Kinetics, third-order aminolysis

Kinetic studies of the reaction of Z-phenyl cyclopropanecarboxylates (1) with X-benzylamines (2) in acetonitrile at 55 °C have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, T, with a hydrogen-bonded four-centre type transition state (3). The results of studies of the aminolysis reactions of ethyl Z-phenyl carbonates (4) with benzylamines (2) in acetonitrile at 25 °C were consistent with a four- (5) and a six-centred transition state (6) for the uncatalysed and catalysed path, respectively. The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25 °C. The reaction was found to be third order in water. The kinetic solvent isotope effect was (A h2o/ D2o) = 2.90 0.12. Proton inventories at each molarity of water studied were consistent with an eight-membered cyclic transition state (7) model. 

Three O-substituted benzophenone oximes (29 X = OMe, F, Cl) have been subjected to aminolysis by pyrrolidine and piperidine, in benzene solution.483 Kinetics were third order in amine, and involved two routes one accelerates with a rise in temperature, the other decelerates. Of the many mechanisms proposed for this reaction in non-polar media, the results support Hirst s mechanism of electrophilic catalysis48b in this instance. 

Kinetic evidence for the involvement of a-hydroxydialkylnitrosamines (142) in the pH-independent solvolysis of the a-(acyloxy)dialkylnitrosamines (141) has been obtained.120 The aminolysis in benzene of 0-(2,4-dinitrophenyl)-/7,/ -disubstituted benzophenone oximes (143) with pyrrolidine and piperidine are third order in amine.121 Hir st s mechanism involving electrophilic catalysis operates and can explain the various effects observed. The bis(pentamethylphenyl)-A-isopropylketenimine (144) undergoes pre-equilibrium /V-protonation in aqueous acetonitrile followed by water attack. An inverse solvent isotope effect and the observation of the diol (145) confirm this.122... 

The intervention of acid catalysis leads to irregular or third-order kinetics in several instances involving alcoholysis, aminolysis, or mercaptolysis (Table X, line 8). Small changes in the basicity of the substrate or of the amine produce large effects ... 

The first step is the formation of H-bonded intermediate 49, in which Ccarbene takes on substantial cationic character. Next, termolecular attack by the amine in the presence of Y provides tetrahedral intermediate 50, which then breaks down into products. The reaction is sensitive to steric hindrance, with ammonia and primary amines reacting rapidly (several orders of magnitude faster than aminolysis of carboxylic acid esters) and secondary amines reacting much more sluggishly. The actual kinetic order associated with the amine is also a function of the solvent. Aprotic solvents such as hexane require a rate law with a third-order contribution from the amine pro tic solvents such as methanol show a mixed first- and second-order contribution from the amine. 

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