Mechanistic insight from thermal activation parameters for oxygenation reactions of different substrates with biomimetic iron porphyrin models for compounds I and II

作者：Fertinger Christoph; Franke Alicja; van Eldik Rudi^*

来源：Journal of Biological Inorganic Chemistry, 2012, 17(1): 27-36.

DOI：10.1007/s00775-011-0822-7

• Iron porphyrin
• Enzyme models
• Activation parameters
• Peroxide
• O-O bond activation

Compound I, an oxo-iron(IV) porphyrin pi-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 Delta H (not equal) and Delta S (not equal) 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., Delta H (not equal) %26gt; -T Delta S (not equal)), 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 (Delta H (not equal) = 42 kJ/mol, Delta S (not equal) = 41 J/Kmol), the same reaction with xanthene shows a large contribution from the activation entropy (Delta H (not equal) = 24 kJ/mol, Delta S (not equal) = -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.

• 出版日期2012-1