Conformational mobility and fast bond shift in the annulenes

Abstract

After a general description of the configuration, of the conformation and of the fast and reversible dynamic processes characteristic of the annulenes, results of an experimental study of [8], [12], [14], [16] and [18]annulenes by N.M.R. spectroscopy are presented. Theoretical analysis of the N.M.R. line shape has allowed a determination of the structures of these compounds and of the thermodynamic and kinetic parameters of their dynamic processes. Study of bond shift in cyclooctatetraene (COT) and of bond shift and ring inversion in alkoxy-COT’s point to a small negative resonance energy in the planar conformation of COT. Two configurations of [16]annulene in 3:1 ratio, coded 85 and 91, have been found in solution. Both of them undergo isodynamical conformational mobility and bond shift and a fast exchange between both is also occurring. The newly synthesized [12]annulene of configuration 21 undergoes extremely fast conformational mobility but no bond shift. Conformational mobility is rather slow in [18]annulene and in the major isomer of [14]annulene, of configuration 43. It is very fast in [14]–21 annulene. Exchange between [14]-43- and [14]-21-annulenes is very slow. The present results agree generally with the calculations of Dewar and Gleicher. © 1971, Walter de Gruyter. All rights reserved.