Racemization of sulphoxides with halide ions anchimerically assisted by a carboxy-group. The different behaviour of chloride and bromide ions

Abstract

The kinetics of the racemization of (–)-2-isopropylsulphinylbenzoic acid (Ib) catalysed by Cl^– and Br^– have been studied in aqueous perchloric acid, and compared with those of the (–)-methyl analogue (Ia). In racemization with Cl^– the steric effect (k(_Ia)/k_(Ib)ca. 50) is similar to those found in the reactions at sulphur centres, whereas steric retardation is minimized in racemization with Br^–(k(_Ia)/k(_Ib)ca. 6). Moreover the reactivity ratios, k_Br/k_Cl[3 100 and 500 for (Ib and a), respectively] are much higher than those found in the racemizations of normal sulphoxides (k_Br/k_Clca. 3). These results seem to indicate that attack by halide ions in the halogenosulphonium intermediate occurs at sulphur or at halogen depending on the nature of Hal^–.