Synthesis, Characterization and Reactivity Studies of Manganese-Oxygen Porphyrin Intermediates in Bioinorganic Oxidation Chemistry

Abstract

In the field of bioinorganic chemistry, the ultimate goal is to develop efficient systems and gain insights into important biological reactions mediated by metalloenzymes such as cytochrome P450, Photosystem II (PSII) and chloroperoxidase. Recent evidence suggests that multiple oxidants, including high-valent metal-oxo and metal-oxidant adduct species, are involved and play distinct roles in these reactions. Building on this foundation, three types of intermediates bearing porphyrin ligands, MnIII-iodosylarene porphyrin adducts, MnIV-oxo porphyrin complexes, and MnIV-hydroxo porphyrin π-cation radical intermediates, were investigated. First, the investigation of MnIII-iodosylarene porphyrin adducts demonstrates remarkable reactivity in both HAT and OAT reactions. The study revealed that substrates with high BDE, such as cyclohexane, were oxidized resulting in a low KIE. Additionally, [MnIII(Porp)(ArIO)]+ exhibited unusual behavior in the Hammett plot when oxidizing 4-X-substituted thioanisoles. The saturation behaviors of reaction rate constants were observed for both reactions, and there was no effect of the electronic nature of porphyrin ligands. These results indicate that there is a pre-equilibrium between the [MnIII(Porp)(ArIO)]+ and substrate before the oxidation reaction. Next, six MnIV-oxo complexes bearing electron-rich and electron-deficient porphyrins were synthesized and the reactivities of these complexes were investigated in diverse oxidation reactions, including HAT, OAT, and ET reactions. The investigation revealed that there are temperature-dependent reversed reactivity patterns in HAT and OAT reactions that can be explained by the Enthalpy-Entropy Compensation Effect (EECE) principle. Additionally, a consistent linear relationship between H‡ and S‡ was identified. Thus, this study could present the first comprehensive mechanistic investigation of the EECE principle in oxidation reactions using synthetic high-valent metal-oxo species. These findings contribute to a better understanding of EECE in biomimetic oxidation chemistry. On the othe hand, the MnIV-hydroxo porphyrin -cation radical complex was generated and characterized and exhibited high reactivity in various reactions such as alkane hydroxylation, OAT, and especially water oxidation. This discovery is significant in that it presents the first example of a manganese-based compound with a porphyrin -cation radical exhibiting high oxidation reactions, including water oxidation. Lastly, the halogenation reactivity of the MnIV-hydroxo porphyrin -cation radical with aromatic compound was investigated., Selective chlorination or bromination of toluene was achieved by switching the counter anion, and it was found that C(sp2)-H bond was prefered over C(sp3)-H bond. The study also investigated the specific halogenation reactions of naphthalene and benzene derivatives, providing a deeper understanding of the underlying mechanisms and potential applications in the chloroperoxidase-related field. Overall, the different reaction behaviors of these three manganese species with porphyrin ligand contribute to a fundamental understanding of biomimetic oxidation reactions mediated by synthetic metal-oxidant adduct and high-valent metal-oxo complexes. The study demonstrates the distinct roles played by multiple oxidants, the influence of the oxidation state of the metal center and the electronic nature of the supporting porphyrin ligands on the reaction behaviors of these intermediates. ;생무기화학 분야에서의 궁극적인 목표는 사이토크롬 P450, 광합성 시스템 II (PSII), 염소페록시다제 등의 금속효소를 매개로하는 중요한 생물학적 반응에 대한 통찰력을 얻는 것이다. 최근의 증거는 고-원자가 금속-옥소 및 금속-산화제 부가물 종을 포함한 다중 산화제가 관련되어 있으며 이러한 반응에서 뚜렷한 역할을 한다는 것을 시사한다. 이러한 기초를 바탕으로, 포르피린 리간드를 지닌 세 가지 유형의 중간체에 대한 연구를 수행하였다. 첫째, MnIII(ArIO)(Porp)에 대한 연구를 통하여 HAT 및 OAT 반응 모두에서 현저한 반응성을 보여줌을 입증하였다. 다음으로, MnIV-oxo 포르피린을 사용한 산화 반응에서 엔탈피-엔트로피 보상 효과(EECE)를 확인하였다. 또한 헴성 MnIV-oxo 포르피린 π-카티온 라디칼 종을 생성하고, 그 분광학적 특성을 규명하였다. 마지막으로, MnIV-oxo 포르피린 π-카티온 라디칼에 의한 선택적인 방향족 할로겐화 반응이 일어남을 알 수 있었다. 그러므로 포르피린 리간드를 가진 세 가지 망간 종의 서로 다른 반응 거동은 합성 금속-산화제 부가물 및 고가 금속-옥소 복합체에 의해 매개되는 생체 모방 산화 반응의 근본적인 이해에 기여한다고 사료된다. 본 연구를 통하여 다중성 산화제, 금속 중심의 산화 상태의 영향 및 이러한 중간체의 반응 거동에 대한 지지 포르피린 리간드의 전자 특성에 의해 수행되는 뚜렷한 역할을 보여주었다.