Pericyclic Reactions: Sigmatropic Rearrangements
Sigmatropic Rearrangements: Examples for [n,m]-Sigmatropic 6- and 8-Electron Systems
6-Electron systems, [3,3] and [2,3] sigmatropic rearrangements
In 1,5-cyclohexadiene, the parent system for [3,3] sigmatropic rearrangements, reaction takes place at 300°C with an activation energy barrier of approximately 34 kcal/mol. These rearrangements of 1,5-diene systems, discovered first by Hurd and later by Cope, are generally called Cope rearrangements. The oxy-Cope rearrangement takes place at lower temperatures.
Formation of a resonance stabilized enolate anion is the the driving force in the reaction. The synthetically more important Ireland-Claisen rearrangement takes place at even milder temperatures to yield the highly stable carboxylate anion.
[3,3] Sigmatropic rearrangement in which one C-atom in the 1,5-diene is substituted by an O-atom are called Claisen rearrangements.
Loss of aromaticity initially leads to a ketone (quinone) which spontaneously tautomerizes to the corresponding aromatic phenol. The key step in the Fischer indole synthesis is a "nitrogen variation" of the Claisen rearrangement.
[2,3] Sigmatropic rearrangements can be observed in sulfur and nitrogen ylids.
Deprotonation of allylic ethers with strong bases leads to [2,3] Wittig rearrangements.
The driving force behind most [2,3] sigmatropic rearrangements is the transformation of an unstable carbanion into a stable product. Utilizing the reduction of ring strain helps to increase the thermodynamic driving force in [3,3] sigmatropic rearrangements and to lower the activation energy. The activation energy barrier is reduced from 34 kcal/mol to 19 kcal/mol if the single bond to be broken in the 1,5-cyclohexadiene system is part of a three-membered ring. The resulting product of the rearrangement is a 7-membered ring.
When bridging the two terminal double bonds with a CH2 -group (homotropylidene) the reaction becomes tautomeric, i.e., product and educt are indistinguishable as in the parent system 1,5-hexadiene. Bridging the two CH2 groups with an additional double bond affords the especially interesting bullvalene. This molecule contains a three-fold axis of symmetry and three symmetrical and equivalent Cope systems.
Each of these 1,5-diene units can undergo a Cope rearrangement again yielding bullvalene. In all, there are more than 1.2 million tautomeric forms possible. At 100°C the reaction is so fast that only one peak is being observed for all H atoms in 1H-NMR. Structures of this type are called fluxional.