Synthesis of polymer/CNTs composites for the heterogeneous asymmetric hydrogenation of quinolines

Abstract

The development of heterogeneous catalytic processes is crucial for the synthesis of chiral compounds for both academic and industrial applications. However, thus far, such achievements have remained elusive. Herein, we report the heterogeneous asymmetric hydrogenation of 2-methylquinoline over solid chiral catalysts, which were prepared by the one-pot polymerization of (1R,2R)-N-(4-vinyl-benzenesulfonyl)-1,2-diphenylethane-1,2-diamine (VDPEN) and divinylbenzene (DVB) in the presence or absence of activated carbon (C) or carbon nanotubes (CNTs), followed by Ru coordination and anion exchange. The solid chiral catalysts were fully characterized by N2 sorption analysis, elemental analysis, TEM, FT-IR spectroscopy, and13C CP-MAS NMR. All the solid chiral catalysts could efficiently catalyze the asymmetric hydrogenation of 2-methylquinoline to afford 2-methyl-1,2,3,4-tetrahydroquinoline with 90% ee. Studies have shown that polymer/C and polymer/CNTs composites are more active than pure polymers. The polymer/CNTs composite exhibited the highest activity among all the solid chiral catalysts under identical conditions, owing to the unique morphology of CNTs. The recycling stabilities of the solid chiral catalysts were greatly improved when ionic liquids (ILs) were employed as solvents; this is mainly attributed to the decreased leaching amount of anions owing to the confinement effect of ILs on ionic compounds.