title: Alkali-metal ion catalysis and inhibition in SNAr displacement: relative stabilization of ground state and transition state determines catalysis and inhibition in SNAr reactivity.
abstract: We report here the first observation of alkali-metal ion catalysis and inhibition in SNAr reactions. The plot of kobsd versus [alkali-metal ethoxide] exhibits downward curvature for the reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene with EtOLi, EtONa, and EtOK, but upward curvature for the corresponding reaction with EtOK in the presence of 18-crown-6-ether (18C6). Dissection of kobsd into the second-order rate constants for the reactions with the dissociated EtO(-) and the ion-paired EtOM (i.e., k EtO - and kEtOM , respectively) has revealed that the reactivity increases in the order EtOLi<EtONa<EtOK<EtO(-)<EtOK/18C6. This indicates that the reaction is inhibited by Li(+), Na(+), and K(+) ions but is catalyzed by 18C6 K(+) ion. The reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes have been proposed to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate-determining step based on the kinetic result that σ(o) constants exhibit a much better Hammett correlation than σ(-) constants. Alkali-metal ion catalysis or inhibition has been discussed in terms of differential stabilization of ground-state and transition-state complexes through a qualitative energy profile. A π-complexed transition-state structure is proposed to account for the kinetic results.
authors.affiliation: Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750 (Korea). ihum@ewha.ac.kr.
authors.forename: Ik-Hwan
authors.initials: IH
authors.lastname: Um
authors.forename: Hyo-Jin
authors.initials: HJ
authors.lastname: Cho
authors.forename: Min-Young
authors.initials: MY
authors.lastname: Kim
authors.forename: Erwin
authors.initials: E
authors.lastname: Buncel
keywords: Pi-complex; SNAr reaction; alkali metals; catalysis; inhibition; nucleofugality