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

• Published in: Journal of the American Chemical Society 2020-06, Vol.142 (22), p.10025-10033
• Main Authors: Yubuta, Ayaka, Hosokawa, Tomoka, Gon, Masayuki, Tanaka, Kazuo, Chujo, Yoshiki, Tsurusaki, Akihiro, Kamikawa, Ken
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.0c01723

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.