Kinetic Study on Aminolysis of 4‐Pyridyl Benzoate and O‐4‐Pyridyl Thionobenzoate in Acetonitrile: Factors Influencing Reactivity and Reaction Mechanism

摘要

A kinetic study on nucleophilic substitution reactions of 4a??pyridyl benzoate (2a) and Oa??4a??pyridyl thionobenzoate (2b) with a series of cyclic secondary amines in acetonitrile at 25.0?°C is reported. Plots of pseudoa??firsta??order rate constant (kobsd) vs. [amine] are linear and pass through the origin for the reactions of 2a but curve upward for those of 2b. The upward curvature observed for the reactions of 2b is typical for reactions that proceed through a stepwise mechanism with a zwitterionic intermediate T?±, which decomposes to the products via uncatalyzed and catalyzed routes competitively. The reaction of 2a has been suggested to proceed through a stepwise mechanism with T?±, in which expulsion of the leaving group occurs in the ratea??determining step on the basis of a linear Br??nsteda??type plot with ?2nuc = 0.77. The catalyzed reaction of 2b from T?± has been proposed to proceed through a concerted mechanism with a sixa??membered cyclic transition state rather than via a stepwise pathway with an anionic intermediate Ta??. Factors influencing reactivity and reaction mechanism are discussed in detail.