Elucidating the Critical Role of Water in Selective Hydrogenation of <i>N</i> ‐heterocycles on a Cobalt Catalyst

Abstract The ambiguous role of water as a solvent in regulating liquid‐phase hydrogenation activity and selectivity is of great significance to modern organic synthesis, yet remains challenging to identify. Here, we present a carbon‐coated cobalt nanoparticle catalyst with a high number of functional groups, synthesized using a simple approach. This catalyst exhibits exceptional water‐promoted N ‐heterocycle hydrogenation activity and selectivity. Remarkably, 100% quinoline conversion and &gt;99% 1,2,3,4‐tetrahydroquinoline selectivity can be achieved at 100 °C and 0.5 MPa H 2 , surpassing the performance of most reported heterogeneous catalysts. Using a combination of advanced mass spectrometry, nuclear magnetic resonance, and theoretical analysis, we elucidate the water‐promoted hydrogenation mechanism. Water is a crucial solvent because it provides protons directly and enhances H 2 diffusion, thereby facilitating a favorable water‐mediated 1–4–2–3 hydrogenation pathway on the surface of this catalyst. Based on this finding, the catalyst exhibits universal water‐promoted hydrogenation performance for a wide range of N ‐heterocycles (14 examples with yields of over 96%). This work highlights the crucial role of water in liquid‐phase hydrogenation reactions and provides a new research paradigm for the future development of such reactions.