Reaction mechanisms Wittig rearrangement

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... 

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. 

Garst, J. F., Smith, C. D. Mechanisms of Wittig rearrangements and ketyl-alkyl iodide reactions. J. Am. Chem. Soc. 1973, 95, 6870-6871. 

Hauser noted that diallyl ether (8) also undergoes Wittig rearrangement upon base treatment and suggested that product formation could involve either a 1,2-shift or a cyclic mechanism (equation 3). Later studies by Schdllkopf and Makisumi with substituted allylic ethers (10,11 and 14-16 equations 4 and 5) pointed to a cyclic (SnO mechanism a process allowed by the Woodward-Hoffmann rules. The diastereoselectivity of the reaction was not determined in these cases, but Schollkopf subsequently found that benzyl rrans-crotyl ether (20 equation 6) affords mainly the anti products upon rearrangement of ether (20) with BuLi in THF. Rautenstrauch observed a 1 1 mixture of syn and anti products upon rearrangement of ether (20) in the presence of TMEDA, whereas the cis isomer (23) gave only the syn product (22 equation 7). ... 

Problem 4.18. Examples of the Stevens rearrangement and the nonallylic Wittig rearrangement are shown. What do the Woodward-Hoffmann rules say about the nature of these reactions Offer two explanations (not necessarily mechanisms) of why these reactions proceed. (Hint Think of the conditions on the applicability of the Woodward-Hoffmann rules.)... 

Scheme 17.2 Reaction mechanism of the r2.31-Wittig rearrangement on a Sp-Qx/Q/ propargyloxy-hexahydroindene ring system. The Newman-like projections in the dashed circles indicate transition state bond formation.

The mechanism of the Stevens rearrangement has been reviewed. The key reaction in the total synthesis of ( )- and (+)-eupomatilone was an asymmetric [2,3]-Wittig rearrangement employing a bis(oxazoline) chiral ligand (Scheme 101). ... 

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. 

The mechanism of the [2,3]-Wittig rearrangement is a concerted, thermally allowed sigmatropic reaction which follows the Woodward-Hoffrnan rules... 

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