Enantioselective Synthesis of Triple Helicenes by Cross-Cyclotrimerization of a Helicenyl Aryne and Alkynes via Dynamic Kinetic Resolution.

Optically active triple helicenes () were prepared via a palladium-catalyzed enantioselective cross-cyclotrimerization of two helicenyl arynes , which are generated from , with dialkyl acetylenedicarboxylate . Enantiomeric ratios of up to 98:2 were obtained when using and ()-QUINAP as the alkyne and chiral ligand, respectively. The absolute stereochemistry of was revealed to be (,,) by a single-crystal X-ray diffraction analysis. Kinetic studies of the racemization of enantiomerically pure at elevated temperatures were conducted based on a high-performance liquid chromatography analysis. The activation energy for the racemization was found to be 29.1 kcal mol. Density functional theory calculations revealed that the palladium-catalyzed enantioselective cross-cyclotrimerization reactions proceed via the dynamic kinetic resolution of a five-membered palladacycle with two [5]helicenes. Several initially formed stereoisomers of eventually isomerize into the most thermodynamically stable palladacycle intermediate (,,)- by inversion of the [5]helicenyl moiety. Then, the insertion of into to form (,,)-, followed by a reductive elimination, leads to the formation of (,,)- in a stereoselective manner. The optical properties of were studied by circular dichroism and circularly polarized luminescence.