By
Shri. Rajendra Prasad

• It is the first Sigmatropic rearrangement, which was discovered.
• When allylic aryl ethers are heated they under go a [3,3]-shift to give o-allyl phenols. This is Claisen rearrangement.
• If ortho as well as para positions are occupied no reaction occurs, migration to meta position has not been observed.
• The following is the mechanism for the o-Claisen process.

• The first is a slow step, which involves a [3,3] shift.
• The second step is fast due to regaining aromaticity through proton transfer.
• This reaction also takes place through a chair like six membered transition state similar to Cope reaction.
• The reaction has been found to be of the, first order through experiment.
• The transfer of allylic group to the new position has been proved through labeling studies.
• Allylic vinylic ethers, also undergo similar rearrangement, but do not involve the proton transfer step, as the products are stable, and there is no loss of aromaticity in the sigmatropic shift.
• Most Claisen rearrangements do not need a catalyst.
• If both ortho positions are occupied by other substituents, the allyl group can migrate to the para position and this is often called p-Claisen rearrangement.
• it involves two [3,3] shifts.

Here is an example of pi-migration through two [3,3] shifts