Abstract
The ring-opening reaction of epoxides is an essential step in CO2 conversion into value-added heterocyclic carbonates. There has been an increasing focus on developing organocatalysts composed by a nucleophile and a hydrogen bond donor (HBD), so that CO2 conversion can occur at ambient conditions, with the rationale that HBD can aid epoxide’s ring-opening. Nevertheless, it is not clear what is the real role of HBDs on this reaction. Herein, it was employed a combined spectroscopic and theoretical approach to obtain mechanistic insight into the ring-opening of a model epoxide, when catalyzed by pyridine and different HBDs. While the pyridine-HBD system decreased the activation barrier from ˜60 to ˜22–23 kcal mol−1, by itself pyridine decreased it to ˜27 kcal mol−1. This shows that HBDs present a small effect on the activation energy. Furthermore, it was found that water molecules can act as HBD with similar results to other molecules. This helps to explain why catalytic amounts of water have found to be advantageous in CO2 conversion. Further work found that the activation barrier is mainly controlled by steric contributions. Subsequent spectroscopic analysis showed that the obtained rate constants follow the same trend found in the theoretical calculations, while these reactions lead to the formation of the same products in similar ratios. It was also found that one pyridine molecule can catalyze the opening of two epoxides, resulting in a trimeric structure. Finally, the addition of electron-donating groups to pyridine increases the catalytic properties of the system.
Published Version
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