Metal Ion Catalysis in Nucleophilic Substitution Reaction of 4-Nitrophenyl Picolinate with Alkali Metal Ethoxides in Anhydrous Ethanol

Bulletin of the Korean chemical society

Ik Hwan Um¹  Song I Kim¹  Yeon Ju Hong¹ 

关键词: Metal ion catalysis;    Ion pair;    Ethanolysis;    Electrophilicity;    Nucleofugality;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

Pseudo-first-order rate constants (k_obsd) were measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl picolinate (6) with alkali metal ethoxides (EtOM, M⁺ = K⁺, Na⁺ and Li⁺) in anhydrous ethanol at 25.0 �? 0.1 ^oC. The plot of k_obsd vs. [EtOM] exhibits upward curvature regardless of the nature of M⁺ ions. However, the plot for the reaction of 6 with EtOK is linear with significantly decreased k_obsd values when 18-crown-6-ether (18C6, a complexing agent for K⁺ ion) is added in the reaction medium. Dissection of k_obsd into k_EtO^- and k_EtOM (i.e., the second-order rate constant for the reaction with dissociated EtO- and ion-paired EtOM, respectively) has revealed that ion-paired EtOM is 3 ~ 17 times more reactive than dissociated EtO^-. The reaction has been proposed to proceed through a 5-membered cyclic transition state, in which M⁺ ion increases the electrophilicity of the reaction site. Interestingly, Na⁺ ion exhibits the largest catalytic effect. The presence of a nitrogen atom in the pyridine moiety of 6 has been suggested to be responsible for the high Na⁺ ion selectivity.

【 授权许可】

Unknown