Activation energies of aminolysis of aliphatic esters in aprotic media
Activation enthalpies and entropies for the third-order aminolysis reaction of n-butyl formate, n-butyl-trichloroacetate and sec-butyl trifluoroacetate were determined in solvents of various polarity and π-basicity. The importance of the steric requirements of the reagents together with high activation entropy values support the assumption of cyclic transition states for the reaction. Electrophilicity of the ester and the solvent properties predetermine the reaction mechanism in the range between the pathway through a zwitterionic intermediate for the formate and the nearly concerted reaction for the trichloroacetate. The negative activation energy of an aminolysis reaction should not necessarily be attributed to a good leaving group, but can be determined by the electrophilicity of the ester and the solvent effects.
Tuulmets, Ants,Talvik, Agu-Tonis
p. 111 - 119
(2007/10/03)
Formation of N-Substituted Trichloroacetamides from Amines and Hexachloroacetone
Procedures are described for converting primary amines into their well-crystalline trichloroacetyl-derivatives by treatment with hexachloroacetone under mild conditions.Although secondary aromatic N-methylamines are unaffected by hexachloroacetone, saturated heterocyclic amines react vigorously.A mechanistic study using 2-amino-4-t-butylthiazole showed that the reaction is first order in hexachloroacetone, second order in amine, and base-catalysed; there is no appreciable kinetic isotope (H/D) effect nor accumulation of intermediates during the reaction.A sequence which accommodates these results is suggested.
Bew, Clive,Joshi, Virginia Otero de,Gray, Jim,Kaye, Perry T.,Meakins, G. Denis
p. 945 - 948
(2007/10/02)
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