1,2-Wittig rearrangement

1,2-Wittig rearrangement

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 [Georg Wittig, L. Löhmann, Ann. 550, 260 (1942)] [G. Wittig, Experientia 14, 389 (1958).] . The reaction is named for Nobel Prize winning chemist Georg Wittig.

The intermediate product is an alkoxy lithium salt and the final product an alcohol. When R2 is a good leaving group and electron withdrawing functional 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 p. 146-150 2002 (vii) [http://www.arkat-usa.org/ARKIVOC/JOURNAL_CONTENT/manuscripts/2002/SK-533EP%20as%20published%20mainmanuscript.pdf link] ] 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 , Pages 4393 - 4397 DOI|10.1002/ejoc.200600304] .

The alkyl group migrates in the order of thermodynamical stability methyl < primary alkyl < secondary alkyl < tertiary alkyl in this is 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 mechanism 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.

ee also

* The 1,3-Wittig rearrangement
* The Smiles rearrangement

References


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