Supramolecular Double-Helix Formation by Diastereoisomeric Conformations of Configurationally Enantiomeric Macrocycles

Solid-state superstructures, resulting from assemblies programmed by homochirality, are attracting considerable attention. In addition, artificial double-helical architectures are being investigated, especially in relation to the ways in which homochiral small molecules can be induced to yield helical forms as a result of chiral induction. Herein, we report the highly specific self-assembly upon crystallization of a double-helical superstructure from an enantiopure macrocyclic dimer which adopts two diastereo­isomeric conformations in a molar ratio of 1.5:1 in dimethyl sulfoxide. These two conformational diastereo­isomers self-organizeand self-sortin the crystalline phase in equimolar proportions to form two single-handed helices which are complementary to each other, giving rise to the assembly of a double helix that is stabilized by intermolecular [C–H···O] and π–π stacking interactions. The observed self-sorting phenomenon occurs on going from a mixed-solvent system containing two equilibrating conformational diastereo­isomers, presumably present in unequal molar proportions, into the solid state. The diastereo­isomeric conformations are captured upon crystallization in a 1:1 molar ratio in the double-helical superstructure, whose handedness is dictated by the choice of the enantiomeric macrocyclic dimer. The interconversion of the two conformational diastereo­isomers derived from each configurationally enantiomeric macrocycle was investigated in CD₃SOCD₃ solution by variable-temperature ¹H NMR spectroscopy (VT NMR) and circular dichroism (VT CD). The merging of the resonances for the protons corresponding to the two diastereo­isomers at a range of coalescence temperatures in the VT NMR spectra and occurrence of the isosbestic points in the VT CD spectra indicate that the two diastereo­isomers are interconverting slowly in solution on the ¹H NMR time scale but rapidly on the laboratory time scale. To the best of our knowledge, the self-assembly of such solid-state superstructures from two conformational diastereo­isomers of a homochiral macrocycle is a rare, if not unique, occurrence