Synthesis and Properties of Cationic (pi)-Conjugated Systems Stabilized by Bicyclo[2.2.2]octene Units

The purpose of this Account is to present the results of our systematic study on the synthesis of a series of cyclic (pi)conjugated systems with the monocyclic six-membered to eight-membered rings as well as polycyclic aromatic compounds, which are fully surrounded by rigid bicyclic (sigma)-frameworks. This structural modification is characteristic in causing elevation of the HOMO levels of neutral (pi)-systems and remarkably stabilizing the corresponding cationic systems by both thermodynamic and kinetic effects. In order to minimize the possible steric strain in (pi)-systems, the relatively strain-free bicyclic system, i.e., bicyclo[2.2.2]octene, was chosen for the structural modification of all the cyclic (pi)-systems, and the successful generation of the cationic species such as radical cation, closed-shell mono- and dications, which are so stable to allow even the X-ray structural analysis, is demonstrated. 1 Introduction 2 Benzenoid Aromatic Hydrocarbons Fully Annelated with Bicyclo[2.2.2]octene (BCO) Frameworks 3 Tropylium Ion with BCO Annelation 4 Silatropylium Ion with BCO Annelation 5 Cyclooctatetraene (COT) Cations with BCO Annelation 6 Sulfur-Containing Cyclic (pi)-Conjugated Systems with BCO Annelation 6.1 Thiophene 6.2 1,2-Dithiin 6.3 1,4-Dithiin 6.4 Oligothiophenes with BCO Annelation 7 Conclusion