Kinetics and mechanism of reactions of substituted (methylthio)benzylidene Meldrum's acids with primary amines in aqueous DMSO

Abstract

AbstractA kinetic study of the aminolysis of substituted (methylthio)benzylidene Meldrum's acids, 2‐SMe‐Z with the aliphatic primary amines, n‐butylamine, glycinamide, methoxyethylamine, and aminoacetonitrile, in aqueous DMSO at 20 °C is reported. With all amines the reaction is strictly second‐order, that is, first‐order in 2‐SMe‐Z and first‐order in the amine. A three steps mechanism has been proposed. The first step is a rate‐limiting attachment of the amine to form the tetrahedral intermediate (T), and the subsequent steps are a fast acid–base equilibrium followed by a fast R‴CH2NH or H2O catalyzed expulsion of the leaving group. The Brönsted plot for k1 for the reaction of 2‐SMe‐H with four primary amines showed a good correlation between log k1 and pK with βnuc = 0.32 ± 0.02, a slightly lower value than that for the reactions of secondary amines (βnuc = 0.41 ± 0.01). The Hammett plot for n‐butylamine yields ρ(k1) = 0.40 ± 0.05 which is smaller than that for the reaction of piperidine with the same substrates (ρ(k1) = 0.72 ± 0.07) and also for the reactions of HOCH2CH2S− (ρ(k1) = 1.18), CF3CH2O− (ρ(k1) = 1.14) and OH− (ρ(k1) = 1.11) addition to 2‐H‐H (ρ(k1) = 1.11). All these observations are consistent with the suggested mechanism. Copyright © 2006 John Wiley & Sons, Ltd.