Electrolysis of carboxylate

Electrolysis of carboxylic acid salts m solution causes decaboxyladve coupling similar to Kolbe reaction. Thus, eleclrolysis of 3,3,3-trifluoro-2-trifluoromethyl-propanoic acid in die presence of some of its potassium salt gives the corresponding fluoroalkane in a satisfactory yield [7d] (equation 67). 

In some cases the polymerization of reactive olefins can be initiated by electrolysis of carboxylic acids. Monomers that have been polymerized this way are styrene [212],... 

The electrolysis of carboxylic acids in aqueous solution can lead to alcohols and esters as major reaction products [5,241]. When electrolyses are performed in methanol or acetic acid methyl ethers or acetates can be found as side or major products. These observations led Walling and others [242] to suggest that in these cases the inter-... 

Non-Kolbe Electrolysis of Carboxylic Acids to Ethers, Esters, and Alcohols... 

Table 9. Preparation of ethers, esters and alcohols by non-Kolbe electrolysis of carboxylates 00... 

Non-Kolbe electrolysis of carboxylic acids in acetonitrile/water leads to acetamides as main products [294] (Table 10). The mechanism has been investigated by using " C-labeled carboxylic acids. The results are rationalized by assuming a reaction layer rich of carboxylate resulting in the formation of a diacylamide that is hydrolyzed... 

Non-Kolbe electrolysis of carboxylic acids can be directed towards a selective fragmentation, when the initially formed carbocation is better stabilized in the y-position by a hydroxy or trimethylsilyl group. In this way the reaction can be used for a three-carbon (Eq. 36) [335] (Table 14, No. 1) or four-carbon ring extension (Eq. 37) [27] (Table 14, Nos. 2-4). Furthermore it can be employed for the stereo-... 

Electrolysis of carboxylate ions, which results in decarboxylation and combination of the resulting radicals, is called the Kolbe reaction or the Kolbe electrosynthesis. 

Eor some common organic electrochemical reactions, for example, the Kolbe electrolysis of carboxylates [13], the adsorption of intermediates has been discussed. 

While anodic amide oxidations have found the most synthetic use to date, the oxidation of nitrogen-containing molecules is not limited to amide substrates. A variety of amine oxidations have been studied, and the Kolbe electrolysis of carboxylic acids has been used to generate nitrogen-based reactive intermediates. Many of these reactions also offer unique synthetic advantages (Sects. 10.2 and 10.3). 

Electrolysis of carboxylate ions, which results in decarboxylation and combination of the resulting radicals, is called the Kolbe reaction. 30 It is used to prepare symmetrical RR, where R is straight- or branched-chained, except that little or no yield is obtained when there is a branching. The reaction is not successful for R = aryl. Many functional groups... 

The electrode material can also influence the product distribution, as shown in the Kolbe electrolysis of carboxylates. With platinum anodes, the Kolbe dimerization of the intermediate radicals predominates strongly (Eq. 22.5). At carbon anodes, however, further oxidation to the carbenium ion (non-Kolbe reaction or Hofer-Moest reaction) becomes the main pathway (Eq. 22.25). 

The electrochemical reaction which is most intensively studied and has the widest synthetic applications is the Kolbe electrolysis of carboxylates (Eq. (152) ) to give dimers (80) ... 

Some results on Kolbe couplings are listed in Table 3 [42,44-51]. The electrolysis of carboxylic acids is usually carried out in an alkaline protic solvent-(Pt electrodes) system. Most frequently, MeOH-MeONa, Me0H-H20-Na0H, MeOH-KOH, MeOH-Py-Et N, and MeOH-Py-Na salt systems are used as electrolysis media. Aprotic media, such as DMF-KOH, MeCN-Et3N, MeCN-Et4NBF4, and Py-H20-Et3N, are also utilized and, in particular cases, acetone-H20-NaOH is usable. 

Electrolysis of carboxylates RCO2 in the presence of olefins (XXVIII) may afford the radical addition products XXX, XXXI, XXXII, and XXXIII. Plausible reaction pathways are illustrated in Eq. (18). The radical R generated by electrodecarboxylation of RCO2 first combines with the olefin (XXVIII) to give the radical intermediates (XXIXa), which may provide the dimers (XXX) or the radical coupling products (XXXI). Further one-electron oxidation of XXIXa may provide cations (XXIXb), which subsequently react with the nucleophiles Nu or liberate to give substituted products (XXXII) or olefins (XXXIII). 

Electrolysis of carboxylic acids in the presence of nitrate [109,110], difluoroamide [111], azide [112], and bromide ions [113] has been examined in which radical species generated from both carboxylic acids and additive anions are formally coupled to give substituted products ... 

Table 8. Some Examples of the Kolbe Electrolysis of 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. ... 

Table 9 Preparation of Ethers, Esters and Alcohols by NonKolbe Electrolysis of Carboxylates...

Decarboxylation of silver carboxylates is a well known thermal process and is involved in the Hunsdiecker76 or Kolbe77 reactions. The Hunsdiecker reaction is the thermal decarboxylation of silver salts of acids and is used for the formation of bromoalkanes and related compounds, while the Kolbe process involves electrolysis of carboxylates as a route to decarboxylated radicals that can dimerize. Silver carboxylates are also photochemically reactive and the irradiation has been described as a facile process for the formation of alkyl radicals, as illustrated in equation 678. Later experimentation has shown that the irradiation of silver trifluoroacetate can serve as a route to trifluoromethyl radicals. This development uses irradiation of silver trifluoroacetate in the presence of titanium dioxide as a photocatalyst. The reaction follows the usual path with the formation of metallic silver and the formation of radicals. However, in this instance the formation of metallic... 

Table 6. Selected examples of mixed Kolbe electrolysis of carboxylic acids...

Formation of symmetrical dimers by the electrolysis of carboxylates (decarboxylative dimerization). The coupling of two distinct carboxylates yields unsymmetrical products ... 

The dimerization reaction, illustrated by the formation of the pinacol in the electroreduction of acetophenone, is an obvious example of a coupling reaction. Such reactions may occur between radicals or radical ions generated in either an anodic or a cathodic process. For example, an electrochemical process in which radicals are generated anodically is the classical Kolbe reaction, the electrolysis of carboxylates ... 

Electrolysis of carboxylates in the presence of olefins may afford the radical addition products [5]. Intramolecular radical addition may furnish a simple and straightforward access to cyclic compounds. Mixed decarboxylative coupling of 6-alkenoic acids and various carboxylic acids giving cyclic compounds is utilized for synthesis of useful chemicals. For instance, a stereoselective synthesis of prostaglandin precursors is achieved successfully by mixed Kolbe-type decarboxylation [b](Fig.2). 

Ono Y, Kim S-H, Yasnda M, Nonaka T (1999) Kolbe electrolysis of carboxylates on a hydrophobic plati-nnm electrode composite-plated with PTFE particles. Electrochemistry 67 1042-1045 (Tokyo)... 

The occurrence of carbocationic intermediates in the Hofer-Moest variant of the Kolbe reaction can lead to numerous rearrangement processes. For example, electrolysis of carboxylic acid 68 in methanol results in the oxidative ring opening of the norbomane skeleton[30]. In a similar manner, GroB-type fragmentation of the decaline derivative 70 affords the 10-membered-ring ketone 71 in good yield (Scheme 12) [31]. 

Electrolysis of carboxylates to afford the coupled products. Homocoupling product is obtained if two carboxylates are the same — decarboxylative dimerization unsymmetrical product will be produced if the two carboxylates are different. 

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