β-Halo-α,β-unsaturated γ-Sultones

The reaction of β-iodo-α,β-unsaturated γ-sultones (i.e., 4-halo-1,2-oxathiole 2,2-dioxides) in aprotic polar solvents such as DMSO or acetone, with ‘soft' nucleophiles such as iodide or thioacetate, yields an allenesulfonate by a very facile halophilic ring-opening E₂-elimination. The ‘harder' nucleophile, azide ion, reacts under the same conditions to yield the corresponding β-azido-α,β-unsaturated γ-sultone (i.e., 4-azido-1,2-oxathiole 2,2-dioxide), displacing the β-halide by an addition−elimination mechanism. In contrast, in the hydroxylic solvent CD₃OD at ambient temperature, various nucleophiles yield neither of the above-mentioned products, but catalyze a rapid replacement of the C_α-H by deuterium. Factors underlying this intriguing rapid exchange are proposed. Interestingly, the β-bromo analogue exhibits similar reactivity except for the halophilic ring-opening. Calculations indicate the relative importance of the β-halogen and the S−O(−C) bonds in enhancing the acidity of the H-C_α-S(O)₂- grouping.