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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 6
BKCSDE 32(6)
June 20, 2011 

 
Title
Metal Ion Catalysis and Inhibition in Nucleophilic Substitution Reactions of 4-Nitrophenyl Nicotinate and Isonicotinate with Alkali Metal Ethoxides in Anhydrous Ethanol
Author
Seo-Young Choi, Yeon-Ju Hong, Ik-Hwan Um *
Keywords
Alkali metal ions, Metal ion catalysis, Inhibition, Ion pair, Cyclic transition state
Abstract
A kinetic study is reported on nucleophilic substitution reactions of 4-nitrophenyl nicotinate 5 and isonicotinate 6 with alkali metal ethoxide EtOM (M = K, Na, and Li) in anhydrous ethanol at 25.0 ± 0.1 oC. Plots of pseudofirst- order rate constant kobsd vs. EtOM concentration exhibit upward curvature for the reactions of 5 and 6 with EtOK and EtONa but are almost linear for those with EtOLi. Dissection of kobsd into kEtO− and kEtOM (i.e., the second-order rate constant for the reaction with dissociated EtO– and ion-paired EtOM, respectively) has shown that kEtOK ≥ kEtONa > kEtO− but kEtOLi < kEtO−. It has been concluded that K+ and Na+ ions catalyze the reactions by increasing the electrophilicity of the carbonyl carbon atom through formation of a 4-membered cyclic transition state TS3 or TS4. However, M+ ion catalysis has been found to be much less significant for the reactions of 5 and 6 than for the corresponding reactions of 4-nitrophenyl picolinate 4, which was reported to proceed through a 5-membered cyclic transition state TS2. Although 5 and 6 are significantly more reactive than 4-nitrophenyl benzoate 3, the reactions of 5 and 6 result in smaller kEtOK/kEtO− ratios than those of 3. The electron-withdrawing ability of the nitrogen atom in the acyl moiety of 5 and 6 has been suggested to be responsible for the increase in reactivity and the decrease in the kEtOK/kEtO− ratio.
Page
1951 - 1956
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