Selective Catalytic C–H Oxygenation Reactions in Metal Organic Frameworks (MOFs)

苯到苯酚的化学选择性单氧化和同一分子中几个不同C-H键之一的位点选择性单氧化是具有挑战性的化学转化。自然界已经开发出了作为催化剂的酶来解决这些挑战：特异性底物结合导致选择性单加氧，底物可以朝向酶活性中心采取一定的方向，这导致位点选择性功能化。然而，酶的结合特性难以在分子催化剂中实现以实现类似的选择性。金属有机框架（MOFs）是非常适合应对化学和位点选择性氧化反应挑战的材料。官能连接体和催化活性金属节点形成具有高表面积的明确限定的多孔催化剂材料。现有的基于铁的M0 F已经显示通过铁（IV）-氧代物质的中间作用与合适的氧化剂组合来氧化烃。具有靠近配位原子的吸电子取代基的新型接头应使铁（IV）-氧代物种更具亲电性，这被提议增加芳烃氧化成电子中性芳烃如苯的反应性。此外，这些新型连接体的特征应该是亲脂性单元，以在催化活性节点附近产生疏水性孔环境，这被认为导致苯氧化为苯酚的高化学选择性：所形成的苯酚比苯更具反应性，但也更具极性，因此应该更快地从亲脂性孔中排出。因此，应防止过度氧化为对苯二酚。为了研究控制氧化反应的位点选择性的重要因素，应采用具有不同类型的C-H键的底物，例如芳族、叔、仲或苄基（如（4-甲基戊基）苯）。应设计和优化新的接头，其分别通过Pi-Pi-堆积或亲脂性相互作用优先与芳基或烷基取代基相互作用。这些相互作用被提出来导致在MOF孔内的底物分子的一定取向：然后实现对某种类型的C-H键的氧化的位点选择性，这取决于分子的哪一部分更接近催化活性节点。新的MOF的表征和催化反应的彻底分析预计将展示MOF材料作为工业和学术界各种应用的氧化催化剂的优化的一般趋势。

The chemoselective monooxygenation of benzene to phenol and the site-selective monooxygenation of one out of several distinct C–H bonds in the same molecule are challenging chemical transformations. Nature has developed enzymes that function as catalysts to address those challenges: specific substrate binding accounts for selective monooxygenation, and the substrate can adopt a certain orientation toward the enzyme active center, which leads to site-selective functionalizations. However, the binding properties of enzymes are difficult to implement in molecular catalysts to achieve similar selectivities. Metal Organic Frameworks (MOFs) are well-suited materials to approach the challenges of chemo- and site-selective oxygenation reactions. Functional linkers and catalytically active metal nodes form well-defined porous catalyst materials with high surface areas. Existing iron based MOFs have been shown to oxidize hydrocarbons in combination with suitable oxidants via the intermediacy of iron(IV)-oxo species. Novel linkers with electron-withdrawing substituents close to the coordinating atoms should render the iron(IV)-oxo species more electrophilic, which is proposed to increase the reactivity of arene oxidation to electron-neutral arenes such as benzene. Moreover, those novel linkers should feature lipophilic units to create a hydrophobic pore environment close to the catalytically active nodes, which is proposed to lead to high chemoselectivity for oxygenation of benzene to phenol: the formed phenol is more reactive but also more polar than benzene and should therefore be expelled from the lipophilic pores more rapidly. Thus, over-oxidation to hydroquinone should be prevented. To study the important factors governing site-selectivity for the oxygenation reaction, substrates that feature different types of C–H bonds such as aromatic, tertiary, secondary, or benzylic as in (4-methylpentyl)benzene should be employed. New linkers should be designed and optimized that preferentially interact with aryl or alkyl substituents via Pi-Pi-stacking or lipophilic interactions, respectively. These interactions are proposed to lead to a certain orientation of the substrate molecule within the MOF pore: site-selectivity for oxygenation of a certain type of C–H bond is then achieved, depending on what part of the molecule is in closer proximity to the catalytically active nodes. The characterization of the new MOFs and the thorough analysis of the catalytic reactions are expected to demonstrate general trends for optimization of the MOF materials as oxidation catalysts for various applications in industry and academia.