Exploring Calixarene-Based Pseudorotaxane Architectures: Threading Ammonium Axles With an Exo-Rim π-Functionalized...

In this study, we present the synthesis of a novel 1,4-π-functionalized calix[6]arene derivative 2, which features two electron-rich 1,2,3-trimethoxybenzene units on the exo rim and six methoxy groups on the endo rim. This design creates an extended and deep aromatic cavity, aimed at enhancing host-guest interactions with dialkylammonium axles. The results show that threading occurs even in the presence of bulky aromatic groups on the exo rim of 2, observed with a series of symmetric and directional ammonium salts. NMR studies confirmed the formation of pseudorotaxane complexes and revealed that exo-rim π-functionalization influences binding affinity and orientational selectivity, depending on the axle's structure. Interestingly, the threading of 2 with the biphenyl axle 3c⁺ results in a pseudorotaxane stabilized by π···π interactions, which is 4.4 times more stable than the analog with hexamethoxy-calix[6]arene 1a that features tert-butyl groups on the exo rim. Comparison of the endo-p-NO₂-benzyl/endo-alkyl stereoselectivity ratios observed with directional axle 3e⁺ and the two macrocycles 1a and 2, indicates that the extended cavity of 2 enhances and promotes C─H···π interactions between the axles' alkyl chain and the enlarged aromatic cavity of 2. This finding is further supported by density functional theory (DFT) and noncovalent interaction (NCI) calculations. The results described here provide valuable insights for designing new wheels with extended and deep aromatic cavities.