Electrosynthesis

• A well-known electrosynthesis is the Kolbe electrolysis, in which two carboxylic acids decarboxylate, and the remaining structures bond together:

• A variation is called the non-Kolbe reaction when a heteroatom (nitrogen or oxygen) is present at the α-position. The intermediate oxonium ion is trapped by a nucleophile, usually solvent.

• Anodic electrosynthesis oxidize primary aliphatic amine to nitrile.[7]
• Amides can be oxidized to N-acyliminium ions, which can be captured by various nucleophiles, for example:

This reaction type is called a Shono oxidation. An example is the α-methoxylation of N-carbomethoxypyrrolidine[8]

• Oxidation of a carbanion can lead to a coupling reaction for instance in the electrosynthesis of the tetramethyl ester of ethanetetracarboxylic acid from the corresponding malonate ester[9]
• α-amino acids form nitriles and carbon dioxide via oxidative decarboxylation at AgO anodes (the latter is formed in-situ by oxidation of Ag₂O):[5][10]

• Cyanoacetic acid from cathodic reduction of carbon dioxide and anodic oxidation of acetonitrile.[11]
• Propiolic acid is prepared commercially by oxidizing propargyl alcohol at a lead electrode.[12].

• In the Markó–Lam deoxygenation, an alcohol could be almost instantaneously deoxygenated by electroreducing its toluate ester.

• In concept, adiponitrile is prepared from dimerizing acrylonitrile:[13]

2 CH₂=CHCN + 2 e⁻ + 2 H⁺ → NC(CH₂)₄CN

In practice,the cathodic hydrodimerization of activated olefins is applied industrially in the synthesis of adiponitrile from two equivalents of acrylonitrile :

• The cathodic reduction of arene compounds to the 1,4-dihydro derivatives is similar to a Birch reduction. Examples from industry are the reduction of phthalic acid:

and the reduction of 2-methoxynaphthalene:

• The Tafel rearrangement, named for Julius Tafel, was at one time an important method for the synthesis of certain hydrocarbons from alkylated ethyl acetoacetate, a reaction accompanied by the rearrangement reaction of the alkyl group:[14][15]

• The cathodic reduction of a nitrile to a primary amine in a divided cell; the cathodic reduction of benzyl cyanide to phenethylamine is shown:[16]

• Cathodic reduction of a nitroalkene can give the oxime in good yield. At higher negative reduction potentials, the nitroalkene can be reduced further, giving the primary amine but with lower yield.[17]

• Azobenzene is prepared in industrial electrosynthesis using nitrobenzene.[13]
• An electrochemical carboxylation of a para-isobutyl benzyl chloride to Ibuprofen is promoted under supercritical carbon dioxide.[18]
• Cathodic reduction of a carboxylic acid (oxalic acid) to an aldehyde (glyoxylic acid, shows as the rare aldehyde form) in a divided cell:[19][20]

• Originally phenylpropanoic acid could be prepared from reduction of cinnamic acid by electrolysis.[21]
• An electrocatalysis by a copper complex helps reduce carbon dioxide to oxalic acid; this conversion uses carbon dioxide as a feedstock to generate oxalic acid.[22]
• It has been reported that formate can be formed by the electrochemical reduction of CO₂ (in the form of bicarbonate) at a lead cathode at pH 8.6:[23]

HCO−3 + H₂O + 2e⁻ → HCO−2 + 2OH⁻

or

CO₂ + H₂O + 2e⁻ → HCO−2 + OH⁻

If the feed is CO₂ and oxygen is evolved at the anode, the total reaction is:

CO₂ + OH⁻ → HCO−2 + ¹/₂ O₂

• Cathodic reduction of carbon dioxide and anodic oxidation of acetonitrile afford cyanoacetic acid.[11]
• An electrosynthesis employing alternating current prepares phenol at both the cathode and the anode.[24]

In organofluorine chemistry, many perfluorinated compounds are prepared by electrochemical synthesis, which is conducted in liquid HF at voltages near 5–6 V using Ni anodes. The method was invented in the 1930s.[25] Amines, alcohols, carboxylic acids, and sulfonic acids are converted to perfluorinated derivatives using this technology. A solution or suspension of the hydrocarbon in hydrogen fluoride is electrolyzed at 5–6 V to produce high yields of the perfluorinated product.