Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

Cyclooctatetraene (COT), the first 4n pi-electron system to be studied, adopts an inherently nonplanar tub-shaped geometry of D-2d symmetry with alternating single and double bonds, and hence behaves as a nonaromatic polyene rather than an antiaromatic compound. Recently, however, considerable 8 pi-antiaromatic paratropicity has been shown to be generated in planar COT rings even with the bond alternated D-4h structure. In this review, we highlight recent theoretical and experimental studies on the antiaromaticity of hypothetical and actual planar COT. In addition, theoretically predicted triplet aromaticity and stacked aromaticity of planar COT are also briefly described.