Formation of Ternary Inclusion Crystal and Enantioseparation of Alkyl Aryl Sulfoxides by the Salt of Urea-Modified l-Phenylalanine and an Achiral Amine

Abstract

Chiral supramolecular hosts comprising urea-modified l-phenylalanines and achiral primary amines were developed, which facilitated enantioselective inclusion of alkyl aryl sulfoxides (3) up to 89% ee owing to the formation of ternary inclusion crystals. From the structural optimization of the components, the combination of N-(phenylcarbamoyl)-l-phenylalanine (1a) and benzhydrylamine (2e) showed the best inclusion ability among the considered supramolecular hosts. The type of the hydrogen-bonding network of ammonium-carboxylate salts 1a·2 was dependent on the steric bulk of combined primary amine 2. The rigid phenylurea moiety of 1a and moderate steric bulk of 2e played an important role in their high inclusion ability. Host 1a·2e afforded two ternary polymorphic inclusion crystals with the inclusion of acetonitrile. Furthermore, the crystal structures of 1a·2e·3 revealed that the inclusion of 3 was assisted by hydrogen bonding to the host and that stereoselectivity reversal occurred for larger sulfoxides, which was attributed to packing variations of one-dimensional hydrogen-bonding networks.