Small cyclophanes: the bent benzene business

Abstract

Short bridges in [njmeta- and [njparacyclophanes with n < 8 bend the benzene ring into a boat shape. Naturally, this influences the properties of the aromatic system, but two seemingly contradictory trends are observed. On the one hand, the chemical reactivity of such small cyclophanes is strongly increased and quite unusual; sometimes, it is remindful of that of a localized cyclohcxatriene-Iike system. On the other hand, aromatic delocalization appears to be hardly impaired as judged from aromatic carbon-carbon bond lengths and from ring current effects such as proton chemical shifts and anisotropy of the magnetic susceptibility. In line with recent theoretical analyses implying that delocalization in benzene is due to the a-system and counteracted by the π-system, it is proposed that bent benzene rings are full-fledged aromatics, and that their sometimes olefin-like reactivity is predominantly a consequence of their high strain energy being dramatically relieved in the initial stages of such reactions. © 1990, IUPAC.