Carbon Dioxide Cycloaddition to Epoxides Promoted by Nicotinamidium Halide Catalysts: A DFT Investigation

Abstract

The utilization of CO2 as building block for the production of cyclic carbonate is a promising route to simultaneously mitigate the global warming issue and obtain valuable commercial chemicals. In this work, the activity of nicotinamidium halide catalysts towards the CO2 conversion into cyclic carbonate has been explored by means of density functional theory (DFT) calculations. DFT calculations support the ability, suggested experimentally, of the pyridium α‐C−H proton of the catalysts to activate the epoxide ring via a hydrogen bond. Interestingly, DFT calculations underline the involvement of the n‐octyl substituent of the pyridyl ring in the epoxide activation, while the hydrogen atom of the amide group N−H is rather involved in the stabilization of the iodide trough electrostatic interactions. Moreover, the replacement of the pyridium α‐C−H proton with the bulkier methyl group leads to a different reaction mechanism. The calculated energy barriers well reproduce the experimental trends of the studied catalysts, and the computed activation barrier of 29.0 kcal/mol, relative to the ring opening step of the most active catalyst, is in line with the experimental working temperature of 80 °C. Those results shed light on the CO2 fixation reaction contributing to the development of more efficient catalytic systems.

A mechanistic study on coupling of CO2 and epoxide mediated by guanidine/TBAI catalysts

“…Additionally, the I − anion had good capability as the leaving group to speed up the ring closure step, as suggested by Butera and co-workers. 41,66 δ E = E (TDTS) − E (TDI) if TDTS appears after TDIδ E = E (TDTS) − E (TDI) + Δ G r if TDTS appears before TDI…”

“…39 The good activity of the nicotinamidium halide catalyst towards CO 2 conversion into cyclic carbonate had been verified by both experimental 40 and theoretical methods. 41 A bidentate H-bond donor model was proposed, in which non-classical α-C–H of the quaternary pyridinium along with one H atom of the n -butyl group was responsible for the ring activation. 40 In the (thio)urea-containing quaternary ammonium salt-catalysed reaction, DFT calculations by Waser et al verified the pathway involving an initial addition of the nucleophilic iodide counter anion to the H-bonding activated epoxide, followed by stepwise CO 2 -fixation and cyclization, which was more favourable kinetically compared to the formation of the S-alkylated intermediate pathway.…”