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

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 >99% 1,2,3,4-tetrahydroquinoline selectivity can be achieved at 100 °C and 0.5 MPa H₂, 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 H₂ 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.