- 1,2-Wittig rearrangement
A 1,2-Wittig rearrangement is a categorization of chemical reactions in
Organic chemistry , and consists of a1,2-rearrangement of anether with an alkyllithium compound [Georg Wittig , L. Löhmann, Ann. 550, 260 (1942)] [G. Wittig, Experientia 14, 389 (1958).] . The reaction is named forNobel Prize winning chemistGeorg Wittig .The intermediate product is an
alkoxy lithium salt and the final product analcohol . When R2 is a goodleaving group and electron withdrawingfunctional group such as acyanide (CN) group, ["Preparation of aryl benzyl ketones by [1,2] -Wittig rearrangement" Alan R. Katritzky, Yuming Zhang, Sandeep K. SinghArkivoc 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 correspondingketone is formed.Reaction mechanism
The
reaction mechanism centers on the formation of afree radical pair with lithium migrating from the carbon atom to the oxygen atom. The R radical then recombines with theketyl "Wittig Rearrangement of Lithiated Allyl Aryl Ethers: A Mechanistic Study" Sven Strunk, Manfred SchlosserEuropean 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 withretention 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 theMeisenheimer 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
* TheSmiles rearrangement References
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