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.^[1][2] The reaction is named for Nobel Prize winning chemist Georg Wittig.

${\displaystyle {\mathsf {R'{-}\!\!\!\!{\overset {\displaystyle H \atop \vert }{\underset {\vert \atop \displaystyle \ \ R''}{C}}}\!\!\!\!{-}O{-}R+{\color {Gray}{R'''{-}Li}}\ \longrightarrow \ R'{-}\!\!\!\!{\overset {\displaystyle R \atop \vert }{\underset {\vert \atop \displaystyle \ \ R''}{C}}}\!\!\!\!{-}O{-}Li+{\color {Gray}{R'''{-}H}}\ \xrightarrow {\displaystyle H^{+}} \ R'{-}\!\!\!\!{\overset {\displaystyle R \atop \vert }{\underset {\vert \atop \displaystyle \ \ R''}{C}}}\!\!\!\!{-}O{-}H}}}$

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

${\displaystyle {\mathsf {R'{-}\!\!\!\!{\overset {\displaystyle H \atop \vert }{\underset {\vert \atop \displaystyle \ \ CN}{C}}}\!\!\!\!{-}O{-}R+{\color {Gray}{R'''{-}Li}}\ \longrightarrow \ {\overset {\displaystyle O \atop \vert \vert }{\underset {\diagup \ \diagdown \atop \displaystyle R'\ \ \ R}{C}}}+{\color {Gray}{R'''{-}H}}+LiCN}}}$

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 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.^[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.

The reaction is a formal dyotropic reaction.

See also[]

• The
• The 2,3-Wittig rearrangement
• The Smiles rearrangement

References[]