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

• Published in: Green chemistry : an international journal and green chemistry resource : GC 2021-08, Vol.23 (16), p.5759-5765
• Main Authors: Yu, Zhaozhuo, Li, Zhengyi, Zhang, Lilong, Zhu, Kaixun, Wu, Hongguo, Li, Hu, Yang, Song
• Format: Article
• 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. An efficient catalytic system for the construction of various formamides and benzimidazoles with good yields of up to 96% and 98% was developed using the C1 source CO 2 and the stable catalyst NHC-CO 2 , and an interesting catalytic mechanism was reported.