A Direct Arylation Approach for Helicene Analogues Composed of Unidirectional Amide Linkages

Amide-bond-containing [5]helicene and [7]helicene analogues were synthesized via palladium-mediated 2- and 3-fold direct intramolecular arylations of halogenated linear diamides and triamides, respectively. The orientation of the amide bonds in these helicene analogues was designed to align with the winding direction of helicene. The introduction of a methyl group at the 3-position of the terminal aniline unit of the linear triamide precursor is crucial to the synthesis of the [7]helicene analogue. The molecular structures of the [5]helicene and [7]helicene analogues in the crystalline state were determined unequivocally by single-crystal X-ray diffraction analysis. The addition of trifluoroacetic acid to a solution of the [7]helicene analogue in dichloromethane increased the fluorescence intensity, whereas the absorption intensity remained unchanged. The enantiomers of the [7]helicene analogue were separated by high-performance liquid chromatography over a chiral stationary phase. The enantiomers exhibited circularly polarized luminescence with relatively large luminescence dissymmetry factors.