Effects of porphyrin composition on the activity and selectivity of the iron(III) porphyrin catalysts for the epoxidation of cyclooctene by hydrogen peroxide

A detailed investigation was carried out of the effects of porphyrin composition on the activity and selectivity of iron(III) porphyrin catalysts used for the epoxidation of cyclooctene by hydrogen peroxide. Under conditions where the formation of μ-oxo-dimers can be avoided, the mechanism of cyclooctene epoxidation and hydrogen peroxide decomposition are identical for all of the porphyrin catalysts investigated. It is observed that as the electron-withdrawing character of the phenyl groups is enhanced through halogenation, the tendency of iron(III) porphyrin chloride to dissociate decreases, but the Lewis acidity of the resulting iron(III) porphyrin cation increases. Increasing the Lewis acidity of the Fe(III) center also enhances the rate of heterolytic versus homolytic cleavage of the O–O bond of coordinated hydrogen peroxide, as well as the rate hydrogen peroxide consumption for olefin epoxidation versus peroxide decomposition. We have also shown that solvent composition affects both the extent of hydrogen peroxide coordination to the iron(III) porphyrin cation, thus affecting its activity, and the reactivity of hydrogen peroxide with the iron(IV) pi-radical cation, thus affecting the selectivity with which hydrogen peroxide is used for epoxidation.

The effect of the nature of phenyl-substituents on the mechanism and individual rate parameters has been determined. The combined effects of individual rate parameters are then used to explain the observed reactivity and selectivity. Overall, the most electronegative porphyrin catalyst, iron(III) [tetrakis(pentafluorophenyl)] porphyrin, was the most reactive and most selective.Figure optionsDownload as PowerPoint slide