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Mechanistic Studies on Nucleophilic Substitution Reactions of Organophosphorus Compounds : Reaction Mechanism and Metal Ion Effects

Title
Mechanistic Studies on Nucleophilic Substitution Reactions of Organophosphorus Compounds : Reaction Mechanism and Metal Ion Effects
Authors
신영희
Issue Date
2007
Department/Major
대학원 나노과학부
Publisher
이화여자대학교 대학원
Degree
Master
Abstract
본 논문은 P=O 또는 P=S 를 친전자 중심으로 갖는 유기인 화합물의 친핵성 치환반응에 관한 것으로, Chapter Ⅰ에서는 aminolysis를 Chapter Ⅱ에서는 alkaline ethanolysis를 다루었다. Ⅰ.(1) Aryl diphenylphosphinates (1a-j)와 aryl benzoates (2a-i)의 가아민 분해반응을 비교해 봄으로써, 반응기질의 친전자 중심을 C=O 에서 P=O 로 변화시키는 것이 반응성과 반응 메커니즘에 미치는 효과에 대하여 살펴보았다. 4-nitrophenyl diphenylphosphinate (2a) 와 일련의 고리형 이차아민류와의 반응,그리고 이탈기의 치환기를 변화시킨 2a-i와 piperidine과의 반응에서 각 Brønsted-type 그래프는 모두 휘어지는 모양을 나타내었으며, 이것은 2a-i의 반응이 RDS 변화를 수반하는 stepwise 메커니즘으로 진행됨을 의미한다. 반면, 이에 상응하는 1a-j의 반응에서 Brønsted-type 그래프는 β_(nuc) = 0.38와 β_(lg) = -0.66 인 직선형태로 나타나, 1a-j의 반응이 concerted 메커니즘으로 진행됨을 알 수 있다. 한편, 1a-j의 반응에 대한 Hammett 그래프는 σ^(-) 보다 σ^(o) 와 더 좋은 직선 상관관계를 보였다. 이와 같이 σo 와 좋은 상관관계를 보이는 Hammett 그래프는 stepwise 메커니즘으로 진행되는 반응이라고 주로 해석되어 앞의 연구내용과 상반되는 결과이다. 그러나 본 실험결과에 Yukawa-Tsuno 방정식을 도입하여 그래프를 도시해 보았을 때, Hammett 그래프보다 훨씬 탁월한 직선 상관관계를 보였다. 이 때 r 값이 0.30으로 비교적 작은 수치를 나타내었으며, 이는 1a-j 반응이 concerted 메커니즘으로 진행되고, 전이상태에서 이탈기의 이탈 정도는 비교적 작다는 것을 의미한다. Ⅰ.(2) Aryl diphenylphosphinothioates (3a-j)와 aryl diphenylphosphinates (1a-j)의 가아민 분해반응을 비교해 봄으로써, 반응기질의 친전자 중심을 P=O 에서 P=S 로 변화시키는 것이 반응성과 반응 메커니즘에 미치는 효과에 대하여 살펴보았다. P=S 화합물(3a-j)의 반응은 이에 상응하는 P=O 화합물(1a-j)의 반응보다 모두 반응성이 작게 나타났다. 3a-j와 1a-j의 반응에서 얻은 Brønstedtype 그래프는 모두 작은 크기의 β_(nuc)와 β_(lg) 값을 가지는 직선형태를 나타내었고, 또한 작은 크기의 r 값을 갖는 Yukawa-Tsuno 그래프가 얻어져, 이들 P=O 화합물과 P=S 화합물의 반응이 모두 concerted 메커니즘으로 진행된다는 것을 알 수 있다. 한편, 3a가 1a보다 더 큰 βnuc 값을 가지고, TΔS^(‡) 값은 더 음의 값을 가진다는 사실을 근거로, 3a가 1a 보다 더 tight 한 전이상태 구조를 갖는다는 결론을 내릴 수 있다. Ⅱ.(1) Phosphoryl 과 thionophosphoryl 화합물의 alkaline ethanolysis를 살펴봄으로써, 알칼리 금속이온이 이들 화합물의 분해 반응에 미치는 효과에 관하여 연구하였다. Ethyl paraoxon (4a), methyl paraoxon (5a)과 EtO^(–)M^(+)과의 반응에서 알칼리 금속이온은 반응에서 촉매제 역할을 하였으며, 금속이온의 크기가 작아질수록 그 촉매효과는 증가하였다. 반면, ethyl parathion(4b), methyl parathion(5b)과 EtO^(–)M^(+)과의 반응에서는 EtO^(–)Li^(+)와 EtO^(–)Na^(+)가 저해효과를 나타내어, P=O 화합물의 반응과 반대의 경향성을 나타내었다. 이와 같은 촉매효과는 4원자 고리형 전이상태 구조로 인한 전이상태 안정화에서 기인한 것으로 설명할 수 있다. 18C6-complexed K^(+)는 EtO^(-)와 약한 이온쌍 형성이 가능하고, 이와 같이 형성된 이온쌍은 5b와의 반응에서는 촉매효과를 나타내었으나, 4b와의 반응에서는 입체장애로 인하여 촉매효과가 나타나지 않았다. Ⅱ.(2) 4-nitrophenyl diphenylphosphinothioate (3d)의 alkaline ethanolysis를 수행하여, 알칼리 금속이온이 thionophosphinyl 기 이동반응에 미치는 효과에 대하여 연구하였다. P=S 화합물인 4-nitrophenyl diphenylphosphinothioate (3d)와 EtO–M+와의 반응에서 EtO–Li^(+)과 EtO^(–)Na^(+)는 저해효과를 나타내었다. 그러나 3d에 상응하는 P=O 화합물인 4-nitrophenyl diphenylphosphinate (1d)와의 반응에서는 EtO^(–)M^(+)이 모두 EtO-보다 더 큰 반응성을 보였고, 이와 같은 촉매효과는 알칼리 금속이온의 크기가 작아질수록 증가하였다. 3d의 반응에서 보인Li^(+)이온의 저해효과와 K+이온의 촉매효과는 Li^(+)과 K^(+) 염을 반응 혼합물에 첨가해 봄으로써 명확히 확인 할 수 있었다. Crown ether-complexed M^(+)는 EtO^(-)과약한 이온쌍 형성이 가능하고 이 이온쌍은 3d와의 반응에서 촉매효과를 나타내었다. 이와 같은 촉매효과는 전이상태에서 soft한 acid인 crown ethercomplexed M^(+)와 soft한 base인 기질 P=S의 S원자 간의 상호작용으로 인해 전이상태가 안정화되어 나타나는 효과이다.;In chapterⅠ, two series of aminolyses have been performed as below. Ⅰ. (1) Aminolysis of Y-Substituted Phenyl Diphenylphosphinates and Benzoates: Effect of Modification of Electrophilic Center from C=O to P=O. The effect of modification of electrophilic center from C=O to P=O on reactivity and reaction mechanism has been investigated for aminolysis of Y-substituted phenyl diphenylphosphinates (1a-j) and benzoates (2a-i). 1a-j are less reactive than 2a-i. The reactions of 2,4-dinitrophenyl diphenylphosphinate (1a) with alicyclic secondary amines resulted in a linear Brønsted-type plot with a β_(nuc) value of 0.38, while the corresponding reactions of 2,4-dinitrophenyl benzoate (2a) yielded a curved Brønsted-type plot. Similarly, a linear Brønsted-type plot with a β_(lg) value of -0.66 was obtained for the reactions of 1a-j with piperidine, while the corresponding reactions of 2a-i gave a curved Brønsted-type plot. The linear Brønsted-type plots for the reactions of 1a-j have been taken as evidence for a concerted mechanism, while the curved Brønsted-type plots for the reactions of 2a-i have been suggested to indicate a change in the rate determining step of a stepwise mechanism. The Hammett plot for the reactions of 1b-j exhibited a poor correlation with σ^(-) constants (R² = 0.962) but slightly better correlation with σ^(˚) (R² = 0.986). However, the Yukawa-Tsuno plot for the same reactions resulted in an excellent correlation (R² = 0.9993) with an r value of 0.30. The aminolysis of 1a-j has been suggested to proceed through a concerted mechanism with an early transition state on the basis of the small β_(nuc) and small r values. (2) Aminolyses of Aryl Diphenylphosphinates and Diphenylphosphinothioates: Effect of Modification of Electrophilic Center from P=O to P=S. A kinetic study is reported for aminolysis of Y-substituted phenyl diphenylphosphinothioates (3a-j). The phosphinothioates 3a-j are less reactive than aryl diphenylphosphinates (1a-j), the oxygen analogues of 3a-j, regardless of the basicity of the leaving aryloxides or the attacking amines. The Yukawa-Tsuno plot for the reactions of 3b-j with piperidine exhibits good linearity with a small r value (r = 0.28), indicating that the leaving group departs at the rate-determining step (RDS) with a small degree of bond fission. Reactions of 2,4-dinitrophenyl diphenylphosphinothioate (3a) with alicyclic secondary amines result in a good linear Brønsted-type plot with β_(nuc) = 0.52, implying that the reactions proceed through a concerted mechanism. The β_(nuc) value determined for the reactions of 3a is slightly larger than that reported for the corresponding reactions of 2,4-dinitrophenyl diphenylphosphinate (1a, i.e., β_(nuc) = 0.38), suggesting that reactions of 3a proceed through a tighter transition state (TS) than that of 1a. The reaction of 3a with piperidine exhibits ca. 0.4 kcal/mol more favorable enthalpy of activation (ΔH^(‡)) than that of 1a. On the contrary, the entropy of activation at 25.0 oC (TΔS^(‡)) is ca. 1.5 kcal/mol more unfavorable for the reaction of 3a than for that of 1a. This result supports the proposal that the reaction of 3a proceeds through a tighter TS than that of 1a and explains why 3a-j are less reactive than 1a-j. In chapterⅡ, two series of alkaline ethanolysis have been performed as below. Ⅱ. (1) Metal ion Catalysis in Nucleophilic Displacement Reactions at Phosphorus Centers. Alkali Metal-Ion Catalysis and Inhibition in the Reactions of Ethoxide with Methyl- and Ethyl-Paraoxon and Parathion. The effect of alkali metal ions on alkaline ethanolysis of ethyl- and methyl-paraoxons (4a and 5a, respectively) and their thio analogues, ethyl- and methyl-parathions (4b and 5b, respectively) has been investigated. The spectrophotometrically measured pseudo-first-order rate constants (kobsd) exhibit an upward curvature in the plots of kobsd vs. [EtO^(-)M^(+)] for the reactions of 4a and 5a, but a downward curvature for those of 4b and 5b except for the reaction with EtO^(-)K^(+). As the size of M^(+) ions decreases, both the upward and downward curvatures become more significant. In the presence of 18-crown-6-ether (18C6), the plot of kobsd vs. [EtO^(-)M^(+)] is linear with significant rate decrease for the reactions of 4a, 4b, and 5a with EtO^(-)K^(+), but is curved upwardly for the corresponding reaction of 5b. The kinetic data have been dissected in terms of ion-pairing treatment to determine the second-order-rate constants for the reactions with dissociated EtO^(-) and ion-paired EtO^(-)M^(+). The reactions of 4a and 5a are catalyzed by ion-paired EtO^(-)M^(+) and the catalytic effect increases as the size of the M^(+) ions decreases, while the reactions of 4b and 5b are inhibited by the ion-paired EtO^(-)Li^(+) and EtO^(-)Na+. A four-membered cyclic transition state (TS) has been suggested to account for the M^(+) ion effect. The 18C6 complexed K^(+) ion forms weak ion-pair with EtO^(-) and the ion-paired species catalyzes the reaction of 5b but not that of 4b. The computational results have revealed that the TS for the reaction of 5b is highly crowded by the three ethyl groups, indicating that steric hindrance is responsible for the absence of the catalytic effect in the reaction of 4b. (2) Kinetic Studies on Alkaline Ethanolysis of 4-Nitrophenyl Diphenylphosphinothioate: Alkali Metal Ion Catalysis and Inhibition. The effect of alkali metal ions on alkaline ethanolysis of 4-nitrophenyl diphenylphosphinate (1d) and its thio analogue, 4-nitrophenyl diphenylphosphinothioate (3d) has been investigated. The reactions of 3d are inhibited by ion-paired EtO^(-) with Li^(+) and Na+ ions. The reactions of 1d are catalyzed by ion-paired EtO^(-)M^(+) and the catalytic effect increases as the size of the M^(+) ions decreases, i.e., K^(+) < Na+ < Li^(+). Addition of Li^(+) salt exhibits an inhibitory effect, while addition of K^(+) salt exerts a catalytic effect on the reactions of 3d with EtO-M^(+). The reactivity of EtO^(-)M^(+) increases in the presence of complexing agents (e. g., 18C6, 15C5, and 12C4), indicating that complexed akali metal ions exhibit stronger interactions with the S atom of the P=S bond than uncomplexed metal ions.
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