A1,2-Wittig rearrangement is a categorization ofchemical reactions inorganic chemistry, and consists of a1,2-rearrangement of anether with analkyllithium compound.[1] The reaction is named forNobel Prize winning chemistGeorg Wittig.[2][3]
The intermediate is analkoxy lithium salt, and the final product analcohol. When R" is a goodleaving group andelectron withdrawing group such as acyanide (CN) group,[4] this group is eliminated and the correspondingketone is formed.
Thereaction 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.[5]
The alkyl group migrates in the order of thermodynamical stabilitymethyl < primary alkyl < secondary alkyl < tertiary alkyl, this is in line with the radical mechanism. The radical-ketyl pair is short lived and due to asolvent cage effect some isomerizations take place withretention of configuration.
With certain allyl aryl ethers a competing reaction takes place.[5] The reaction ofallyl phenyl ether1 withsec-butyllithium at −78 °C gives the lithiated intermediate2 which on heating to −25 °C only shows the rearranged product5 but not4 after trapping the lithium alkoxide withtrimethylsilyl chloride. This result rules out a radical-ketyl intermediate3a in favor of theMeisenheimer complex3b. 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 formaldyotropic reaction.