Base-catalyzed rearrangements of a-haloketones

Consideration of the energetics of the attack of a carbonium ion on an alkene reveals that such reactions are not likely to be efficient unless the intermediate which is formed, which is itself some type of carbonium ion, is more stable than the attacking reagent. In fact, most of the reactions which have attained synthetic importance involve alkenes which are capable of specific subsequent stabilization of the intermediate carbonium ion. 

One group of alkenes which react efficiently with electrophilic carbon species are the allylsilanes. The carbon-silicon bond is broken as the reaction proceeds, thus providing a low-energy product.Among the electrophilic carbon species 

Chan and I. Fleming, Synthesis 761 (1979) P. Magnus, Aldrichimica Acta 13, 43 (1980). 114a. A. Hosomi and H. Sakurai, Tetrahedron Lett., 1295 (1976). 

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The base-catalyzed rearrangement of a-haloketones (chloro and bromo) to carboxylic acid derivatives is called Favorskii rearrangement. 

Favorskii rearrangement. Base-catalyzed rearrangement of a-haloketones to acids or esters. The rearrangement of a,a -dibromocyclohexanones to 1 -hydroxycyclopentanecarboxylie acids, followed by oxidation to the ketones, is known as the Wallach degradation. 

Favorskii rearrangement is the base-catalyzed transformation of a-haloketones into the corresponding carboxylic acid or carboxylic acid derivative. Alexey Favorskii described the rearrangement that has his name more than a century ago. Since then, this reaction has been used in a variety of different manners, particularly in the synthesis of highly branched systems. Herein, examples regarding the ring contraction will be discussed. 

The Favorskii rearrangement is a base-catalyzed conversion of an a-haloketone into an acid or acid derivative such that the ketone a and a carbons come to share a bond, and the carbonyl carbon of the ketone becomes the carbonyl carbon of the carboxylic acid derivative tScheme 7T). The reaction can be regioselective, and its outcome is dependent not only on structural features of the starting material but on reaction conditions as well. 

Transformation of a-haloketones to esters via base-catalyzed rearrangement. 

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