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

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.