A Key Fragment in Carbon Schwarzite Unit Cells and Its Triple [6]Helicene Precursor

Abstract

This study explores two structurally related π-skeletons. The π-skeleton of compounds 1a–e containing three heptagons represents a key fragment in theoretical carbon schwarzites, while that of 2a–b is a triple [6]helicene. Compounds 1a–e were synthesized via Scholl reactions, and using a weaker acid allowed the reaction to stop at an intermediate stage, yielding 2a–b. X-ray crystallography revealed not only distinct stereochemistry of 1b and 2a but also unique supramolecular assemblies in the clathrate of 2a with chloroform. Compound 1b adopts a saddle-like geometry, while 2a exhibits a propeller-like structure with C3 symmetry, consistent with density functional theory (DFT) calculations. The π-skeleton of 1a–e is flexible, enabling rapid enantiomerization, whereas that of 2a–b is rigid, allowing resolution of 2b into optically pure forms with an absorption dissymmetry factor as high as 0.015. Comparative analysis shows that presence of seven-membered rings in 1a–e does not significantly alter the local aromaticity of the triple [6]helicene framework.