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

A 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 degrees 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-A(+/-)), and the subsequent steps are a fast acid-base equilibrium followed A by a fast R'''CH2NH3+ or H2O catalyzed expulsion of the leaving group. The Bronsted plot for k(1) for the reaction of 2-SMe-H with four primary amines showed a good correlation between log k(1) and pK(a)(AH) with beta(nuc) = 0.32 +/- 0.02, a slightly lower value than that for the reactions of secondary amines (beta(nuc) = 0.41 +/- 0.01). The Hammett plot for n-butylamine yields p(k(1)) = 0.40 +/- 0.05 which is smaller than that for the reaction of piperidine with the same substrates (p(k(1)) = 0.72 +/- 0.07) and also for the reactions of HOCH2CH2S- (rho(k(1)) = 1.18), CF3CH2O- (rho(k(1)) = 1.14) and OH-(rho(k(1)) = 1.11) addition to 2-H-H (rho(k(1)) = 1.11). All these observations are consistent with the suggested mechanism. Copyright (c) 2006 John Wiley & Sons, Ltd.