:1,2-Wittig rearrangement
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A 1,2-Wittig rearrangement is a categorization of chemical reactions in organic chemistry, and consists of a 1,2-rearrangement of an ether with an alkyllithium compound.{{cite book |doi=10.1002/0470084960|title=March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure |year=2006 |last1=Smith |first1=Michael B. |last2=March |first2=Jerry |isbn=9780470084960|pages=1624–1625 }} The reaction is named for Nobel Prize winning chemist Georg Wittig.Georg Wittig, L. Löhmann, Ann. 550, 260 (1942)G. Wittig, Experientia 14, 389 (1958)
File:1,2_WITTIG_REARRANGMENT.svg
The intermediate is an alkoxy lithium salt, and the final product an alcohol. When R" is a good leaving group and electron withdrawing group such as a cyanide (CN) group,Preparation of aryl benzyl ketones by [1,2]-Wittig rearrangement Alan R. Katritzky, Yuming Zhang, Sandeep K. Singh Arkivoc pp. 146–50 2002 (vii) [http://www.arkat-usa.org/ARKIVOC/JOURNAL_CONTENT/manuscripts/2002/SK-533EP%20as%20published%20mainmanuscript.pdf link] {{Webarchive|url=https://web.archive.org/web/20060928190900/http://www.arkat-usa.org/ARKIVOC/JOURNAL_CONTENT/manuscripts/2002/SK-533EP%20as%20published%20mainmanuscript.pdf |date=28 September 2006 }} this group is eliminated and the corresponding ketone is formed.
Reaction mechanism
The reaction mechanism centers on the formation of a free radical pair with lithium migrating from the carbon atom to the oxygen atom. The R radical then recombines with the ketyl.Wittig Rearrangement of Lithiated Allyl Aryl Ethers: A Mechanistic Study Sven Strunk, Manfred Schlosser European Journal of Organic Chemistry Volume 2006, Issue 19, pp. 4393–97 {{doi|10.1002/ejoc.200600304}}
Image:WittigRearrangementMechanism.png
The alkyl group migrates in the order of thermodynamical stability methyl < primary alkyl < secondary alkyl < tertiary alkyl, this is in line with the radical mechanism. The radical-ketyl pair is short lived and due to a solvent cage effect some isomerizations take place with retention of configuration.
With certain allyl aryl ethers a competing reaction takes place. The reaction of allyl phenyl ether 1 with sec-butyllithium at −78 °C gives the lithiated intermediate 2 which on heating to −25 °C only shows the rearranged product 5 but not 4 after trapping the lithium alkoxide with trimethylsilyl chloride. This result rules out a radical-ketyl intermediate 3a in favor of the Meisenheimer complex 3b. Additional evidence for this mechanism is provided by the finding that with a para tert-butyl substituent the reaction is retarded.
Image:WittigRearrangementAltMech.png
The reaction is a formal dyotropic reaction.
See also
References
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{{Organic reactions}}
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