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-Synthetic cyclophane hosts form highly structured complexes with aromatic substrates in aqueous and organic solutions. Solvation-desolvation processes are recognized as a central factor that determines the stability of these complexes. From comparative studies it is evident that the driving force for complexation is much larger in aqueous solutions than in organic solvents. Investigations into the nature of the special driving force for molecular complexation in aqueous solution show that the binding of neutral benzene guests in deep, apolar cyclophane cavities is predominantly enthalpically driven and that the entropic contributions are very unfavorable. The nature of this enthalpic hydrophobic effect is discussed in terms of an increase in cohesive interactions between water molecules and in attractive dispersion interactions upon complexation. Binding studies in a series of organic solvents covering the entire polarity range demonstrate that the complexation strength is highly solvent dependent. Differences in complex stability are discussed in terms of the ability of the solvent to compete effectively with the guest for the cavity kinding site. The different solvation requirements of cation versus apolar binding are demonstrated with a novel ditopic host. By changing from a water/methanol (60/40) mixture to pure methanol, this host can be switched from an efficient binder of neutral aromatics to a good host for potassium cations.

Derivatives of 2,2',7,7' -tetrahydroxy-I, I'-binaphthyl (5) are synthetically readily available chiral shapes with a major and a minor groove. Previous work by Cram et aI. has shown that efficient chiral crown ether binding sites can be constructed at the binaphthyl minor groove. The major groove is almost twice as wide and possesses suitable dimensions to shape flat apolar binding sites in chiral cyclophane hosts. The eight-step syntheses of the two novel cyclophanes 6 and 7 are described. The apolar cavities of these hosts are formed by bridging with two linear four-carbon chains the 7,7'-oxygens at the major groove of 5 to the two phenolic oxygens of a 4,4-bis(4-hydroxy-3,5-dimethylphenyl)piperidine unit. 'H NMR complexation studies show that 1:1 complexes of 6 and 7 with 6-methoxy-2-naphthonitrile and other naphthalene derivatives in aqueous methanol are as stable as the previously reported complexes of bis(diphenylmethane)-hosts. The different solvation characteristics of cation and apolar binding are demonstrated with the ditopic host 7. In addition to the apolar binding site at the major groove, 7 also possesses a crown ether binding site at the minor groove of the binaphthyl spacer. By changing the methanol content of a water-methanol mixture, this host can be switched from an efficient binder of neutral naphthalene guests to a good host for potassium cations. The activities of the two binding sites in 7 are almost entirely independent of each other; consequently, the binding of a naphthalene guest in the 1:1:1 (host-naphthalene-K+) complex is nearly as strong as in the I: I complex. The methyl ester of (S)-naproxen forms stable complexes with the racemic hosts 6 and 7. The 'H NMR spectra of these complexes show considerably different shifts for the aromatic resonances of the host enantiomers. These differences indicate the formation of diastereomeric complexes of different geometries or stabilities. COMPLEXATION STUDIES WITHCYCLOPHANES 6 AND 7 Binding ofNaphthalene Derivatives in Aqueous MethanolOur first IH NMR complexation studies with 6 and 7 were designed to determine whether the major groove Hester, Uyeki and Diederich / Chiral Spacer