Frozen Dissymmetric Cavities in Resorcinarene-Based Coordination Capsules

By introducing slight structural modifications to a D₄ -symmetric coordination capsule, we succeeded in isolating the nearly enantiopure capsules (P)- and (M)-2 a(BF₄ )₄ . Chiral guest, dibenzyl 4,4'-diacetoxy-6,6'-dimethyl-[1,1'-biphenyl]-2,2'-dicarboxylate (3) was encapsulated within the dissymmetric cavity of 2 a(BF₄ )₄ , resulting in a high diastereoselectivity of >99 % de. The encapsulated guest was successfully removed from the complex without racemization through precipitation of the empty capsule. CD spectra confirmed that the chirality of the capsule was maintained in THF and 1,4-dioxane for long periods, whereas a small amount of acetonitrile accelerated racemization of the empty capsule. The activation parameters of the racemization reaction were determined in dichloromethane and 1,2-dichloroethane, resulting in positive enthalpic contributions and large negative entropic contributions, respectively. Accordingly, the racemization fits a first-order kinetic model. Mechanically coupled Cu⁺ -2,2'-bipyridine coordination centers were responsible for the high-energy barrier of racemization and led to the unique chiral memory of the dissymmetric cavity, which was turned off by the addition of acetonitrile.