A substituent- and temperature-controllable NHC-derived zwitterionic catalyst enables CO2 upgrading for high-efficiency construction of formamides and benzimidazoles

Abstract

Chemocatalytic upgrading of the greenhouse gas CO₂ to valuable chemicals and biofuels has attracted broad attention in recent years. Among the reported approaches, N-formylation of CO₂ 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-CO₂) was facilely prepared and could be used as a recyclable zwitterionic catalyst for efficient CO₂ reductive upgrading via either N-formylation or further coupling with cyclization under mild conditions (25 °C, 1 atm CO₂) 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-CO₂ was found to evidently affect the thermostability and nucleophilicity of the zwitterionic catalyst, which is directly correlated with its catalytic activity. Moreover, NHC-CO₂ could supply CO₂ 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-CO₂ was not only a nucleophilic center, but also a C1 source which rapidly captures or substitutes ambient CO₂ 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.