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

点击翻译按钮获取中文摘要

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.