Decarboxylative coupling polarity

The alkyl groups of two identical carboxylic acids can be coupled to symmetrical dimers in the presence of a fair number of functional groups (equation 1). Since free radicals are the reactive intermediates, polar substituents need not be protected. This saves the steps for protection and deprotection that are necessary in such cases when electrophilic or nucleophilic C—C bond-forming reactions are involved. Furthermore, carboxylic acids are available in a wide variety from natural or petrochemical sources, or can be readily prepared from a large variety of precursors. Compared to chemicd methods for the construction of symmetrical compounds, such as nucleophilic substitution or addition, decomposition of azo compounds or of diacyl peroxides, these advantages make the Kolbe electrolysis the method of choice for the synthesis of symmetrical target molecules. No other chemical method is available that allows the decarboxylative dimerization of carboxylic acids. 

Crabtree prepared rhodium, iridium, ruthenium, and palladium complexes e.g. 30) bearing NHCs by heating the carboxylate compound and an appropriate metal precursor in acetonitrile [eqn (2.7)]. It was also shown that the carboxylate of l,3-dimethylimidazol-2-ylidene (IMe) could be prepared from the direct reaction of 1-methylimidazole with dimethyl carbonate. Subsequently, Delaude showed that these species can be used in lieu of free NHCs for the synthesis of alkene metathesis pre-catalysts and to form palladium cross-coupling catalysts in situ while Olszewski has used these to prepare silver(i) and eopper(i) complexes. Falvey has shown that solvent choice is important, beeause the rate of decarboxylation is higher in less polar solvents. ... 

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