DFT study of N-Heterocyclic Olefins-catalyzed carboxylative cyclization of CO2 with alkynol: A CO2-promoted hydrogen abstraction mechanism

DFT calculations have been carried out to study the detailed mechanisms of the carboxylative cyclization reaction between propargylic alcohols and CO2 catalyzed by N-Heterocyclic Olefins (NHO), as well as the molecular orbital theory. Results indicated that this type of reaction prefers a three steps mechanism controlled by free NHO rather than to be catalyzed by the NHO-CO2 adducts. For the first step, CO2 promotes the hydrogen transfer from alkynol to NHO to form the carboxylate, in which propargylic alcohols was deprotonated by the free NHO acted as the catalyst precursor to form the alkynol anion; meanwhile, alkynol anion captures carbon dioxide to form the carboxylate. We found this CO2 promoting Hydrogen abstraction mechanism would decrease the reaction energy barrier and increase releasing heat of this reaction. Secondly, a five-membered-ring intermediate is easily formed to generate carboxylate via an intramolecular ring-closing reaction. Finally, the production generated through a protonating process.