Asymmetric Hydrogenation of Phenanthridines with Chiral Boranes

The asymmetric hydrogenation of N-heteroarenes provides an efficient method for the synthesis of optically active cyclic secondary amines. In this paper, we described an asymmetric hydrogenation of phenanthridines using a chiral mono-alkene-derived borane. A variety of dihydrophenanthridines were furnished in high yields with up to 93% ee. The current catalytic system was very sensitive for the steric hindrance of phenanthridines. Bulky substituents at one phenyl group of phenanthridines were required to obtain the high enantioselectivity. But large substituents on the carbon of the C=N bonds would diminish the reactivity sharply. A metal-free asymmetric hydrogenation of phenanthridines was successfully accomplished by using a chiral monoene-derived borane as a catalyst. A variety of optically active dihydrophenanthridines were furnished in high yields with up to 93% ee. The bulky substituents at one phenyl group was required to enhance the difference between the two sides of the N-ring.image