Mechanistic insights into iron porphyrin-catalyzed olefin epoxidation by hydrogen peroxide: Factors controlling activity and selectivity

Abstract

Iron porphyrins are well known for their ability to catalyze the oxidation of hydrocarbons by hydrogen peroxide and by organic peroxides in general. While many mechanistic studies have been reported, a complete description of the reaction pathway by which the olefin epoxidation occurs has emerged only recently as a result of the work reported by the authors. The aim of this review is to present a summary of the authors’ research and to place it into perspective with previously published studies. What emerges is a complete mechanistic picture for the epoxidation of olefins by hydrogen peroxide catalyzed by iron porphyrins that is consistent with all experimental evidence. Rate parameters associated with elementary processes in the reaction mechanism have been determined from experimental measurements of cyclooctene epoxide formation and hydrogen peroxide consumption as a function of the composition of the solvent, axial ligand, porphyrin, and substrate. Several notable findings emerge from this effort. The first is that only iron(III) porphyrin cations are catalytically active. These species are formed by dissociation of the neutral complex, consisting of an iron(III) porphyrin cation and an anion serving as the axial ligand, into solvated cations and anions. Weakly bound axial ligands, such as triflate anions, dissociate in aprotic solvent, whereas a protic solvent is necessary to dissociate strongly bound ligands such as chloride anions. The role of solvent composition on the dissociation of iron porphyrin complex is fully described by a model of the thermodynamics of the process. The selectivity of hydrogen peroxide towards epoxidation versus decomposition is determined by two competitive processes, heterolytic and homolytic cleavage of the O–O bond of the iron(III)-coordinated hydrogen peroxide molecule. The former process leads to the production of an iron(IV) pi-radical cation which is active for olefin epoxidation, while the later process leads to an iron(IV)-hydroxo species that is active exclusively for peroxide decomposition. A competition also occurs between olefin and hydrogen peroxide for reaction with the iron(IV) pi-radical cation species. Substrate composition does not affect the individual rate parameters as long as the olefin does not interact electronically with the iron porphyrin. Solvent alcohol coordinates to the iron(III) porphyrin in the axial position, thereby modifying the electronic properties of the iron. A second effect of alcohols is to facilitate the heterolytic cleavage of the oxygen–oxygen bond of hydrogen peroxide. The quantity, position, and electronegativity of halogen substituents attached to the phenyl groups at the meso-position of the porphyrin ring also affect the activity and selectivity of the porphyrin for olefin epoxidation. All of these effects are well explained by the mechanism that we have proposed. The rate parameters associated with the proposed mechanism vary in a systematic and physically meaningful fashion with changes in the composition of the porphyrin, the axial ligand associated with the porphyrin, and the solvent in which the porphyrin is dissolved.