Radical mechanisms 1,2 -Wittig rearrangement

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. 

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 ... 

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. 

An interesting example of chemistry involving a radical dissociation-recombination mechanism is the 1,2-Wittig rearrangement [17]. The general scheme for the Wittig rearrangement is shown in Eq. (12). 

Concerted and multistep radical mechanisms have been proposed (68, 69). Lithium complexation appears to be involved in the aza-Wittig rearrangement (70). 

This type of carbanion rearrangement is recognised to proceed by means of a radical dissociation-recombination mechanism. In 1983, Mazaleyrat and Wel-vart reported the kinetic resolution of a racemic binaphthyl ether, which involves an enantioselective variant of the [1,2]-Wittig rearrangement [Eq.(2)j [3]. 

The [1,2]-Wittig rearrangement is believed to proceed via a radical mechanism ... 

The [1,2]-Wittig rearrangement is believed to proceed via a radical mechanism,which is illustrated in the example shown below. The reaction of 11 with MeLi provides organolithium intermediate 12, which imdergoes C-0 bond homolysis to form radicals 13 and 14. Radical 13 is converted to 15 via 1,2-lithium migration, and a subsequent intermolecular radical coupling of 15 with 14 yields alkoxide 16. The alcohol product 17 is obtained after an aqueous workup. The homolysis/recombination events are believed to occur inside a solvent cage. 

Knowledge of the cyclization rate of the 5-hexenyl radical has also been used to study cage reactions of alkyl radicals generated in solution from diacyl peroxides or to distinguish between intra- and intermolecular pathways in the Wittig rearrangement. It has also been used to study the mechanism of the deshalogenation of 1,4-, 1,5-, and 2,3-dihaloalkanes by alkali... 

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