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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 kEtOand 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.

bulletin of the korean chemical society

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