Sigmatropic Rearrangements

By
Shri. Rajendra Prasad

Examples of other sigmatropic rearrangements.

The [1,5]-shift

• 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

• 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:

Mechanism of benzidine rearrangement

In the following mechanism shown the [5,5]-shift is evident