Host-Guest Chemistry of a Chiral Cyclohexanediamine-Viologen Cyclophane in Solution and in the Solid State

Two π-accepting viologen substituants were coupled with the chiral barrier (1R,2R)-(+)- 1,2-cyclohexanediamine to synthesize a rigid, C₂-symmetric cyclophane host for aromatic guest molecules. UV-visible spectroscopic titrations show an association constant of 3.3 × 10³ M^-1 with indole, a hydrophobic guest, and constants in the range of 10¹-10² M^-1 for more hydrophilic indole derivatives in water. Tryptophan methyl ester complexes this host in aqueous base with an enantioselectivity ratio of 3.3, and an association constant of 2.0 × 10² M^-1 for the preferred L-enantiomer. Protonation of tryptophan methyl ester causes both the L-and D-enantiomers to have the same affinity for the host, which is, within experimental error, the same as that of the D-enantiomer in base. The cyclophane host can be intercalated into α-zirconium phosphate, a lamellar solid acid, by first swelling the latter with tetranbutylammonium hydroxide. Because the intercalated cyclophane (layer spacing = 14.7 Å) has a pre-organized binding pocket, indole intercalates from solution to give a 1:1 complex in the solid, with no change in interlamellar spacing. Adsorption isotherms corresponding to indole intercalation and complexation were Langmuirian, in contrast to the strongly cooperative binding found previously for complexation of π-donors with noncyclophane chiral hosts in α-zirconium phosphate.