After completing this section, you should be able to
- write an equation to represent the Claisen rearrangement of allyl phenyl ester.
- account for the formation of a specific product from a Claisen rearrangement, without giving mechanistic details.
Make certain that you can define, and use in context, the key term below.
A Claisen rearrangement is a reaction which is specific to ally aryl ethers and allyl vinyl ethers. Heating an allyl aryl ether to 250 oC causes an intramolecular rearrangement to produce an o-allylphenol.
The Claisen rearrangement takes place through a concerted mechanism in which a C-C bond forms between the C3 of the allyl group and the ortho position of the benzene ring at the same time that the C-O bond of the ether breaks. This rearrangement initially produces the non-aromatic 6-allyl-2,4- cyclohexadienone intermediate which quickly undergoes a proton shift to reform the aromatic ring in the o-allylphenol product. Claisen rearrangement occurs in a six-membered, cyclic transition state involving the concerted movement of six bonding electrons in the first step. The presence of six electrons in a ring suggests that the transition state may have aromatic characteristics. The Claisen rearrangement is part of a broader class of reactions called sigmatropic rearrangements which will be discussed in more detail in Section 30-8.
Evidence for this mechanism was provided by performing the rearrangement with allyl group with a 14C label at C3. The product of this reaction was shown to have the 14C labeled carbon exclusively bonded to the ring.
Allyl vinyl ethers can also undergo a Claisen rearrangement when heated to form gamma, delta -unsaturated ketones or aldehydes.
Claisen rearrangements are rare in biological chemistry. One example is the chorismate mutase catalyzed Claisen rearrangement of chorismate (a allylic vinyl ether) to form prephenate. Prephenate is a precursor in the biosynthetic pathway of aromatic amino acids phenylalanine and tyrosine.
1) Show how you could synthesize allyl phenyl ether from allyl bromide and phenol.
2) What would be the expected product of the following Claisen rearrangement?