Closed Pentaaza[9]helicene and Hexathia[9]/[5]helicene: Oxidative Fusion Reactions of ortho‐Phenylene‐Bridged Cyclic Hexapyrroles and Hexathiophenes

Oxidative fusion reactions of ortho‐phenylene‐bridged cyclic hexapyrroles and hexathiophenes furnished novel closed helicenes in a selective manner. X‐Ray diffraction analysis unambiguously revealed the structures to be a closed pentaaza[9]helicene, the longest azahelicene reported so far, and an unexpected double‐helical structure of hexathia[9]/[5]helicene, whose formation was assumed to result from multiple oxidative fusion along with a 1,2‐aryl shift. The pentaaza[9]helicene exhibited well‐defined emission with high fluorescence quantum yield (ΦF=0.31) among the known [9]helicenes. Chiral resolution of the racemic pentaaza[9]helicene and hexathia[9]/[5]helicene were achieved by chiral‐phase HPLC and the enantiomers were characterized by circular dichroism spectra and DFT calculations. Case closed: Oxidative fusion reactions of ortho‐phenylene‐bridged cyclic oligomers furnished novel closed heterohelicenes in a selective manner. X‐ray diffraction analysis revealed the structures to be the closed pentaaza[9]helicene and hexathia[9]/[5]helicene, rare examples of a heteroaromatic closed helicene and an asymmetric double helicene, respectively. Their optical properties were studied and the enantiomeric separation was performed by chiral‐phase HPLC.