Electronegatively substituted iron(III) porphyrin chlorides such as tetrakis(pentafluorophenyl)porphyrin react with iodosylbenzenes, peracids, hydroperoxides, and hydrogen peroxide in hydroxylic solvents to form a high-valent oxene that is capable of carrying out further oxidations. According to the yields and stereochemistry of epoxidations, the oxene is formed exclusively via a heterolytic mechanism. Structure-reactivity studies show evidence for continual changes in transition-state structure rather than a change of mechanism from heterolysis with peracids to homolysis with hydroperoxides, as had previously been proposed. These changes can be described by the coefficients partial derivative rho/partial derivative pK(a) or partial derivative beta(1g)/partial derivative sigma and partial derivative beta(1g)/partial derivative pK(a). The observed partial derivative rho/partial derivative pK(a) or partial derivative beta(1g)/partial derivative sigma, as well as the increase in beta(1g) with increasing pK(a)(ROH), can be interpreted with a reaction-coordinate diagram in which both catalyst and oxidant are varied. On the basis of the heterolytic mechanism, a method for efficient, catalytic, and stereoselective epoxidation using simple and inexpensive hydroperoxides has been developed.