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

苯的化学选择性单氧化为苯酚，以及同一分子中几个不同的C-H键中的一个的位置选择性单氧合是具有挑战性的化学转化。自然界已经开发出起催化剂作用的酶来解决这些挑战：特定的底物结合导致选择性单氧合，底物可以朝着酶活性中心采取一定的取向，从而导致位置选择性功能化。然而，酶的结合特性很难在分子催化剂中实现类似的选择性。金属有机骨架(MOF)是一种非常适合解决化学选择性氧化反应和位置选择性氧化反应挑战的材料。功能连接体和催化活性金属节点形成了具有高比表面积的定义良好的多孔催化剂材料。现有的铁基MOF已被证明通过铁(IV)-氧物种的中介作用与合适的氧化剂结合来氧化碳氢化合物。具有接近配位原子的吸电子取代基的新型连接体应该使铁(IV)-氧代物种更亲电，这被认为可以提高芳烃氧化为电子中性芳烃的反应活性，如苯。此外，这些新的连接体应该具有亲油单元，以在催化活性节点附近创造疏水的孔环境，这被认为是导致苯氧化为苯酚的高化学选择性：形成的苯酚比苯更具活性，但也更具极性，因此应该更快地从亲脂的孔中排出。因此，应防止对苯二酚的过度氧化。为了研究控制氧化反应中心选择性的重要因素，应使用具有不同类型的C-H键的底物，如芳香族、叔基、仲基或苯基，如(4-甲基戊基)苯。应该设计和优化新的连接物，分别通过PI-PI堆积或亲油作用优先与芳基或烷基取代基相互作用。这些相互作用被认为是导致底物分子在MOF孔内的特定取向：然后实现对某种类型的C-H键的氧化的位置选择性，这取决于分子的哪一部分更接近催化活性节点。新的MOF的表征和对催化反应的彻底分析有望揭示MOF材料作为氧化催化剂在工业和学术界各种应用中的优化趋势。

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.