High diastereoselection of a dissymmetric capsule by chiral guest complexation

Encapsulation of chiral guests in the dissymmetric capsule 1·4 BF₄ formed diastereomeric supramolecular complexes G?1·4 BF₄ (G: guest). When chiral guests 2 a-q were encapsulated within the dissymmetric space of the self-assembled capsule 1·4 BF₄, circular dichroism (CD) was observed at the absorption bands that are characteristic of the π-π* transition of the bipyridine moiety of the capsule, which suggests that the P and M helicities of the capsule are biased by the chiral guest complexation. The P helicity of diastereomeric complex (S)-2 l?1·4 BF₄ was determined to be predominant, based on CD exciton coupling theory and DFT calculations. The diastereoselectivity was highly influenced by the ester substituents, such that benzyl ester moieties were good for improving the diastereoselectivity. A diastereomeric excess of 98 was achieved upon the complexation of 2 j. The relative enthalpic and entropic components for the distereoselectivity were obtained from a van't Hoff plot. The enthalpic components were linearly correlated with the substituent Hammett parameters (σ_p ⁺). The electron-rich benzyl ester moieties generated donor-acceptor π-π stacking interactions with the bipyridine moiety, which resulted in a significant difference in energy between the predominant and subordinate diastereomeric complexes.