Kinetic Resolution of Azomethine Imines by Brønsted Acid Catalyzed Enantioselective Reduction

Azomethine imines are valuable substrates in asymmetric catalysis, and can be precursors to β‐amino carbonyl compounds and complex hydrazines. However, their utility is limited because complex and enantioenriched azomethine imines are often unavailable. Reported herein is a kinetic resolution of N,N′‐cyclic azomethine imines by enantioselective reduction (s=13–43). This resolution was accomplished using a Brønsted acid catalyst, and represents the first example of the asymmetric reduction of azomethine imines. The pyrazolidinone product (up to 86 % ee) and the recovered azomethine imine (up to 99 % ee) can both be used to access the opposite enantiomers of valuable products.