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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 33, Number 5
BKCSDE 33(5)
May 20, 2012 

Kinetics and Reaction Mechanism of Aminolyses of Benzyl 2-Pyridyl Carbonate and t-Butyl 2-Pyridyl Carbonate: Effect of Nonleaving Group on Reactivity and Reaction Mechanism
Ji-Sun Kang, Jae-In Lee, Ik-Hwan Um*
Aminolysis, Steric hindrance, H-bonding interaction, Nonleaving group, Brønsted-type plot
Second-order rate constants (kN) have been measured spectrophotometrically for the reactions of benzyl 2- pyridyl carbonate 7 and t-butyl 2-pyridyl carbonate 8 with a series of alicyclic secondary amines in H2O at 25.0 oC. Substrate 8 is less reactive than 7. Steric hindrance exerted by the bulky t-Bu group of 8 has been suggested to be responsible for the decreased reactivity. The Brønsted-type plots for the reactions of 7 and 8 are linear with βnuc = 0.49 and 0.44, respectively, which is typical for reactions reported previously to proceed through a concerted mechanism. Aminolyses of 7 and 8 were expected to proceed through a zwitterionic tetrahedral intermediate T±, which would be stabilized through an intramolecular H-bonding interaction. However, the kinetic results suggest that the reactions proceed through a concerted mechanism. The H-bonding interaction in T± has been suggested to accelerate the rate of leaving-group expulsion from T±. Another factor that might accelerate expulsion of the leaving group is the “push” provided by the RO group in T± through resonance interactions. Thus, it has been concluded that the enhanced nucleofugality through the H-bonding interaction and the “push” provided by the RO group forces the reactions to proceed through a concerted mechanism.
1551 - 1555
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