Neighboring-group participation: a "quasi-SNi" mechanism in the acetolysis and thioacetolysis of 1-(phenylthio)-2-[(p-tolylsulfonyl)oxy]ethane

The acetolysis and the thioacetolysis of the title compound, selectively deuteriated at one of the two methylenes, was studied. The excess of the unrearranged acetate in the acetolysis at 40°C has been explained in terms of coordination of the acid to the sulfur, followed by a "quasi-SNi" displacement of the leaving group. This pathway is concurrent with the equilibration of the two isomeric tosylates (through the agency of the episulfonium ion 2) and with the acetolysis of 2, resulting in a 1:1 mixture of 4 and 5. The thioacetolysis of 1 and 3 at 40°C gave in turn a 1:1 mixture of the thiolacetates 8 and 9, along with the thionoacetate 6 in the reaction of 1 and the thionoacetate 7 in that of 3. When the acetolysis and the thioacetolysis of 1 were run at the boiling temperature of the solvent, the reactions afforded only a 1:1 mixture of 4 and 5 and 8 and 9, respectively, showing that at higher temperature coordination of the acid is not at work. In the dichloroacetolysis of 1, a 1:1 mixture of the isomeric dichloroacetates 10 and 11 was always obtained. The results show that in the case of 2-(arylthio)ethyl tosylates there are several competing solvolytic pathways, whose relative importance depends upon the acidity and nucleophilicity of the solvent.