Metal-Free N-Formylation of Amines with CO₂ and Hydrosilane by Nitrogen-Doped Graphene Nanosheets

N-Formylation of amines with carbon dioxide (CO2) as a carbonyl source is emerging as an important way for CO2 transformation into high-value-added chemicals; however, the developed catalytic systems mainly focused on transition-metal-based homogeneous catalysts. Herein, we reported rationally designed nitrogen-doped graphene nanosheets (NG) as metal-free catalysts for N-formylation of various amines with CO2 and hydrosilane to formamide under mild conditions. The NG catalyst displayed a wide amine scope with the desired formamide yields up to >99%, demonstrating its comparable catalytic performance to the reported transition-metal-based catalysts. Our experimental research reveals that the N-formylation of aniline involves an initial NG-promoted CO2 hydrosilylation with PhSiH3 to silyl formate and a subsequent nucleophilic attack of the aniline to give N-formanilide. Moreover, the key step of CO2 hydrosilylation can be simplified to a pseudo-first-order reaction under a high CO2 concentration with an observed reaction rate constant (kobs) of 226 h-1 at 40 °C and an apparent activation energy (Ea) of 34 kJ mol-1. In sharp contrast, a kobs of 23 h-1 and Ea of 47 kJ mol-1 were observed under catalyst-free conditions. Our theoretical investigation indicates that NG-promoted CO2 hydrosilylation corresponds to an exergonic reaction (ΔG = -0.53 eV), which is much lower in energy state than that of catalyst-free conditions (ΔG = -0.44 eV). Finally, the NG showed outstanding recyclability in the N-formylation reaction with almost unchanged catalytic performance during twelve-time recycling. This research thus represented a breakthrough in metal-free transformation of CO2 into fine chemicals with low-cost, environment-friendly, and carbon-based catalysts to replace the scarce and expensive transition-metal-based catalysts.