Asymmetric hydrogenation of quinolines with recyclable and air-stable iridium catalyst systems

The iridium complex-catalyzed asymmetric hydrogenation of quinolines in a DMPEG/hexane biphasic system was studied. Catalysts with C2-symmetric ligands such as Xyl-P-Phos, Cl-MeO-BIPHEP, SYNPHOS, and DifluorPhos are highly effective for this type of reaction. Most of the catalysts tested can be retained in DMPEG (Mn= 500), and the asymmetric hydrogenation of various quinoline substrates can be carried out in DMPEG/hexane biphasic system with up to 92% ee. The catalysts and the products can be separated via simple phase separation, and the reactivity/stereoselectivity of the catalysts can be retained for at least three reaction cycles.