Dimer formation of cinchonidine in liquid phase: relevance to the heterogeneous catalytic enantioselective hydrogenation of ethyl pyruvate

Literature data related to the possible dimer formation of cinchona alkaloids in the liquid phase are collected and analyzed. These data are correlated with experimental results obtained in the heterogeneous catalytic enantioselective hydrogenation of ethyl pyruvate. In this reaction, the addition of achiral tertiary amines (ATAs) resulted in an increase in both the reaction rates and enantioselectivity. The positive influence of ATAs was attributed to the suppression of dimer formation in aprotic solvents. The results of circular dichroism spectroscopy and ab initio calculations provided further proof for dimer formation. Four possible cinchonidine dimer configurations were found with approximate 11–13 kcal/mol stabilization energies.