Reaction Mechanism and Regioselectivity of Cu(I)-Catalyzed Hydrocarboxylation of 1-Phenyl-propyne with Carbon Dioxide

Abstract

Density functional theory (DFT) calculations have been used to conduct a detailed study of the mechanism involved the copper(1)-catalyzed hydrocarboxylation of 1-phenyl-propyne using CO2 and hydrosilane. Theoretical calculations suggested that the activated catalyst Cl2IPrCuH is initially generated in situ by the reaction of Cl2IPrCuF with (EtO)(3)SiH, and that the entire catalytic reaction involves three steps, including (1) the addition of Cl2IPrCuH to 1-phenyl-propyne to afford two isomeric copper alkenyl intermediates, which lead to the formation of the corresponding final alpha,beta-unsaturated carboxylic acid derivatives; (2) CO2 insertion to give the corresponding copper carboxylate intermediate; and (3) sigma-bond metathesis of the copper carboxylate intermediate with a hydrosilane to provide the corresponding silyl ester with the regeneration of the active catalyst. The results of our calculations show that the rate-limiting steps for the two paths leading to the two alpha,beta-unsaturated carboxylic acid derivatives are different. In Path a, the alkyne and CO2 insertion steps were both identified as possible rate-limiting steps, with free energy barriers of 68.6 and 67.8 kJ.mo1(-1), respectively. However, in Path b, the alkyne insertion step was identified as the only possible rate-limiting step with an energy barrier of 78.7 kJ.mol(-1). These results were in agreement with the experimental observations. It was also found that the alkyne insertion step controlled the regioselectivity of the products, and that electronic effects were responsible for the experimentally observed regioselectivity.