Trends in Aromatic Oxidation Reactions Catalyzed by Cytochrome P450 Enzymes: A Valence Bond Modeling

The mixed density functional theory (DFT) and valence bond study described herein focuses on the activation of 17 benzene derivatives by the active species of Cytochrome P450, so-called Compound I (Cpd I), as well as by the methoxy radical, as a potentially simple model of Cpd I (Jones, J. P.; Mysinger, M.; Korzekwa, K. R. Drug Metab. Dispos. 2002, 30, 7-12). Valence bond modeling is employed to rationalize the P450 mechanism and its spin-state selectivity from first principles of electronic structure and to predict activation energies independently, using easily accessible properties of the reactants: the singlet-triplet excitation energies, the ionization potentials of the aromatics, and the electron affinity of Cpd I and/or of the methoxy radical. It is shown that the valence bond model rationalizes all the mechanistic aspects and predicts activation barriers (for 35 reactions) with reasonable accuracy compared to the DFT barriers with an average deviation of ±1.0 kcal·mol(-1) (for DFT barriers, see: Bathelt, C. M.; Ridder, L.; Mulholland, A. J.; Harvey, J. N. Org. Biomol. Chem. 2004, 2, 2998-3005). The valence bond modeling also reveals the mechanistic similarities between the P450 Cpd I and methoxy reactions and enables one to make predictions of barriers for reactions from other studies.