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Metal Ion Effects on Nucleophilic Substitution Reactions of Aryl2-Furoates and 2-Thiophenates

Metal Ion Effects on Nucleophilic Substitution Reactions of Aryl2-Furoates and 2-Thiophenates
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대학원 화학과
Metal ion effectsnucleophilic substitution reactionsaryl2-furoates2-thiophenates
이화여자대학교 대학원
The rate of the nucleophilic substitution reactions of aryl 2-furoates and 2-thiopbenates with ethoxide in ethanol at 25.0 ℃ has been studied by spectrophotometric techniques. Hammett treatment of leaving group effects results in good correlation of rate data with σ˚ substituent constants and large ρ˚ values of 1.90 (furoate system), 2.06 (thiophenate system) but poor σ^(-) correlation. A rate-determining transition state having well-advanced EtzO-C bond formation but little C-OAr bond breakage is proposed. For the reactions of alkali metal ethoxides with p- or m-nitrophenyl 2-furoate and p- or m-nitrophenyl 2-thiophenate, the order of reactivity is EtO^(-) ≤ EtOLi < EtONa ≤ EtOK. The effect of added 18-crown-6 ether complexing agent was also investigated. Addition of 18C6 to the reaction of EtOK results in the rate decreasing to a minimum value corresponding to the reaction of free ethoxide. The involvement of metal ions in these reactions is proposed to occur via free metal ions and reactive alkali metal ethoxide ion pairs. The kinetic data are analyzed in terms of an ion pairing treatment that allows the calculation of second-order rate constants for catalyzed and uncatalyzed pathways; the rate constants increase in the order EtO^(-) ≤ EtOLi < EtONa ≤ EtOK, indicating that alkali metal ions studied are all catalysts in most cases. Possible roles of the metal ions are discussed in terms of ground state and transition state stabilization. Assuming that only ion pairs are catalysts, equilibrium constants for the association of the various metal ions with the transition state are calculated using a thermodynamic cycle, and are compared to association constants in the ground state; Li^(+), Na^(+), and K^(+) a11 stabilize the transition state more than the ground state in most cases. p-Nitrophenyl ester is more reactive than m-nitrophenyl ester toward alkali ethoxides, but metal ion catalysis is more significant in m-nitro system. The results rationalized in terms of the extent of charge polarization of C=O bond in the two transition states. The trend in the magnitude of the transition state stabilization is interpreted in terms of interactions of the transition state with bare or solvated metal ions. It is concluded that the transition state for the reaction of furoates with ethoxide forms solvent separated ion pairs with alkali metal ions and that of thiophenates forms contact ion pairs. The difference is attributed to a greater localization of negative charge in the thiophenate transition state, leading to stronger interactions with metal ions, which overcome metal ion-solvent interactions.;25.0℃ ethanol에서 aryl 2-furoates 흑은 aryl 2-thiophenates와 ethoxide와의 친핵성 치환 반응의 속도를 분광학적으로 관찰하였다. 반응 이탈기 효과에 대해 Hammett treatment를 해 본 결과, σ^(-)보다는 σ^(0) 상수와 더 좋은 상관관계를 보였고 비교적 큰 P^(0) 값 (furoate system에서는 1.90, thiophenate system에서는 2.06)을 가졌다. 따라서 속도 결정 단계의 전이상태는, EtO-C bond 형성은 많이 진행되고 C-OAr bond 파괴는 거의 진행되지 않은 구조인 것으로 생각되었다. Alkali metal ethoxides와 p- or m-nitrophenyl 2-furoate 혹은 p- or m-nitrophenyl 2-thiophenate와의 반응에서는, EtO^(-)≤EtOLi < EtONa ≤EtOK의 순서로 반응성이 증가하였다. 또한, 18-crown-6 ether complexing agent를 첨가하여 그 효과를 관찰했는데, EtOK의 반응에 18C6가 첨가될수록 반응 속도는 점차 감소하여 결국 free ethoxide의 반응성으로까지 감소하는 것을 볼 수 있었다. 금속 이온은 free metal ion 혹은 reactive한 alkali metal ethoxide ion pair의 형태로 반응에 참여하는 것으로 생각되었는데, ion pairing treatment를 이용해서 kinetic data를 분석함으로써 촉매화된 경로와 촉매화되지 않은 경로에서의 이차 속도 상수를 계산하였다. 계산 결과, 속도 상수는 EtO^(-)≤EtOLi
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