Tunneling E ff ect That Changes the Reaction Pathway from Epoxidation to Hydroxylation in the Oxidation of Cyclohexene by a Compound I Model of Cytochrome P 450

Content Provider	Semantic Scholar
Author	Gupta, Ranjana Li, Xiao-Xi Cho, Kyung-Bin Guo, Mian Lee, Yong-Min Wang, Yong Fukuzumi, Shunichi Nam, Wonwoo
Copyright Year	2017
Abstract	The rate constants of the CC epoxidation and the C−H hydroxylation (i.e., allylic C−H bond activation) in the oxidation of cyclohexene by a high-valent iron(IV)−oxo porphyrin π-cation radical complex, [(TMP•+)FeIV(O)(Cl)] (1, TMP = meso-tetramesitylporphyrin dianion), were determined at various temperatures by analyzing the overall rate constants and the products obtained in the cyclohexene oxidation by 1, leading us to conclude that reaction pathway changes from the CC epoxidation to C−H hydroxylation by decreasing reaction temperature. When cyclohexene was replaced by deuterated cyclohexene (cyclohexene-d10), the epoxidation pathway dominated irrespective of the reaction temperature. The temperature dependence of the rate constant of the C−H hydroxylation pathway in the reactions of cyclohexene and cyclohexene-d10 by 1 suggests that there is a significant tunneling effect on the hydrogen atom abstraction of allylic C−H bonds of cyclohexene by 1, leading us to propose that the tunneling effect is a determining factor for the switchover of the reaction pathway from the CC epoxidation pathway to the C−H hydroxylation pathway by decreasing reaction temperature. By performing density functional theory (DFT) calculations, the reaction energy barriers of the CC epoxidation and C−H bond activation reactions by 1 were found to be similar, supporting the notion that small environmental changes, such as the reaction temperature, can flip the preference for one reaction to another. H iron(IV)−oxo porphyrin π-cation radical species, called Compound I (Cpd I), have been spectroscopically characterized and well accepted as reactive intermediates in the catalytic cycles of heme enzymes, such as cytochrome P450 (P450), horseradish peroxidase (HRP), catalase, and chloroperoxidase. Biomimetic studies using synthetic iron porphyrin complexes have provided valuable insights into the reactivities and reaction mechanisms of the intermediates in various oxidation reactions as well as in electron-transfer reactions. For example, it has been reported that the regioselectivity of CC epoxidation versus C−H hydroxylation in the oxidation of cyclohexene by Cpd I models changes dramatically depending on reaction temperatures, substrates, and the electronic nature of iron porphyrins. However, activation parameters of the rate constants of the CC epoxidation versus the C−H hydroxylation of the same substrate, including the deuterium kinetic isotope effect (KIE), has yet to be clarified in comparison with density functional theory (DFT) calculations. In addition, factors that determine the reaction pathways (e.g., CC epoxidation versus C−H hydroxylation) in the oxidation of cyclohexene by Cpd I models need to be clarified, although we have discussed recently the regioselectivity switch (e.g., CC epoxidation versus C−H hydroxylation) in the oxidation of cyclic olefins by nonheme metal(IV)−oxo complexes, and Shaik and coworkers have shown the tunneling effect on the counterintuitive hydrogen atom (H atom) abstraction reactivity of nonheme iron(IV)−oxo complexes. We report herein the temperature effect on the rate constants of the CC epoxidation and C−H hydroxylation pathways in the oxidation of cyclohexene and deuterated cyclohexene (cyclohexene-d10) by a Cpd I model compound by determining both the overall rate constants and products at various reaction temperatures. The switchover of the reaction pathway from the CC epoxidation to the C−H hydroxylation in the oxidation of cyclohexene by a Cpd I model compound was observed by decreasing the reaction temperature. The temperature dependence of the deuterium KIE revealed the significance of the Received: February 25, 2017 Accepted: March 16, 2017 Published: March 16, 2017 Letter
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Language	English
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Resource Type	Article