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Kinetics and mechanism of reactions of substituted (methylthio)benzylidene Meldrum's acids with primary amines in aqueous DMSO
Author(s) -
Ali Mahammad,
Biswas Supriya,
Rappoport Zvi,
Bernasconi Claude F.
Publication year - 2006
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1109
Subject(s) - chemistry , aminolysis , piperidine , amine gas treating , aqueous solution , medicinal chemistry , tetrahedral carbonyl addition compound , morpholine , reaction rate constant , base (topology) , kinetics , nucleophile , stereochemistry , organic chemistry , catalysis , mathematical analysis , physics , mathematics , quantum mechanics
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 °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 by a fast R‴CH 2 NH 3 +or H 2 O catalyzed expulsion of the leaving group. The Brönsted plot for k 1 for the reaction of 2‐SMe‐H with four primary amines showed a good correlation between log k 1 and p K a AHwith β nuc = 0.32 ± 0.02, a slightly lower value than that for the reactions of secondary amines (β nuc = 0.41 ± 0.01). The Hammett plot for n ‐butylamine yields ρ( k 1 ) = 0.40 ± 0.05 which is smaller than that for the reaction of piperidine with the same substrates (ρ( k 1 ) = 0.72 ± 0.07) and also for the reactions of HOCH 2 CH 2 S − (ρ( k 1 ) = 1.18), CF 3 CH 2 O − (ρ( k 1 ) = 1.14) and OH − (ρ( k 1 ) = 1.11) addition to 2‐H‐H (ρ( k 1 ) = 1.11). All these observations are consistent with the suggested mechanism. Copyright © 2006 John Wiley & Sons, Ltd.