Brown-Walker electrolysis

The Brown- Walker Electrolysis is useful for dimerizing half-acid-esters ... 

The dimerization of half esters to l,n-diesters is also called Brown-Walker electrolysis. Thereby valuable intermediates for the synthesis of medium-sized rings or l,n-difunctionalized compounds can be prepared (Table 2, entry 4). This reaction is also of industrial interest since in this way sebacic acid can be prepared from adipic acid half ester. This process has been scaled-up in Germany, the USSR and Japan, and yields as high as 93% have been reported. Reaction conditions and yields for the coupling of other half esters have also been studied in detail. 1, -Polyethylene- or polydifluoromethylene-dicar-boxylic acids are reported to be formed by electrolysis of azelaic acid or perfluoroglutaric acid. Ketocarboxylic acids can be coupled to 1,4-, 1,6- or 1,14-diketones (Table 2, entries 9 and 10). Aldehydes must be dimerized in the form of their acetals to obtain good yields, as has been shown for (17) and (18). The arrow on (10)-(18) indicates the location of dimerization, along with the yield and reaction conditions. 

The Brown-Walker 1 method has been found to be of excellent service in the electrolysis of the potassium salts of the mono-esters of malonie acid. The formation of the diethyl ester of succinic acid from ethyl potassium malonate has already been mentioned fp. 103). 

Monomethyl adipate is fed into the electrolysis loop. This specific variation of the Kolbe reaction is known as the Brown-Walker reaction. The electrodes are bipolar (the anode is platinum-plated titanium). 

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. 

The first successful experiments in the electrolysis of aliphatic carboxylic acids were made by Kolbe.1 These experiments are supplemented by the researches of Kekule,2 Brown and Walker,3 Mulliken,4 and Weems,5 who amplified our knowledge regarding this subject which, still further investigated in the most varied directions by a number of investigators, has yielded valuable results. 

Benzylmalonic Acid.—When this acid in the form of its ethyl-potassium salt was submitted to electrolysis by Brown and Walker 8 it exhibited a behavior materially different from that of malonic acid. The solution became dark-colored, but contained no new compound. If oxidation occurred, it was a complete oxidation into carbon dioxide and carbon monoxide, such as has been observed in the case of unsaturated acids. 

Many years ago. Brown and Walker found that by electrolysis of mono-esters of dibasic acids, carbon dioxide was removed, and the residues condensed to form a dibasic ester of higher molecular weight. 

Electrosyntheses.—The same investigators recently added to our knowledge on this subject by submitting to electrolysis solutions containing the potassium salts of mono-basic acids and the ethyl esters of the mono-potassium salts of dibasic acids dissolved in equi-molecular proportion. This was in accordance with the experiments made by Brown and Walker. 

---