A detailed investigation has been made into the antiaromaticity and chemical reactivity of a planar cyclooctatetraene (COT) molecule fully annelated with bicyclo[2.1.1]hexane units 2. In spite of its planar 8pi-electronic structure, theoretical calculations have indicated that the antiaromaticity of COT 2 is considerably decreased in comparison with a planar COT 16 with D 4h 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 sigma-pi* orbital interaction between the strained bicyclic framework and the COT pi system. The two-electron reduction of 2 required the use of potassium mirror or a combination of lithium/corannulene in highly dried [D8]THF at -78 degrees C under vacuum. In contrast, the [4+2] cycloaddition of 2 with tetracyanoethylene (TCNE) proceeded quite smoothly owing to the high-lying HOMO. Reaction 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 afforded isomers of dicyclopropanated derivatives 25 and 26. Yet, the reactivity 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.

An entirely new naphthalene having one six-membered ring, whose bond lengths are equalized, and another ring, whose bond lengths are alternated, has been prepared via one-electron oxidation of a benzene annelated with three bicyclo[2.1.1]hexene units. The structure has been determined by X-ray crystallography and the aromaticity of two benzene rings estimated by theoretical calculations. Although the former six-membered ring has a higher degree of aromaticity, it has been found to be more reactive toward singlet oxygen owing to the greater strain contained in it. These results have been interpreted by means of theoretical calculations.

An overview of magnetic circular dichroism (MCD) spectroscopy of π‐electron systems derived from a 4N‐electron perimeter is provided, with emphasis on the hypothetical parent cycloocta‐1,3,5,7‐tetraene of D8h symmetry (1) and its D4h symmetry derivatives. UV‐visible absorption and MCD spectra of 2, a D4h symmetric cycloocta‐1,3,5,7‐tetraene planarized by the effect of 4 bicyclo[2.1.1]hexeno units fused to its 8‐membered ring, are reported and interpreted. The perimeter model is applied to obtain an understanding of the nature of electronic states in 1 and 2 and to predict general trends in the spectra. The electronic excitation patterns are found to be different in the antiaromatic D8h and unaromatic D4h species, and their states cannot be unequivocally correlated. The results of time‐dependent density functional theory and extended multistate complete active space second‐order perturbation theory (XMS‐CASPT2) computations agree with the algebraic perimeter model analysis and reproduce the spectra of 2 well, including three of the four observed MCD signs of A and B terms.

A novel cyclooctatetraene (COT) radical cation fully annelated with bicyclo[2.1.1]hexene units was prepared as SbCl(6)(-) salt, and planarity of the octagonal ring was clarified by ESR and theoretical calculations. Its longest wavelength absorption (630 nm) is blue-shifted from that (745 nm) of COT radical cation annelated with bicyclo[2.2.2]octene units due to the widening of the HOMO-SOMO gap accompanying the flattening of the COT ring. [structure: see text]