STEREOELECTRONIC EFFECT IN EQUILIBRATION AND METHOXY EXCHANGE OF CYCLIC METHOXYSULFONIUM SALTS

The conversion of alkylthiolanes and alkylthianes by tert-butyl hypoch lorite in methanol yields an equilibrium mixture of cis- and trans-met hoxysulfonium chlorides. The corresponding methoxysulfonium tetrafluor oborates prepared from sulfoxides by O-alkylation show equilibration a nd methoxy exchange in methanol containing chloride ions. The rate con stants have been determined by H-1 NMR measurements, and an S(N)(Ad+E) mechanism is proposed for these reactions. Both the greater thermodyn amic stability of the cis isomers and the greater reactivity of the tr ans analogues are explained by stereoelectronic effect of sigma(C-H) - sigma(S-X) type with X = OMe or Cl.