Cyclic Electron Delocalization in Pericyclic Reactions

Since in 1938 Evans and Warhurst established the analogy between electron delocalization in the transition structure of the Diels- Alder reaction and in benzene, many studies have been carried out to combine the concept of aromaticity with the Woodward- Hoffmann rules for thermally allowed pericyclic reactions. This review includes first a brief survey of the main aromaticity descriptors that have been applied successfully to the analysis of electron delocalization in the transition structures associated with pericyclic reactions. In the remaining sections pericyclic reactions such as cycloadditions, electrocyclizations, cheletropic reactions, the Claisen and Cope rearrangements and the ene reaction are covered. It is concluded that contemporary computational tools are reliable enough to quantify the magnitude of electron delocalization in thermally allowed pericyclic reactions. However, chemical variables such as regioand stereoselectivity are more difficult to correlate with aromaticity. In general, pericyclic and pseudopericyclic reactions can be distinguished using magnetic aromaticity descriptors: Transition structures associated with thermally allowed reactions are always Huckel or Mobius aromatic, whereas those associated with pseudopericyclic reactions exhibit low aromaticity, nonaromaticity or even low antiaromaticity.