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.