Sulfonium ylides 2,3- sigmatropic rearrangement

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. 

Scheme 6.18. Carbon-Carbon Bond Formation via [2,3]-Sigmatropic Rearrangements of Sulfonium and Ammonium Ylides...

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

It is interesting to note that asymmetric induction was also observed (308) during generation of ylide 288 from achiral sulfonium salt 287a by means of chiral lithium 2,2,2-trifluoromethyl-a-phenylethoxide. The [2,3]sigmatropic rearrangement of the chiral ylide 288 obtained in situ in this way leads to optically active sulfide 289 of 5% optical purity. 

The major reaction pathways for sulfonium ylide formation generated from a metal carbene complex and sulfide are [2,3]-sigmatropic rearrangement and [l,2]-shift, similar to those of the oxonium ylide formation. 

Recent study on sulfonium ylide [2,3]-sigmatropic rearrangement has been focused on the development of new catalytic systems, including new catalysts and alternative carbene precursor other than commonly used a-diazocarbonyl compounds. Besides the most commonly used Cu(i) and Rh(ii) catalysts, Fe com-... 

Further study by Katsuki, McMillen, Hashimoto, and Wang improved the enantioselectivity up to a moderately high level.Wang and co-workers further extended the asymmetric catalysis to the [2,3]-sigmatropic rearrangement of propargyl sulfonium ylide to give allenic products with up to 81% ee (Equation (18))." ... 

We have mentioned previously as a stable ylide dimethyl sulfonium fluorenide (see Section 2.9). It yielded the first example (1938) of a [2,3] sigmatropic rearrangement of a sulfur ylide [505, 506] (Sommelet-Hauser rearrangement). 

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]. 

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... 

As a C - C bond-forming process, [2,3]-sigmatropic rearrangements of sulfonium ylides merit particular interest. Indeed, since their discovery in the late 1960s [172-176], this type of yhde rearrangement has become the most extensively studied and applied. This activity is well dociunented in a niunber of excellent review articles, many of them published quite recently [9,177-179]. This demonstrates a vivid interest by the scientific community in these rearrangements which is strongly related to the advent of asymmetric and catalytic variants, as will be discussed later. 

Sigmatropic Rearrangements of Propargylic Sulfonium Ylides and Related Systems... 

Scheme 1 Butadienyl sulfides via [3,2]sigmatropic rearrangement of sulfonium ylides...

The [2,3]sigmatropic rearrangement of sulfonium ylide (24) derived from rhodium-catalyzed decomposition of ethyl 3,3,3-trifluoro-2-diazopropanoate (23) in the presence of propargyl sulfide (22a) affords the corresponding functionalized CF3-containing carboxylic ester sulfide (25a). The latter was... 

Scheme 4 [2,3]Sigmatropic rearrangement of acetylenic and allenic sulfonium ylides via copper salt catalyzed thermal decomposition of diazomalonates...

Schemes [2,3]Sigmatropic rearrangement of sulfonium ylides via rhodium-catalyzed decomposition of ethyl 3,3,3-trifluoro-2-diazopropionate...

Scheme 6 Iron-catalyzed carbene generation/[2,3]sigmatropic rearrangement of propargyl sulfonium ylides...

Scheme 7 [2,3]Sigmatropic rearrangement of propargyl sulfonium ylides as a ring forming reaction...

Allenic sulfides (37) have been obtained with moderate to good enantio-selectivities (up to 81%) in sigmatropic rearrangements of sulfonium ylides generated from propargyl aryl sulfide (35) and aryldiazoacetates (36) as carbenoid source catalyzed by chiral Rh(ll) (38a,b) and Cu(l) (38c) catalysts [34] (Scheme 8). 

A similar approach has been applied to the [2,3]sigmatropic rearrangements of sulfonium ylides derived from allenic sulfides (39) [35]. Rh(II) catalysts 38a,b are found to be more effective than Cu (I) catalysts 38c. Ho-mopropargyhc sulfides (40) were obtained in moderate to good yields and moderate enantiomeric excess in optimized conditions in n-hexane at 0 °C (Scheme 8). 

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