Kinetics and mechanisms of hydrolysis of cyclic sulphinamidates. Part 2. The breakdown of the intermediate ions formed through the ring opening of 3-phenyl-3,4-dihydro-1,2,3-benzoxathiazin-2-one

Abstract

The first step in the ring opening of 3-phenyl-3,4-dihydro-1,2,3-benzoxathiazin-2-one (Ib) is very fast and gives rise to two different intermediate ions depending on the pH of the solutions. Above pH 11, this step proceeds through the cleavage of the ring sulphur–oxygen bond and leads to the formation of N-(o-hydroxybenzyl)-N-phenylsulphinamidate ion (IIb). Below pH 9, the S–N bond is the easiest to break and α-anilino-o-tolyl hydrogensulphite ion (IVb) is obtained. The results for the hydrolysis of sulphinamidate ion (IIb) show that the phenol is much more reactive than the phenolate ion and that the reaction occurs via hydroxide ion attack on the sulphinyl group. Two mechanisms account for the decomposition of the hydrogensulphite ion: (i) in slightly alkaline media (7 < pH < 9), hydroxide ion and base-assisted water attack on the species possessing a non-protonated nitrogen; (ii) at lower pH, acid attack at the oxygen atom bound to the aromatic ring, concerted with the opening of its bond to the sulphur atom. This mechanism is operative for both species of (IVb), which have similar reactivity.