Sulfonium ylides, allylic rearrangements

Concerted [2,3]-sigmatropic rearrangements of allyl and benzyl ammonium ylides, allyl sulfonium ylides, allyl sulfoxides, selenoxides, amine oxides and anions of allyl ethers have been reported. These are illustrated by die following general examples ... 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

A more direct access to the unstable and non isolated sulfonium ylides 58a- c is the reaction of diisopropyl diazomethylphosphonate 57 with allylic sulfides, catalyzed by Cu(II), Rh(II) [39], or ruthenium porphyrins.[40] For example, the a-phosphorylated y,d-unsaturated sulfides 59-61 are obtained through the [2,3] -sigmatropic rearrangement of 58a-c. This method allows the use of a greater variety of starting allylic sulfide substrates, such as 2-vinyl tetrahydrothiophene, or propargylic sulfides (Scheme 15). 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Allylic sulfonium ylides readily undergo [2,3]-sigmatropic rearrangement.280 The ylides are usually formed by deprotonation of the S-allyl sulfonium salts. 

Iodonium ylides (136), generated in situ with bisacetoxyiodobenzene, are converted to allyl- or benzyl-substituted oxonium or sulfonium ylides (137) via rhodium- or copper-catalysed carbene transfer.115 Such ylides undergo [1,2]- or [2,3]-rearrangement to the corresponding 2-substituted heterocycles (138). An example of the rhodium-catalysed reaction is reported in Scheme 36. 

Andrews, G., Evans, D. A. Stereochemistry of the rearrangement of allylic sulfonium ylides. New method for the stereoselective formation of asymmetry at quaternary carbon. Tetrahedron Lett. 1972, 5121-5124. 

Allyl sulfides do not form cyclopropanes when they react with dichlorocarbene. Instead, the sulfonium ylide undergoes a [2,3] sigmatropic rearrangement to give a 1,1-dichloro derivative, which affords 1-alkylsulfanyl-l -chlorobuta-1,3-diene 4 after elimination of hydrogen chlor-ide.2 ... 

A modification called tandem [2,3] sigmatropic rearrangement of sulfonium ylide—bromine allylic rearrangement has been reported (88JOC5149). Thus, reaction of the C5 brominated 2-pyrone 224 with ethyl diazoacetate under rhodium catalysis results not only in transfer of the ester moiety to C5, as described earlier, but also in the transfer of the bromine atom from C5 to the side chain at C6 in such a way that the functional group remaining at that side chain, as in 225, can be further elaborated (89JHC1205). 

Sulfonium ylides arc the most widely studied of the allylic dipolar species which undergo [2,3] sigmatropic rearrangement1188. Both stabilized and unstabilized sulfur ylides undergo this rearrangement, although rearrangements of the latter substrates are often complicated by... 

Alkylative rearrangement Sulfonium ylide formation from allylic sulfides and diazo compounds (particularly trimethylsilyldiazomethane) and subsequent [2.3]sigma-tropic rearrangement are induced by (dppe)FeCl2 in 1,2-dichloroethane. 

Homologation of allyl sulfides. Sulfonium ylides formed at low temperature are liable to undergo a [2,3]sigmatropic rearrangement on treatment with a mild base (e.g., DBU). 

Ylide formation can be used to good effect for a variety of transformations. With an allylic sulfide, the resulting sulfonium ylide undergoes [2,3]-sigmatropic rearrangement (4.106). This type of rearrangement (see Section 3.7) can be effected with allylic amines, ethers and even allylic halides. 

Synthetically valuable [2,3]-sigmatropic rearrangements include those of allyl sulfonium and ammonium ylides and a -carbanions of allyl vinyl ethers. 

At first, the asymmetric [2,3]sigmatropic rearrangement of allylic sulfur ylides is surveyed. The first successful asymmetric [2,3]sigmatropic rearrangement involving sulfur ylides by Trost and Hammen appeared in 1973 [44]. The treatment of optically active sulfonium salt afforded the corresponding homo-allylic sulfide with 94% enantiomeric excess, in which the chiral information has been transferred from the sulfur atom to the carbon atom with excellent control (Scheme 28). Trost and Biddlecom also reported another approach to generate... 

In a seminal paper published in 1973, Trost reported on the preparation of highly enantioenriched sulfonium tetrafluoroborates by resolution of the corresponding dibenzoylhydrogen tartrates (Scheme 94) [220]. When the resolved allyl sulfonium salt 382 was treated with base the rearrangement product 383 was obtained and its absolute configuration was determined by chemical correlation. Furthermore, it was deduced that 383 had 94% ee, which denotes that the rearrangement should be fast on the timescale of ylide inversion and indicates at least 94% asymmetric induction. 

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