Annulenes, 1980–2000

摘要

The first fully conjugated carbocyclic compound to benrecognised and described, early in the nineteenth century, wasnbenzene 1. The differences between the properties of thisncompound and those of other unsaturated compounds, alkenesnand alkynes puzzled early organic chemists. Consequently theynassociated this unusual character with the complete cyclicnconjugation in the carbon ring. Further confusion arose whenncyclooctatetraene 2 was prepared and it showed no chemicalnresemblance to benzene. A major contribution to the under-nstanding of the nature of cyclic conjugated compounds camenfrom Hückeln1n whose famous rule predicted that only thosencompounds whose conjugated systems involved (4nu00012) π-nelectrons (n = an integer) would have special properties resem-nbling those of benzene and could be classified as benzene-like.nThey could be described as aromatic, the term used to describenbenzene since the early nineteenth century. It was laternsuggested2n that analogous compounds having 4n π-electronsnmight be termed antiaromatic and could have contrastingnproperties to those of benzene. Confusion reigned however,nover the precise meaning of these terms.