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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 33, Number 2
BKCSDE 33(2)
February 20, 2012 

Alkali-Metal Ion Catalysis in Alkaline Ethanolysis of 2-Pyridyl Benzoate and Benzyl 2-Pyridyl Carbonate: Effect of Modification of Nonleaving Group from Benzoyl to Benzyloxycarbonyl
Ik-Hwan Um*, Ji-Sun Kang, Chae Won Kim, Jae-In Lee
Metal ion catalysis, Ethanolysis, Electrophilicity, Nucleofugality, Transition state
A kinetic study is reported on nucleophilic displacement reactions of benzyl 2-pyridyl carbonate 6 with alkalimetal ethoxides, EtOM (M = Li, Na, and K), in anhydrous ethanol at 25.0 ± 0.1 oC. The plots of pseudo-firstorder rate constant kobsd vs. [EtOM] curve upward, a typical phenomenon reported previously for alkaline ethanolysis of esters in which alkali-metal ions behave as a Lewis-acid catalyst. The kobsd value for the reaction of 6 with a fixed EtOK concentration decreases rapidly upon addition of 18-crown-6-ether (18C6), a complexing agent for K+ ion up to [18C6]/[EtOK] = 1.0 and then remains constant thereafter, indicating that the catalytic effect exerted by K+ ion disappears in the presence of excess 18C6. The reactivity of EtOM towards 6 increases in the order EtO– < EtOLi < EtONa < EtOK, which is contrasting to the reactivity order reported for the corresponding reactions of 2-pyridyl benzoate 4, i.e., EtO– < EtOK < EtONa < EtOLi. Besides, 6 is 1.7 and 3.5 times more reactive than 4 towards dissociated EtO– and ion-paired EtOK, respectively. The reactivity difference and the contrasting metal-ion selectivity are discussed in terms of electronic effects and transition-state structures.
519 - 523
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