The literature on the chemistry of α,β-unsaturated γ-sultones in general, and of their β-halo derivatives in particular, is very sparse. Following our previous reports of convenient paths to such β-bromo and β-iodo compounds, [1, 2] we have now embarked on a systematic study of the chemistry of such systems. We hereby report some intriguing observations.
The treatment of 4-iodo-1-oxa-2-thiaspiro[4.5]dec-3-ene 2,2-dioxide (1a) with excess NaI in acetone (or DMSO) at RT for ~3 hrs lead to their quantitative conversion to the corresponding allenesulphonate salt. This halophilic E₂ elimination is not unexpected, and is noteworthy only for the mild conditions under which it occurs. A similar elimination occurs, albeit more slowly (2 days, acetone, 56 ⁰C, or DMSO, RT with NaI or NaCN or thioacetate) in the case of 4-iodo-5-methyl-5H-1,2-oxathiole 2,2-dioxide (1b) whose γ-carbon is secondary, whereas in the case of 4-iodo-5-phenyl-5H-1,2-oxathiole 2,2-dioxide (1c) elimination is immediate at RT with either NaI or NaBr. Displacement of the β-halogen was achieved only when using the 'harder' nucleophile, azide ion. Thus sodium azide in DMSO (RT) converted both 1b and 4-bromo counterpart, 1d, to the 4-azido-5-methyl-5H-1,2-oxathiole 2,2-dioxide (1e). The fact that the bromo sultone 1d reacted about twice as fast as the iodo sultone 1b (t_1/2 15 vs 29 min) indicates that the reaction proceeds via an addition-elimination mechanism in which the first step is rate determining. In the reaction of the compound 1a with azide ion in DMSO elimination to the corresponding allenesulphonate salt competed with the formation of the 4-azido-1-oxa-2-thiaspiro[4.5]dec-3-ene 2,2-dioxide and a mixture of products was isolated and separated. Our most surprising finding is that 1b in CD₃OD solution at RT, in the presence of various nucleophiles, undergoes neither elimination nor displacement reactions, but rapidly exchanges its α-hydrogen for deuterium. Thus exchange is complete with azide or triethylamine (<1 min), acetate (30 min), thioacetate (3 hrs), iodide (9 hrs). 1c similarly exchanged its α- but not its benzylic hydrogen. The possible mechanisms of this intriguing rapid exchange will be discussed.

* This work has been supported by a grant from the Israel Science Foundation (Grant No.919-05).

[1] S. Braverman, T. Pechenick-Azizi, M. Sprecher, J. Org. Chem. 2006, 71, 2147-2150.
[2] S. Braverman, D. Reisman, J. Am. Chem. Soc. 1977, 99, 605-607.

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