Large Water‐Soluble Cyclophanes with Convergent Intracavity Functionality

Abstract

AbstractNew tricyclic spacers, readily available through fourfold Mannich reaction of substituted dibenzyl ketones, were introduced into a series of ten H2O‐soluble cyclophanes with spacious preorganized cavity binding sites. These spacers provide H2O‐solubility with amine or crown‐ether functionality remote from the cyclophane cavity while directing functional groups such as keto or OH groups in a precise geometrical array inside the cavity. The cyclophanes were designed to include organic substrates via a combination of apolar and specific polar functional group interactions. The X‐ray crystal‐structure analysis of the tritopic receptor 18 with one potential neutral‐molecule and two cation‐binding sites showed a large rectangular open cavity with dimensions of roughly 9 × 14 Å and a spacing of 9.7 Å between the O‐atoms of two convergent CO groups. Despite the binding‐site preorganization, cyclophanes incorporating two of the new spacers did not show any substrate binding in aqueous solutions. The failure of these systems to function as receptors is mainly due to steric hindrance to important cyclophane aromatic ring‐guest interactions. Also, the favorable solvation of the intracavity functionality may prevent the formation of complexes. Hybrid receptors constructed from the novel spacers and diphenylmethane units were found to bind flat aromatic substrates as well as bulky [4.2]paracyclophanes. The observed large differences in stability (ΔΔG°> 2 kcal mol−1) of the complexes formed by three structurally closely related hybrid receptors with convergent CO, OH or CH2 groups and 6‐hydroxynaphthalene‐2‐carbonitrile as guest can be explained by a strong solvation effect of the convergent functional groups on apolar inclusion complexation.