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

摘要

A kinetic study is reported for S_nAt reaction of 1-fluoro-2,4-dinitrobenzene (5a) and 1-chloro-2,4-dinitro-benzenc (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 thatof 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.