Carboxylic esters anodic oxidation

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Various nucleophiles other than methanol can be introduced onto the carbonyl carbon. Anodic oxidation of acylsilanes in the presence of allyl alcohol, 2-methyl-2-propanol, water, and methyl /V-methylcarbamate in dichlorometh-ane affords the corresponding esters, carboxylic acid, and amide derivatives (Scheme 24) [16]. Therefore, anodic oxidation provides a useful method for the synthesis of esters and amides under neutral conditions. 

Anodic oxidation of j9-methylbenzyl-sulfonic ester, -carboxylic ester, and -nitrile in Et3N-3HF/CH3CN affords fluorides and acetamides at tbe metbyl (Me) and substituted (CH2E) benzyl position Me/CH2E = 24/76 (E = C02Et), 9/91(CN), 69/31 (SOsEt). In tbe radical bromination of these compounds, substitution at CH3 is enhanced [20],... 

Steckhan and coworkers found that the indirect anodic oxidation of N-pro-tected dipeptide esters 56, in which the C-terminal amino acid is a-branched, can afford methyl imidazolidin-4-one-2-carboxylate 57 in 45-84% yields [86], This reaction can be performed at a Pt-anode by using Et4NCl as an electrolyte in the presence of 5% methanol in an undivided cell (Scheme 30). 

Anodic oxidation of fiirans in acetic acid leads to the 2,5-diacetoxy-2,5-dihydro-furan 58 [185, 186]which is readily converted to 2-acetoxyfiiran, This has proved a valuable intermediate for the synthesis of butenolides [187]. Reactions in moist acetonitrile yield the 2,5-dihydro-2,5-dihydroxyfurans which can be oxidised to the maleic anhydride 59 [188], Oxidation of furan-2-carboxylic acid in methanol and sulphuric acid is a route to the ester of a-ketoglutaric acid [189]. 

If these anions are oxidized at carbon anodes instead of Pt anodes, the main product is not the Kolbe dimer but an ester of the original carboxylic acid. This reaction (Hofer-Moest) is explained by the inherent instability of C radicals at highly anodic potentials, which are necessary for the anodic oxidation of carboxylate anions. At these potentials, the C radicals are oxidized to carbonium ions that react with carboxylate anions forming esters ... 

The electrochemical behavior of malonyl-a-aminopyridines 661 was investigated by Gullu et al. in acetonitrile or a mixture of trifluoroacetic acid and dichloromethane containing tetrabutylammonium tetrafluoro-borate or triethylammonium trifluoroacetate in a water-jacketed, two-compartment glass cell equipped with a platinum disk anode at 1.50 V (Ag/ Ag+) and a carbon-rod secondary electrode (91T675). Controlled potential anodic oxidation of 661 afforded labile coupled carboxylic acids 662 (R2 = COOH), which easily decarboxylated to compounds 662 (R2 = H) under the work-up conditions. Sometimes, the carboxylic acid 662 (R2 = COOH) could be isolated or when the reaction mixture was treated with methanol, methyl ester 662 (R = H, R1 = Bu, R2 = COOMe) was obtained in 40% yield. 

Simons process — Electrochemical polyfluorination reactions of organic compounds are the only efficient way to industrial production of perfluorinated compounds. The reaction proceeds in the solution of KF in liquid HF (b.p. 19.5 °C), where the starting substances as alcohols, amines, ethers, esters, aliphatic hydrocarbons and halo-hydrocarbons, aromatic and heterocyclic compounds, sulfo- or carboxylic acids are dissolved. During anodic oxidation, splitting of the C-H bonds and saturation of the C=C bonds occur and fluorine atoms are introduced. 

Imidazole carboxylic acids are readily converted into hydrazides,436 acid halides,437 amides,437-439 and esters,439-440 and they may be reduced to alcohols with lithium aluminum hydride,441 and to aldehydes by controlled potential reduction.442 Anodic oxidation of l-methylimidazole-5-acetic acid (94) using cooled platinum electrodes yields l,2-bis(l-methylimidazol-5-yl)ethane (95).443... 

The electrochemistry of amino acids has been studied in strong acid solutions. In general, the compounds are decomposed to carboxylic acids, aldehydes, ammonia, and carbon dioxide. The results are reviewed by Weinberg [35]. The anodic oxidation mechanism has been studied in pH 10 buffer solution. Decarboxylation accompanied by C-N bond cleavage is the main reaction process [182]. The synthetically interesting Hofer-Moest decarboxylations of A/ -protected amino acids and a-amino malonic half esters under the formation of A/ -acyliminium ions is treated in the following section. 

S-tert-buty thioates can be deprotected by indirect electrochemical oxidation using bromide/bromine as mediator the reaction probably goes through a bromosulfonium intermediate. By direct anodic oxidation, 4-methoxyphenylthiomethyl esters are readily converted to carboxylic acids. The initially formed radical cation cleaves on reaction with water to the acylated hemiacetal and further to the acid [144,145]. 

Allyl and benzyl silanes may be anodically oxidized in the presence of an alcohol, a carboxylic acid, or water to the corresponding ether, ester, or alcohol [208]. The initially formed radical cation is believed to lose the silicon moiety by attack by the nucleophile on silicon ... 

Germain and Commeyras have found that perfluorosulfonic esters and fluorosulfates are formed in high yields by direct anodic oxidation of Rfl in perfluoroalkane sulfonic acids and fluorosulfuric acid (Eqs. 34 and 35). With diiodo compounds, the mono- and diesters can be selectively obtained (Eq. 36). These are useful precursors to valuable perfluoro carboxylic acids. 

Besides the aforementioned Kolbe dimers, alcohols, esters or ethers can become the major products in the electrolysis of carboxylic acids. These results have suggested that in anodic decarboxylation the intermediate radicals were further oxidized to carbocations that yielded solvolysis and elimination prod-ucts. °2 This part of the anodic decarboxylation, which leads to carbenium ions, is frequently called nonKolbe electrolysis. Applications of the nonKolbe electrolysis to synthesis and to mechanistic investigation of carbocations are summarized in refs. 8,19,20 and 23. ... 

Methyl esters are produced in the electrolytic oxidation of methanolic solutions of aldehydes in the presence of sodium cyanide at a platinum anode the eight examples that have been studied show yields of 38 to 80%. Carefully purified butyraldehyde reacts with RuHgfPPha) to give butyl butyrate aliphatic and aromatic aldehydes undergo this dimerization process, which may have considerable synthetic potential." The electrolysis of furan-2-carboxylic acid (66) provides an efficient synthesis of 4,4-dimethoxy-esters (67), typically in 77 % yield (Scheme 35). "... 

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