Enantioselective Synthesis of 1,12-Disubstituted [4]Helicenes.

A highly enantioselective synthesis of 1,12-disubstituted [4]carbohelicenes is reported. The key step for the developed synthetic route is a Au-catalyzed intramolecular alkyne hydroarylation, which is achieved with good to excellent regio- and enantioselectivity by employing TADDOL-derived (TADDOL=α,α,α,α-tetraaryl-1,3-dioxolane-4,5-dimethanol) α-cationic phosphonites as ancillary ligands. Moreover, an appropriate design of the substrate makes the assembly of [4]helicenes of different substitution patterns possible, thus demonstrating the synthetic utility of the method. The absolute stereochemistry of the newly prepared structures was determined by X-ray crystallography and characterization of their photophysical properties is also reported.