Antiaromaticity and Reactivity of a Planar Cyclooctatetraene Fully Annelated with BicycloA Units

A detailed investigation has been made into the antiaromaticity and chemical reactivity of a planar cyclooc- tatetraene (COT) molecule fully anne- lated with bicycloA units 2. In spite of its planar 8p-electronic structure, theoretical calculations have indicated that the antiaromaticity of COT 2 is considerably decreased in comparison with a planar COT 16 with D4h symmetry. This behavior appears to be related to the wider HOMO- LUMO gap of 2 relative to 16, which is caused by the raised LUMO level as a result of the effective s-p* orbital in- teraction between the strained bicyclic framework and the COT p system. The two-electron reduction of 2 required the use of potassium mirror or a com- bination of lithium/corannulene in highly dried (D8)THF at � 788C under vacuum. In contrast, the (4 + 2) cyclo- addition of 2 with tetracyanoethylene (TCNE) proceeded quite smoothly owing to the high-lying HOMO. Reac- tion of 2 with meta-chloroperbenzoic acid gave all-trans tetraepoxide 23 in the same way as the corresponding benzene derivative 3. While the Simons-Smith-type cyclopropanation of benzene 3 gave tricyclopropanated derivative 21, the reaction of 2 only af- forded isomers of dicyclopropanated derivatives 25 and 26. Yet, the reactivi- ty of 2 is higher than the parent COT, which does not show any reactivity under the same conditions. On the basis of homodesmic reactions, it was concluded that release of strain is also an important factor for such relatively high reactivity in the epoxidation and cyclopropanation of bicycloannelated COT 2 as well as benzene 3.