Reactions of Iron(III) Porphyrins with Oxidants. Structure—Reactivity Studies

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 highvalent 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 ∂ϱ/∂pK_a or ∂β_1g/∂σ and ∂β_1g/∂pK_a. The observed ∂ϱ/∂pK_aor ∂β_1g/∂σ, as well as the increase in β_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.