Chiral Molecular Glass: Synthesis and Characterization of Enantiomerically Pure Thiophene-Based [7]Helicene

Graphical Abstract

Chiral molecular glass: Synthesis of highly-functionalized thiophene-based[7]helicenes is based upon kinetic resolution of the intermediate diketone, followed by the annelation step forming the center benzene ring (intramolecular McMurry reaction). One such enantiopure [7]helicene, which adopts a tetrahedral-like shape in the racemic crystal (illustrated here), would not crystallize but formed a rare chiral molecular glass.

Abstract

Synthesis of thiophene-based [7]helicenes, which are functionalized for both design of organic chiral glasses with strong chiroptical properties and for further homologation to higher [n]helicenes, is reported. The key synthetic transformations are kinetic resolution of the intermediate diketone and the annelation step forming the center benzene ring by means of an intramolecular McMurry reaction. Based upon X-ray crystallographic determinations of the absolute configurations for (+)-enantiomers of the diketone and the [7]helicene, stereochemical correlation between the (R) axial chirality of the diketone and the (M) helical chirality of the [7]helicene is established. One such enantiopure trimethylsilyl-substituted [7]helicene possesses enchanced chiroptical properties and forms a chiral molecular glass.