Sulfonium ylides generation

Interestingly, sulfonium ylides generated from electrophilic carbene complexes and sulfides can react with carbonyl compounds, imines, or acceptor-substituted alkenes to yield oxiranes [1320-1325], aziridines [1321,1326,1327] or cyclopropanes [1328,1329], respectively. In all these transformations the thioether used to form the sulfonium ylide is regenerated and so, catalytic amounts of thioether can be sufficient for complete conversion of a given carbene precursor into the... 

Sulfonium ylides generated through base-promoted deprotonation of sulfonium salt have been extensively studied. The reaction of sulfides with a diazo carbonyl compound in the presence of a transition metal catalyst is an alternative approach to obtain sulfonium ylides. Sulfonium ylides are more stable than the corresponding oxonium ylides. Stable sulfonium ylides generated by the reaction of an Rh(ii) carbene complex with thiophene have been reported (Figure 5). ... 

Sulfonium ylides, generated from sulfonium bromides 625, react with arylidenecyanothio-acetamides 624 providing 4,5-dihydrothiophenes 627 via a cyclization of the intermediates 626 (Scheme 93) <1997S623>. Similar cyclizations of 628 or 629 afford the dihydrothiophenes 630 <2001SC1647>. 

Sulfonium Ylides Generated via Carbene or Carbenoid Precursors. 72... 

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). 

Sulfonium ylides can also be generated by in situ alkylation with diazo compounds. The alkylation can be carried out by reaction of a diazo compound with HBF4 and DBU.281 The reagents are added alternately in small portions and the reaction presumably proceeds by trapping of the carbocation generated by dediazonization and deprotonation. 

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. 

The sulfonium ylide derived chemistry of penicillins continues to meet the interest of several research groups. It is well known that intermolecular carbenoid attack at the sulfur atom generates a sulfonium ylide which undergoes spontaneous opening of the thiazolidine ring to furnish a l,2-sm>-penicillin 326). Novel examples of this reaction type were found upon Rb2(0Ac)4-catalyzed decomposition of diazomalonic esters in the presence of various penicillins this transformation constituted the opening step of a synthetic sequence directed towards 2-alkoxycarbonyl-cephems 345 a) or modified penicillins 345 b). Similar to its reaction with 4-thio-2-azetidinone... 

If acceptor-substituted carbene complexes are generated in the presence of thioethers, ylide formation is generally the mostly favored process. The resulting sulfonium ylides are often sufficiently stable to be isolated [975,1307-1309]. Typical reactions of sulfonium ylides include 1,2-alkyl migration, leading to products of... 

Several examples are of enantioselective rearrangement reactions of oxonium, sulfonium, or selenonium ylides, generated by the catalytic decomposition of diazo compounds (199-203), are known. The first reports of enantioselective dipolar... 

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. 

Interaction of carbene 43 with thioethers leads first to a sulfonium ylide a subsequent 1,2-R shift provides the formal C,S insertion product 203 and, if possible, -elimination generates (alkylthio)acetate 204116. 

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. 

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.

Thiophene sulfonium salts have also been used for alkylation of phenols, thiophenols, and other nucleophiles (Equation 70) <2001T2871>. Ylides generated from THT and alkyl or allyl halides are known to react with aldehydes to form oxiranes. However, a modified procedure has been developed in which only a catalytic amount of THT is used for the preparation of vinyloxiranes from allyl bromides and aldehydes. In most of the cases, a m-/ra r-mixture of vinyloxiranes was obtained. Optically pure C2-symmetric /ra r-2,5-dimethyltetrahydrothiophene has also been used for the asymmetric version of this reaction, but the enantioselectivity was poor (25% ee) (Equation 71) <2003CC2074>. 

The alkylidenecarbene generated from alkenyl(phenyl)iodonium tetrafluoroborate by base adds irreversibly to THT to form a sulfonium ylide, which further gives the sulfonium salt along with a small amount of a thioether (Scheme 40). This was in contrast to the corresponding oxonium ylide where the reaction was found to be reversible <1996JA10141>. 

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