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

For many decades, selective hydrogenation N-heterocycles using H2 has been considered the exclusive domain of d-block metals. Herein, a series of solid-state s-block metal hydrides has been demonstrated to function as transition-metal-free catalysts for 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 when compared with the existing methodology. Mechanistic studies reveal that quinoline substrate can activate BaH2 in situ, generating a kind of catalytically active organobarium hydride species that effects the hydrogenation process. With the aid of theoretical modeling, the synergistic scenario between Lewis acidic barium cation and nucleophilic hydride anion in regioselective hydrogenation of quinoline is delineated, which shows a distinct working principle 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 the transition metals.