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Kinetic Studies on Nucleophilic Substitution Reactions of Aryl Benzoates, Aryl Carbonates, Aryl Benzenesulfonates and O-Ethyl O-4-Nitrophenyl Phenylphosphonothioate, and Michael-type Reaction of X-Substituted β-Nitrostyrenes

Title
Kinetic Studies on Nucleophilic Substitution Reactions of Aryl Benzoates, Aryl Carbonates, Aryl Benzenesulfonates and O-Ethyl O-4-Nitrophenyl Phenylphosphonothioate, and Michael-type Reaction of X-Substituted β-Nitrostyrenes
Authors
강지선
Issue Date
2013
Department/Major
대학원 화학·나노과학과
Publisher
이화여자대학교 대학원
Degree
Master
Advisors
엄익환
Abstract
1. Effect of Alkali Metal Ions on Alkaline Ethanolysis of 2-Pyridyl and 4-Pyridyl Benzoates in Anhydrous Ethanol Pseudo-first-order rate constants (kobsd) have been measured for nucleophilic substitution reactions of 2-pyridyl benzoate 1c with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol. The plots of kobsd vs. [EtOM] 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 1c with a given EtONa concentration decreases steeply upon addition of 15-crown-5-ether (15C5) to the reaction medium up to ca. [15C5]/[EtONa] = 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 2, which cannot form such a 6-membered cyclic transition state. 2. Metal Ion Catalysis in Nucleophilic Displacement Reactions of 2-Pyridyl X-Substituted Benzoates with Potassium Ethoxide in Anhydrous Ethanol A kinetic study on nucleophilic displacement reactions of 2-pyridyl X-substituted benzoates 1a-e with potassium ethoxide (EtOK) in anhydrous ethanol is reported. Plots of pseudo-first-order rate constants (kobsd) vs. [EtOK] exhibit upward curvature. The kobsd value at a fixed [EtOK]o decreases steeply upon addition of 18-crown-6-ether (18C6) to the reaction mixture up to [18C6]/[EtOK] = 1 and then remains nearly constant thereafter. In contrast, kobsd increases sharply upon addition of LiSCN or KSCN. Dissection of kobsd into kEtO– and kEtOM has revealed that ion-paired EtOK is more reactive than dissociated EtO–, indicating that K+ ion acts as a Lewis acid catalyst. Hammett plots for the reactions of 1a-e with dissociated EtO– and ion-paired EtOK result in excellent linear correlation with ρ values of 2.98 and 2.67, respectively. The kEtOK/kEtO– ratio increases as the substituent X in the benzoyl moiety becomes a stronger electron-donating group. K+ ion has been concluded to catalyze the current reaction by stabilizing the transition state through formation of a 6-membered cyclic complex. 3. A Kinetic Study on Nucleophilic Displacement Reaction of Y-Substituted-Phenyl 2-Methoxybenzoates with Potassium Ethoxide: Effect of Leaving Group on Reaction Mechanism and Role of K+ Ion on Transition State A kinetic study on nucleophilic displacement reactions of Y-substituted-phenyl 2-methoxybenzoates 3a-h with potassium ethoxide (EtOK) in anhydrous ethanol is reported. Plots of pseudo-first-order rate constants (kobsd) vs. [EtOK] exhibit upward curvature. Dissection of kobsd into kEtO– and kEtOK, i.e., the second-order rate constants for the reaction with the dissociated EtO– and the ion-paired EtOK, respectively has revealed that the ion-paired EtOM is more reactive than dissociated EtO–. Brønsted-type plots for the reactions of 3a-h with the dissociated EtO– and ion-paired EtOK are linear with βlg = −0.55 and −0.44, respectively, which is typical for reactions reported previously to proceed through a concerted mechanism. The Yukawa-Tsuno plots result in an excellent linear correlation with ρY = 2.12 and r = 0.22 for kEtO− , and ρY = 1.61 and r = 0.18 for kEtOK. It has been concluded that the reactions proceed through a concerted mechanism with a 6-membered cyclic transtion state. 4. Effect of Alkali Metal Ions and Reaction Mechanism for Alkaline Ethanolysis of Y-Substituted-Phenyl Phenyl Thionocarbonates in Anhydrous Ethanol Pseudo-first-order rate constants (kobsd) have been measured for reactions of Y-substituted- phenyl phenyl thionocarbonates 4a-h with alkali metal ethoxides (EtOM, M = Li, Na, and K). The plots of kobsd vs. [EtOM] curve downward for the reactions of 4a with EtOLi and EtONa. In contrast, the corresponding plot curves upward for that with EtOK in the presence of 18-crown-6-ether (18C6). The kobsd values for the reactions with EtOM have been dissected into kEtO– and kEtOM, i.e., the second-order rate constants for the reactions of 4a-h with dissociated EtO– and ion-paired EtOM, respectively. The reactivity of EtOM toward 4a increases in the order EtOLi < EtONa
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