zum Directory-modus

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.

Fig.1
Suprafacial
Fig.2
Antarafacial

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.

Fig.3
[1,5] Sigmatropic

  • 6 Electrons
  • Hückel aromatic
  • Suprafacial

Fig.4
[1,7] Sigmatropic

  • 8 Electrons
  • Möbius aromatic
  • Antarafacial

3D Animation of a suprafacial [1,5] sigmatropic H-migration

3D Animation of a antarafacial [1,7] sigmatropic H-migration

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.

Fig.5
Retention
Fig.6
Inversion
Fig.7
[1,5] Sigmatropic

  • 6 Electrons
  • Hückel aromatic
  • Retention

Fig.8
[1,3] Sigmatropic

  • 4 Electrons
  • Möbius aromatic
  • Inversion

3D Animation of alkyl migration with retention

3D Animation of alkyl migration with inversion

General rules.

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.
Page 2 of 8