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Please login to access the full content or check if you have access via Variation 3: Ylide Generation by Decarboxylation of a Sulfonium Salt

DOI: 10.1055/sos-SD-037-00188

Aggarwal, V. K.; Crimmin, M.; Riches, S.Science of Synthesis, (200837347.

In a variation similar to that described in Section, sulfonium methylides may be generated by the thermal decarboxylation of a carboxymethylsulfonium betaine (thetin salt).[‌124‌,‌144‌] Carrying out the reaction in the presence of a carbonyl compound provides an in situ method for carbonyl epoxidation that obviates the need for base (Scheme 45). The rate of decarboxylation, and therefore the efficiency of methylidene transfer, is strongly solvent dependent and the use of a nonpolar, aprotic solvent facilitates decarboxylation. The lipophilic sulfonium betaine 49 may be employed as a methylide precursor to increase solubility in nonpolar solvents. In turn, 49 may be readily prepared from the corresponding thetin bromide 48 with silver(I) oxide in methanol. The betaine 49 undergoes facile decarboxylation at 60°C in 1,2-dichloroethane.[‌124‌] The generated sulfonium methylide is both thermally and aerobically labile, and it appears that decomposition (possibly by an α,β-elimination reaction) effectively competes with carbonyl epoxidation at these temperatures. Carbonyl epoxidation has been demonstrated for a series of electron-deficient aromatic aldehydes and an aliphatic aldehyde giving monosubstituted epoxides 50.[‌124‌] Electron-rich aromatic aldehydes and ketones do not undergo epoxidation under these conditions. One further drawback of this variation is that the sulfide byproduct of epoxidation, octyl methyl sulfide, is a nonvolatile liquid and must be removed by either chromatography or reduced-pressure distillation.

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