Sulfonium compounds, elimination reactions

Sulfonium compounds (—C— SR2) undergo elimination similar to that of their ammonium counterparts (17-6 and 17-7) in scope and mechanism but this reaction is not of great synthetic importance. These syn-elimination reactions are related to the Cope elimination (17-8) and the Hofmann elimination (17-6). ... 

The simplest sulfur ylids are formed from sulfonium salts 69 by deprotonation in base. These ylids react with carbonyl compounds to give epoxides.18 Nucleophilic attack on the carbonyl group 70 is followed by elimination 71 of dimethylsulfide 72 and formation of the epoxide 73. You should compare diagram 71 with diagram 23 in chapter 15. The phosphonium ylid reacted by formation of a P-0 bond and an alkene in the Wittig reaction. The sulfonium compound reacts by formation of a C-O bond 71 as the S-O bond is much weaker than the P-0 bond. The sulfonium salt 69 can be reformed by reaction of 72 with Mel. 

One procedure for the synthesis of these title ring systems appeared recently <2003S1079>. Yadav and Kapoor described that the transformation of some oxadiazole and thiadiazole derivatives bearing specially substituted methylsulfinyl side chain 131, when reacted with thionyl chloride, give ring-closed compounds 134. The reaction was carried out in pyridine under reflux conditions in 74-79% yield. As shown in Scheme 25, the authors assume that the first step is the formation of the sulfonium salt 132 which undergoes cyclization with hydrogen chloride and sulfur dioxide elimination to 133 and, finally, demethylation of this intermediate leads to the final product 134. 

Irrespective of whether the initially formed sulfonium ion B or the subsequently formed sulfonium ion D reacts with the alcohol, the alcohol is taken up by such a sulfonium ion with formation of sulfuranes A (first case) or C (second case). Any of these sulfuranes would yield the sulfonium salt E after dissociation. Once this sulfonium salt has formed, five equivalents ofNEtj are added to the reaction mixture, which then is allowed to warm up from -60 to -45°C. Under these conditions, the sulfonium salt E is deprotonated to give the sulfonium ylide F. This ylide undergoes a /3-elimination via a cyclic transition state to form the desired carbonyl compound and dimethyl sulfide as a side-product. 

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