Claisen Rearrangement

Claisen Rearrangement

Claisen rearrangement is also [3,3] sigmatropic rearrangement of an allyl vinyl ether. It involves the formation of six-membered ring transition state. Under thermal conditions, Claisen rearrangement also takes place through suprafacial pathway. Following example shows aliphatic Claisen rearrangement reaction.

The rearrangement takes place in concerted fashion i.e., the breakdown and formation of bonds takes place simultaneously.

This reaction proceeds via chair transition state. The products formed in this reaction are highly optically pure. For example,

Claisen rearrangement can also take place with aromatic ring system. The carbonyl functionality tautomerizes to hydroxy group to restore the aromaticity.

If the -ortho positions in above example are substituted, then re-aromatization is not possible. The allyl group first undergoes Cope rearrangement to the -para position followed by tautomerization to produce para allyl phenol ring.

The Ireland-Claisen rearrangement is slightly modified model of Claisen rearrangement reaction. This reaction used allyl ester instead of ally vinyl ether. In this reaction the allyl ester is first reacted with base to form enolate. The enolate is trapped by reacting it with trimethylsilylchloride. The resulting trimethylsilyl ether upon mild heating undergoes Claisen rearrangement reaction as shown below.

Following are two other variations of Claisen rearrangement reaction.