Deprotonation Favorskii rearrangement

The Favorskii [213] and Ramberg-Backlund [214] rearrangements are mechanistically akin to the above. The first step is deprotonation (although in highly strained systems the Favorskii rearrangement proceeds by donor addition to the carbonyl group) which is followed by internal displacement of the a-halogen. 

While the RBR does not encompass a true rearrangement of the carbon skeleton, it has been likened, mechanistically, to the Favorskii rearrangement, which does. Indeed, both involve a rate-limiting intramolecular ring-forming step that follows an acid-base equilibrium. The Favorskii intermediate is a cyclopropanone that is formed by a-deprotonation of an a-haloketone, analogous to the episulfone invoked in the RBR mechanism. 

Ketones are readily deprotonated at the a-position and will undergo a Favorskii rearrangement if the other side of the ketone is suitably activated. 

The canonical formulation of the mechanism of the Favorskii rearrangement involves initial deprotonation of the a-carbon to generate an enolate, intramolecular displacement of the leaving group on the a -carbon by the enolate to generate a cyclopropanone, addition of a nucleophile to the cyclopropanone ketone followed by elimination to generate the more stable of two possible carbanions, and protonation to yield the rearranged carboxylic acid derivative. 

---