Consecutive Asymmetric Transfer Hydrogenation of C2-Acylated Quinolines and Quinoxalines: A Diastereodivergent Synthesis of Enantioenriched Tetrahydroquinolines and Tetrahydroquinoxalines Bearing Endo- and Exocyclic Chirality

Abstract

Consecutive asymmetric hydrogenation offers a direct and convenient approach to synthesizing complex C(sp³)-enriched products with multiple chirality. Herein, we report an asymmetric synthesis of chiral 1,2,3,4-tetrahydroquinolines (THQs) and tetrahydroquinoxalines bearing both endo- and exocyclic vicinal chirality through the consecutive transfer hydrogenation of easily accessible C2-acylated quinolines and quinoxalines. The method features mild conditions, easy operation, broad substrate scope (42 examples), and excellent asymmetric control (generally >90% ee and 20/1 dr). The key to success is the use of a water-soluble chiral aminobenzimidazole Ir catalyst. Mechanistic experiments support that the reaction involves the sequential reduction of the carbonyl group and then the quinoline core, with the asymmetric control of each step dominated by the catalyst. Remarkably, a diastereodivergent synthesis of all four stereoisomers of a chiral THQ has been successfully implemented.