Unprecedented chiral molecular recognition in a C-3-symmetric environment

Abstract

The enantiomeric recognition of alpha-chiral primary ammonium ions has been studied with benzene-based tripodal tris(oxazoline) receptors. Contrary to the literature and our expectation, a good level of chiral discrimination is observed with one of the tripodal receptors, which provides a C-3-symmetric chiral environment on guest binding. The chiral discrimination has been found to be general in the case of alpha-aryl substituted guests, suggesting pi-pi interactions as an important factor. This result raises a question with respect to the origin of the chiral discrimination since little steric or electronic difference is expected between the diastereomeric inclusion complexes, Binding studies by NMR titration and isothermal titration calorimetry show that the chiral discrimination results from the different thermodynamic stabilities between the diastereomeric complexes and that the host-guest complex formation is driven by favorable enthalpy changes with a minor negative contribution by entropy changes. The X-ray crystal structures for both of the diastereomeric inclusion complexes are resolved, which unambiguously show the binding mode and provide clues on the origin of the chiral discrimination. Bond angle analyses indicate that the minor complex experiences a larger steric strain, which is discernible when it is viewed from "three-body" interactions between the host and the guest. The guest and oxazoline phenyl rings are well stacked, indicating interplay of the pi-pi interactions. The pi-pi interactions are believed to stabilize host-guest complexes, thereby endowing the highly flexible receptors with a substantial enantio-discrimination.