Twist along Central C−C Bonds in a Series of Fully π‐Fused Propellanes

Without stereogenic carbon centers, organic molecules can be chiral when they have large energy barriers for conformational inversion. In this work, conformational behaviors are investigated for a series of tricyclic propellane skeletons with increasing 6-membered-ring peripheral moieties fused with aromatic rings. According to theoretical calculations, trinaphtho[3.3.3]propellane has three vertical naphthalene rings like triptycene shape without torsion along the central C-C single bond. On the other hand, hexabenzo[4.4.4]propellane shows hexaphenylethane-like ca. 60° twist along the bond with large activation energy of 64 kcal mol^-1 for twist inversion because of the high congestion caused by three 6-membered-ring loops. Indeed, the [4.4.4]propellane gives a stable pair of chiroptical enantiomers toward heating at 146 °C. By contrast, a hybrid [4.3.3]propellane exhibits fast interconversion between two twisted conformations even at -80 °C.