What Factors Influence the Ratio of CH Hydroxylation versus CC Epoxidation by a Nonheme Cytochrome P450 Biomimetic?

Abstract

Density functional calculations on a nonheme biomimetic (Fe=O(TMCS+) have been performed and its catalytic properties versus propene investigated. Our studies show that this catalyst is able to chemoselectively hydroxylate C=H bonds even in the presence of C=C double bonds. This phenomenon has been analyzed and found to occur due to Pauli repusions between protons on the TMCS ligand with protons attached to the approaching substrate. The geometries of the rate determining transition states indicate that the steric hindrance is larger in the epoxidation transition states than in the hydroxylation ones with much shorter distances; hence the hydroxylation pathway is favored over the epoxidation. Although, the reactant experiences close lying triplet and quintet spin states, the dominant reaction mechanism takes place on the quintet spin state surface; i.e., Fe=O(TMCS)+ reacts via single-state reactivity. Our calculations show that this spin state selectivity is the result of geometric orientation of the transition state structures, whereby the triplet ones are destabilized by electrostatic repulsions between the substrate and the ligand while the quintet spin transition states are aligned along the ideal axis. The reactivity patterns and geometries are compared with oxoiron species of dioxygenase and monoxygenase enzymes. Thus, Fe=O(TMCS)+ shows some similarities with P450 enzyme reactivity: it chemoselectively hydroxylates C=H bonds even in the presence of a C=C double bond and therefore is an acceptable P450 biomimetic. However, the absolute barriers of substrate oxidation by Fe=O(TMCS)+ are higher than the ones obtained with heme enzymes, but the chemoselectivity is lesser affected by external perturbations such as hydrogen bonding of a methanol molecule toward the thiolate sulfur or a dielectric constant. This is the first oxoiron complex whereby we calculated a chemoselective hydroxylation over epoxidation in the gas phase.