Generation of 1,2-oxathiolium ions from (arysulfonyl)- and (arylsulfinyl)allenes in Brønsted acids. NMR and DFT study of these cations and their reactions.

In strong Brønsted acids (CF3SO3H, FSO3H, D2SO4), (arysulfonyl)allenes (ArSO2-CR1=C=CR2R3) and (arylsulfinyl)allenes (ArSO-CR1=C=CR2R3) undergo cyclization into the corresponding stable 1,2-oxathiolium ions, which were studied by means of NMR and DFT calculations. Quenching of solutions of these cations with low nucleophilic media, aqueous HCl, leads to their deprotonation with a stereoselective formation of (arysulfonyl)butadienes (for instance, ArSO2-CR1=C-C(Me)=CH2, for R2 = R3 = Me, yields of 87-98%). Reactions of (arysulfonyl)allenes in the system TfOH (0.1 equiv)-HFIP (hexafluoropropan-2-ol) followed by hydrolysis give rise to allyl alcohols (ArSO2-CR1=CH-C(OH)R2R3, yields of 78-99%). Reflux of solutions of (arysulfonyl)allenes in the presence of TfOH (1 equiv) in 1,2-dichlorobenzene leads to the cyclization into thiochromene 1,1-dioxides in high yields. Under the action of TfOH or AlX3 (X = Cl, Br) followed by hydrolysis of reaction mixtures, (arylsulfinyl)allenes give allyl alcohols (ArSO2-CR1=CH-C(OH)R2R3). Plausible reaction mechanisms have been proposed for all studied reactions.