Building Cyclic Planar Chirality via Intermolecular [2+2] Photocycloaddition

Comprehensive Summary Planar–chiral compounds have garnered significant interest due to their unique stereochemical features and promising applications in asymmetric catalysis, materials science, and molecular recognition. To date, only a few representative classes have been discovered and synthesized, including planar‐chiral cyclophanes, ferrocenes, and macrocyclic hosts such as pillararenes and calixarenes. The construction of novel planar‐chiral architectures therefore continues to pose a formidable challenge. Herein, we present a simple but highly effective approach for constructing a new form of planar chirality through the formation of three‐dimensional double‐layered cyclic molecular architectures. Photocycloaddition of C=C double bonds embedded within benzo[ b ]thiophene 1,1‐dioxide affords trans Z‐type ( Trans ‐1 ) and cis ‐∏‐type ( Cis ‐ Rac ‐1 ) dimers. While Trans ‐1 is achiral owing to its central symmetry, Cis ‐ Rac ‐1 adopts a distinctive bisplanar geometry that gives rise to an unprecedented type of planar chirality. The enantiomers of Cis ‐ Rac ‐1 are successfully resolved via chiral high‐performance liquid chromatography. Circular dichroism spectroscopy and single‐crystal X‐ray diffraction unequivocally confirm the planar–chiral nature of these enantiomers. Moreover, both experimental and computational results show that the planar chirality is highly stable, with a racemization barrier exceeding 35 kcal/mol. These findings establish a general and straightforward strategy for construction of planar chirality, opening new avenues for the design of planar–chiral functional materials.