Favorskii

The Favorskii rearrangement is a class of organic reactions in which α-halo carbonyl compounds subjected to basic conditions undergo a rearrangement that displaces the halogen and reorganizes the carbon skeleton. The most classical case involves α-halo ketones, but α-halo esters and related derivatives can also participate. The outcome is typically a carboxylate or carboxylic acid derivative in which a migration of a substituent accompanies cyclization and ring contraction or rearrangement.

Mechanistically, the reaction is thought to proceed by deprotonation at the α-position to form an enolate, followed by intramolecular attack on the carbonyl and departure of the halide. This sequence generates a highly strained cyclopropanone-type intermediate that opens and rearranges to a product bearing a migrated carbonyl fragment. The rearranged product can be trapped by nucleophiles or hydrolyzed on workup to give the final carboxylic acid or its derivatives. The exact path can depend on the substrate and the nucleophile present.

Substrates include α-halo ketones and, in related variants, α-halo esters. The reaction often leads to ring-contracted or otherwise reorganized products, particularly with cyclic substrates, and the halogen is effectively displaced by the nucleophile present under basic conditions. Common reagents include bases such as hydroxide or alkoxide, but other nucleophiles can participate, yielding a range of subsequent products after aqueous workup.

History and naming: the reaction is named after the Russian chemist Yevgeny S. Favorskii, who described the transformation in the early 20th century. Since then, the mechanism and scope have been extended, and Favorskii-type rearrangements are recognized as a useful tool for constructing reorganized carboxylate-containing products in organic synthesis.