Kinetics and mechanism of ring transformation of S-[1-(4-methoxyphenyl)pyrrolidin-2-on-3-yl]isothiuronium bromide to 2-methylimino-5-[2-(4-methoxyphenylamino)ethyl]thiazolidin-4-one

Abstract

The kinetics and mechanism of transformation reaction of S-[1-(4-methoxyphenyl)pyrrolidin-2-one-3-yl]-N-methylisothiuronium bromide into 2-methylimino-5-[2-(4-methoxyphenylamino)ethyl)]thiazolidin-4-one have been studied in aqueous solutions of amine buffers (pH 8.1–11.5) and sodium hydroxide solutions (0.005–0.5 mol l⁻¹) at 25 °C and at I = 1 mol l⁻¹ at pseudo-first-order reaction conditions. The kinetics observed shows that the transformation reaction is subject to general base, general acid, and hydroxide-ion catalyses. The rate-limiting step of transformation is the splitting-off a proton from the tetrahedral intermediate In. The value of pK_a for S-[1-(4-methoxyphenyl)pyrrolidin-2-one-3-yl]-N-methylisothiuronium bromide has been determined from the kinetic data (pK_a = 8.75 ± 0.10) and by potentiometric titration (pK_a = 8.90 ± 0.05). With increasing pK_a value of the acid buffer component, the value of Brønsted coefficient β gradually decreases from about 0.7 to almost zero. The value of pK_a ≈ 10 for the intermediate of base-catalysed transformation has been found from this dependence. In the N-methylpyrrolidine and triethylamine buffers, the rate-limiting step of transformation is changed into ring opening of In⁻−−, and the general-base-catalysed reaction changes into a specific-base-catalysed one.