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

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.

Activation Parameters as Mechanistic Probes in the TAML Iron(V)–Oxo Oxidations of Hydrocarbons

“…The measured value of 19 kJ mol −1 is physically realistic. It is also worth noting that this study supports a previous assertion by Fertinger, Franke, and van Eldik in a related study that “a close correlation between bond strength and reaction rate…︁no longer exists” 22…”

“…It is also worth noting that this study supports a previous assertion by Fertinger, Franke, and van Eldik in a related study that "a close correlation between bond strength and reaction rate…no longer exists". [22] Further evidence that 9,10-dihydroanthracene follows a different pathway is presented in Figure 5, which demonstrates an isokinetic plot in which all hydrocarbons (including [D 10 ]ethylbenzene) other than 9,10-dihydroanthracene show a linear correlation between DH°and DS°. Once again, the 9,10-dihydroanthracene point lies significantly off the straight line.…”

Iron(IV) or iron(V)? Heterolytic or free radical? Oxidation pathways of a TAML activator in acetonitrile at −40 °C

“…Various mechanistic aspects of the formation of such high-valent iron species have been a subject of careful investigation by multiple researchers, including Rudi van Eldik, to whom this issue is devoted [12][13][14][15][16]. Noticeable attention has been paid recently to TAML activators of peroxides [17][18][19][20][21][22].…”

Highly Reactive Manganese(IV)-Oxo Porphyrins Showing Temperature-Dependent Reversed Electronic Effect in C–H Bond Activation Reactions

“…This phenomenon is called the “enthalpy–entropy compensation effect”, which is ubiquitous in various fields, such as micellization, microemulsion, and solution thermodynamics . Moreover, it has been discussed by Fujii, van Eldik, and their co-workers that the oxidation reactions of Cpd I and Cpd II models can be controlled by the large contribution of the entropy term (i.e., − T Δ S ‡ ) to the free energy of activation. , …”