Pericyclic Reactions: Sigmatropic Rearrangements
Sigmatropic Rearrangements: Examples for [1,n] Sigmatropic 2- and 4-Electron systems
2-Electron systems (Wagner-Meerwein rearrangement)
The Wagner-Meerwein rearrangement represents a 1,2-migration of an alkyl or aryl residue to a carbocation center. It represents the most simple and smallest system in which a sigmatropic rearrangement can take place.
In principle, several other [1,2] rearrangements could be considered sigmatropic, but it is not always obvious whether the reactive intermediate (cation, carbene, nitrene) really exists or whether formation and migration occur simultaneously.
4-Electron systems, [1,3] sigmatropic
According to the Woodward-Hoffman rules, [1,3] sigmatropic rearrangements are allowed thermally either antarafacial or with inversion of the migrating center. Antarafacial [1,3] hydrogen migrations have not been observed as yet but alkyl migrations with inversion are known.
Photochemical [1,3] hydrogen migrations are allowed suprafacially.
Anionic 1,2 rearrangements such as the Wittig or Stevens rearrangement formally can also be considered 4 electron [1,2] sigmatropic processes:
Because of the small size of the system, the rearrangement can neither proceed through an antarafacial allowed process nor with inversion. However, since a geometrically possible suprafacial rearrangement is forbidden, these reactions are two step processes.