Alkali‐Metal Ion Catalysis and Inhibition in SNAr Displacement: Relative Stabilization of Ground State and Transition State Determines Catalysis and Inhibition in SNAr Reactivity |
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Authors: | Prof Ik‐Hwan Um Hyo‐Jin Cho Min‐Young Kim Prof Erwin Buncel |
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Institution: | 1. Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120‐750 (Korea);2. Department of Chemistry, Ducksung Women's University, Seoul 132‐714 (Korea);3. Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6 (Canada), Fax: (+1)?613‐533‐6669 |
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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 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 σo 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. |
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Keywords: | alkali metals catalysis inhibition nucleofugality SNAr reaction Pi‐complex |
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