Modern Electrochemical Aspects for the Synthesis of Value-Added Organic Products

Abstract

The use of electricity instead of stoichiometric amounts of oxidizers or reducing agents in synthesis is very appealing for economic and ecological reasons, and represents a major driving force for research efforts in this area. To use electron transfer at the electrode for a successful transformation in organic synthesis, the intermediate radical (cation/anion) has to be stabilized. Its combination with other approaches in organic chemistry or concepts of contemporary synthesis allows the establishment of powerful synthetic methods. The aim in the 21st Century will be to use as little fossil carbon as possible and, for this reason, the use of renewable sources is becoming increasingly important. The direct conversion of renewables, which have previously mainly been incinerated, is of increasing interest. This Review surveys many of the recent seminal important developments which will determine the future of this dynamic emerging field.

Keywords: electrolysis; flow electrochemistry; organocatalysis; renewable resources; synthetic methods.

Figure 1

Different operation modes of electrodes in electrosynthetic applications.

Scheme 1

Electrochemical organocatalyzed α‐oxyamination of aldehydes.12

Scheme 2

Electrochemical organocatalyzed α‐alkylation of aldehydes.14 Bn=benzyl.

Scheme 3

Electrochemical organocatalyzed synthesis of meta‐substituted anilines.16 TMS=trimethylsilyl, Ts=tosyl.

Scheme 4

Anodic oxidation of aldehydes in methanolic NaCN solution.17

Scheme 5

NHC‐mediated electrochemical oxidation of aldehydes to esters, thioesters, or amides.19, 20, 22 DIPP=2,6‐diisopropylphenyl, DMAP=4‐(dimethylamino)pyridine, DMF=N,N‐dimethylformamide, DBU=1,8‐diazabicyclo[5.4.0]undec‐7‐ene, Mes=mesityl, PVDF=polyvinylidene difluoride, TBAB=tetrabutylammonium fluoride, Tf=triflyl.

Scheme 6

Electrochemical activation via the ionic liquid.27, 28

Scheme 7

General steps of the “cation‐pool” concept.29, 30

Scheme 8

Potential reaction pathways and accumulated cations.

Scheme 9

Stabilized “cation‐pool” method for an integrated reaction sequence.37 DMSO=dimethyl sulfoxide.

Scheme 10

Stabilized “cation‐pool” for cross‐coupling between aromatic and benzylic groups.39

Scheme 11

Accumulation and conversion of thionium ions.42

Scheme 12

Indirect electrochemical regeneration of the cofactor.62 bpy=2,2′‐bipyridine, Cp*=C₅Me₅.

Scheme 13

Proposed mechanism for the [2+2] cycloaddition of enol ethers and terminal olefins. The oxidation potentials E _Ox were measured versus the Ag/AgCl reference electrode.70

Scheme 14

Proposed mechanism for olefin metathesis as a result of the ineffective redox tag.71

Scheme 15

Comparison of olefins in electrocatalytic olefin metathesis.72

Scheme 16

Anodic SET‐triggered Diels–Alder reaction with trans‐anethole and 1‐propenylbenzene as well as the proposed mechanism reaction involving an intramolecular SET process. The oxidation potentials E _Ox were measured versus the Ag/AgCl reference electrode.74 Q=amount of charge.

Scheme 17

Electrochemical reduction of carbon dioxide to various products.79, 80, 81

Scheme 18

Electrochemical reduction of carbon dioxide to formaldehyde at boron‐doped diamond anodes in seawater.85

Scheme 19

Carboxylation of methional at BDD cathodes.86

Scheme 20

Electrochemical degradation of lignosulfonate at nickel anodes.89

Scheme 21

Highly selective electro‐depolymerization of Kraft lignin to vanillin on porous Ni/P‐foam electrodes.92

Scheme 22

Products from the electrochemical depolymerization of bamboo lignin: vanillin (left), syringaldehyde (center), and p‐coumaric acid (right).98

Scheme 23

Selective anodic TEMPO‐mediated oxidation of the primary hydroxy group of a glycoside (left), methyl‐l‐sorbopyranose (middle), and d‐maltose (right).

Scheme 24

Simplified pathway of the electrochemical deoxygenation of xylolactone to δ‐valerolactone.105 DSA=dimensionally stable anode.

Scheme 25

Electrochemical reduction of glucose to sorbitol.106

Scheme 26

Combined method of bio‐ and electrochemical transformation for the conversion of glucose into bio‐based nylon‐6,6.108

Scheme 27

Electrochemical glycosylation and sequential one‐pot cleavage of the fluorenylmethoxycarbonyl (Fmoc) group.110 Bz=benzoyl.

Scheme 28

Electrochemical conversion of furanic compounds.112

Scheme 29

Electrochemical dialkoxylation of a furan derivative and synthesis of a pyridoxine derivative.115

Scheme 30

Homocoupling of a ricinoleic acid derivative by Kolbe electrolysis.119

Scheme 31

Top: Anodic addition of a Kolbe radical from monomethyladipate to ethylene. Bottom: Unsymmetrical electrochemical coupling of two fatty acids to the precursor to generate a muscone precursor.117, 119

Scheme 32

Anodic diacetoxylation of methyl oleate.120

Scheme 33

Electrochemical diacetoxylation of a doubly unsaturated fatty acid derived from linoleic acid ester.101

Scheme 34

TEMPO‐mediated anodic oxidation of methyl linolenoate.121

Scheme 35

Anodic decarboxylation of oleic acid to diesel‐like compounds.122

Scheme 36

Hydrodimerization by cathodic reduction of a fatty acid enone.123

Scheme 37

Sequential electrochemical synthesis of adiponitrile from glutamic acid.126

Scheme 38

Electrochemical decarboxylation of lysine to the corresponding nitrile, amine, or amide.127

Scheme 39

Electrochemical α‐cyanation of an N‐protected proline derivative.128 Tr=trityl.

Scheme 40

Continuous electrochemical α‐allylation of carbamates using allylsilanes as electrophiles.47

Scheme 41

Electrochemical oxidation of different para‐substituted toluene derivatives in flow cells.131, 133

Scheme 42

Electrodimethoxylation of furan without a supporting electrolyte in a flow cell.134

Scheme 43

α‐Methoxylation of N‐formylpyrrolidine in a flow cell.135

Scheme 44

Formation of aryl thioethers by electrooxidation of catechol.138

Scheme 45

Continuous C−C cross‐coupling reaction between naphthalenes and methylbenzenes.139

Scheme 46

Electrosynthesis of diaryliodonium salts in flow cells.140

Scheme 47

Continuous electrochemical introduction of CF₃ and CF₂H groups in electron‐deficient alkenes.141 EWG=electron‐withdrawing group.

Scheme 48

Electrochemical anodic aryl‐phenol cross‐coupling in a flow cell.142

Scheme 49

Oxidation of alcohols by electrogeneration of an oxammonium cation in a flow cell.143

Scheme 50

Electrosynthesis of licarin A from isoeugenol.144

Scheme 51

Electrosynthetic generation of metabolites from different commercial drugs in flow cells.145

Scheme 52

Continuous formation of a 2‐pyrrolidone anion by electroreduction and subsequent reactions.147, 148

Scheme 53

Anions as nucleophiles in the synthesis of alcohols or carboxylic acids in flow cells.149, 150 HMPA=hexamethylphosphoric triamide.

Scheme 54

Cathodic coupling of benzyl bromides with acetic anhydride and activated olefins in a flow cell.151, 152, 153

Scheme 55

Electrochemical double dehalogenation of a cyclopropane derivative in a flow cell.154 Boc=tert‐butoxycarbonyl.

Scheme 56

Electrosynthesis of copper‐NHC complexes in a flow cell.155

Scheme 57

Domino oxidation‐reduction sequence of an oxime to the corresponding nitrile in a flow cell.156

Scheme 58

Examples of currently used technical electrochemical processes.