Factors Affecting Hydrogen-Tunneling Contribution in Hydroxylation Reactions Promoted by Oxoiron(IV) Porphyrin π-Cation Radical Complexes

摘要

Hydrogen atom transfer with a tunneling effect (H-tunneling) has been proposed to be involved in aliphatic hydroxylation reactions catalyzed by cytochrome P450 and synthetic heme complexes as a result of the observation of large hydrogen/deuterium kinetic isotope effects (KIEs). In the present work, we investigate the factors controlling the Htunneling contribution to the H-transfer process in hydroxylation reaction by examining the kinetics of hydroxylation reactions at the benzylic positions of xanthene and 1,2,3,4- tetrahydronaphthalene by oxoiron(IV) 5,10,15,20-tetramesitylporphyrin π-cation radical complexes ((TMP~(+?))Fe~(IV)O(L)) under single-turnover conditions. The Arrhenius plots for these hydroxylation reactions of H-isotopomers have upwardly concave profiles. The Arrhenius plots of D-isotopomers, clear isosbestic points, and product analysis rule out the participation of thermally dependent other reaction processes in the concave profiles. These results provide evidence for the involvement of H-tunneling in the rate-limiting H-transfer process. These profiles are simulated using an equation derived from Bell's tunneling model. The temperature dependence of the KIE values (k_H/k_D) determined for these reactions indicates that the KIE value increases as the reaction temperature becomes lower, the bond dissociation energy (BDE) of the C-H bond of a substrate becomes higher, and the reactivity of (TMP~(+?))Fe~(IV)O(L) decreases. In addition, we found correlation of the slope of the ln(k_H/k_D) - 1/T plot and the bond strengths of the Fe=O bond of (TMP~(+?))Fe~(IV)O(L) estimated from resonance Raman spectroscopy. These observations indicate that these factors modulate the extent of the H-tunneling contribution by modulating the ratio of the height and thickness of the reaction barrier.