Electrosynthesis

Electrosynthesis in organic chemistry is the synthesis of chemical compounds in a electrochemical cell ^{[1] [2]} The main advantage of electrosynthesis over an ordinary redox reaction is avoidance of the potential wasteful other half-reaction and the ability to precisely tune the required potential. Electrosynthesis is actively studied as a science and also has many industrial applications.

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Experimental setup

The basic setup in electrosynthesis is a galvanic cell, a potentiostat and two electrodes. The reaction solvent usually is methanol, acetonitrile or dichloromethane. An electrolyte is added often lithium perchlorate or tetrabutylammonium acetate. The electrode can be platinum, carbon rod, magnesium, mercury (as a liquid pool in the reactor), stainless steel or reticulated vitreous carbon. In many reactions a sacrificial electrode is used which is consumed during the reaction like zinc or lead. The two basic cell types are undivided cell or divided cell type (connected through a semiporous membrame).

Electrosynthesis is carried out with constant potential or constant current.

Reactions

Organic oxidations take place at the anode with initial formation of radical cations as reactive intermediates. Compounds are reduced at the cathode to radical anions. The initial reaction takes place at the surface of the electrode and then the intermediates diffuse into the solution where they participate in secondary reactions.

Anionic oxidations

• The most well-known electrosynthesis is the Kolbe electrolysis
• 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.

• In the so-called Crum Brown-Walker reaction an aliphatic dicarboxylic acid is oxidized forming the elongated di-acid, for example the formation of the dimethyl ester of decanedioic acid from methyl hydrogen hexanedioate ^[3]
• Amides can be oxidized through a N-acyliminium ion which can be captured by a nucleophile:

This reaction type is called a Shono oxidation. An example is the α-methoxylation of N-carbomethoxypyrrolidine ^[4]

• 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 ^[5]

Cathodic reductions

• The cathodic hydroisomerization of activated olefins is applied industrially in the synthesis of adiponitrile from 2 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-methoxy naphtalene:

References

1. ^ The application of cathodic reductions and anodic oxidations in the synthesis of complex molecules Jeffrey B. Sperry and Dennis L. Wright Chem. Soc. Rev., 2006, 35, 605 - 621, doi:10.1039/b512308a
2. ^ Topics in current chemistry. Electrochemistry, Vol. 3 (Topics in Current Chemistry, Vol. 148) E. Steckhan (Ed), Springer, NY 1988.
3. ^ Organic Syntheses, Coll. Vol. 7, p.181 (1990); Vol. 60, p.1 (1981) Links.
4. ^ Organic Syntheses, Coll. Vol. 7, p.307 (1990); Vol. 63, p.206 (1985). Link
5. ^ Organic Syntheses, Coll. Vol. 7, p.482 (1990); Vol. 60, p.78 (1981) Link