1,2-Wittig rearrangement

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A 1,2-Wittig rearrangement is an organic reaction and a 1,2-rearrangement of an ether with an alkyllithium compound [1] [2].

The 1,2-Wittig rearrangement

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, [3] this group is eliminated and the corresponding ketone is formed.

The 1,2-Wittig rearrangement

[edit] 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 [4] .

The 1,2-Wittig rearrangement reaction mechanism

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 [4]. 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.


1,2-Wittig rearrangement competing mechanism

[edit] See also

[edit] References

  1. ^ G. Wittig, L. Löhmann, Ann. 550, 260 (1942)
  2. ^ G. Wittig, Experientia 14, 389 (1958).
  3. ^ Preparation of aryl benzyl ketones by [1,2]-Wittig rearrangement Alan R. Katritzky, Yuming Zhang, Sandeep K. Singh Arkivoc p. 146-150 2002 (vii) link
  4. ^ a b 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
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