Favorskii rearrangement

The Favorskii rearrangement (not to be confused with the Favorskii reaction), named for the Russian chemist Alexei Yevgrafovich Favorskii , is most principally a rearrangement of cyclopropanones and α-halo ketones which leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorski rearrangement constitutes a ring contraction. This rearrangement takes place in the presence of a base, sometimes hydroxide, to yield a carboxylic acid but most of the time 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 [Favorskii, A. E. "J. Russ. Phys. Chem. Soc." 1894, "26", 590.] [Favorskii, A. E. "J. Russ. Phys. Chem. Soc." 1905, "37", 643.] [Favorskii, A. E. "J. Prakt. Chem." 1913, "88", 658.] [Kende, A. S. "Org. React." 1960, "11", 261-316. (Review)] [Organic Syntheses, Coll. Vol. 6, p.368 (1988); Vol. 56, p.107 (1977). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0368 Article] )] [Shioiri, T.; Kawai, N. "J. Org. Chem." 1978, "43", 2936.] [Organic Syntheses, Coll. Vol. 7, p.135 (1990); Vol. 62, p.191 (1984). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv7p0135 Article] )] .

In the case of cylic α-halo ketones, the rearrangement occurs as depicted below [Organic Syntheses, Coll. Vol. 4, p.594 (1963); Vol. 39, p.37 (1959). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv4p0594 Article] )] :

Reaction mechanism

The reaction mechanism is thought to involve the formation of an enolate on the side of the ketone away from the chlorine atom. This enolate cyclizes to a cyclopropanone intermediate which is then attacked by the hydroxide nucleophile.

Usage of alkoxide anions such as sodium methoxide, instead of sodium hydroxide, yields the ring-contracted ester product.

Wallach degradation

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 ["Zur Kenntnis der Terpene und der ätherischen Öle. Über das Verhalten zweifach gebromter hexacyclischer Ketone in Abhängigkeit von der Stellung der Bromatome" Justus Liebig's Annalen der Chemie Volume 414, Issue 3 , Pages 271 - 296 O. Wallach 1918 DOI|10.1002/jlac.19184140303] ["Zur Kenntnis der Terpene und der ätherischen Öle" (p 296-366) O. Wallach Justus Liebig's Annalen der ChemieVolume 414, Issue 3 , Pages 296 - 366 1918 DOI|10.1002/jlac.19184140304]

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 so-called "p-hydroxyphenacyl" groups ["New Photoactivated Protecting Groups. 6. p-Hydroxyphenacyl: A Phototrigger for Chemical and Biochemical Probes" Chan-Ho Park and Richard S. Givens J. Am. Chem. Soc.; 1997; 119(10) pp 2453 - 2463; (Article) DOI|10.1021/ja9635589] . The deprotection proceeds through a triplet diradical (3) and a dione spiro intermediate (4) although the latter has thus far eluded detection ["The Photo-Favorskii Reaction of p-Hydroxyphenacyl Compounds Is Initiated by Water-Assisted, Adiabatic Extrusion of a Triplet Biradical" Richard S. Givens, Dominik Heger, Bruno Hellrung, Yavor Kamdzhilov, Marek Mac, Peter G. Conrad, II, Elizabeth Cope, Jong I. Lee, Julio F. Mata-Segreda, Richard L. Schowen and Jakob Wirz J. AM. CHEM. SOC. 2008, 130, 3307-3309 DOI|10.1021/ja7109579] .

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ee also

* A classic cubane synthesis contains two Favorskii rearrangements.

References