The Kolbe electrolysis is an effective and simple method for the coupling of carboxylates. Experimental conditions and substituents in the carboxylate that favour dimerisation are specified and representative preparative examples are given. Cross-coupling of carboxylates with halfesters of dicarboxylic acids affords an easy access to fatty acids; additive dimerisation with butadiene yields unsaturated diesters. Some recent applications of the Kolhe reaction are the preparation of 1,5-dienes by dimerisation of ~, ~/-unsaturated acids and the synthesis of pheromones, e.g. muscalure, looplure, brevicomin or disparlure by mixed coupling of unsaturated carboxylic acids with halfesters of dicarboxylic acids.
Scope of the method
The Kolbe electrolysis [1-6] affords a simple way for C-C coupling of carboxylates, which are oxidised anodically at +2.3 V (versus. NHE), or higher, to acyloxy radicals. These are decarboxylated to alkyl radicals, which dimerise or can be further oxidised to carbenium ions, that form alkenes, esters or alcohols. Conditions that favour dimerisation (path a) are, the use of a smooth platinum anode, especially in aqueous medium and in non-aqueous electrolytes such as methanol, acetic acid, acetonitrile or dimethylformamide, also, platinised titanium, gold or baked graphite [71. 0
Scheme 1. Reaction pathways of the Kolbe electrolysis.
The choice of the cathode material is uncritical; platinum, graphite, mercury, and steel have been employed. The preferred solvent is methanol; in water the dimer yields are lower. Aprotic solvents that have been sucessfully used are dimethyiformamide, -though compared to methanol it is less resistant toward oxidation,and acetonitrile . The supporting electrolyte is the earboxylate; because low pH is favourable, only 2-10% of the acid is neutralised with sodium or potassium hydroxides, or methoxides. The current density should be high (> 0.25 A/cm2), the