SOME ASPECTS OF STRAINED SYSTEMS. [4]ANNULENE AND ITS CH+ ADDUCT

Abstract

The application of a set of criteria based on quantum chemistry very often predicts the possible existence of unique organic species that are never conceived in classical organic chemistry. The square pyramidal adduct of [4]annulene of C4v symmetry is a good example and has been well characterized. In contrast, the quantitative evaluation of strained systems by means of the most recent theoretical treatments appears to be still inadequate, particularly in the case of unstable species surrounded by low energy barriers. [4]Annulene, a century-old problem, illustrates the current state of theoretical development. The properties of a derivative of [4]annulene, perturbed slightly by substituents, methyl tri-tert-butyl[4]annulenecarboxylate, has now been fully elucidated, including its x-ray crystallographic analysis. Thus, the first unambiguous direct evidence has been secured to demonstrate that a rectangle, distorted slightly by substituents, corresponds to the most stable conformation of this [4]annulene system. It is now almost certain that the ground state of the parent [4]annulene is singlet. Evaluating all the experimental evidence available at present, it is most likely that [4]annulene is rectangular with two short bonds of approximately 1.37 å and two longer ones of the order of 1.51 å. Should this compound turn out to be square, both square and rectangle must reside in a region of a flat potential surface.