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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 8
BKCSDE 35(8)
August 20, 2014 

Alkali-Metal Ion Catalysis and Inhibition in SNAr Reaction of 1-Halo-2,4-dinitrobenzenes with Alkali-Metal Ethoxides in Anhydrous Ethanol
Min-Young Kim, Gyu Ho Ha, Ik-Hwan Um*
Alkali-metal ion, Lewis acid catalyst, Inhibitor, Nucleofugality, Electrophilicity
A kinetic study is reported for SNAr reaction of 1-fluoro-2,4-dinitrobenzene (5a) and 1-chloro-2,4-dinitrobenzene (5b) with alkali-metal ethoxides (EtOM, M = Li, Na, K and 18-crown-6-ether complexed K) in anhydrous ethanol. The second-order rate constant increases in the order kEtOLi < kEtO− < kEtONa < kEtOK < kEtOK/18C6 for the reaction of 5a and kEtOLi < kEtONa < kEtO− < kEtOK < kEtOK/18C6 for that of 5b. This indicates that M+ ion behaves as a catalyst or an inhibitor depending on the size of M+ ion and the nature of the leaving group (F– vs. Cl–). Substrate 5a is more reactive than 5b, although the F– in 5a is ca. 10 pKa units more basic than the Cl– in 5b, indicating that the reaction proceeds through a Meisenheimer complex in which expulsion of the leaving group occurs after the rate-determining step (RDS). M+ ion would catalyze the reaction by increasing either the nucleofugality of the leaving group through a four-membered cyclic transition state or the electrophilicity of the reaction center through a π-complex. However, the enhanced nucleofugality would be ineffective for the current reaction, since expulsion of the leaving group occurs after the RDS. Thus, it has been concluded that M+ ion catalyzes the reaction by increasing the electrophilicity of the reaction center through a π-complex between M+ ion and the π-electrons in the benzene ring.
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