Sulphonium ylids

Sulphonium ylids also react as nucleophiles towards carbonyl groups but the intermediate sulphonium betaines decompose to give oxiranes (epoxides) fioo] through an internal... 

The Stevens rearrangement of sulphonium salts is known to proceed through the intermediacy of the corresponding sulphonium ylid . There appears to be two distinct mechanistic pathways, depending upon the structure of the ylid, leading to rearranged product. Rearrangement of... 

A second type of rearrangement involves ylids derived from non-allyl sulphonium salts. Baldwin and coworkers have reported that rearrangement of the sulphonium ylid 115 in toluene at reflux temperatures occurs by a radical pair mechanism (equation 94), in which the benzyl group migrates with predominant retention of configuration to yield 116. 

Sulphonium salts, rearrangement of 561-566 allyl 562-564 non-allyl 564-566 Sulphonium ylid, intermediate in... 

The following reactions have been excluded as being beyond the scope of this chapter the variety of synthetically useful reactions of dimethyl sulphoxide (DMSO) and dimethyl sulphide, which include reactions involving dimsyl anion, oxidation reactions involving DMSO, methylene transfer reactions of corresponding sulphonium methylides, and reaction of stabilized sulphonium ylids normally prepared from dimethyl sulphide. ... 

For a review of nitrogen ylids, see Musker, W.K. Fortschr. Chem. Forsch., 1970,14, 295. For a monograph on sulfur ylids, see Trost, B.M. Melvin Jr, L.S. Sulfur Ylids Academic Press NY, 1975. For reviews, see Fava, A. in Bemardi Csizmadia Mangini Organic Sulfur Chemistry Elsevier NY, 1985, p. 299 Belkin, Yu.V. Polezhaeva, N.A. Russ. Chem. Rev., 1981, 50,481 Block, E. in Stirling The Chemistry of the Sulphonium Group,... 

The reductive cleavage of sulphonium salts in aprotic solvents leads to the generation of radical and then carbanions in a further electron transfer step. Protonation of the carbanion by extraneous water leaves a hydroxide ion. Basic species formed in this way can abstract a proton from sulphonium ion to give the ylid, which is not reducible. A good example is the reduction of 9 in dimethylsulphox-ide, which consumes only one Faraday and follows the course shown [58]. 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

The reaction of vinylsulphonium salts (356) with active methylene functions also gives electrophilic cyclopropanes via the formation of an unstable ylid which undergoes proton shift to an intermediate 1,4-sulphonium betaine (375) followed by expulsion of dimethyl sulphide (equation 115) . In addition MIRC reactions of substrates with... 

However, unlabelled dimethyl sulphide is obtained from the reaction of 2-phenylethyl-2//2-dimethylsulphonium ion with hydroxide in water . Elimination from tri(ethyl-l,l-//2)-sulphonium ion with triphenylmethylsodium in ether gives triphenylmethane, 75% of which contains deuterium, indicating that the predominant route for the decomposition is via an ylid mechanism. The analyses of the other products are also in agreement with this conclusion... 

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