Stereochemistry of molecular figures-of-eight

A trans isomer of a figure-of-eight (Fo8) compound was prepared from an electron-withdrawing cyclobis(paraquat-p-phenylene) derivative carrying trans-disposed azide functions between its two phenylene rings. Copper(I)-catalyzed azide-alkyne cycloadditions with a bispropargyl derivative of a polyether chain, interrupted in its midriff by an electron-donating 1,5-dioxynaphthalene unit acting as the template to organize the reactants prior to the onset of two click reactions, afforded the Fo8 compound with C _i symmetry. Exactly the same chemistry is performed on the cis-bisazide of the tetracationic cyclophane to give a Fo8 compound with C ₂ symmetry. Both of these Fo8 compounds exist as major and very minor conformational isomers in solution. The major conformation in the trans series, which has been characterized by X-ray crystallography, adopts a geometry which maximizes its C-H⋯O interactions, while maintaining its π⋯π stacking and C-H⋯π interactions. Ab initio calculations at the M06L level support the conformational assignments to the major and minor isomers in the trans series. Dynamic ¹H NMR spectroscopy, supported by 2D ¹H NMR experiments, indicates that the major and minor isomers in both the cis and trans series equilibrate in solution on the ¹H NMR timescale rapidly above and slowly below room temperature. The use of topological concepts and the application of molecular symmetry arguments are crucial when it comes to identifying the structure and dynamics of higher-order topologies, such as those exemplified by molecular figures-of-eight. Using topological concepts, the ability to design and synthesize this particular molecular representation of a mathematical topology has been demonstrated.