Pericyclic Reactions: Sigmatropic Rearrangements
Sigmatropic Rearrangements: Examples for [1,n] Sigmatropic 6- and 8-Electron Systems
6-Electron systems, [1,5] sigmatropic
The parent system reacts at approximately 250°C with an activation energy barrier of 36 - 38 kcal/mol.
The [1,5] hydrogen shift in cyclopentadien can be obserevd already at roomtemperature. At 60°C, migration is so fast that only one signal appears for all hydrogens in 1H-NMR.
The situation becomes more complicated when different conformers of the same compound react. In order to test the Woodward-Hoffmann rules, Roth synthesized a chiral 1,3-pentadiene system containing different substituents:
Only compounds with (E) configuration at the terminal double bond and (S) stereochemistry at the terminal stereocenter together with the corresponding (Z),(R)-isomer were found. Antarafacial hydrogen shift was not observed.
[1,5] sigmatropic migrations of alkyl residues (suprafacial, retention) are relatively rare.
It is known that 7,7-substituted cycloheptatrienes isomerize via a cascade of electrocyclic and [1,5] sigmatropic rearrangements.
8-Electron systems ([1,7] sigmatropic)
Antarafacial [1,7] sigmatropic rearrangements are only possible by twisting the reacting system. The best known example is the formation of vitamin D3 in the skin from dehydrocholesterol by UV irradiation. The photochemical electrocyclic ring opening is followed by a spontaneous [1,7] sigmatropic hydrogen shift.
The twisted conformation already exists in the steroid framework.