Structure and reactivity of polycyclic cross-conjugated π-electron systems

Abstract

In contrast to the well-known monocyclic conjugated systems with (4n + 2) and 4nπ-electrons, non-benzenoid polycyclic conjugated π-electron system which should not obey Hückers rule contain the element of cross-conjugation. In bicyclic and linear annelated polycyclic conjugated systems this cross-conjugation is not associated with branching of the π-electron system, but this will however occur in pericondensed tri- and polycyclic compounds. The participation of the element of cross-conjugation in the π-electron systems of such polycycles should affect their properties and result in characteristic differences in bonding character and reactivity compared with monocyclic conjugated compounds with the same number of π-electrons. To obtain experimental support for these theoretical predictions, several non-benzenoid bicyclic as well as tri- and tetracyclic pericondensed π-electron systems were synthesized and studied with respect to the magnetic criteria of aromaticity and the connection between structure and reactivity. The successful preparation of most of the described hydrocarbons has centred on a single basic and rational synthetic principle. © 1971, Walter de Gruyter. All rights reserved.