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

Abstract

The purpose of this Account is to present the results of our systematic study on the synthesis of a series of cyclic π-conjugated systems with the monocyclic six-membered to eight-membered rings as well as polycyclic aromatic compounds, which are fully surrounded by rigid bicyclic σ-frameworks. This structural modification is characteristic in causing elevation of the HOMO levels of neutral π-systems and remarkably stabilizing the corresponding cationic systems by both thermodynamic and kinetic effects. In order to minimize the possible steric strain in π-systems, the relatively strain-free bicyclic system, i.e., bicyclo[2.2.2]octene, was chosen for the structural modification of all the cyclic π-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 π-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

Key words

C-C hyperconjugation - cations - one-electron oxidation - X-ray crystallography - aromatic stabilization

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Nishinaga, T.; Matsuura, A.; Komatsu, K. unpublished.