Sigmatropic Rearrangements

Abstract

Sigmatropic rearrangements are another important class of pericyclic reactions, in which one sigma bonded atom or group is shifted from its allylic type position to the distant end of the adjacent $π$ system followed by reorientation of the $π$ electrons in the carbon skeletal system. These reactions are described by the order [i, j], where i specifies the number of atom or atoms shifted and j specifies the number of atoms in the $π$ system that are directly involved in the bonding changes. These rearrangements are described as suprafacial and antarafacial shifts, according to the shift of the sigma bonded atom or group on the same surface and opposite surface of the conjugated $π$ system, respectively. Woodward and Hoffmann predicted the rules for orbital symmetry allowed thermal and photochemical sigmatropic rearrangement reactions for the total number of 4ne and [4n+2]e systems. The wide applications of [3,3]- sigmatropic rearrangements such as the Cope and Claisen rearrangements and their useful modifications including the oxy-Cope, anionic-oxy-Cope, aza-Cope, thio-Claisen, aza-Claisen rearrangements have been highlighted in details. The regio-, stereo-, and enantioselectivities as well as versatile applications of [1.3]-, [1,5]-, [1,7]- H and alkyl shifts, [3,3]- and [2,3]- rearrangements including the Wittig, aza-Wittig, Meisenheimer, Mislow- Evans, Sommelet- Hauser rearrangements as well as [5,5]-, [3,5]-, [4,5]- and [9,9]-rearrangements have been discussed.