Battered benzene and twisted ethene

Abstract

Strained compounds, besides providing valuable information about the structure of ’normal’ compounds, are of significance as materials for the discovery of new synthetic methods and procedures (ref. 1). We have been interested for some time in the preparation of benzenoid systems strained by in-plane distortions (ref. 2) and have shown that such compounds have unusual, but predictable, chemistry (refs. 3,4). These compounds, cyclopropabenzocyclobutenes and related systems, were initially prepared to determine whethr bond delocalization towards one cyclohexatriene structure might be observed, the modern interpretation of the Mills-Nixon effect (ref. 5), but the physical properties of the compounds indicated that they were still benzenoid in character. The lack of bond alternation has also been observed in annelated 15,16-dimethyldihydropyrenes (ref. 6), but bond alternation is observed in extended biphenylenes (ref. 7). No systems have so far been reported in which benzene is strained by both in-plane and out-of-plane distortions and such systems, besides being intrinsically structurally interesting, may have a rich and distinctive chemistry. This paper describes our present approach to such systems and also the application of the incremental strain approach to the synthesis of highly hindered alkenes (ref. 8). © 1990, IUPAC.