From triangles to squares - chiral, overcrowded polyimine macrocycles of significant chiroptical response enable the...

A series of highly strained macrocycles has been synthesized via condensations of chiral diamines with linear dialdehydes bearing Ph₃X groups (X = C, Si, Ge). The initially formed, optically active (3 + 3) triangular products expanded into unusual square-shaped macrocyclic structures following treatment with ethyl acetate. The resultant overcrowded (4 + 4) macrocycle remains stable and does not undergo ring contraction. When racemic diamine was used, the anticipated triangular products were formed, driven by the propensity of the (3 + 3) product to precipitate. The presence of triple carbon-carbon bonds attached to the aromatic segments of the macrocycle resulted in circular dichroism spectra with exceptionally high amplitudes, a phenomenon uncommon in this class of compounds. The observed chiroptical response is attributable to both intra- and interchromophoric interactions, which operate independently. The chiral octaimine, embellished with eight trityl groups, selectively adsorbs dichloromethane and comparable solvents in the crystal lattice. Nevertheless, even when immersed in the solution, the crystals remain stable as long as the chlorinated solvent is present in the mixture.