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Sigmatropic Rearrangements

By
Shri. Rajendra Prasad

Examples of other sigmatropic rearrangements.

The [1,5]-shift

Sigmatropic rearrangement [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

Sigmatropic rearrangement [2,3] shift
  • 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

Mechanism of benzidine rearrangement

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

Benzidine Rearrangement