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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. The reaction is named for Nobel Prize winning chemist Georg Wittig.

R C | R H | O R + R Li     R C | R R | O Li + R H   H +   R C | R R | O H

The intermediate product is an alkoxy lithium salt and the final product an alcohol. When R" is a good leaving group and electron withdrawing functional group such as a cyanide (CN) group, 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.

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.

The reaction is a formal dyotropic reaction.

References

1,2-Wittig rearrangement Wikipedia


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