X-ray Structures and Anionotropic Rearrangements of Di-tert-butyl-Substituted Thiiranium and Thiirenium Ions. A Structure−Reactivity Relationship

The X-ray structures of c-2,t-3-di-tert-butyl-r-1-methylthiiranium 8 BF₄^-, t-2,t-3-di-tert-butyl-r-1-methylthiiranium ion 10 BF₄^-, and 2,3-di-tert-butyl-1-methylthiirenium 11 BF₄^- have been determined. The ΔG^⧧₂₉₈ values for the rearrangements from the cis and the trans tert-butyl groups of 8 SbCl₆^- to thietanium ion (two intramolecular S_N2 displacements) and for the rearrangement of 11 SbCl₆^- to thietium ion (an intramolecular S_N2-Vin displacement) are linearly correlated with the strengths of the C−S breaking bonds, suggesting that the two mechanisms are, in the absence of steric hindrance, uniquely governed by the nucleofugality of the sulfonium leaving group.