Résumé
We recently reported highly enantioselective hydrogenation of 2-substituted quinolininium salts catalyzed by a halogen-bridged iridium dinuclear complex bearing chiral diphosphine ligand. Herein we report a development of iridium-catalyzed asymmetric hydrogenation of 2-alkyl-substituted or 2-aryl-substituted quinoxalines. After optimization of reaction conditions including halogen ligands and diphosphine ligands of iridium complex, various 2-substituted quinoxalines were efficiently catalyzed by chloride-bridged iridium dinuclear complex bearing difluorphos to afford the corresponding 2-substituted 1,2,3,4-tetrahydroquinoxalines in high enantioselectivity. To the best of our knowledge, this is the first report of a highly enantioselective asymmetric hydrogenation of 2-aryl substituted quinoxalines.