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 or the epoxidation of cyclooctene by hydrogen peroxide. Under conditions where the formation of -oxo-dimers can be avoided, the mechanism f cyclooctene epoxidation and hydrogen peroxide decomposition are identical for all of the porphyrin catalysts investigated. It is observed that s the electron-withdrawing character of the phenyl groups is enhanced through halogenation, the tendency of iron(III) porphyrin chloride to issociate decreases, but the Lewis acidity of the resulting iron(III) porphyrin cation increases. Increasing the Lewis acidity of the Fe(III) center lso enhances the rate of heterolytic versus homolytic cleavage of the O–O bond of coordinated hydrogen peroxide, as well as the rate hydrogen eroxide consumption for olefin epoxidation versus peroxide decomposition. We have also shown that solvent composition affects both the extent f hydrogen peroxide coordination to the iron(III) porphyrin cation, thus affecting its activity, and the reactivity of hydrogen peroxide with the ron(IV) pi-radical cation, thus affecting the selectivity with which hydrogen peroxide is used for epoxidation. 2007 Elsevier B.V. All rights reserved.