β-Halo-α,β-unsaturated γ-sultones

The reaction of beta-iodo-alpha,beta-unsaturated gamma-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 E-2-elimination. The 'harder' nucleophile, azide ion, reacts under the same conditions to yield the corresponding beta-azido-alpha,beta-unsaturated gamma-sultone (i.e., 4-azido-1,2-oxathiole 2,2-dioxide), displacing the beta-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-alpha-H by deuterium. Factors underlying this intriguing rapid exchange are proposed. Interestingly, the beta-bromo analogue exhibits similar reactivity except for the halophilic ring-opening. Calculations indicate the relative importance of the beta-halogen and the S-O(-C) bonds in enhancing the acidity of the H-C-alpha-S(O)(2)- grouping.