Thiirane-1-oxide, protonation

A mechanism analogous in many ways to that of the acid-catalyzed ring opening reaction was advanced for the reaction of the thiirane oxide with alkyl chloromethyl ethers S . The first step is the displacement of the chloride by the sulfoxy oxygen (equation 24). In view of the above mechanistic interpretation, it is quite surprising that the parent thiirane oxide (16a) was found to be protonated on sulfur and not at oxygen in FSOjH-SbFfi at — 78 °C, according to NMR studies s . 

Alkyl chloromethyl ethers react with thiiran 1-oxide to give sulphenic esters (26), probably by way of an intermediate sulphoxonium salt (27). Thiiran oxide in FSOsH-SbF at — 78 C was protonated at sulphur and not at oxygen, according to n.m.r. studies. ... 

Acids are poor catalysts for ring cleavage of thiirane 1,1-dioxides but are good catalysts for reactions of thiirane 1-oxides with nucleophiles. These reactions of episulfoxides are believed to proceed by protonation of the oxygen atom (but see the NMR evidence cited above for 5-protonation in fluorosulfonic acid) and will be treated in the section on nucleophilic reactions. 

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