Chirality Switching in the Crystallization of 1-(4-Chlorophenyl)ethylamine with Binaphthoic Acid by Ketimine Formation

Abstract

Axially chiral binaphthoic acid (BNA) was studied as a resolving agent for a stereoselective crystallization of 1-(4-chlorophenyl)ethylamine (CPEA). The diastereomeric pair of (R)-BNA/(S)-CPEA crystallizes in methylene chloride, on the other hand, the pair of (S)-BNA/(S)-CPEA crystallizes in acetone. The switch of the solubility of the diastereomeric pair is due to the imine formation with acetone. The very low solubility of the BNA/imine pair appears to be responsible for the fast and complete imine formation. The crystal structure of the BNA part in both crystals of the diastereomers maintains a same feature. Asymmetric chiral channels and pockets composed by intermolecular packing of BNA molecules appear in the crystal structures, and the robustness of them seem to contribute to the recognition of the chirality of CPEA with high selectivity.