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 (CF(3)SO(3)H, FSO(3)H, D(2)SO(4)), (arysulfonyl)allenes (ArSO(2)–CR(1)=C=CR(2)R(3)) and (arylsulfinyl)allenes (ArSO–CR(1)=C=CR(2)R(3)) 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, ArSO(2)–CR(1)=C–C(Me)=CH(2), for R(2) = R(3) = 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 (ArSO(2)–CR(1)=CH–C(OH)R(2)R(3), 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 AlX(3) (X = Cl, Br) followed by hydrolysis of reaction mixtures, (arylsulfinyl)allenes give allyl alcohols (ArSO(2)–CR(1)=CH–C(OH)R(2)R(3)). Plausible reaction mechanisms have been proposed for all studied reactions.