Alkali-Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Picolinate with Alkali Metal Ethoxides: Effect of Modification of Nonleaving Group from Benzoyl to Picolinyl on Reactivity and Transition State Structure

Abstract

A kinetic study on nucleophilic substitution reaction of 5-nitro-8-quinolyl picolinate (6) with alkali-metal ethoxides (EtOM; M = K, Na, and Li) in anhydrous ethanol is reported. The plot of <TEX>$k_{obsd}$</TEX> vs. [EtOM] curves upward in the absence of crown ethers but is linear with significantly decreased reactivity in the presence of crown ethers. Dissection of <TEX>$k_{obsd}$</TEX> into <TEX>$k_{EtO}$</TEX>- and <TEX>$k_{EtOM}$</TEX> (i.e., the second-order rate constants for the reactions with the dissociated <TEX>$EtO^-$</TEX> and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is significantly more reactive than the dissociated <TEX>$EtO^-$</TEX> (e.g., <TEX>$k_{EtOM}/k_{EtO^-}$</TEX> = 33.4-141). This indicates that the reaction of 6 is catalyzed by <TEX>$M^+$</TEX> ions in the order <TEX>$Na^+$</TEX> > <TEX>$Li^+$</TEX> > <TEX>$K^+$</TEX> and the catalytic effect disappears in the presence of a proper crown ether. Picolinate ester 6 is much more reactive and is more strongly catalyzed by <TEX>$M^+$</TEX> ions than 5-nitro-8-quinolyl benzoate (5). It has been concluded that <TEX>$M^+$</TEX> ions catalyze the reaction of 6 by increasing electrophilicity of the reaction center through a cyclic transition state, which is structurally not possible for the reaction of 5.