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

Abstract

Two π-accepting viologen substituents 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 tetran-butylammonium hydroxide. Because the intercalated cyclophane (layer spacing = 14.7 A) has a pre-organized bindi...