Asymmetric ruthenium-catalyzed hydrogenation of 2- and 2,9-substituted 1,10-phenanthrolines.

Asymmetric hydrogenation of heteroaromatic compounds has captured considerable attention because it offers straightforward and environmentally benign routes to optically active compounds with chiral heterocyclic skeletons. Recently, various heteroarenes such as quinolines, isoquinolines, quinoxalines, indoles, pyrroles, (benzo)furans, pyridines, imidazoles, and (benzo)thiophenes have been successfully hydrogenated with high enantiomeric excesses. However, despite achievements made in this field, many challenges still remain and polycyclic heteroarenes (containing more than one heterocycle) are particularly difficult substrates. 1,10-Phenanthroline (Phen; 1) and its derivatives containing two pyridyl rings are one of the most versatile bidentate ligands for transition-metal catalysis. Much less attention has been directed toward the partially reduced 1,2,3,4-tetrahydroand 1,2,3,4,7,8,9,10-octahydro-1,10-phenanthroline [TPhen (2) and OPhen (3), respectively] derivatives, which are two kinds of heterocycle-containing compounds with potential bioactivity and can also be used as new ligands such as vicinal diamines and benzimidazole-based N-heterocyclic carbenes. So far, few reports have focused upon heterogeneous metal-catalyzed hydrogenation or reduction with stoichiometric reducing agents of 1,10-phenanthroline and its derivatives, and all these methods suffered from low stereoselectivities and poor reaction yields. Moreover, as far as we know, homogeneous transition-metal catalyzed hydrogenation of such substrates has never been reported, probably because of the aromaticity, as well as the strong coordination and poisoning ability of the substrate or the reduced product. For example, the cationic half-sandwich ruthenium complex 4 containing a 1,10-phenanthroline ligand was found to be an effective catalyst for the transfer hydrogenation of ketones. Expectedly, it is more challenging to realize the asymmetric reduction of substituted 1,10-phenanthrolines to selectively provide chiral TPhen and OPhen (Scheme 1). To the best of our knowledge, only one example of asymmetric transfer hydrogenation of 2and 2,9substituted 1,10-phenanthrolines catalyzed by chiral Bronsted acid has been reported. However, several obvious limitations remain, such as low reactivity or selectivity, and