Pericyclic Reactions: Sigmatropic Rearrangements
Sigmatropic Rearrangements: Stereochemistry of [1,n] Sigmatropic Rearrangements
In principle, there are two stereochemical pathways by which the hydrogen atom can migrate.
In suprafacial rearrangements the hydrogen atom moves along the top or bottom of the π-plane while in antarafacial mode the hydrogen moves across the π-system.
Suprafacial sigmatropic rearrangements proceed through Hückel aromatic transition states and antarafacial rearrangements via Möbius transition states. Because of the strong deformation required of the conjugated system Möbius topology can only be realized in large molecules. During the migration of organic residues inversion at the migrating carbon atom can take place. In this case the transition state also displays Möbius topology but it is easier to realize geometrically in smaller systems than it is during antarafacial migration.
- Stereochemical rules for [1,n] sigmatropic reactions
- 4n+2 Electrons: supra and retention, respectively
- 4n Electrons: antara and inversion, respectively
- The opposite behavior is observed in photochemical reactions.