Asymmetric Hydrogenation of Triazolo[1,5-a]-, Imidazo[1,2-a]-, and Pyrazolo[1,5-a]pyridines

The asymmetric hydrogenation of a series of triazolo[1,5-a]-, imidazo[1,2-a]-, and pyrazolo[1,5-a]pyridines, which have received little coverage in the literature, was investigated. The hydrogenation was achieved in high yields and moderate to good enantioselectivities using Ru precatalysts and bis(naphthylethyl)-substituted NHC ligands (SINpEt). Several new ligands of the SINpEt family were developed, which proved crucial for an enantioselective transformation. Particularly, an alkoxy group at the C2 position of the 1-naphthyl group led to a considerably higher selectivity. Remarkably, hydrogenation at room temperature with H₂ pressures as low as 5 bar could be achieved.