Laterally π-Extended Dithia[6]helicenes with Heptagons: Saddle-Helix Hybrid Molecules

A laterally π-extended dithia[6]helicene 1, representing an interesting saddle-helix hybrid molecule containing an unusual heptagon, has been synthesized by MoCl 5 -mediated oxidative stitching of tetrakis(thienylphenyl)naphthalene precursor 2 involving reactive-site capping by chlorination and subsequent Pd-mediated dechlorination of tetrachlorinated intermediate 1-Cl 4 . Highly distorted, wide helical structures of dithia[6]helicenes ( 1 and 1-Cl 4 ) were clarified by single-crystal X-ray diffraction analyses where heterochiral slipped π–π stacking was displayed in a one-dimensional fashion. Notably, theoretical studies on the thermodynamic behavior of 1 predicted an extraordinarily high isomerization barrier of 49.7 kcal·mol –1, which enabled optical resolution and chiroptical measurements. Electronic structures of these huge helicenes were also examined by photophysical and electrochemical measurements.