of other sigmatropic rearrangements.
- This example involves a migration of the sigma bond from [1,1] to
- 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.
Conversion of sulphur ylide
with an allylic group to unsaturated sulphides
- 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
- Benzidine rearrangement is a good example of a [5,5]-shift.
- Hydrazobenzene in acid medium rearranges to benzidine (4,4’-diamino
The net reaction is:
Mechanism of benzidine rearrangement
In the following mechanism
shown the [5,5]-shift is evident