Mechanistic insights into the Cu(ii)/DBU-catalyzed incorporation of CO2 into homopropargylic amines

Abstract

The mechanisms of Cu(II)/DBU-catalyzed carboxylation of homopropargylic amines with CO₂ to construct 6-methylene-1,3-oxazin-2-ones are studied via density functional theory. The calculations suggest that Cu(II) catalysts exhibit a positive effect on the process of intramolecular cyclization, and the catalytic activities of which are determined by their coordination abilities ((DBU)CuCl₂ < CuCl₂ < [(DBU)CuCl]⁺). Furthermore, three roles of DBU are revealed in this study: (a) DBU acts as the proton-acceptor to capture a proton of homopropargylic amines. (b) DBU attacks CO₂ to form DBU-CO₂ as the nucleophilic agent, and then DBU-CO₂ assists the H⁺ shift as the proton shuttle. (c) DBU serves as the ligand replacing one Cl⁻ anion of CuCl₂, forming the activated species [(DBU)CuCl]⁺. Besides, the theoretical study well explains the selectivity of Cu(II)-catalyzed reactions (6- vs. 7-membered cyclization), and the 6-membered products are found to be formed selectively. Overall, our work reproduces the experimental results well and provides an indispensable understanding of CO₂-conversion to synthesize various high-value chemicals with the assistance of a catalytic system combining transition-metal catalysts and organic bases.