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Mechanistic Studies on the Aromatic Hyd roxylation by High-Valent Iron(Ⅳ)-Oxo Porphyrin π-Cation Radical Complexes

Mechanistic Studies on the Aromatic Hyd roxylation by High-Valent Iron(Ⅳ)-Oxo Porphyrin π-Cation Radical Complexes
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대학원 나노과학부
이화여자대학교 대학원
Aromatic hydroxylation은 Cytochrome P450, nonheme iron mono- and dioxygenase, 그리고 copper enzyme에 의한 detoxification과 drug metabolism의 실험 결과에서 보여지듯이, enzymatic reaction에서 굉장히 중요한 화학 반응들 중의 하나이다. Aromatic ring hydroxylation은 cytochrome P450와 iron porphyrin model 분야 중에서 지난 30년 동안 많은 연구가 진행되었던 대표적인 반응이다. 이 반응의 메커니즘을 규명하고 이해함으로써, drug synthesis와 environmental pollutants의 생분해에 유용한 인공 촉매를 개발해낼 수 있기 때문이다. High-valent 철(Ⅳ) 옥소 포르피린 π-cation radical 종에 의한 aromatic hydroxylation의 메커니즘 규명 연구를 통해 aromatic ring의 π-system에 electrophilic attack으로 인한 tetrahedral radical이나 cationic σ-complex의 생성을 수반한다는 결론을 내렸다. 이렇게 도출한 결론은 굉장히 큰 음의 값을 갖는 Hammett ρ 값과 inverse kinetic isotope effect (KIE) 값과 같은 실험 결과들에 근거를 두고 있다. 생성물 분석을 통해서 주생성물이 2-methylbenzoquinone이라는 것을 밝혀냈다. 이는 이 반응이 aromatic ring hydroxylation이라는 사실의 확실한 증거이다. Isotope labeling study도 함께 수행한 결과, 산화된 생성물 중간체의 산소가 공기 중의 산소 분자에서 온 것이 아니라 철 옥소 포르피린 중간체에서 온 것이라는 것을 확인할 수 있었다.;In the aromatic hydroxylation by high-valent iron(Ⅳ)-oxo porphyrin π-cation radicals, I could conclude that this reaction involves the formation of a tetrahedral radical or cationic sigma-complex on the π-system of aromatic ring by an initial electrophilic attack (Scheme 1). To find out the mechanism of the aromatic hydroxylation, I have performed several experiments including kinetic studies and product analysis. This mechanism is mainly based on the experimental results such as a very large negative Hammett ρ value and an inverse kinetic isotope effect (KIE) value. Next, activation parameter of benzene hydroxylation was obtained by plotting first-order-rate constants on a graph versus various temperatures ranging from 183K to 208K. Experimental activation enthalpy and entropy are ΔH^(‡) = 4.6(3) kcal·mol^(-1) and ΔS^(‡) = -29(2) cal·mol^(-1)·K^(-1), respectively. In the process of product analysis with GC and GC-MS, I could get a benzoquinone as a major product. That is the direct evidence that this reaction is the aromatic ring hydroxylation. Also, isotope labeling study has been done and I could find out that the oxygen in oxygenated products came from not the oxygen in the air, but the iron-oxo porphyrin intermediates. ◁표 삽입▷ (원문을 침조하세요) Scheme 1. Formation of a tetrahedral radical or cationic ρ-complex on the π-system of the aromatic ring.
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