Carboxylic acids cathodic reduction

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

On the other hand, it has been found that the electrochemical reduction is a very unique and useful tool in synthetic organic chemistry when magnesium is used as the material of the electrode. The cathodic reduction of 1,3-dienes with magnesium electrode gives very unique products, i.e. 3-cyclopentenol derivatives when it is carried out in the presence of a carboxylic acid ester (equation 23)17. 

Aliphatic carboxylic acids are difficult to reduce electroehemically. Reduction of a 10% oxalic acid in 10% H2SO4. at 15 °C at a mercury cathode (Refs. [494, 532] in Ref. [29]), a lead or amalgamated lead cathode (Ref. [495] in Ref. [29]) or at a sodium amalgam (Na(Hg) cathode (Ref. [497] in Ref. [29]) produces glyoxylic acid with a material yield of 88% and a current efficiency of 70%. The glyoxylic acid formed is stabilized by hydration [29]. 

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

Scheme 23 Cathodic reduction of aromatic carboxylic acids to benzyl alcohols or benzaldehydes.

Scheme 25 Cathodic reduction of activated aliphatic carboxylic acids to aldehydes (R alkyl, yields 70-82%) and ketones (R benzyl, yields 66- 72%).

Scheme 26 Cathodic reduction of aliphatic carboxylic acids in the presence of triphenylphosphineto aldehydes R alkyl, aryl, yields 36 -100%.

Recently, Ohmori and coworkers have used an anodic oxidation reaction to promote the reduction of an acid [34]. In this experiment, the anodic oxidation of triphenyl- or tributylphosphine in the presence of a carboxylic acid led to the formation of an acyl phosphonium ion. The acyl phosphonium ion was then reduced at the cathode to form an ylide which then trapped a second carbonyl... 

This review describes the electrochemical behavior of compounds containing the C=C, C=0 and C=N functional group. The review covers both anodic oxidation and cathodic reduction of such compounds. The electrochemistry of these functionalities was reviewed in an earlier volume of this series1 this article updates the previous one but does not include the material included there. The Kolbe oxidation of carboxylic acids has... 

Also in the cathodic reduction of carboxylic acids, electrolysis is in competition with catalytic methods. However, catalytic hydrogenations in this area do not always proceed so smoothly that electrochemical processes are without any prospects from the outset. 

Aromatic and heteroaromatic esters can be electrochemically reduced to benzyl alcohols, similar to the carboxylic acids. One example is the cathodic reduction of dimethyl terephthalate531 . 

Carboxylic acids can be reduced in acid solution to alcohols, aldehydes, or hydrocarbons 7<9 Polarographic and epe studies on the cathodic reduction of isonicotinic acid in weakly acid solution (pH about 3.1 M aqueous potassium chloride) have shown that the product, 4-pyridinealdehyde, exists as a hydrate in aqueous medium and that the hydrate does not undergo further cathodic reduction 137 The same applies to reduction of picolinic acid 137 imidazole-2-car-boxylic acid 138 and 2-thiazolecarboxylic acid 13 9. In the last case the yield of aldehyde was fairly low, probably due to competing reduction of the thiazole ring. It was concluded 139 that the following requirements must be fulfilled for the facile reduction of a carboxylic acid to the aldehyde stage to take place ... 

According to this model, the first stage in the treatment of nitrophenols aqueous wastes was the release of the nitro group from the aromatic ring. As a consequence, phenols or quinones were formed. These organic compounds were oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Also the cathodic reaction steps were considered in the global process when the electrochemical cell was undivided at the cathode, the reduction of the nitro to the amine group and the transformation of nitrate into ammonia were observed. In alkaline media, aminophe-nols were polymerised and transformed into a dark brown solid. 

When the reduction is performed without control of the potential, a mixture of products results. Thus, reduction in sulfuric acid at lead or mercury cathodes of the pyridine carboxylic acids leads to a mixture of picolines andtetra- and hexahydropyridine derivatives.284-287 Derivatives of nicotinic acid are more apt to be reduced in the ring than the 2- and 4-carboxypyridines. Reduction of the corresponding pyridylcarbinoles under identical conditions produces a similar reaction mixture.287... 

Salicylaldehydes and some o-hydroxyaryl ketones react with Meldrum s acid to give coumarin-3-carboxylic acids <03TL1755>. 3-Cyanocoumarins, and thus the 3-carboxylic acids, are available in high yield from a Knoevenagel reaction between salicylaldehydes and malononitrile in water <0382331>. 3-Chlorocoumarins result from the cathodic reduction of trichloroacetyl esters of o-hydroxyacetophenones <03T9161>. 

This chapter is concerned with the cathodic reduction of carboxylic acids and their derivatives, that is, esters, amides, anhydrides, acyl halides, hydrazides, nitriles, and corresponding thio derivatives. Cyclic derivatives of substituted carboxylic and polycarboxylic acids, such as lactones, lactams, imides, and anhydrides, are also included. Only those transformations in which the functional group itself is involved are discussed. Reductive coupling of carboxylic acids and derivatives is covered in Chapter 22, and there is some overlap with reduction of heterocycles in Chapter 18. 

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