Metal Ion Catalysis and Inhibition in Nucleophilic Substitution Reactions of 4-Nitrophenyl Nicotinate and Isonicotinate with Alkali Metal Ethoxides in Anhydrous Ethanol

Bulletin of the Korean chemical society

Ik-Hwan Um¹  Seo-Young Choi¹  Yeon-Ju Hong¹ 

关键词: Alkali metal ions;    Metal ion catalysis;    Inhibition;    Ion pair;    Cyclic transition state;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

A kinetic study is reported on nucleophilic substitution reactions of 4-nitrophenyl nicotinate 5 and isonicotinate 6 with alkali metal ethoxide EtOM (M = K, Na, and Li) in anhydrous ethanol at 25.0 ± 0.1 oC. Plots of pseudofirst- order rate constant kobsd vs. EtOM concentration exhibit upward curvature for the reactions of 5 and 6 with EtOK and EtONa but are almost linear for those with EtOLi. Dissection of kobsd into kEtO− and kEtOM (i.e., the second-order rate constant for the reaction with dissociated EtO– and ion-paired EtOM, respectively) has shown that kEtOK ≥ kEtONa > kEtO− but kEtOLi < kEtO−. It has been concluded that K+ and Na+ ions catalyze the reactions by increasing the electrophilicity of the carbonyl carbon atom through formation of a 4-membered cyclic transition state TS3 or TS4. However, M+ ion catalysis has been found to be much less significant for the reactions of 5 and 6 than for the corresponding reactions of 4-nitrophenyl picolinate 4, which was reported to proceed through a 5-membered cyclic transition state TS2. Although 5 and 6 are significantly more reactive than 4-nitrophenyl benzoate 3, the reactions of 5 and 6 result in smaller kEtOK/kEtO− ratios than those of 3. The electron-withdrawing ability of the nitrogen atom in the acyl moiety of 5 and 6 has been suggested to be responsible for the increase in reactivity and the decrease in the kEtOK/kEtO− ratio.

【 授权许可】

Unknown