Alkali-Metal Ion Catalysis and Inhibition in S_NAr Reaction of 1-Halo-2,4-dinitrobenzenes with Alkali-Metal Ethoxides in Anhydrous Ethanol

Abstract

A kinetic study is reported for S N Ar 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 k EtOLi < k EtO − < k EtONa < k EtOK < k EtOK/18C6 for the reaction of 5a and k EtOLi < k EtONa < k EtO − < k EtOK < k EtOK/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 pK a 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.