Unambiguous Assessments of Reaction Paths for Selected Pericyclic Reactions

Three major approaches have been developed to predict outcomes for concerted reactions controlled by orbital symmetry analysis using (i) correlation diagrams1, (ii) frontier orbital interactions in transition states2 and (iii) incipient aromatic/ antiaromatic character in transition states3-5. The latter approach (Zimmerman-Dewar or Hückel-Möbius) is particularly simple to teach to undergraduate students. However, as it was originally advanced, the Zimmerman-Dewar approach relied on so-called Möbius transition states. The notion of “aromatic” Möbius annulenes is problematic both from the pedagogical standpoint and from the standpoint of modern semiempirical molecular orbital results. For those reasons, I have previously proposed a modification of the Zimmerman-Dewar method which removes its dependence on Möbius structures6. The earlier paper6, demonstrated that such a modification of the Zimmerman-Dewar approach (ending its dependence on Möbius structures) does not impair its predictive capacity. Indeed, my proposed shift to assessing transition state aromaticity using kinetic-stability-based aromaticity arguments removed the ambiguity which marred pre-existing Zimmerman-Dewar analyses for some electrocyclic and some cycloaddition reactions. This report presents four examples of familiar concerted chemical reactions and compares the performance of the established approaches with that of the modified ZimmermanDewar method.