A substituent- and temperature-controllable NHC-derived zwitterionic catalyst enables CO 2 upgrading for high-efficiency construction of formamides and benzimidazoles

• Published in: Green chemistry : an international journal and green chemistry resource : GC Vol. 23; no. 16; pp. 5759 - 5765
• Main Authors: Yu, Zhaozhuo, Li, Zhengyi, Zhang, Lilong, Zhu, Kaixun, Wu, Hongguo, Li, Hu, Yang, Song
• Format: Journal Article
• Language: English
• Published: 16.08.2021
• ISSN: 1463-9262; 1463-9270
• DOI: 10.1039/D1GC01897C

Chemocatalytic upgrading of the greenhouse gas CO 2 to valuable chemicals and biofuels has attracted broad attention in recent years. Among the reported approaches, N -formylation of CO 2 with an amine is of great significance due to its versatility in the construction of N-containing linear and cyclic skeletons. Herein, a stable N-heterocyclic carbene-carboxyl adduct (NHC-CO 2 ) was facilely prepared and could be used as a recyclable zwitterionic catalyst for efficient CO 2 reductive upgrading via either N -formylation or further coupling with cyclization under mild conditions (25 °C, 1 atm CO 2 ) using hydrosilane as a hydrogen source. More than 30 different alkyl and aromatic amines could be transformed into the corresponding formamides or benzimidazoles with remarkable yields (74%–98%). The electronic effect of the introduced substituent on NHC-CO 2 was found to evidently affect the thermostability and nucleophilicity of the zwitterionic catalyst, which is directly correlated with its catalytic activity. Moreover, NHC-CO 2 could supply CO 2 by in situ decarboxylation at a specific temperature that is dependent on the introduced substituent type. Experimental and computational studies showed that the carboxyl species on NHC-CO 2 was not only a nucleophilic center, but also a C1 source which rapidly captures or substitutes ambient CO 2 during hydrosilylation. In addition, a simple and green conceptual process was designed for the product purification and catalyst recycling, with a good feasibility for small-scale production.