Sulfonium ylides, allylic

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 ... 

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

The Stockman group has also studied reactions between the same imines and allyl sulfonium ylides (first reported by Hou and Dai [45]) [46], N-Sulfinyl vinylazir-... 

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. 

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

Corey s sulfonium ylide methodology32 was then adopted. As shown in Scheme 7-51, conversion of ketone 161 to spiroepoxide 165 proceeded with high yield, and a Lewis acid-induced epoxide opening gave the desired allylic alcohol 164 with perfect yield. 

In addition to 1,2-alkyl shifts, sulfonium ylides with P-hydrogen can also undergo fragmentation into an olefin and a thioether [1317,1318]. When allylic thioethers are treated with two equivalents of ethyl diazoacetate in the presence of a catalyst for diazodecomposition, S-alkylation and elimination of the thioalkyl group from the initially formed a-alkylthio-4-alkenoic esters occurs to yield 2,4-dienoic esters [1319]. 

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. 

S lyl- sulfomidines 10 S lyl- suffinamkles 11 S lyl- imidosulfinates 12 S lyl-sulfinafos 13 Allyl- sulfoxides 14 sulfimines 1 Sulfonium ylides 16... 

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). 

Alkylation of sulfides. In a study of the possible role of sulfonium ylides in the coupling of allyl units to give squalene and related terpenes, Baldwin el al. examined the alkylation of the sulfide (1). The only product was the symmetrical salt (2). 

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). 

Cycloadditions. 2-Vinylaziridines can be prepared directly from allyl-dimethylsulfonium bromide and A -sulfonylimines. The sulfonium ylides are generated in situ. 

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. 

---