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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.

Fig.1
Wagner-Meerwein rearrangement

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.

Fig.
Wagner-Meerwein rearrangement
Fig.
Beckmann rearrangement
Fig.
Curtius / Hofmann degradation
Fig.
Wolff rearrangement
Fig.
Baeyer-Villiger rearrangement
Fig.
Carbene rearrangement

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.

Fig.2
Alkyl group shift with inversion

2D Animation of [1,3] sigmatropic rearrangement with inversion

Photochemical [1,3] hydrogen migrations are allowed suprafacially.

Fig.3
Photochemical suprafacial [1,3] sigmatropic H-shift

Anionic 1,2 rearrangements such as the Wittig or Stevens rearrangement formally can also be considered 4 electron [1,2] sigmatropic processes:

Fig.4
Wittig rearrangement
Fig.5
Stevens rearrangement

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.

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