Asymmetric Transfer Hydrogenation of 3‐Substituted 2H‐1,4‐Benzoxazines under Tethered Cp*Rh(III)‐Diamine Catalysis with Unexpected Reversal of Enantioselectivity

Abstract

The asymmetric transfer hydrogenation of 3‐substituted 2H‐1,4‐benzoxazines with an azeotropic mixture of HCO2H/NEt3 (5/2) using tethered Cp*Rh(III)‐diamine catalysis has been realized. This process allows access to a broad range of chiral 3,4‐dihydro‐2H‐1,4‐benzoxazines in high yields with up to 99% ee, and tolerates a variety of functional groups. The enantiocontrol is achieved by the judicious choice of catalyst and hydrogen source. This reaction proceeds with unexpected reversal of enantioselectivity, which is attributed to the acidic reaction conditions and the hydrogen bond between the N−H of the rhodium species and the O atom in the substrate.