Carbanions 2.3 -Wittig rearrangement

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

Nevertheless, another possibility remained for the formation of insertion products, that they might be formed from the O-H insertion product, e.g., dichloromethyl benzyl ether 5, by the Wittig rearrangement of dichloromethoxy-carbanion 4, (Scheme 5, Eq. I).17 However, treatment of independently prepared benzyl dichloromethyl ether 5 with the same base solely gave benzyl chloride, but the insertion product was not obtained (Eq. 2). Hence, a Wittig-type rearrangement process was excluded. 

An interesting problem of the periselectivity arises in the rearrangement involving an a-oxyallylic carbanion as the terminus. In this particular case, the [l,4]-shift may compete with the [1,2]-Wittig rearrangement (see Section n.C). For example, the rearrangement of 5 affords a mixture of the [l,4]-product 6 and the [l,2]-product 7 (equation 5) °. 

There are two aspects to the stereospecificity of the [1,2]-Wittig rearrangement the steric course (inversion vs. retention) at the migrating carbon atom and at the Li-bearing carbanion center (equation 6). 

The pioneering work on thia-[2,3]-Wittig rearrangement was reported by Rautenstrauch in 1971 °. The reaction of allyl sulfides 137 with n-BuLi at —30°C for 1.5-4 h gave homoallyl sulfide or thiol 138 ([2,3]-product) exclusively (equation 81). In contrast, a similar reaction of vinyl sulfide 139 gave no [1,2]-rearrangement product 141 albeit a-thio-carbanion was generated as shown by its methylation to 140 (equation 82) °. 

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

Wittig Rearrangement. There is a continuing controversy over the role of carbanion intermediates in this process. The reaction involves the formation of a carbanion at the ot-carbon of an ether leading to a rearrangement that produces an alkoxide (Eq. 19) ... 

With allylic alcohols, there is the possibility of a [2,3] variant of the Wittig rearrangement that can compete with the [1,2] rearrangement described above (Eq. 22, the indicated electron flow is for the [2,3] rearrangement). The reaction is expected to be a one-step, pericyclic process without a distinct carbanion intermediate. This rearrangement has proven to be useful synthetically because its concerted nature can lead to high stereoselectivity. ... 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Review articles have addressed advances in photochemical generation and reactions of carbanions,165 the [l,2]-WMg rearrangement stereochemistry and synthetic application,167 and the aza-Wittig rearrangement.168... 

The pattern of results suggest that, if the substituent at the acyl group is alkyl, and hence relatively carbanion-destabilizing, rearrangement is inhibited relative to the alternative normal Wittig reaction pathway when the substituent is aromatic, however, and therefore capable of stabilizing incipient carbanion formation as the silicon-carbon bond... 

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 ... 

Kunishima, M. Hioki, K Kono, K Kato, A. Tani, S. Sml2-Induced 2,3-Wittig rearrangement regioselective generation of a-allyloxy carbanions via 1,5-hydrogen transfer of vinyl radicals./. Org. Chem. 1997, 62, 7542-7543. 

Hoffmann, R. Ruckert, T. Bruckner, R. [1,2]-Wittig rearrangement of a lifhioalkyl benzyl ether with inversion of configuration at the carbanion C atom. Diastereoselective reductions of cydohexyl radicals with Li arerie. Tetrahedron Lett. 1993, 34, 297-300. 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

Chiral lithium bases have been used for enantioselective deprotonation to yield configurationally stable a-oxy carbanions. This holds potential for asymmetric [2,3]-Wittig rearrangement in stereoselective synthesis. Thus, treatment of propargylic ether 72 with (S,S)-3 in THF at — 70 °C to —15 °C afforded propargylic alcohol 73 in 82% yield and in 69% ee of the shown enantiomer96,97. This product was successfully employed as a precursor of (-l-)-Aristolactone (Scheme 55). 

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