The nucleofuge in the pyridinolysis of O-(4-nitrophenyl) S-aryl thio and dithiocarbonates~?

摘要

A kinetic investigation is undertaken on the pyridinolysis of S-phenyl O-(4-nitrophenyl) dithiocarbonate (1), S-(4-nitrophenyl) O-(4-nitrophenyl) dithiocarbonate (2) and S-phenyl O-(4-nitrophenyl) thiocarbonate (3) in aqueous ethanol. The Br?nsted-type plots (log k_N versus pK_a) obtained are linear and are explained by a stepwise mechanism with the existence of a tetrahedral intermediate (T~±) and its breakdown to products as the rate determining step. The high-performance liquid chromatography analysis of the products of these reactions and that for the reaction of S-(4-chlorophenyl) O-(4-nitrophenyl) dithiocarbonate (4) with 4-oxypyridine shows that 4-nitrophenoxide is the principal (or the sole) leaving group in these reactions. From the results obtained in the reaction of compound 2, it can be concluded that 4-nitrophenoxide is a better nucleofuge than 4-nitrobenzenethiolate from the same tetrahedral intermediate, although the former is three pKa units more basic than the latter. Reasons for this behaviour are given.