Abnormal Claisen rearrangement

What is abnormal Claisen rearrangement?

The first recorded occurrence of abnormal Claisen rearrangements was reported by Lauer and Filbert in 1936. Unlike the regular Claisen rearrangement, which involves a [3,3] migration, the abnormal Claisen rearrangement predominantly occurs in allyl aromatic ethers. The thermal rearrangement of cyclopropyl ketones to homoallylic ketones also undergoes a similar reaction. The abnormal Claisen rearrangement is thought to occur in two sequential steps, starting with the standard ortho Claisen rearrangement of γ-alkylallyl aryl ether to form an o-(α-alkylallyl) phenol, followed by the isomerization of the resulting phenol.

Abnormal Claisen rearrangement
Abnormal Claisen rearrangement

Typically, this form of abnormal Claisen rearrangement occurs with difficulty, except in the presence of Lewis acids like FeCl3, although other Lewis acids such as HfCl4, GaCl3, and ZrCl4 have limited potential to expedite the reaction. It has been reported that 1,1,1,3,3,3-hexamethyldisilazane and N,O-bis(trimethylsilyl)acetamide can prevent the abnormal Claisen rearrangement.

References

The Rearrangement of Phenyl Allyl Ethers1
Walter M. Lauer and William F. Filbert
Journal of the American Chemical Society 1936 58 (8), 1388-1392
DOI: 10.1021/ja01299a024

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