Kolbe electrolytic coupling

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. 

Blanka, M. Guttman, A. Kollmann, H. Leitner, K. Mayhofer, G. Roven-szky, F. Winkler, K. Tetrahedron 1998, 54, 2059. 

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It is used as base in Kolbe electrolytic coupling in methanol solution a catalytic amount suffices since it is regenerated at the cathode. Commercial sodium methoxide... 

The new 23-hydroxy-epimers of cholesterol have been prepared by boro-hydride reduction of the 23-ketone, and Grignard reactions on the cyanohydrin of pregnenolone acetate have been used to prepare both epimers of 20a,22-dihydroxycholesterol. Using optically pure half-esters of methyl succinic acid in Kolbe electrolytic coupling reactions with various bile acids the corresponding 25-d- and 25-L-cholestanoic acids have been prepared. ... 

Electrolytic coupling of two molecules of a fliiorocarboxylic acid accompanied by decarboxylation, according to Kolbe, is a general method for the preparation of suitable fluoro derivatives and the early literiiture on this has been dealt with in Houben-Wcyl, Vol. 5/3. 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

Kolbe radicals can be added to olefins that are present in the electrolyte. The primary adduct, a new radical, can further react by coupling with the Kolbe radical to an additive monomer I (Eq. 9, path a), it can dimerize to an additive dimer II (path b), it can be further oxidized to a cation, that reacts with a nucleophile to III (path c), or it can disproportionate (path d). 

Finally, we note that the photocorrosion process is strongly pH-dependent, occurring most readily in strongly acid solutions, and that the presence of a carboxylic acid is required for the occurrence of severe photocorrosion. In Table II we present analytical results, based on inductively coupled argon plasma (ICP) emission spectroscopy, for representative electrolyte solutions after 6-8 hr. of photo-Kolbe electrolysis with n-SrTiC anodes. It can be seen that the formation of soluble strontium and titanium species is... 

Electrolytic generation of perfluoroalkyl radicals can also lead to coupling (Kolbe Reaction), and when carried out in the presence of an addend, like an olefin, can lead to coupling, reduction and disproportionation-type products of the adduct radicals, as well as occasionally-decent yields of simple adducts [183-188]. 

Electrolytic decarboxylative coupling of sodium salts of carboxylic acids takes place during their electrolysis. Carbon dioxide is eliminated, and the free radicals thus generated couple to form hydrocarbons or their derivatives. The reaction is referred to as the Kolbe electrosynthesis and is exemplified by the synthesis of 1,8-difluorooctane from 5-fluorovaleric acid (equation 469) [574]. Yields of homologous halogenated acids range from 31% to 82% [574]. 

By anodic decarboxylation carboxylic acids can be converted simply and in large variety into radicals. The combination of these radicals to form symmetrical dimers or unsymmetrical coupling products is termed Kolbe electrolysis (Scheme 1, path a). The radicals can also be added to double bonds to afford additive monomers or dimers, and in an intramolecular version can lead to five-membered heterocycles and carbocycles (Scheme 1, path b). The intermediate radical can be further oxidized to a carbenium ion (Scheme 1, path c). This oxidation is favored by electron-donating substituents at the a-carbon of the carboxylic acid, a basic electrolyte, graphite as anode material and salt additives, e.g. sodium perchlorate. The carbocations lead to products that are formed by solvolysis, elimination, fragmentation or rearrangement. This pathway of anodic decarboxylation is frequently called nonKolbe electrolysis. 

The electrolysis products of different carboxylates have been compared with the ionization potentials of the intermediate radicals. From this it appeared that alkyl radicals with gas-phase ionization potentials smaller than 8 eV mainly lead to carbenium ions. Accordingly, a-substituents such as carboxy, cyano or hydrogen support the radical pathway, whilst alkyl, cycloalkyl, chloro, bromo, amino, alkoxy, hydroxy, acyloxy or aryl more or less favor the route to carbenium ions. Besides electronic effects, the oxidation seems also to be influenced by steric factors. Bulky substituents diminish the extent of coupling. The main experimental factors that affect the yield in the Kolbe electrolysis are the current density, the pH of the electrolyte, ionic additives, the solvent and the anode material. 

Ionic additives to the electrolyte can influence the Kolbe electrolysis in a negative way. Anions other than the carboxylate should be excluded, because they hinder the formation of a carboxylate layer at the anode, that seems to be a prerequisite for the decarboxylation. In the electrolysis of phenyl acetate the coupling to dibenzyl is totally suppressed when sodium perchlorate is present in concentrations of 5 x 10" mol 1" benzyl methyl ether, the nonKolbe product, is formed instead. This shift from the radical... 

Another source of single electrons is the anode of an electrolytic cell. In the Kolbe reaction, an alkylcarboxylate salt, RC02, is decarboxylated and the resulting alkyl radicals couple to form the dialkyl product, R-R. Suggest the pathway that is followed by this reaction. 

A polymer electrolyte membrane (PEM) reactor is described by Hicks and Fedkiw [2.454] for use during Kolbe electrolysis, which involves the anodic oxidation of an alkyl carboxylic acid, and its subsequent decarboxylation and coupling to produce a dimer. 

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