Scheme 2

Oxidative cyclization of oximes with different cathode materials. RVC: reticulated vitreous carbon.

Scheme 3

Synthesis of 2‐allylpyrrrolidines by the cation pool method.

Scheme 4

Catalytic asymmetric coupling of tertiary amines and ketones. TFE: 1,1,1‐trifluoroethanol.

Scheme 5

Examples of iodine(III) reagents obtained by the electrolysis of iodoarenes. HFIP: 1,1,1,3,3,3‐hexafluoro‐2‐propanol.

Scheme 6

Synthesis of N‐cyanosulfilimines by dehydrogenative coupling.

Scheme 7

Electrochemical Hofmann rearrangement.

Scheme 8

Strong electrogenerated bases from weak acidic solvents. The pKa data refer to the protonated form in DMSO at 25 °C and for [dmim]. [dmim]: N,N‐dimethylimidazolium.

Scheme 9

The electrochemical synthesis of β‐lactams, promoted by the deprotonation of MeCN at the cathode.

Scheme 10

Pathways for the electrochemical degradation of quaternary ammonium salts or halogenated solvents.

Scheme 11

Oxydihalogenation of alkynes using halogenated solvents as the halide source.

Scheme 12

Oxidative formation of benzimidazole paired with the reduction of CO₂. Ar: 4‐hydroxy‐3,5‐dimethoxybenzene. gC: glassy carbon. CAN: ceric ammonium nitrate.

Scheme 13

Electrochemical Birch reduction at room temperature.

Scheme 14

Electrochemical Horner–Wadsworth–Emmons reaction. [a] Potential vs. Ag/AgCl.

Scheme 15

Electrochemical Sm^II‐enabled coupling of aldehydes by in situ preparation of the catalyst from an Sm sacrificial anode.

Scheme 16

Application of Ni anodes in either alkaline or acidic conditions. Ni²⁺ can be applied for electroplating and the following electrocatalytic hydrogenation.

Scheme 17

Supporting‐electrolyte‐free electrocatalytic hydrogenation of cyanamide at Ni‐foam cathodes with the OER as the counter reaction. [a] 0.5 m H₂SO₄ at j=10 mA cm⁻² obtained from Ref. .

Scheme 18

Cathodic synthesis of benzotriazoles, with the oxidation of MeOH as the counter reaction.

Scheme 19

Reductive addition of styrenes and aliphatic carbonyl compounds. GDL: gas diffusion layer. MTBS: methyltributylammonium methyl sulfate.

Scheme 20

Electrochemical reduction of ketones to pinacols and alcohols.

Scheme 21

Electrochemical reduction of phthalimides using a sterically demanding amine as the electron source.

Scheme 22

Electrochemical deoxygenation of phtalimides by rAP. Boc: tert‐butoxycarbonyl. Piv: Pivalyol.

Scheme 23

Carboxylation of halides with CO₂ as well as bromide oxidation.

Scheme 24

Electrochemical synthesis of N‐bromoamino acids.

Scheme 25

Common types of paired electrolyses. Parallel (A), consecutive (B), convergent (C), and divergent (D).

Scheme 26

Anodic methoxylation of tert‐butyltoluene paired with the reductive synthesis of phthalide.

Scheme 27

Consecutive paired synthesis for the electron‐shuttle transfer of halo substituents.

Scheme 28

Convergent synthesis of sulfenamides using alternating current.

Scheme 29

Ni‐catalyzed amination of aryl halides. Ligand: 4,4′‐di‐tert‐butyl‐2,2′‐dipyridyl.