Electrochemical reduction carboxylic acids

Hi) Electrochemical reactions and reactions with free electrons Electrochemical oxidation of 3-methyl-l-phenylpyrazole gave the 3-carboxylic acid whereas electrochemical reduction (Section 4.04.2.1.6(i)) of l,5-diphenyl-3-styrylpyrazole produced the A -pyrazoline (B-76MI40402) with concomitant reduction of the exocyclic double bond (343). 

The electrochemical reduction of 3-nitrophthalic acid at controlled potentials gave 2,1-benzisoxazole-3-carboxylic acid. Cyclization is presumed to proceed via an intermediate oxime (67AHC(8)277). Treating 5-iodoanthranilic acid with acetic anhydride gave 3-acetoxy-5-iodo-2,l-benzisoxazole (596) (65JMC550). 

Electrochemical reduction of oxazolinium salts 36 gives the anions 37, which add efficiently to alkyl halides or, in the presence of McsSiCl, to methyl acrylate, methyl vinyl ketone, and acrylonitrile. Simple acid hydrolysis then gives the ketone products 38 and 39, and this method is quite general since the starting salts are readily prepared from carboxylic acids, R C02H (87TL4411). 

Elving, Rosenthal, Hayes and Martin 41> studied the electrochemical reduction of bromofumaric acid(27) and bromomaleic acid(28)in aqueous solution over a wide pH range. It was claimed that reduction of 27 proceeds stereo-specifically to fumaric acid, and that reduction of 28 affords mixtures of maleic and fumaric acids. Because of the polar and hydrogen-bonding properties of the carboxyl groups in 27 and 28, the relation of these results to those of Fry and Mitnick l6) is unclear. 

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. 

Monsanto [117] has developed a way to electrosynthesize by reductive carboxylation the optically active precursor to Naproxen, (S)-2-(6 -methoxy-2 -naphthyl)propionic acid, a drug used to treat arthritis. A more economical route was needed since the US patent expires in 1993 while the market is growing. The electrochemical process is said to cut manufacturing costs by over 50%. Since it uses CO 2 instead of the hazardous HCN used in conventional synthesis, it is also safer. 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

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. 

The addition of carbanions, generated electrochemically by reduction of the carbon-halogen bond, to carbon dioxide has been examined under a variety of experimental conditions. Direct electrosynthesis of carboxylic acids in a divided cell using an aprotic solvent and a tetraalkylammonium salt as electrolyte is most sue-... 

Cyclic voltammetry has been also used for estimation of the rate constants for oxidation of water-soluble ferrocenes in the presence of HRP (131). There is a perfect match between the data obtained spectrophotometrically and electrochemically (Table IV), which proves that the cyclic voltammetry reveals information on the oxidation of ferrocenes by Compound II. It is interesting to note that an enzyme similar to HRP, viz. cytochrome c peroxidase, which catalyzes the reduction of H202 to water using two equivalents of ferrocytochrome c (133-136), is ca. 100 times more reactive than HRP (131,137). The second-order rate constant equals 1.4 x 106 M-1 s 1 for HOOCFc at pH 6.5 (131). There is no such rate difference in oxidation of [Fe(CN)e]4- by cytochrome c peroxidase and HRP (8). These comparisons should not however create an impression that the enzymatic oxidation of ferrocenes is always fast. The active-R2 subunit of Escherichia coli ribonucleotide reductase, which has dinuclear nonheme iron center in the active site, oxidizes ferrocene carboxylic acid and other water-soluble ferrocenes with a rate constant of... 

Creating acidic groups (carboxylic acids and phenol groups) on the internal soot surface by chemical oxidation with, e.g., chromic acid, fixation of cationic complexes of the catalyst (e.g., amino complexes of Pt(II) or Pt(IV) at the inner surface of the soot particles by ion exchange, followed by chemical or electrochemical reduction of these complexes... 

The electrochemical reduction of thiophene-2-carboxylic acid to the 2,5-dihydro derivative (198) in 76% yield has been reported the tetrahydro derivative is obtained as a by-product in 15% yield (75IZV888). The reduction is carried out in 2M LiOH on Hg electrodes. 

Raney nickel reduction of 2-benzyl-5-ethylselenophene (71) yields 1-phenylheptane (72), a conversion analogous to the much used reductive desulfurization of thiophenes (73JGU871). The electrochemical reduction of selenophene-2-carboxylic acid gives a mixture of dimeric products the major product is compound (73). This is in contrast to the 2,5-dihydro derivatives obtained by electrochemical reduction of thiophene and furan carboxylic acids (82CS( 19)95). Wolff-Kishner reduction of 2-selenienyl 2 -thienyl ketone gives, in addition to the expected methylene derivative, 2-(pentenyl)thiophene (72ZOB1780). 

Selective dehalogenation of halopyridines is an important industrial process for the same reason that reduction of carboxylic acids, esters, amides, and nitriles are also important. There is a dearth of selective oxidation technologies whether by conventional or electrochemical methods. Therefore, many intermediate oxidation stage products are made by overoxidation, i.e., overhalogenation, followed by selective reduction. 

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... 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

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. 

Industrial work on the reduction of carboxylic acids was focused on the search for an electrochemical synthesis of glyoxylic add from oxalic acid ... 

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