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Mechanistic Studies on the Iron Porphyrin Complex-Catalyzed Oxidations of Organic Substrates

Title
Mechanistic Studies on the Iron Porphyrin Complex-Catalyzed Oxidations of Organic Substrates
Authors
이경아
Issue Date
1996
Department/Major
대학원 화학과
Keywords
Mechanistic Iron PorphyrinComplex-Catalyzed OxidationsOrganic Substrates
Publisher
이화여자대학교 대학원
Degree
Master
Abstract
본 연구에서는 철 포르피린 착물을 촉매로 하여 여러 종류의 산화제 - MCPBA, PhIO, H_(2)O_(2), t-BuOOH - 를 사용한 유기 물질의 산화 반응을 행하였고, 이러한 반응에서 반응성 중간체의 구조와 산소 전달 반응의 메카니즘에 대해 연구하였다. 동위원소 물, H_(2)^(18)O을 사용한 철 포르피린 착물 촉매에 의한 유기 물질의 산화 반응에서 H_(2)O_(2), t-BuOOH, MCPBA가 산화제인 경우 생성된 산화물은 같은 양의 ^(18)O을 포함하고 있었고, 그 결과 이 반응의 중간체는 동일한 high-valent iron oxo 포르피린 착물임을 알 수 있었다. 이 중간체는 동위원소 물과 산소 원자 교환이 가능한데 이 속도는 중간체로부터 올레핀으로의 산소 원자 전달 속도와 경쟁적이고 중간체의 축 위치에 리간드가 배위되어 있을 때는 물과의 산소 원자 교환이 크게 방해 받았다. 저온에서 진행된 반응으로부터 전자를 끄는 치환체로 치환된 철 포르피린 종은 저온에서 상대적으로 안정한 (Porp)Fe(III)-OOR 착물을 형성하고, 이 중간체가 올레핀을 에폭시화할 수 있다는 것을 알았다. 동위원소 물을 사용한 반응의 생성물에서 ^(18)O의 포함여부 관찰로 중간체 추정이 가능했지만, 반응조건에 따라 포함되는 ^(18)O의 양이 변하므로 실험시 모든 영향을 고려해 주어야 했다. 다양한 반응성을 가진 여러 기질의 경쟁반응을 시행하였을 때의 결과를 분석함으로써 H_(2)O_(2), t-BuOOH, MCPBA가 산화제인 경우에는 같은 경향성을 나타내고, PhIO가 산화제인 경우에만 다른 경향성을 나타냄을 보았고, 이로부터 H_(2)O_(2), t-BuOOH, MCPBA가 산화제인 경우에는 동일한 중간체를 형성하지만 PhIO의 반응의 경우에는 다른 형태의 중간체가 형성됨을 알았다. PhIO 반응의 경우에는 다른 산화제를 사용했을 때보다 반응성이 큰 중간체가 형성되었고, 이 반응의 분광학적 연구로부터 다른 산화제의 반응에서 생성되지 않는 철(Ⅳ)oxo가 생성됨을 알았다. PhIO 반응에서 보이는 다른 반응성은 이 중간체가 PhIO와 다시 반응함으로써 또 다른 반응성 증간체를 형성하기 때문인 것으로 추정하였다. 올레핀의 에폭시화 반응에서 여러 반응 조건을 변화시켜가며 산소-산소 결합 분리에 미치는 영향을 연구하여, Fe(III)-OOR 종의 산소-산소 결합 분리는 용매, 축 방향 리간드, 포르피린 리간드의 구조에 의한 electronic effect에 의해 성질이 결정됨을 알았다. 용액 중에 존재하는 양성자성 용매는 축에 배위하여 전자를 밀어줌으로써 산소-산소 결합의 불균일 분리를 돕고, 산소-산소 결합 사이의 전자 밀도에 의해 불균일과 균일 분리가 결정되는데, 전자 밀도가 낮을 경우에는 불균일 분리, 전자 밀도가 높을 경우에는 균일 분리가 일어나는 것으로 제안하였다. 실험에 사용한 산화제는 그 성질에 따라 MCPBA는 electronic effect의 영향을 거의 받지 않음을 알았고 t-BuOOH가 그 영향을 가장 많이 받는데 이 역시 산소-산소 결합 사이의 전자 밀도의 차이로 설명할 수 있었다.;Iron porphyrin complex-catalyzed oxygenations of hydrocarbons by several oxidants, i.e., hydrogen peroxide (H_(2)O_(2)), tert-butyl hydroperoxide (t-BuOOH), m-chloroperbenzoic acid (MCPBA) and iodosylbenzene (PhIO), have been studied in order to understand the nature of the reactive intermediates responsible for oxygen atom transfer. We have studied iron porphyrin complex-catalyzed oxygenations of hydrocarbons by several oxidants, i.e., hydrogen peroxide, tert-butyl hydroperoxide, and m-chloroperbenzoic acid, in the presence of H^(18)_(2)O. In the olefin epoxidation and alkane hydroxylation reactions catalyzed by (meso-tetrakis(pentafluorophenyl)porphinato) iron(Ⅲ) chloride [Fe(F_(20)TPP)Cl], the percentages of ^(18)O incorporated into the oxygenated products were found to be the same in all the reactions of hydrogen peroxide, tert-butyl hydroperoxide, and MCPBA, leading us to conclude that a common high-valent iron oxo complex was the reactive intermediate responsible for oxygen atom transfer. When the epoxidation of cyclooctene by MCPBA and H_(2)O_(2) was carried out at low temperature in the Presence of H^(18)_(2)O, it was found that there was no ^(18)O-incorporation from labeled water into cyclooctene oxide. We interpreted the lack of ^(18)O-incorporation in these reactions with that an electronegatively-substituted iron porphyrin complex forms a relatively stable (Porp) Fe(Ⅲ)-OOR species and this intermediate transfers its oxygen to olefin prior to the O-O bond cleavage at low temperature. As the reaction temperature raised from -78 ℃ to room temperature, the amount of ^(18)O incorporated into the oxide product gradually increased in the reactions of cyclooctene epoxidation. This was attributed to the fast conversion of Fe(Ⅲ)-OOR to the high-valent iron oxo complex via the O-O bond cleavage at higher temperature. We found, by studying the effects of the olefin and H^(18)_(2)O concentrations on the amount of ^(18)O incorporated into the oxide product, that oxygen exchange between high-valent iron oxo complex and labeled water was comparable in rate to the reaction of the intermediate with organic compounds for oxygen atom transfer in catalytic oxygenation reactions. Blocking an axial position of iron porphyrin complex with imidazole prevented ^(18)O-incorporation from labeled water into the oxygenated Products, explaining the Phenomenon of no oxygen exchange in cytochrome P-450 systems. From the studies of competitive reactions using organic substrates which have different reactivities towards oxygenation reactions, we obtained results that reactive intermediates generated in the reactions of H_(2)O_(2), t-BuOOH, and MCPBA showed the same reactivities, whereas the intermediate formed in the PhIO reaction gave different reactivity patterns. On the basis of these results, it was concluded that the active species generated in the PhIO reaction was different from that formed in the reactions of H_(2)O_(2), t-BuOOH and MCPBA. The active species generated in the PhIO reaction had a greater reactivity toward the functionalization of hydrocarbons than that formed in H_(2)O_(2), t-BuOOH and MCPBA reaction. Spectroscopic studies carried out in the PhIO reaction showed that Fe^(Ⅳ)=O species was generated. We therefore suggest that different reactivity observed in the PhIO reaction might arise from the formation of a more reactive intermediate by the reaction between the iron(Ⅳ) porphyrin complex and PhIO. In the olefin epoxidation reaction, effects of reaction conditions on the O-O bond cleavage of peroxides such as H_(2)O_(2) and t-BuOOH were investigated. The type of the bond cleavage of Fe(Ⅲ)-OOR species was found to depend on electronic effects such as the kinds of solvents, the presence of imidazole in the reaction solution and the porphyrin ligands bound to iron. Protic solvent in the reaction solution encouraged the O-O bond heterolytic cleavage by coordination at axial position of iron porphyrln by donating electron density to the O-O bond through iron. The type of O-O bond cleavage was influenced by the degree of electron density between O-O bond. In the case of low electron density, heterolytic cleavage occurred, whereas homolytic cleavage was suggested to occur in the case of high electron density. We also found out that MCPBA was insensitive with these electronic effects and that t-BuOOH was the most sensitive.
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