Sigmatropic Rearrangements
Examples of other sigmatropic rearrangements.
The [1,5]-shift
![Sigmatropic rearrangement [1,5] shift](images/sigma4.GIF)
- This example involves a migration of the sigma bond from [1,1] to [1,5] position
- It is a [1,5]-sigmatropic shift. Migration is over five atoms.
- The hydrogen has not changed its position, yet there is the migration of the sigma bond of that hydrogen.
The[2,3]-shift
Conversion of sulphur ylide with an allylic group to unsaturated sulphides
![Sigmatropic rearrangement [2,3] shift](images/sigma5.GIF)
- The sigma bond that migrates is between S1-C1 to C2-C3
- Migration is over two atoms. New sigma bond is created between C2-C3. This is a [2,3]-shift.
- Note: Ylide is a compound, which has positive and negative charges on adjacent atoms
The [5,5]-shift
- Benzidine rearrangement is a good example of a [5,5]-shift.
- Hydrazobenzene in acid medium rearranges to benzidine (4,4’-diamino biphenyl)
The net reaction is:
![Sigmatropic rearrangement [5,5] shift](images/sigma6.GIF)
Mechanism of benzidine rearrangement
In the following mechanism shown the [5,5]-shift is evident
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Author: Rajendra Prasad