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The Favorskii rearrangement is principally a rearrangement of cyclopropanones and α-halo ketones that leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorskii rearrangement constitutes a ring contraction. This rearrangement takes place in the presence of a base, sometimes hydroxide, to yield a carboxylic acid, but usually either an alkoxide base or an amine to yield an ester or an amide, respectively. α,α'-Dihaloketones eliminate HX under the reaction conditions to give α,β-unsaturated carbonyl compounds.[1][2][3][4][5] Note that trihalomethyl ketone substrates will result in haloform and carboxylate formation via the haloform reaction instead.
The second step has also been proposed to be stepwise process, with chloride anion leaving first to produce a zwitterionic oxyallyl cation before a disrotatory electrocyclic ring closure takes place to afford the cyclopropanone intermediate.[10]
Usage of alkoxide anions such as sodium methoxide, instead of sodium hydroxide, yields the ring-contracted ester product.
When enolate formation is impossible, the Favorskii rearrangement takes place by an alternate mechanism, in which addition to hydroxide to the ketone takes place, followed by concerted collapse of the tetrahedral intermediate and migration of the neighboring carbon with displacement of the halide. This is sometimes known as the pseudo-Favorskii rearrangement or quazi-Favorskii rearrangement, although previous to labeling studies, it was thought that all Favorskii rearrangements proceeded through this mechanism.
In the related Wallach degradation (Otto Wallach, 1918) not one but two halogen atoms flank the ketone resulting in a new contracted ketone after oxidation and decarboxylation[11][12]
Photo-Favorskii reaction
The reaction type also exists as a photochemical reaction. The photo-Favorskii reaction has been used in the photochemical unlocking of certain phosphates (for instance those of ATP) protected by p-hydroxyphenacyl groups.[13] The deprotection proceeds through a triplet diradical (3) and a dionespiro intermediate (4) although the latter has thus far eluded detection.[14]
