Mild and selective synthesis of imines and N-Heterocyclic compounds via formic acid-mediated reductive coupling strategy over Nitrogen-Coordinated cobalt Single-Atom catalysts

Abstract

The selective formation of C = N bonds through the formic acid-mediated reductive coupling of nitro groups with carbonyl groups over heterogeneous catalysts poses challenges due to the corrosion of metal catalysts by formic acid and the rapid further hydrogenation of C = N into C-N bonds. In this study, we demonstrate that nitrogen-coordinated cobalt single-atom catalysts exhibit robust activity in this transformation. Various imines and N-heterocyclic compounds, including benzimidazoles, quinazolines, and pyrroles, can be synthesized at a modest temperature of only 70 °C. The high catalytic activity is attributed to the synergistic roles of nitrogen and cobalt atoms, serving as basic and active sites, respectively. Experimental data, complemented by density functional theory (DFT) calculations, clearly indicate that the transfer hydrogenation of nitrobenzene preferentially proceeds via the “direct” pathway rather than the “condensation” pathway. Moreover, the reduction of C = N bonds into C-N bonds demands much higher energy than the reduction of nitro groups, thereby enabling high selectivity towards C = N bonds. The findings of this study are expected to provide a novel approach for selective C = N bond formation and contribute to a deeper understanding of the mechanism underlying the reductive amination process.