Ligand-Directed Divergent Selectivities in Cobalt-Catalyzed Transfer Hydrogenation of Quinolines: Insights from Experiment and Theory

The selective hydrogenation of quinolines is a valuable transformation for accessing functionalized N-heterocycles, but achieving control over the degree of reduction under mild and sustainable conditions remains challenging. Herein, we report the transfer hydrogenation of quinoline using 1 equiv of ammonia–borane (NH3BH3) at room temperature, catalyzed by two well-defined cobalt hydride complexes: HCo(PMe3)4 and H3Co(PPh3)3. The Co(I) hydride complex HCo(PMe3)4 selectively affords 1,2-dihydroquinoline (1,2-DHQ), while the Co(III) trihydride H3Co(PPh3)3, for which we report the first X-ray crystal structure, yields doubly reduced tetrahydroquinoline (THQ). A detailed mechanistic study combining DFT calculations and deuterium labeling experiments reveals that the phosphine ligand plays a decisive role in directing the reaction selectivity: the bulky and less donating PPh3 ligand facilitates the isomerization of 1,2-DHQ to 1,4-DHQ, whereas the PMe3 ligand favors the halting at the 1,2-DHQ stage. These results provide a rare case of ligand-controlled divergence in base-metal-catalyzed transfer hydrogenation and highlight the potential of cobalt hydride complexes for tunable and selective N-heteroarene reductions.