Ruthenium-Catalyzed Solvent-Controlled Chemoselective Asymmetric Hydrogenation of 2,8′-Bisquinoline Derivatives

We report the first solvent-controlled chemoselective asymmetric hydrogenation of 2,8'-bisquinolines using chiral Ru/<i>N</i>-sulfonylated diamine complexes. By simply switching the solvent from dichloromethane to methanol, either chiral 1',2',3',4'-tetrahydrobisquinolines or 1,1',2,2',3,3',4,4'-octahydrobisquinolines were obtained in high yields with excellent enantioselectivities. The methodology was successfully applied to scale-up syntheses and the preparation of a new <i>C</i>₁-symmetric <i>N</i>-heterocyclic carbene ligand. Preliminary mechanistic study revealed that an intramolecular hydrogen bond in the tetrahydroquinoline intermediate is the key factor governing the chemoselectivity, which can be disrupted in protic solvents.