Mechanistic Insight into the Gold-Catalyzed Carboxylative Cyclization of Propargylamines

Abstract

DFT calculations have been carried out to study the detailed mechanisms for the carboxylative cyclization of propargylamine using CO2 catalyzed by NHC-gold(I) complexes. The calculation results indicate that the reaction starts with an N-coordinated species, [(NHC)Au(propargylamine)]Cl, which undergoes isomerization to an alkyne-coordinated species. An amine–carbon dioxide interaction gives a carbamate ion species, from which a nucleophilic attack of the in-plane lone pair of electrons in the carbamate anion moiety on one of two coordinated alkyne carbons leads to formation of a five-membered-ring intermediate. The final product is generated through deprotonation and protonation processes. Through a detailed mechanistic study, we found that the substrate propargylamine assists (catalyzes) the deprotonation and protonation processes. Careful study of the solvent effect indicates that solvents, which are polar and capable of hydrogen bonding, promote the catalytic reactions through stabilizing the carbamate ion intermediate species.