Simple s-block metal hydrides for selective hydrogenation of quinoline compounds

For many decades, selective hydrogenation of N-heterocycles using H₂ has been considered the exclusive domain of d-block metals. Herein, a series of solid-state s-block metal hydrides is demonstrated to function as transition-metal-free catalysts for the hydrogenation of quinoline, affording high yields of 1,2,3,4-tetrahydroquinoline. Under relatively mild conditions, the catalytic performance of the representative barium hydride (BaH₂) is good enough to compete with some recently developed Fe, Co, Ni catalysts. Moreover, this transformation is remarkably simple and does not need any sophisticated ligands, co-catalysts, and additives compared with the existing methodology. Mechanistic studies reveal that the quinoline substrate can activate BaH₂ in situ, generating a catalytically active organobarium hydride species that effects the hydrogenation process. With the aid of theoretical modeling, the synergistic scenario between the Lewis acidic barium cation and the nucleophilic hydride anion in regioselective hydrogenation of quinoline is delineated, which shows a working principle distinct from the conventional transition metal catalysis. These findings provide insights into the discovery of new catalysts and/or reactivity that is complementary to that of transition metals.