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.