Indole-3-sulfonium Ylides and Related Sulfonium Salts. Chemical and Physical Properties

Article abstract of DOI:10.1021/jo01293a003

Acid-base titration of the sulfonium salt-sulfonium ylide pair dimethyl(1H-indol-3-yl)sulfonium 3-(dimethylsulfonio)indolide, the corresponding 2-methyl or 2-phenyl analogues, or the homologous 3-diethylsulfonium compounds resulted, in each case, in a hysteresis; i.e., titration of the sulfonium ylide with acid gave a different set of pH values from those observed upon titration of the sulfonium salt with base.In related studies comparison of ultraviolet spectra of the sulfonium salts and ylides in anhydrous dioxane and in water revealed significant differences. 1H-NMR spectra of sulfonium salts in aqueous or aqueous trifluoroacetic acid solutions revealed the formation of a new species which (a) exhibited an acid-base titration hysteresis indistinguishable from that of the precursor salt, (b) exhibited ions in the mass spectra corresponding to a sulfonium salt plus a molecule of water, and (c) reverted to the precursor salt upon attempted purification.These results are consistent with covalent hydration across the highly polarized C-2, C-3 double bond of the indole ring.