Rhodium-Catalyzed Enantioselective Synthesis and Chiroptical Properties of Helicenes

The enantioselective synthesis of helicenes have been achieved via the rhodium-catalyzed [2+2 +2] cycloaddition. The rhodium-catalyzed enantioselective intramolecular [2+2+2] cycloaddition of 2-naphthol-linked triynes afforded [7]helicenes with good yields and ee values. The rhodium catalyzed enantioselective double intramolecular [2+2+2] cycloaddition of a 2-naphthol-linked hexayne also proceeded to give a [11] helicene-like molecule with high ee value, although the product yield was low. Not only the intramolecular reactions but also intermolecular ones were accomplished by combination of electron-rich tetraynes and electron-poor diynes to give [7] and [9] helicenes, and hetero [7] helicenes in varying yields and ee values. Fluorescence quantum yields of thus synthesized helicenes were higher than unfunctionalized carbohelicenes and some of them exhibited excellent circular polarized luminescence (CPL) activity.