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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 5
BKCSDE 35(5)
May 20, 2014 

 
Title
Alkali-Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Picolinate with Alkali Metal Ethoxides: Effect of Modification of Nonleaving Group from Benzoyl to Picolinyl on Reactivity and Transition State Structure
Author
Seong Hoon Jeon, Jung Hwan Yoon, Min-Young Kim, Ik-Hwan Um*
Keywords
Metal-ion catalysis, Crown ether, Electrophilicity, Nucleofugality, Transition state
Abstract
A kinetic study on nucleophilic substitution reaction of 5-nitro-8-quinolyl picolinate (6) with alkali-metal ethoxides (EtOM; M = K, Na, and Li) in anhydrous ethanol is reported. The plot of kobsd vs. [EtOM] curves upward in the absence of crown ethers but is linear with significantly decreased reactivity in the presence of crown ethers. Dissection of kobsd into kEtO− and kEtOM (i.e., the second-order rate constants for the reactions with the dissociated EtO– and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is significantly more reactive than the dissociated EtO– (e.g., kEtOM/kEtO− = 33.4-141). This indicates that the reaction of 6 is catalyzed by M+ ions in the order Na+ > Li+ > K+ and the catalytic effect disappears in the presence of a proper crown ether. Picolinate ester 6 is much more reactive and is more strongly catalyzed by M+ ions than 5-nitro- 8-quinolyl benzoate (5). It has been concluded that M+ ions catalyze the reaction of 6 by increasing electrophilicity of the reaction center through a cyclic transition state, which is structurally not possible for the reaction of 5.
Page
1506 - 1510
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