Frozen Dissymmetric Cavities in Resorcinarene‐Based Coordination Capsules

Abstract

By introducing slight structural modifications to a D -symmetric coordination capsule, we succeeded in isolating the nearly enantiopure capsules (P)- and (M)-2 a(BF ) . 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(BF ) , 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.

A Regulable Internal Cavity inside a Resorcinarene‐Based Hemicarcerand

“…The dihedrala ngles of the bipyridyl arms of the methyl-and p-methoxyphenyl-substituted capsules were determined by single-crystal XRD to be 98.6 and 124.48,r espectively. [34,35] The UV/Vis absorption spectra of 2 and 3 showed MLCT bands at l max = 450 nm for 2 and 456 nm for 3 ( Figure 3b), both of which were similar to the MLCT band of the methyl-substituted capsule. Hence, the trans form was likelyt obef ormed by olefin metathesis.…”

“…Metal-ligand coordination of 4 induced changes in the chemical shifts of the protons on the 2,2'-bipyridyl arms similar to those reported previously (Figure 2a and b). [35][36][37][38] Thef ormationo f3 was confirmed by NOESY NMR spectroscopy and ESI-MS.…”

“…The UV/Vis absorption spectra were consistent with the trans stereoisomer.T he metal-to-ligand charget ransfer (MLCT) band of the coordination capsules was found to be sensitive to the coordination environment of the metal centers. [35,40] For example, the MLCT band of the methyl-substitutedc apsule was observed at l max = 462 nm (green line in Figure 3b), while that of the p-methoxyphenyl-substituted one was shifted to l max = 547 nm (blue line in Figure 3b). The dihedrala ngles of the bipyridyl arms of the methyl-and p-methoxyphenyl-substituted capsules were determined by single-crystal XRD to be 98.6 and 124.48,r espectively.…”

“…In dichloromethane,t he reaction gave complex mixtures, whereas the reactioni nT HF produced 2 as the main product.P reviously,w er eported that the rate of helicityi nversion of Cu-coordinated capsules in THF was much slower than that in dichloromethane. [35] Hence, the slow inversiono f3 in THF is likely indicative of olefin metathe- sis. In addition, preorganization is of vital importance for successful carbon-carbon bond formation;t he olefin metathesis of 4 in THF under similar reaction conditions gave no desired product.…”

“…Although the degree of expansion was not very large, the p-methoxyphenyl-substituted capsule hostedal ong alkyl chain that cannot be encapsulated by its methyl-substituted counterpart. [35] These resultsi ndicate that the molecularr ecognition capability of the capsule can be reversibly switched when the cavity size is regulated by external species.T hese findings led us to design an ew molecular capsule with ar egulable internalc avity.…”