Kolbe reactions reaction conditions, dimerization

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. 

The Kolbe dimerization is believed to be favored by the following reaction conditions high concentration of caiboxylic acid, low pH value, absence of foreign anions, high cunent density and use of a platinum anode. 

Experimental variables affecting the course of the electrolytic decarboxylation of carboxylic acids are summarized in Table 2. For the Kolbe dimerization, the conditions specified for a one-electron process are recommended otherwise the reaction through carbenium ion (non-Kolbe reaction) may occur predominantly. It should be emphasized that even under the conditions most favorable for the Kolbe dimerization, the cation-derived products are usually formed to some extent or, in particular cases, as a major product, depending on the structure of the employed carboxylic acid. 

In 1834, while studying the conductivity of acetates, M. Faraday observed that an inflammable gas was produced at the anode [1]. However, being more interested in physics than in chemistry, he reported this phenomenon but did not identify the gas. Fifteen years later, in 1849, W.H. Kolbe reinvestigated this transformation [2]. He characterized ethane as the product formed at the anode and recognized the nature and utility of this electrochemical process. The Kolbe reaction, the electrochemical oxidative decarboxylation-dimerization of carboxylic acids, is a powerful method for the generation of C-C bonds under particularly mild conditions (Scheme 1). 

In the past three decades, more sophisticated electrolysis conditions suited for the Kolbe dimerization, for hydrogen abstraction, for substitution, for rearrangement, and for other radical- or cation-induced reactions have accumulated for a number of carboxylic acids and have been used for many synthetic purposes. Most recently, in the field of the... 

Shortly after our publications on the sonoelectrochemical oxidation of pheny-lacetate [186,187], a parallel study was reported by Japanese workers [191] who employed crossed Kolbe electrolyses of phenylacetates and succinates, variously deuterated, to produce deuterated derivatives of 4-phenylbutyric acids. In control experiments to produce deuterated bibenzyls from phenylacetate without succinate present, they obtained 11% of dimer with pyridine present. Under normal conditions, this rose to 47% yield of and 41% of dimer under ultrasound from a cleaning bath, although here the reaction time is stated to be reduced threefold. However, it is ambiguous whether this shortened reaction-time benefit also applies to the reaction with pyridine but without ultrasound. The authors state that ultrasound helps to keep the electrode surface clean and it would seem that in their conditions, which employ aqueous solution instead of methanol, the electrode is not completely switched off by the insulating film under normal conditions. The authors did not examine the system with both pyridine present and ultrasound, but the observed yield drop from 47 to 41% might suggest the same trend towards the two-electron pathway under ultrasound, although other products were not identified and quantified. 

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