Equilibrating (L)FeIII–OOAc and (L)FeV(O) Species in Hydrocarbon Oxidations by Bio-Inspired Nonheme Iron Catalysts using H2O2 and AcOH
Equilibrating (L)FeIII–OOAc and (L)FeV(O) Species in Hydrocarbon Oxidations by Bio-Inspired Nonheme Iron Catalysts using H2O2 and AcOH
复制标题
使用 H2O2 和 AcOH 通过仿生非血红素铁催化剂平衡烃氧化中的 (L)FeIII–OOAc 和 (L)FeV(O) 物质
DOI：
10.1021/acs.jacs.7b06246
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发表时间：
2017
影响因子：
15
通讯作者：
Que, L.
中科院分区：
文献类型：
--
作者：
Oloo, W. N.;Banerjee, R.;Lipscomb, J. D.;Que, L.
受 Rieske 加氧酶家族卓越化学特性的启发，四齿 N4 配体的非血红素铁复合物已被开发出来，可在添加羧酸的情况下使用 H2O2 催化烃氧化反应。观察到氧化产物的立体选择性和对映选择性可以通过酸的电子和空间性质来调节，这暗示了包含羧酸酯部分的氧化物质。这些催化混合物的冷冻溶液通常表现出来自twoS= 1/2中间体的EPR信号，一个高度各向异性的g2.7子集（gmax= 2.58至2.78和Δg= 0.85–1.2），我们将其分配给FeIII-OOAc物种和一个较低各向异性的g2.07子集（g= 2.07、2.01和1.96和Δg≈） 0.11) 我们与 FeV(O)(OAc) 物种相关联。对 TPA（三（吡啶基-2-甲基）胺）配体家族支持的铁配合物与 H2O2/AcOH 或 AcOOH 在 -40 °C 下反应的动力学研究表明，在 460 nm 处形成了可见的发色团，该发色团持续存在于稳态阶段，然后在过氧氧化剂耗尽时呈指数衰减，其速率常数与底物无关。值得注意的是，这种稳态阶段的持续时间可以通过底物的性质及其浓度来调节，这是一种很少观察到的现象。这种行为作为底物类型和浓度的函数的数值模拟提供了一个动力学模型，其中两个 S= 1/2 中间体存在于动态平衡中，该动态平衡由支持配体的电子性质调节。这一观点得到了反应混合物 EPR 研究的支持。重要的是，这些研究明确表明 g2.07 物质（而不是 g2.7 物质）负责 (L)FeII/H2O2/AcOH 催化系统中的底物氧化。相反，g2.7 物种似乎偏离了路径，并充当 g2.07 物种的储存库。这些发现不仅有助于设计区域和立体特异性非血红素铁氧化催化剂，而且有助于深入了解自然界最有效的加氧酶形成的用途广泛的氧化剂的机制。
Inspired by the remarkable chemistry of the family of Rieske oxygenase enzymes, nonheme iron complexes of tetradentate N4 ligands have been developed to catalyze hydrocarbon oxidation reactions using H2O2in the presence of added carboxylic acids. The observation that the stereo- and enantioselectivity of the oxidation products can be modulated by the electronic and steric properties of the acid implicates an oxidizing species that incorporates the carboxylate moiety. Frozen solutions of these catalytic mixtures generally exhibit EPR signals arising from twoS= 1/2 intermediates, a highly anisotropic g2.7 subset (gmax= 2.58 to 2.78 and Δg= 0.85–1.2) that we assign to an FeIII–OOAc species and a less anisotropic g2.07 subset (g= 2.07, 2.01, and 1.96 and Δg≈ 0.11) we associate with an FeV(O)(OAc) species. Kinetic studies on the reactions of iron complexes supported by the TPA (tris(pyridyl-2-methyl)amine) ligand family with H2O2/AcOH or AcOOH at −40 °C reveal the formation of a visible chromophore at 460 nm, which persists in a steady state phase and then decays exponentially upon depletion of the peroxo oxidant with a rate constant that is substrate independent. Remarkably, the duration of this steady state phase can be modulated by the nature of the substrate and its concentration, which is a rarely observed phenomenon. A numerical simulation of this behavior as a function of substrate type and concentration affords a kinetic model in which the twoS= 1/2 intermediates exist in a dynamic equilibrium that is modulated by the electronic properties of the supporting ligands. This notion is supported by EPR studies of the reaction mixtures. Importantly, these studies unambiguously show that the g2.07 species, and not the g2.7 species, is responsible for substrate oxidation in the (L)FeII/H2O2/AcOH catalytic system. Instead the g2.7 species appears to be off-pathway and serves as a reservoir for the g2.07 species. These findings will be helpful not only for the design of regio- and stereospecific nonheme iron oxidation catalysts but also for providing insight into the mechanisms of the remarkably versatile oxidants formed by nature’s most potent oxygenases.