Aminolysis of Benzyl 4-Pyridyl Carbonate in Acetonitrile: Effect of Modification of Leaving Group from 2-Pyridyloxide to 4-Pyridyloxide on Reactivity and Reaction Mechanism

作者：Bae Ae Ri; Um Ik Hwan^*

来源：Bulletin of the Korean Chemical Society, 2012, 33(8): 2719-2723.

DOI：10.5012/bkcs.2012.33.8.2719

• Aminolysis
• Bronsted-type plot
• Nucleofuge
• Reaction mechanism
• Intramolecular H-bonding interaction

A kinetic study is reported for nucleophilic substitution reactions of benzyl 4-pyridyl carbonate 6 with a series of alicyclic secondary amines in MeCN. The plot of pseudo-first-order rate constant (k(obsd)) vs. [amine] curves upward, which is typical for reactions reported previously to proceed through a stepwise mechanism with two intermediates (i.e., a zwitterionic tetrahedral intermediate T-+/- and its deprotonated form T-). Dissection of k(obsd) into the second- and third-order rate constants (i.e., Kk(2) and Kk(3), respectively) reveals that Kk(3) is significantly larger than Kk(2), indicating that the reactions proceed mainly through the deprotonation pathway (i.e., the k(3) process) in a high [amine] region. This contrasts to the recent report that the corresponding aminolysis of benzyl 2-pyridyl carbonate 5 proceeds through a forced concerted mechanism. An intramolecular H-bonding interaction was suggested to force the reactions of 5 to proceed through a concerted mechanism, since it could accelerate the rate of leaving-group expulsion (i.e., an increase in k(2)). However, such H-bonding interaction, which could increase k(2), is structurally impossible for the reactions of 6. Thus, presence or absence of an intramolecular H-bonding interaction has been suggested to be responsible for the contrasting reaction mechanisms (i.e., a forced concerted mechanism for the reaction of 5 vs. a stepwise mechanism with T-+/- and T- as intermediates for that of 6).

• 出版日期2012-8-20