Theoretical Insights into the Catalytic Mechanism of N-Heterocyclic Olefins in Carboxylative Cyclization of Propargyl Alcohol with CO2

The mechanism of carboxylative cyclization of propargyl alcohol with CO2 catalyzed by N-heterocyclic olefins (NHOs) has been studied by density functional theory calculations. The calculations reveal that the catalytic reaction tends to proceed via the NHO-mediated basic ionic pair mechanism, in which free NHO primarily acts as a basic precursor to trigger the carboxylation of propargyl alcohol with CO2, leading to a [NHOH](+)[carbonate](-) ion pair intermediate. Then, the catalytic cycle proceeds, including isomerization of the [NHOH](+)[carbonate](-) ion pair intermediate, intramolecular nucleophilic addition of the carbonate oxygen anion to the alkynyl group, and protonation of the alkenyl carbon anion with an external propargyl alcohol molecule. Molecule orbital and nature population analysis discloses that the preference for the basic ionic pair mechanism is due to the favorable orbital and charge interactions between the a-carbon atom of NHO and the hydroxyl hydrogen of propargyl alcohol. The [NHOH](+) cation has proven to be crucial for stabilizing the [carbonate](-) anion, which allows the reaction to proceed through a more thermodynamically stable pathway. The investigations of the effect of substituents of NHOs predict that N-substituents with a strong electron donating effect and a bulky steric effect might improve the catalytic activity of NHOs for the reaction.