Kinetic Study on Nucleophilic Displacement Reactions of Y-Substituted-Phenyl 2-Methylbenzoates with Cyclic Secondary Amines in Acetonitrile: Effects of Modification of 2-MeO in Benzoyl Moiety by 2-Me on Reactivity and Reaction Mechanism

Abstract

The second-order rate constants () have been measured spectrophotometrically for nucleophilic substitution reactions of Y-substituted-phenyl 2-methylbenzoates (6a-e) with a series of cyclic secondary amines in MeCN at . Comparison of the values for the reactions of 4-nitrophenyl 2-methylbenzoate (6d) with those reported previously for the corresponding reactions of 4-nitrophenyl 2-methoxybenzoate (5) reveals that 6d is significantly less reactive than 5, indicating that modification of 2-MeO in the benzoyl moiety of 5 by 2-Me (i.e., ) causes a significant decrease in reactivity. This supports our previous report that aminolysis of 5 proceeds through a six-membered cyclic intermediate, which is highly stabilized through intramolecular H-bonding interactions. The Brnsted-type plot for the reactions of 6d with a series of cyclic secondary amines is linear with , which appears to be a lower limit of for a stepwise mechanism with breakdown of an intermediate () being rate-determining step (RDS). The Brnsted-type plot for the reactions of 6a-e with piperidine is curved, i.e., the slope of Brnsted-type plot () decreases from -1.05 to -0.41 as the leaving-group basicity decreases. The nonlinear Brnsted-type plot has been taken as evidence for a stepwise mechanism with a change in RDS (e.g., from the step to the process as the leaving-group basicity decreases). Dissection of into the microscopic rate constants associated with the reactions of 6a-e with piperidine (e.g., and ratio) also supports the proposed mechanism.