The "innocent" role of Sc(3+) on a non-heme Fe catalyst in an O2 environment

Pathways for Arene Oxidation in Non-Heme Diiron Enzymes: Lessons from Computational Studies on Benzoyl Coenzyme A Epoxidase

Oxygenation of aromatic rings using O₂ is catalyzed by several non-heme carboxylate-bridged diiron enzymes. In order to provide a general mechanistic description for these reactions, computational studies were carried out at the ONIOM(B3LYP/BP86/Amber) level on the non-heme diiron enzyme benzoyl coenzyme A epoxidase, BoxB. The calculations revealed four possible pathways for attacking the aromatic ring: (a) electrophilic (2e^-) attack by a bis(μ-oxo)-diiron(IV) species (Q pathway); (b) electrophilic (2e^-) attack via the σ* orbital of a μ-η²:η²-peroxo-diiron(III) intermediate (Pσ* pathway); (c) radical (1e^-) attack via the π*-orbital of a superoxo-diiron(II,III) species (Pπ* pathway); (d) radical (1e^-) attack of a partially quenched bis(μ-oxo)-diiron(IV) intermediate (Q' pathway). The results allowed earlier work of de Visser on olefin epoxidation by diiron complexes and QM-cluster studies of Liao and Siegbahn on BoxB to be put into a broader perspective. Parallels with epoxidation using organic peracids were also examined. Specifically for the BoxB enzyme, the Q pathway was found to be the most preferred, but the corresponding bis(μ-oxo)-diiron(IV) species is significantly destabilized and not expected to be directly observable. Epoxidation via the Pσ* pathway represents an energetically somewhat higher lying alternative; possible strategies for experimental discrimination are discussed. The selectivity toward epoxidation is shown to stem from a combination of inherent electronic properties of the thioacyl substituent and enzymatic constraints. Possible implications of the results for toluene monooxygenases are considered as well.