The Mysticism of Pericyclic Reactions: A Contemporary Rationalisation of Organic Reactivity Based on Electron Density Analysis

The molecular mechanism of the five most representative pericyclic reactions has been studied by applying molecular electron density theory (MEDT). The different phases into which the reaction paths are topologically divided can be regrouped into four well-characterised steps: 1) the rupture of the C-C double bonds, 2) the formation of pseudoradical centres at the interacting carbons, 3) the formation of new C-C single bonds and 4) the formation of new C-C double bonds in the final products. Consequently, the bonding changes in these reactions are neither concerted nor cyclic. As the transition-state structures are located at the end of the large first group of phases or in the narrow second group, the high activation enthalpies found in these reactions are mainly associated with the rupture of C-C double bonds. The present MEDT study makes it possible to rule out the definition of pericyclic reactions, made by Woodward and Hoffmann in 1969, in which all first order changes in bonding relationships take place in concert on a closed curve.