Anodic carboxylates

In the case of benzal chloride, the carboxylation in conventional diaphragm systems fails, leading to poor yields in phenylacetic and mandelic add [180], At an Al anode, carboxylation occurs because the self-esterification of the first carboxylate anion onto the second chloride group is hindered by the formation of Al complex salts [181]. Yields of phenylmalonic and chlorophenylic acetic acids up to 30% each have been obtained [178],... 

In the Kolbe reaction and in anodic carboxylation reactions, salts of carboxylic acids function as both substrate and electrolyte. Usually the presence of other anions diminishes the yield of the Kolbe reaction, whereas the presence of anions that are oxidizable with difficulty, like bicarbonate or perchlorate ions, favors the related Hofer-Moest reaction. 

Meanwhile, on reaching the anode, carboxylate anions are neutralized by hydrogen ions from the electrolysis of water. As resist solids are removed from the bath at the cathode, there is a gradual build up of ionizer in the bath. Therefore, to maintain bath chemistry, free acid must be removed by ultrafiltration, or drag-out, or the use of semipermeable membranes (anolyte boxes) [3]. 

A classic reaction involving electron transfer and decarboxylation of acyloxy radicals is the Kolbe electrolysis, in which an electron is abstracted from a carboxylate ion at the anode of an electrolysis system. This reaction gives products derived from coupling of the decarboxylated radicals. 

The anodic oxidation of the carboxylate anion 1 of a carboxylate salt to yield an alkane 3 is known as the Kolbe electrolytic synthesis By decarboxylation alkyl radicals 2 are formed, which subsequently can dimerize to an alkane. The initial step is the transfer of an electron from the carboxylate anion 1 to the anode. The carboxyl radical species 4 thus formed decomposes by loss of carbon dioxide. The resulting alkyl radical 2 dimerizes to give the alkane 3 " ... 

Suitable starting materials for the Kolbe electrolytic synthesis are aliphatic carboxylic acids that are not branched in a-position. With aryl carboxylic acids the reaction is not successful. Many functional groups are tolerated. The generation of the desired radical species is favored by a high concentration of the carboxylate salt as well as a high current density. Product distribution is further dependend on the anodic material, platinum is often used, as well as the solvent, the temperature and the pH of the solution." ... 

A further effect which has been known for many years is that of anions, which are specifically adsorbed at high anodic potentials on platinum, on the products of the oxidation of carboxylate ions. For example, carbonium ion-derived products can be obtained in the presence of such specific adsorption and this demands a complete change in reaction route (Fioshin and Avrutskaya, 1967 Glasstone and Hickling, 1934). 

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. 

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

It has been shown by employing the radioactive tracer method with C-labeled carboxylic acids [79] and with rotating disc electrode experiments [80] that carbo-xylates are adsorbed at the anode surface. 

Two equal carboxylates can be coupled to symmetrical dimers (Eq. 4). In spite of the high anode potential, that is necessary for Kolbe electrolysis, a fair number of... 

Hydroxy- and amino carboxylic acids can be dimerized in good to moderate yields, when the substituents are not in the a- or P-position and when they are additionally protected against oxidation by acylation (Table 2, No. 17-19). 2-Alkenoic acids cannot be dimerized but lead to more or less extensive passivation of the anode due to the formation of polymer films [136]. 3- and 4-Alkenoic acids give moderate yields when they are neutraUzed with BU3N or EtjN [136]. 3-Alkenoic acids with the structure... 

Non-Kolbe electrolysis of alicyclic p-hydroxy carboxylic acids offers interesting applications for the one-carbon ring extension of cyclic ketones (Eq. 35) [242c]. The starting compounds are easily available by Reformatsky reaction with cyclic ketones. Some examples are summarized in Table 13. Dimethylformamide as solvent and graphite as anode material appear to be optimal for this reaction. 

Pseudo-Kolbe electrolysis is the name given to anodic decarboxylations where the electron transfer does not occur from the carboxylate but from a group attached to it [31]. These oxidations are characterized by potentials that are much lower than the critical potential for the Kolbe electrolysis. The salt of p-methoxyphenylacetic acid can be oxidized in methanol to afford the corresponding methyl ether as the sole product. The low oxidation potential of 1.4 V (see) suggests, that the electron is being transferred from the aromatic nucleus (Eq. 39) [31]. 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Anodic Oxidation of Carboxylic Acids Without Decarboxylation... 

Carboxylic acids can be converted by anodic oxidation into radicals and/or carbo-cations. The procedure is simple, an undivided beaker-type cell to perform the reaction, current control, and usually methanol as solvent is sufficient. A scale up is fairly easy and the yields are generally good. The pathway towards either radicals or carbocations can be efficiently controlled by the reaction conditions (electrode material, solvent, additives) and the structure of the carboxylic acids. A broad variety of starting compounds is easily and inexpensively available from natural and petrochemical sources, or by highly developed procedures for the synthesis of carboxylic acids. 

Methyl 2-furoate was dimethoxylated using methanol in sulfuric acid to give methyl-2,5-dihydro-2,5dimethoxy-2-furan carboxylate [70]. The reaction mechanism at the electrodes is not completely known. However, the anodic reaction is said to be the oxidation of methanol. A two-electron process is assumed and hydrogen production is observed at the cathode. 

The shrinking of a PAANa gel touching the anode in an NaOH solution has been analyzed by Doi et al. [14], They have calculated the osmotic pressure at the anode side using Eq. 16. At the anode, H+ ions are produced by the electrolysis of water, and this suppresses the dissociation of carboxyl groups near the anode. As a result, n at the anode side decreases very quickly. Thus the gel shrinks when it is kept in contact with the anode. 

Radicals, (34), that subsequently dimerise, are also obtained through the anodic oxidation of carboxylate anions, RCO20, in the Kolbe electrolytic synthesis of hydrocarbons ... 

Photoluminescence (PL) of anodic aluminum oxides was first investigated in films formed in organic acids, the most intense PL being in those formed in oxalic acid. Tajima, in his comprehensive review315 on electro- and photoluminescence in anodic oxide films, concluded that PL centers are carboxylate anions incorporated into the oxide. On the other hand, Eidel berg and Tseitina316 proposed... 

Anodic decarboxylation proceeds via a C—C bond scission of carboxylate anions to afford the Kolbe dimer,197 i.e.,... 

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