1 research outputs found
Rationalization of the Barrier Height for <i>p</i>‑Z-styrene Epoxidation by Iron(IV)-Oxo Porphyrin Cation Radicals with Variable Axial Ligands
A versatile class of heme monoxygenases
involved in many vital functions for human health are the cytochromes
P450, which react via a high-valent ironÂ(IV) oxo heme cation radical
species called Compound I. One of the key reactions catalyzed by these
enzymes is Cî—»C epoxidation of substrates. We report here a
systematic study into the intrinsic chemical properties of substrate
and oxidant that affect reactivity patterns. To this end, we investigated
the effect of styrene and para-substituted styrene epoxidation by
Compound I models with either an anionic (chloride) or neutral (acetonitrile)
axial ligand. We show, for the first time, that the activation enthalpy
of the reaction is determined by the ionization potential of the substrate,
the electron affinity of the oxidant, and the strength of the newly
formed C–O bond (approximated by the bond dissociation energy,
BDE<sub>OH</sub>). We have set up a new valence bond model that enables
us to generalize substrate epoxidation reactions by ironÂ(IV)-oxo porphyrin
cation-radical oxidants and make predictions of rate constants and
reactivities. We show here that electron-withdrawing substituents
lead to early transition states, whereas electron-donating groups
on the olefin substrate give late transition states. This affects
the barrier heights in such a way that electron-withdrawing substituents
correlate the barrier height with BDE<sub>OH</sub>, while the electron
affinity of the oxidant is proportional to the barrier height for
substrates with electron-donating substituents