S<inf>N</inf>Ar reactions of 1-halo-2,4-dinitrobenzenes with alkali-metal ethoxides: Differential stabilization of ground state and transition state determines alkali-metal ion catalysis or inhibition

Abstract

A kinetic study on S<inf>N</inf>Ar reactions of 1-halo-2,4-dinitrobenzenes (6a-6d) with alkali-metal ethoxides (EtOM; M = Li, Na, K and 18-crown-6-ether-complexed K) is reported. The plots of pseudo-first-order rate constant (k<inf>obsd</inf>) vs. [EtOM] curve upward or downward depending on the size of M+ ions. The reactions are catalyzed or inhibited by the M+ ions, e.g., the large K+ ion complexed by 18-crown-6-ether (18C6) acts as a catalyst while the small Li+ and Na+ ions behave as an inhibitor. Reactivity of 6a-6d toward EtOM decreases linearly as the halide ion becomes less basic regardless of the size of M+ ions, indicating that expulsion of the leaving group occurs after the rate-determining step (RDS). Thus, the reactions have been proposed to proceed through a stepwise mechanism with formation of a Meisenheimer complex being the RDS. Computational studies using B3LYP density functional theory have revealed that Mulliken charge density of the electrophilic center decreases as the halogen atom becomes less electronegative. Thus, it has been concluded that the S<inf>N</inf>Ar reactivity of 6a-6d toward EtOM is governed by electrophilicity of the reaction center but not by nucleofugality of the leaving group. A π-complexed transition-state structure has been proposed to account for the experimental and computational results. © 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.