Krapcho decarboxylation mechanism

The reaction is a decarboxylation of the Krapcho type,6 in the course of which one of the two ester groups of the starting malo-nate is removed and replaced by a proton. What results is ester 12. The only aspect of the reaction mechanism that has been established is that it does not proceed via the free acid. Krapcho himself has proposed the following mechanism ... 

The mechanism of the Krapcho dealkoxycarbonylation is dependent on the structure of the substrate ester and the type of anion used. In the case of a,a-disubstituted diesters (especially the methyl esters), the anion from the salt (cyanide ion in the scheme) attacks the alkyl group of the ester in an Sn2 fashion and the decarboxylation results in the formation of a carbanionic intermediate that is quenched by the water. In the case of a-monosubstituted diesters the cyanide attacks the carbonyl group to form a tetrahedral intermediate, which breaks down to give the same carbanionic intermediate and a cyanoformate, which is hydrolyzed to give carbon dioxide and an alcohol. 

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