Sulfonium salts reagents

The ionic species 5, as well as 6, represent the so-called activated dimethyl sulfoxide. Variants using reagents other than oxalyl chloride for the activation of DMSO are known. In the reaction with an alcohol 1, species 5, as well as 6, leads to the formation of a sulfonium salt 7 ... 

Sulfur ylides have several applications as reagents in synthesis.282 Dimethylsul-fonium methylide and dimethylsulfoxonium methylide are particularly useful.283 These sulfur ylides are prepared by deprotonation of the corresponding sulfonium salts, both of which are commercially available. 

Sulfur ylides are a classic reagent for the conversion of carbonyl compounds to epoxides. Chiral camphor-derived sulfur ylides have been used in the enantioselective synthesis of epoxy-amides <06JA2105>. Reaction of sulfonium salt 12 with an aldehyde and base provides the epoxide 13 in generally excellent yields. While the yield of the reaction was quite good across a variety of R groups, the enantioselectivity was variable. For example benzaldehyde provides 13 (R = Ph) in 97% ee while isobutyraldehyde provides 13 (R = i-Pr) with only 10% ee. These epoxy amides could be converted to a number of epoxide-opened... 

In addition to routes involving resolution and asymmetric synthesis, optically active sulfonium salts have been prepared in a stereospecific way by Andersen (158,159). Thus, synthesis of the optically active dialkyl-p-tolylsulfonium salts 114 from the optically active ethoxy-sulfonium salt 115 was accomplished by the addition of alkyl Grig-nard or dialkylcadmium reagents. This reaction occurs with inversion... 

Sulfonium salts are not often employed as reagents for quater-nization. It is therefore interesting to note that the primary biological methylating reagent is the sulfonium salt S-adenosylmethionine (9).30,31... 

More reactive than Helquist s reagent, it can be used at room temperature for cyclopropanation of alkenes. Thus, ds-cyclooctene was converted to the cyclopropane in an 85% yield in 3 h at 22°C [472], Although probably oversimplified, the pathway given here accounts for the cyclopropanation property of the sulfonium salts [471]. 

Both reagents transfer a methylene group in efficient and selective pathways. So it is not surprising that sulfur ylides have been widely used as synthetic tools for the preparation of epoxides. The reactions can make use of sulfonium salts under phase transfer catalytic conditions, and the cheap and easily accessible trimethyl sulfonium methyl sulfate and triethylsulfonium ethyl sulfate were found to show a high reactivity under such conditions [450]. 

With methyl iodide as the alkylating reagent, Link and Stark (133) prepared a monosubstituted sulfonium salt. Methionine 29 was strongly indicated as the principal site of modification. The activity of the derivative toward C>p was the same as RNase-A, and the methylation reaction was not affected by competitive inhibitors such as 2 (3 )-UMP. 

In a convenient modification of this procedure wltloh gives the furan in 70 75% yield, the sulfonium salt is prclumicd In acetonitrile and, without isolation, the other reagents are added. 

G. Grewal, N. Kaila, and R. W. Frank, Arylbis(arylthio)sulfonium salts as reagents for the synthesis of 2-deoxy-jS-glycosides, J. Org. Chem., 57 (1992) 2084-2092. 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

Fig. 17.13. Mechanism of the Swern oxidation of alcohols. The actual reagent is an "activated DMSO" (compound B or D), which reacts with an alcohol with formation of A or C, respectively. Dissociation leads to the sulfonium salt E, which is then converted into the sulfonium ylide F after NEt3 addition and raising the temperature from -60 to -45 °C. /3-Elimination via a cyclic transition state generates the carbonyl compound and dimethyl sulfide from F.

Indium-mediated allylation of a nitro group is achieved in aqueous media to give A,A-diallylamine and 7V,0-diallylhydroxylamine (Equation (81)) 316 In situ generated sulfonium salts derived from a,/3-enones undergo a nucleophilic substitution with allylindium reagents to give the corresponding Michael addition products (Scheme 83).317... 

Enamittones. The usual method for conversion of a S-diketone such as 1 results in low yields (10-40%). The reaction is improved considerably by use of the Corey-Kim reagent in the presence of trifluoroacetic acid. Undoubtedly an intermediate sulfonium salt is involved, as in Corey-Kim oxidation (5, 129-130). 

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