Factors Affecting Hydrogen-Tunneling Contribution in Hydroxylation Reactions Promoted by Oxoiron(IV) Porphyrin π-Cation Radical Complexes
Factors Affecting Hydrogen-Tunneling Contribution in Hydroxylation Reactions Promoted by Oxoiron(IV) Porphyrin π-Cation Radical Complexes
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DOI：
10.1021/ic501737j
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发表时间：
2014-10-06
影响因子：
4.6
通讯作者：
Fujii, Hiroshi
中科院分区：
文献类型：
--
作者：
Cong, Zhiqi;Kinemuchi, Haruki;Fujii, Hiroshi
由于观察到大的氢/氘动力学同位素效应（KIEs），已经提出具有隧穿效应（H-tunneling）的氢原子转移参与细胞色素P450和合成血红素复合物催化的脂肪族羟基化反应。在本工作中，我们研究了氧合铁（IV）5，10，15，20-四均三甲苯卟啉π-阳离子自由基配合物（TMP+中心点）（FeO）-O-IV（L））在氧杂蒽和1，2，3，4-四氢萘苄位羟基化反应的动力学，考察了氢隧道效应对氢转移过程的影响. H-同位素异构体的这些羟基化反应的Arrhenius图具有向上凹的轮廓。D-同位素的Arrhenius图，清晰的等吸光度点，和产品分析排除了热依赖的其他反应过程中的凹轮廓的参与。这些结果为氢隧穿参与限速氢转移过程提供了证据。这些配置文件使用来自贝尔的隧道模型的方程进行模拟。对于这些反应测定的KIE值（k（H）/k（D））的温度依赖性表明，KIE值随着反应温度变低而增加，基板的C-H键的键离解能（BDE）变高，并且（TMP+中心点）（FeO）-O-IV（L）的反应性降低。此外，我们发现1 n（k（H）/k（D））- 1/T曲线的斜率与由共振拉曼光谱估算的（TMP+中心点）（FeO）-O-IV（L）的Fe=O键的键强度相关。这些观察结果表明，这些因素通过调节反应势垒的高度和厚度的比率来调节H-隧穿贡献的程度。
Hydrogen atom transfer with a tunneling effect (H-tunneling) has been proposed to be involved in aliphatic hydroxylation reactions catalyzed by cytochrome P450 and synthetic heme complexes as a result of the observation of large hydrogen/deuterium kinetic isotope effects (KIEs). In the present work, we investigate the factors controlling the H-tunneling contribution to the H-transfer process in hydroxylation reaction by examining the kinetics of hydroxylation reactions at the benzylic positions of xanthene and 1,2,3,4-tetrahydronaphthalene by oxoiron(IV) 5,10,15,20-tetramesitylporphyrin pi-cation radical complexes ((TMP+center dot)(FeO)-O-IV(L)) under single-turnover conditions. The Arrhenius plots for these hydroxylation reactions of H-isotopomers have upwardly concave profiles. The Arrhenius plots of D-isotopomers, clear isosbestic points, and product analysis rule out the participation of thermally dependent other reaction processes in the concave profiles. These results provide evidence for the involvement of H-tunneling in the rate-limiting H-transfer process. These profiles are simulated using an equation derived from Bell's tunneling model. The temperature dependence of the KIE values (k(H)/k(D))) determined for these reactions indicates that the KIE value increases as the reaction temperature becomes lower, the bond dissociation energy (BDE) of the C-H bond of a substrate becomes higher, and the reactivity of (TMP+center dot)(FeO)-O-IV(L) decreases. In addition, we found correlation of the slope of the 1n(k(H)/k(D)) - 1/T plot and the bond strengths of the Fe=O bond of (TMP+center dot)(FeO)-O-IV(L) estimated from resonance Raman spectroscopy. These observations indicate that these factors modulate the extent of the H-tunneling contribution by modulating the ratio of the height and thickness of the reaction barrier.