Background and Originality Content
Optical active 5,6-dihydrophenanthridines are important functional moieties widely presented in natural products and biologically active molecules.[1] Despite numerous reported methods, there are only limited asymmetric catalytic approaches for their synthesis have been disclosed.[2] In 2015, Lautens and co-workers described a Pd-catalyzed asymmetric homocoupling reaction of o -bromobenzylamines, giving 5,6-dihydrophenanthridins with up to 99% ee.[3a] Recently, Liu reported an iron-catalyzed dehydrogenative kinetic resolution of 5,6-dihydrophenanthridines.[3b] Notably, the asymmetric hydrogenation of phenanthridines stands as an atomic economy and straightforward route to access optically active 5,6-dihydrophenanthridines. A significant progress has been achieved in recent years. In 2017, Fan and co-workers disclosed a Ru-catalyzed asymmetric hydrogenation of phenanthridines, delivering the corresponding products in excellent yields with 75–92% ee’s (Scheme 1).[4a] One year later, Zhou and co-workers reported an Ir-catalyzed asymmetric hydrogenation of phenanthridines with high enantioselectivities (Scheme 1).[4b]However, to the best of our knowledge, the metal-free asymmetric hydrogenation of phenanthridines has rarely been reported.
Scheme 1 Asymmetric hydrogenation of phenanthridines