-Substitution Activates Oxoiron(IV) Porphyrin π-Cation Radical Complex More Than Pyrrole-β-Substitution for Atom Transfer Reaction.

There have been two known categories of porphyrins: a -substituted porphyrin like -tetramesitylporphyrin (TMP) and a pyrrole-β-substituted porphyrin like native porphyrins and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). To reveal the chemical and biological function of native hemes, we compare the reactivity of the oxoiron(IV) porphyrin π-cation radical complex (Compound I) of TMP () with that of TMTMP () for epoxidation, hydrogen abstraction, hydroxylation, sulfoxidation, and demethylation reactions. Kinetic analysis of these reactions indicated that is much more reactive than when the substrate is not sterically bulky. However, as the substrate is sterically bulkier, the difference of the reactivity between and becomes smaller, and the reactivity of is comparable to that of for a sterically hindered substrate. Since the redox potential of is almost the same as that of , we conclude that is intrinsically more reactive than for these atom transfer reactions, but the steric effect of is stronger than that of . This is contrary to the previous result for the single electron transfer reaction: is faster than . DFT calculations indicate that the orbital energies of the Fe═O moiety for are higher than those for . The difference in steric effect between and is explained by the distance from the mesityl group to the oxo ligand of Compound I. Significance of the pyrrole-β-substituted structure of the hemes in native enzymes is also discussed on the basis of this study.