Electrolytic decarboxylative coupling

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]. 

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. 

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. 

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... 

The radicals generated by anodic decarboxylation of carboxylic acids can be intercepted by alkenes that are present in the electrolyte. The adduct radical can couple with the radical generated from the car-boxylate to form an additive monomer I (Scheme 8 path a), it can dimerize to form an additive dimer II (path b), it can be further oxidized to a cation, which reacts with a nucleophile to form III (path c), or it can disproportionate (path d). 

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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