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

摘要

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 kobsd 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 kobsd into kEtO− and kEtOM (i.e., the second-order rate constants for the reactions with the dissociated EtO– and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is significantly more reactive than the dissociated EtO– (e.g., kEtOM/kEtO− = 33.4-141). This indicates that the reaction of 6 is catalyzed by M+ ions in the order Na+ Li+ K+ 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 M+ ions than 5-nitro- 8-quinolyl benzoate (5). It has been concluded that M+ 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.