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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 31, Number 12
BKCSDE 31(12)
December 20, 2010 

 
Title
Metal Ion Catalysis in Nucleophilic Displacement Reactions of 2-Pyridyl X-Substituted Benzoates with Potassium Ethoxide in Anhydrous Ethanol
Author
Jae In Lee*, Ji Sun Kang, Li Ra Im, Ik Hwan Um*
Keywords
2-Pyridyl benzoate, Metal ion catalysis, Ion-pair, Transition state, Hammett plot
Abstract
A kinetic study on nucleophilic displacement reactions of 2-pyridyl X-substituted benzoates 1a-e with potassium ethoxide (EtOK) in anhydrous ethanol is reported. Plots of pseudo-first-order rate constants (kobsd) vs. [EtOK]o exhibit upward curvature. The kobsd value at a fixed [EtOK]o decreases steeply upon addition of 18-crown-6-ether (18C6) to the reaction mixture up to [18C6]/[EtOK]o = 1 and then remains nearly constant thereafter. In contrast, kobsd increases sharply upon addition of LiSCN or KSCN. Dissection of kobsd into kEtO‒ and kEtOM has revealed that ion-paired EtOK is more reactive than dissociated EtO, indicating that K+ ion acts as a Lewis acid catalyst. Hammett plots for the reactions of 1a-e with dissociated EtO and ion-paired EtOK result in excellent linear correlation with ρ values of 3.01 and 2.67, respectively. The kEtOK/kEtO ratio increases as the substituent X in the benzoyl moiety becomes a stronger electron-donating group. K+ ion has been concluded to catalyze the current reaction by stabilizing the transition state through formation of a 6-membered cyclic complex.
Page
3543 - 3548
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