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

Optically active triple helicenes (TH-1) were prepared via a palladium-catalyzed enantio­selective cross-cyclotrimerization of two helicenyl arynes 5, which are generated in situ from 3, with dialkyl acetylene­dicarboxylate 4. Enantiomeric ratios of up to 98:2 were obtained when using 4a and (S)-QUINAP as the alkyne and chiral ligand, respectively. The absolute stereo­chemistry of TH-1a was revealed to be (M,P,M) by a single-crystal X-ray diffraction analysis. Kinetic studies of the racemization of enantio­merically pure TH-1a 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–1. Density functional theory calculations revealed that the palladium-catalyzed enantio­selective cross-cyclotrimerization reactions proceed via the dynamic kinetic resolution of a five-membered pallada­cycle 6a with two [5]­helicenes. Several initially formed stereo­isomers of 6a eventually isomerize into the most thermodynamically stable pallada­cycle intermediate (M,P,M)-6a by inversion of the [5]­helicenyl moiety. Then, the insertion of 4 into 6a to form (M,P,M)-12a, followed by a reductive elimination, leads to the formation of (M,P,M)-TH-1a in a stereo­selective manner. The optical properties of TH-1a were studied by circular dichroism and circularly polarized luminescence.