Sulfonium salts sulfur ylides from

Sulfonium, cyclopropyldiphenyl tetrafluoroborate, 54, 28 Sulfonium salts, acetylenic, furans from, 53, 3 Sulfonium ylides, 54, 32 Sulfur, reaction with organo-lithium compounds, 50, 105 Sulfuryl chloride, with 1,1-cyclobutanedicarboxylic acid to give 3-chloro-l,1-cyclobutanedicarboxylic acid, 51, 73... 

Chiral induction can also be quite effective when the locus of asymmetry is attached to the sulfur ylide itself The sulfonium salt 637, derived from Eliel s oxathiane, can be used to deliver a benzylic center to tosylimines (e.g., 638) and efficiently produces phenylaziridines with a very high degree of asymmetric induction. The method is amenable to gram-quantity synthesis, and the chiral auxiliary can be easily recovered. In general, cisitrans-mxxtmcs are obtained, depending upon the steric bulk of the imine substituent (Scheme 156) <2004JOC1409>. 

Sulfur ylides contain a carbanion, which is stabilizea oy an adjacent positively-charged sulfur. Ylides derived from alkylsulfonium salts are usually generated and utilized at low temperatures. Oxosulfonium ylides are, however, stable near room temperature. The most common method of ylide formation is deprotonation of a sulfonium salt. What has been said... 

The sulfur ylide is formed from the sulfonium salt,... 

Apart from their fundamental role in sulfur ylide chemistry, sulfonium salts have found applications as soft electrophiles. In alkylation of ambident nucleophiles such as the enolates of [3-dicarbonyl compounds they led to selective C-alkylation [205],... 

Finally, Sato and co-workers have added a twist to the known preparation of epoxides from the action of sulfur ylides on carbonyl compounds. In this version, the requisite sulfur ylides are formed by desilylation of [(trimethylsilyl)methyI]sulfonium salts (e.g., 45) in DMSO. This avoids the strongly basic conditions typically encountered in the preparation of sulfur ylides [95SYN649],... 

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... 

Several groups have investigated the use of complex sulfur ylide reagents for a key carbon-carbon bond formation step in leukotriene synthesis (equation 4). For example, tetraene (6) reacted with methyl S-oxopentanoate to give an 8S% yield of the desired epoxides as a mixture of cis and trans isomers. Significantly, only small amounts of by-products resulting from elimination or rearrangement of the sulfonium salt were observed. 

Several examples of the alkylation of sulfur ylides are known. " " This reaction is particularly useful for the synthesis of j-alkyl diphenylsulfonium salts which cannot otherwise be prepared from diphenyl sulfide, 5-alkyl iodides and silver tetrafluoroborate, since the latter reaction usually leads to a mixture of regioisomeric sulfonium salts. Diphenylsulfonium methanide on reaction with [ C]methyl iodide allows the synthesis of [ C]isopropyl diphenyl sulfonium salt which has been used for the synthesis of [ C]-2,3-epoxysqualene (Scheme 130, entry a). This reaction unfortunately is not general... 

Warming the sulfonium salt (5) may cause elimination of a p-hydrogen to yield a mixture of the sulfide and the alkene, i.e. route (iv) in Scheme 3. On the other hand, proton abstraction from (5) by treatment with a strong base gives the sulfur ylide (6) which can undergo molecular rearrangement to form the sulfide (7). Sulfonium salts are unstable in basic media as a result of their tendency to undergo nucleophilic substitution reactions. 

Some specific examples of the formation of sulfur ylides by the deprotonation of sulfonium salts are shown in Scheme 2. The abstraction of a proton from the corresponding sulfonium salt as shown in Schemes 1 and 2, is the most common procedure for the generation of sulfur or sulfonium ylides however, the most direct synthesis of sulfur ylides (2a) involves addition of a sulfide to a carbene (5),... 

A single sulfonium ylide is beUeved to be formed as alkylation of oxathiane 3a gave the equatorial sulfonium salt exclusively [29]. Ylide conformation has been studied by X-ray, NMR, and computation [30]. All of these studies indicate that the preferred conformation of sulfur yUdes is one in which the filled orbital on the ylide carbon is orthogonal to the lone pair on sulfur. The barrier to rotation around the C-S bond of the semi-stabilized ylide, dimethylsulfonium fluorenide, has been found to be 42 1.0 kJmol [30]. This impHes that the ylide will adopt conformations 6a and 6b and that these will be in rapid equilibrium at room temperature. Of these two, conformation 6b will be favored as 6a suffers from... 

Sulfur-to-carbon chirality transmission is also observed for the rearrangement of ylides derived from sulfonium salts 3111. While chirality transfer for the rearrangement is apparently very efficient, absolute configurations were not assigned to either starting materials or products in this study and further details regarding the nature of asymmetric induction for these systems are unavailable. 

Sulfur ylides are useful reagents in organic synthesis. The ylide is formally a zwitterion in which a carbanion is stabilized by interaction with an adjacent sulfonium centre. They are usually prepared by proton abstraction from a sulfonium salt with a suitable base or by reaction of a sulfide with an alkylating agent such as Me30+BF4 or a carbene formed, for example, by metal-catalysed or photolytic decomposition of a diazo compound (1.103). 

---