X-Ray Diffraction Evidence for a Cyclohexa-triene Motif in the Molecular Structure of Tris(bicyclo[2.1.1]hexeno)benzene: Bond Alternation after the Refutation of theMills-Nixon Theory

摘要

For over 60 years, chemists have attributed bond length alternation in.various types of benzenoid structures to the Mills-Nixon effect. Originally this theory was an attempt to account in structural terms for the observed regioselectivity of electrophilic substitution in tetralin and indan. A historical analysis of the development of this effect has shown that Mills and Nixon based their proposal on the incorrect Kekule oscillation model for benzene, in which two D_(3h)-symmetric cyclohexa-irienes are presumed to be in equilibrium; appropriate substitutions could then, in principle, shift the equilibrium to one side. Because the bond lengths and angles in this model are assumed to be dictated by van't Hoff tetrahedra, and the strain energy is thought to be described by Baeyer's theory, the Mills-Nixon postulate has strong structural implications. Specifically, indan and related annelated benzenes are predicted to be cyclohexa-triene-like, with double bonds positioned exocyclic to the five-rnemberedring; however, experimental structural data has never been available to support such speculations.