Effect of Alkali Metal Ions on Alkaline Ethanolysis of 2-Pyridyl and 4-Pyridyl Benzoates in Anhydrous Ethanol

Bulletin of the Korean chemical society

Ik Hwan Um¹  Song I Kim¹  Jae In Lee¹  Ji Sun Kang¹ 

关键词: Ethanolysis;    Metal ion catalysis;    Electrophilicity;    Nucleofugality;    Transition state;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

Pseudo-first-order rate constants (kobsd) have been measured for nucleophilic substitution reactions of 2-pyridyl benzoate 5 with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol. The plots of kobsd vs. [EtOM]o are curved upwardly but linear in the excess presence of 18-crown-6-ether (18C6) with significant decreased kobsd values in the reaction with EtOK. The kobsd value for the reaction of 5 with a given EtONa concentration decreases steeply upon addition of 15-crown-5-ether (15C5) to the reaction medium up to ca. [15C5]/[EtONa]o = 1, and remains nearly constant thereafter, indicating that M+ ions catalyze the reaction in the absence of the complexing agents. Dissection kobsd into kEtO- and kEtOM, i.e., the second-order rate constants for the reaction with the dissociated EtO- and the ion-paired EtOM, respectively has revealed that ion-paired EtOM is 3.2 - 4.6 times more reactive than dissociated EtO-. It has been concluded that M+ ions increase the electrophilicity of the reaction center through a 6-membered cyclic transition state. This idea has been examined from the corresponding reactions of 4-pyridyl benzoate 6, which cannot form such a 6-membered cyclic transition state.

【 授权许可】

Unknown