Synergistic Ion-Pair/Lewis Acid Catalysis Enables Enantioselective Synthesis of Helically Chiral Oxa[6]Helicenes

Helicenes are privileged chiral architectures with exceptional optical and electronic properties, underpinning their importance in asymmetric catalysis, optoelectronics, and supramolecular chemistry. Despite significant progress in helicene synthesis, strategies for extending peripheral aromatic rings to access structurally diversified helicenes remain elusive. Here we report a synergistic catalytic system integrating a bifunctional phosphonium salt (BPS) with silver, which enables a highly efficient [4 + 2] cyclization/aromatization cascade to directly construct a new family of oxa[6]helicenes. This cooperative catalysis exhibits broad substrate scope, delivering chiral helicenes in excellent yield and enantioselectivity. Mechanistic studies reveal that Ag⁺ activates the cyano group, while the BPS catalyst engages through hydrogen bonding and ion-pairing interactions to form a unique sandwich-type transition state. This dual activation mode provides a powerful framework for constructing helicenes with tailored structural diversity, paving the way for future applications in chiroptical materials and enantioselective catalysis.