Complexation and Structure Elucidation of the Axial Conformers of Mono- and (±)-trans-1,2-Disubstituted Cyclohexanes by Enantiopure Alleno-Acetylenic Cage Receptors

Abstract

Single crystal X-ray diffraction is a powerful method to unambiguously characterize the structure of molecules with atomic resolution. Herein, we review the molecular recognition of the (di)axial conformers of Mono- and (±)-trans-1,2-disubstituted cyclohexanes by enantiopure alleno-acetylenic cage receptors in solution and in the solid state. Single crystals of the host–guest complexes suitable for X-ray diffraction allow for the first time to study the dihedral angles of a series of Mono- and (±)-trans-1,2-disubstituted cyclohexanes in their (di)axial chair conformation. Theoretical studies indicate negligible influence of the host structure on the guest conformation, suggesting that the structural information obtained from the host–guest complexes give insight into the innate structures of Mono- and (±)-trans-1,2-disubstituted cyclohexanes. Strong deviation of the dihedral angles ?a,a(X–C(1)–C(2)–X) from the idealized 180° are observed, accompanied by substantial flattening of the ring dihedral angles ?(X–C(1)–C(2)–C(3)).