P‐450 Type Activation of Dioxygen by Heterogenized Metal Porphyrins: Comparison with the Corresponding Homogeneous Systems

The protons of silanol groups on a silica or silica‐alumina surface are replaced by metal(III) porphyrin cations, FeTPP⁺, MnTPP⁺, and CoTPP⁺ (TPP = tetraphenylporphyrin). The heterogenized iron(III) or manganese(III) porphyrin supported on silica thus prepared can activate dioxygen in the presence of excess amounts of NaBH₄ and cyclohexene, affording cyclohexanol and cyclohex‐2‐ene‐1‐ol in a 4:1 ratio, which is the same as the ratio observed in the known P‐450 type oxidation of cyclohexene catalyzed by a homogeneous manganese porphyrin (MnTPPCl) in the presence of NaBH₄ used as an electron source. The heterogenized iron(III) porphyrin shows a reactivity comparable with that of the corresponding heterogenized manganese(III) porphyrin. On the other hand, substitution of the heterogenized iron(III) porphyrin by the corresponding homogeneous catalyst, FeTPPCl, results in drastic change in the product ratio; the ratio of cyclohexanol to cyclohex‐2‐ene‐l‐ol is reversed, suggesting that the oxidation of cyclohexene proceeds via radical chain autoxidation reactions. The catalytic activity for the P‐450 type oxidation is diminished while FeTPPCl is converted to the μ‐oxo‐dimer (FeTPP)₂O. Cobalt(III) porphyrin in a homogeneous system also affords the autoxidation products. Such a mechanistic difference between the homogeneous and heterogeneous metal porphyrins in the oxidation of cyclohexene by dioxygen has been confirmed by kinetic studies in which the P‐450 type oxidation and autoxidation obey different kinetics.