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Theoretical Study of the Ring-Opening of Epoxides Catalyzed by Boronic Acids and Pyridinic Bases

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journal contribution
posted on 14.07.2017, 00:00 by Luís Pinto da Silva
The ring-opening reaction of epoxides is a key step in the conversion of carbon dioxide (CO2), a greenhouse waste product of fossil fuel combustion, into value-added heterocyclic carbonates. Being an essential step in CO2 conversion, controlling its activation energy is essential in the development of methods in which heterocyclic carbonates can be obtained under mild conditions. We have employed a theoretical approach to obtain mechanistic insight into the ring-opening of a model epoxide, when catalyzed by both 2,6-phenylboronic acids and pyridinic bases. The presence of both catalysts decreases the activation energy of this process from ∼60 to ∼20 kcal mol–1. The electronic character of the substituent at the C2,6-position of phenylboronic acid has little effect on the activation energy. Nevertheless, electron-withdrawing substituents decrease the activation energy by increasing the acidity of the boronic acid group. It was also found that nonhalogenated pyridinic bases present similar results to those provided by halide nucleophiles. Thus, two-component systems composed by phenylboronic acids and pyridinic bases appear to be a good choice for catalyzing the ring-opening reaction of epoxides under mild, metal-free, and nonhalogenated conditions.