Mechanistic Studies of Olefin Epoxidation by Hydrogen Peroxide Catalyzed by Iron Porphyrins
Ned A. Stephenson, University of California - Berkeley, 201 Gilman Hall, Berkeley, CA 94720 and Alexis T. Bell, Chemical Engineering, University of California - Berkeley, 201 Gilman Hall, Berkeley, CA 94720.
Recently, the authors have presented a mechanism for the epoxidation of olefins by hydrogen peroxide catalyzed by iron(III) [tetrakis(pentafluorophenyl)] porphyrin in methanol-containing solvents. The proposed mechanism and derived rate constants have now been shown to explain well the rates of hydrogen peroxide consumption and cyclooctene epoxide generation over a wide range of oxidant, substrate, and catalyst concentrations. Studies have also been completed to show that the proposed mechanism is independent of the nature of the olefin substrate provided that the olefin does not affect the electronic properties of the porphyrin catalyst. In more recent work, it has been shown that methanol has two functions when iron(III) [tetrakis(pentafluorophenyl)] porphyrin chloride is dissolved in a methanol-containing solvent. The first function of methanol is to enable the porphyrin to dissociate into cations and anions by solvating both ionic species. The thermodynamics of this equilibrium reaction were determined quantitatively using 1H NMR and UV-visible spectroscopy. The second function of methanol is to act as a proton transfer agent in the heterolytic cleavage of the oxygen-oxygen bond of coordinated hydrogen peroxide. The combination of these two mechanistic functions of methanol has been quantified, and the combined result explains well the change in the observed rate of cyclooctene epoxidation as a function of methanol concentration. The mechanism identified for the iron(III) [tetrakis(pentafluorophenyl)] porphyrin catalyzed epoxidation of cyclooctene by hydrogen peroxide in methanol-containing solvent mixture is also applicable when solvent mixtures containing longer chain-length alcohols are used. The chain length of the alcohol coordinated to the iron cation in the porphyrin affects the electron density on the cation and, hence, the rate constants for the hemolytic and heterolytic cleavage of the oxygen-oxygen bond of coordinated hydrogen peroxide. The systematic change in the magnitude of these variables is clearly evidenced as is their effect on the rate of peroxide consumption and the selectivity of peroxide utilization for epoxidation. Recently completed studies showing how the mechanism and individual rate constants are affected by structural modifications to the porphyrin ring will also be discussed.