Wittig rearrangement mechanism

Asymmetric induction of the same kind controls the configuration at the allylic methine carbon in compound (134) (134) results from a chain-elongating Wittig rearrangement (mechanism Scheme 6) of sulfone (133 equation 35). Surprisingly, the sense of the asymmetric induction in the rearrangement of silyl ether (135 equation 36) is opposite to that of more than a dozen other ( )-ethers. ... 

Several mechanisms had been proposed for the [1,2]-Wittig rearrangement, including (i) a concerted process taking place through a pentacoordinate transition state, (ii) a... 

It is generally accepted that the aza-[l,2]-Wittig rearrangement proceeds via a radical dissociation-recombination mechanism, whereas the aza-[2,3]-rearrangement proceeds... 

The discoveiy that a mixture of (Z)-(94a) and (Z)-(94b), on treatment with LDA, forms only (Z)-(95a) and (Z)-(95b) is consistent with an intramolecular mechanism (e.g. Scheme 6) for this new imino-Wittig rearrangement of hydroximates alternative radical pathways have also been discussed for reactions of Z- and. E-isomers.169... 

The lithium enolate formed from methyl S-trityl mercaptoacetate (8) has been C-alkylated in high yield at or below —40 °C (Scheme 4).37 At higher temperatures, the [ 1,2]-thio-Wittig rearrangement of the enolate was the predominant process. ESR evidence has indicated that the rearrangement occurred by a radical mechanism. 

This stability order has been used in a reevaluation of the mechanism of the Wittig rearrangement of alkyl benzyl ethers (Eq. (82)). The migration... 

The [1,2]-Wittig Rearrangement is a carbanion rearrangement that proceeds via a radical dissociation-recombination mechanism. The lithiated intermediate forms a ketyl radical and a carbon radical, which give an alcoholate after fast recombination within the solvent cage ... 

Antoniotti, P. Tonachini, G. Mechanism of the anionic Wittig rearrangement. An ab initio theoretical study. /. Org. Chem. 1998, 63, 9756— 9762. 

Tomooka, K. Inoue, T. Nakai, T. Stereochemistry and mechanism of vinyl-migrating [1,2]-Wittig rearrangement of a-lithioalkyl vinyl ethers. Chem. Lett. 2000, 418-419. 

Wittig rearrangements proceed with predominant inversion, with radicals intervening in the mechanism.27 For example, stannane (/ )-30 of 88% ee rearranges, on transmetallation with alkyllithiums, to the alcohol (/ )-31 of 42% ee, a reaction demonstrating 74% invertive stereospecificity. 

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