Solvent-dependent, formic acid-mediated, selective reduction and reductive n‑formylation of n‑heterocyclic arenes with sustainable cobalt-embedded n‑doped porous carbon catalyst

Abstract

The usefulness of formic acid is demonstrated here as a sustainable source of H2 and H2 + CO for the selective reduction and reductive N-formylation, respectively, of N-heterocyclic arenes. We synthesized a cobaltembedded porous N-doped carbon catalyst for this purpose. The formation of the Co-embedded porous N-doped carbon framework is confirmed using powder X-ray diffraction, N2-sorption, Raman spectrometry, transmission electron microscopy, and XPS measurements. The catalyst exhibits a formic acid-mediated selective reduction of N-arenes in the fused aromatic heterocyclic ring system in water, whereas the catalyst exclusively produces a reductive N-formylated product in toluene. A very low amount of surface Co (1.35 wt %)-embedded N-doped carbon framework provides 92.1% quinoline conversion with 87% selectivity of 1,2,3,4-tetrahydroquinoline in transfer hydrogenation (with TOF of 16.8 h−1 ) using formic acid as the H2 source. Moreover, the catalyst exclusively catalyzes the formation of N-formyltetrahydroquinoline with 98% yield using formic acid as the H2 + CO source in the reductive N-formylation reaction (with TOF of 13.4 h−1 ). The structure−activity relationship is established using quinoline adsorption, CO2-temperature program desorption, control reactions, and other spectroscopic measurements. After five recycles, 6.8 ppm of Co is lost from the catalyst. Only a marginal loss [selective reduction: quinoline conversion (fresh catalyst = 92.1%; after five recycles = 87.4%); reductive N-formylation: (fresh catalyst = 98.0%; after five recycles = 92.4%)] in the catalyst activity is observed. Moreover, the catalyst framework remains stable after five recycles. Further, the heterogeneity and the catalytic efficiency of the process is confirmed from hot filtration and KSCN poisoning tests, respectively. Owing to the development of a stable and recyclable porous catalyst in this study, and also its success in the synthesis of pharmaceutically important synthetic intermediates, we expect to fulfill the requirements for its commercial implementation in various industrial processes.