Enantioselective catalysis, C. Decarboxylation of malonic acids in the presence of copper(I) compounds — Not a copper(I) catalysis but a base effect

The catalytic decarboxylation of malonic acids, claimed to be catalyzed by copper(I) compounds, has been investigated. Decarboxylation of different malonic acid derivatives (1–5) in acetonitrile was far more effective with Cu2O than with CuCl. Thus, the decarboxylation is obviously influenced by the basicity of the anion. In the decarboxylation of phenylmalonic acid (3),bis(tricyclohexylphosphane)copper(I) hydrogenphenylmalonate (6) and potassium hydrogenphenylmalonate (7) show nearly identical rate constants. It is concluded that the monoanions of the malonic acid derivatives are the reactive species undergoing decarboxylation. Further experiments are presented which demonstrate that everything that increases the concentration of the monoanions also increases the rate of decarboxylation. In the enantioselective decarboxylation of the monoethyl ester of methylphenylmalonic acid (2), the enantiomeric excess of (S)-(+)-ethyl 2-phenylpropionate could be raised to 34.5%ee using the alkaloid cinchonine.