Mechanistic insight from thermal activation parameters for oxygenation reactions of different substrates with biomimetic iron porphyrin models for compounds I and II |
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Authors: | Christoph Fertinger Alicja Franke Rudi van Eldik |
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Institution: | (1) Inorganic Chemistry, Department of Chemistry and Pharmacy, University of Erlangen-Nuremberg, Egerlandstrasse 1, 91058 Erlangen, Germany; |
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Abstract: | Compound I, an oxo–iron(IV) porphyrin π-cation radical species, and its one-electron-reduced form compound II are regarded as key intermediates in reactions catalyzed
by cytochrome P450. Although both reactive intermediates can be easily produced from model systems such as iron(III) meso-tetra(2,4,6-trimethylphenyl)porphyrin hydroxide by selecting appropriate reaction conditions, there are only a few thermal
activation parameters reported for the reactions of compound I analogues, whereas such parameters for the reactions of compound
II analogues have not been investigated so far. Our study demonstrates that ΔH
≠ and ΔS
≠ are closely related to the chemical nature of the substrate and the reactive intermediate (viz., compounds I and II) in epoxidation
and C–H abstraction reactions. Although most studied reactions appear to be enthalpy-controlled (i.e., ΔH
≠ > −TΔS
≠), different results were found for C–H abstractions catalyzed by compound I. Whereas the reaction with 9,10-dihydroanthracene
as a substrate is also dominated by the activation enthalpy (ΔH
≠ = 42 kJ/mol, ΔS
≠ = 41 J/Kmol), the same reaction with xanthene shows a large contribution from the activation entropy (ΔH
≠ = 24 kJ/mol, ΔS
≠ = −100 J/kmol). This is of special interest since the activation barrier for entropy-controlled reactions shows a significant
dependence on temperature, which can have an important impact on the relative reaction rates. As a consequence, a close correlation
between bond strength and reaction rate—as commonly assumed for C–H abstraction reactions—no longer exists. In this way, this
study can contribute to a proper evaluation of experimental and computational data, and to a deeper understanding of mechanistic
aspects that account for differences in the reactivity of compounds I and II. |
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