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

作者：Kim Min Young; Ha Gyu Ho; Um Ik Hwan^*

来源：BULLETIN OF THE KOREAN CHEMICAL SOCIETY, 2014, 35(8): 2438-2442.

DOI：10.5012/bkcs.2014.35.8.2438

• 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 k(EtOLi) %26lt; k(EtO-) %26lt; k(EtONa) %26lt; k(EtOK) %26lt; k(EtOK/18C6) for the reaction of 5a and k(EtOLi) %26lt; k(EtONa) %26lt; k(EtO-) %26lt; k(EtOK) %26lt; 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 pi-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 pi-complex between M+ ion and the pi-electrons in the benzene ring.

• 出版日期2014-8-20