Induced-Fit Molecular Recognition with Water-Soluble Cavitands

Synthesis of novel water‐soluble cavitands 1 and 2 and their complexes—the caviplexes—is described. The solubility in water derives from four primary ammonium groups on the lower rim and eight secondary amide groups on the upper rim. Cavitands 1 and 2 exist as D2d velcraplex dimers in aqueous solution but the addition of lipophilic guests 15–24 induces conformational changes to the vase‐like structures. The internal cavity dimensions are 8×10 Å, and the exchange rates of guests in the caviplexes are slow on the NMR timescale (room temperature and 600 MHz). The direct observation of bound species and the stoichiometry of the complexes is reported. The association constants (Ka) between 0.4×101 (−ΔG 295=0.7 kcal mol−1) and 1.4×102 M−1 (−ΔG 295=2.9 kcal mol−1) in D2O and 1.4×101 (−ΔG 295=1.7 kcal mol−1) and 2.8×104 M−1 (−ΔG 295=6.0 kcal mol−1) in [D4]methanol for aliphatic guests 16–24 were determined. Guest exchange rates of the new hosts 1 and 2 are considerably slower than rates observed for typical open‐ended cavities in aqueous solution.