Synthesis and Characterization of Kinetically Stabilized Octadehydrobisthieno[12]annulenes

This study presents the synthesis and characterization of kinetically stabilized <i>b</i>- and <i>c</i>-isomers of octadehydrobisthieno[12]annulene (<b>ODTA</b>) derivatives. The synthesis of <i><b>c</b></i>- and <i><b>b</b></i><b>-ODTA</b> derivatives was attempted with four 3,5-di-<i>tert</i>-butylphenyl groups on the thiophene rings. A tetraaryl-substituted <i><b>c</b></i><b>-ODTA</b> derivative was successfully isolated and characterized, whereas tetraaryl-substituted <i><b>b</b></i><b>-ODTA</b> derivatives could not be synthesized despite utilizing the same reaction conditions. By replacing the aryl groups at the β-position of the thiophene rings with smaller <i>n</i>-butyl groups, <i><b>anti-b</b></i><b>-ODTA</b> derivative was successfully isolated and characterized, whereas <i><b>syn-b</b></i><b>-ODTA</b> derivative could not be detected. Intramolecular coupling of the linear precursor produced <i><b>syn-b</b></i><b>-ODTA</b> which decomposed during solvent evaporation. The different reactivities of <i>anti</i>- and <i>syn</i>-isomers of <i><b>b</b></i><b>-ODTA</b> derivatives under ambient conditions are attributed to the different degrees of steric protection afforded by substituent groups around the strained butadiynylene units. Ultraviolet-visible absorption spectroscopy and cyclic voltammetry of <i><b>c</b></i>- and <i><b>anti-b</b></i><b>-ODTA</b> derivatives confirmed the narrower HOMO-LUMO energy gap and higher HOMO energy level of <i><b>anti-b</b></i><b>-ODTA</b> compared to those of <i><b>c</b></i><b>-ODTA</b>, which is attributed to the stronger antiaromatic character of the 12-membered ring of <b><i>anti-</i></b><i><b>b</b></i><b><i>-</i>ODTA</b>.