Efficient Asymmetric Hydrogenation of Quinolines over Chiral Porous Polymers Integrated with Substrate Activation Sites
Lin Tao, Yiqi Ren, Chunzhi Li, He Li, Xuelian Chen, Lina Liu, Qihua Yang
The heterogeneous asymmetric hydrogenation of quinolines for the production of optically active tetrahydroquinoline derivatives still remains a difficult task due to the aromatic stability of quinolines. Herein, we reported the efficient heterogeneous asymmetric hydrogenation of quinolines over chiral porous polymers integrated with both chiral active sites (VDPEN-RuOTs) and substrate activation sites (TsOH). The porous polymer integrated with TsOH is 10 times more active than that without TsOH in the asymmetric hydrogenation of 2-methylquinoline. The volcano curve of TOF with TsOH/Ru ratio confirms the synergistic catalysis of VDPEN-RuOTs and TsOH. Comparison results with homogeneous catalytic system imply that the synergy between chiral centers and acid sites is greatly enhanced in the polymer network. Under optimized conditions, the chiral porous polymer afforded up to 90% ee with 90 h-1 TOF, which is one of the best solid catalysts for asymmetric hydrogenation of quinoline derivatives ever reported. Furthermore, the bi-functional porous polymers realized the asymmetric cascade hydrogenation/reductive amination reaction to obtain benzo-quinolizidines. Our primary results suggest that the incorporation of substrate activation sites near chiral centers is an efficient strategy for the synthesis of high-performance solid catalysts for heterogeneous asymmetric catalysis.
Circular dichroism, Solid state, Stereochemistry, Polymers