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

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.