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

A laterally pi-extended dithia[6]helicene 1, representing an interesting saddle-helix hybrid molecule containing an unusual heptagon, has been synthesized by MoCl5-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 pi-pi 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.