title: Factors affecting hydrogen-tunneling contribution in hydroxylation reactions promoted by oxoiron(IV) porphyrin π-cation radical complexes.
abstract: 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 H-tunneling 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.
authors.affiliation: Department of Chemistry, Biology and Environmental Science, Faculty of Science, Nara Women's University , Kitauoyanishi, Nara 830-8506, Japan.
authors.forename: Zhiqi
authors.initials: Z
authors.lastname: Cong
authors.forename: Haruki
authors.initials: H
authors.lastname: Kinemuchi
authors.forename: Takuya
authors.initials: T
authors.lastname: Kurahashi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujii
mesh_terms: D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006900:Hydroxylation; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008665:Metalloporphyrins / Q000737:chemistry*; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D013764:Tetrahydronaphthalenes / Q000737:chemistry; D014966:Xanthenes / Q000737:chemistry
chemical_list: D002412:Cations; D005609:Free Radicals; D008665:Metalloporphyrins; D013764:Tetrahydronaphthalenes; D014966:Xanthenes; D006859:Hydrogen; D007501:Iron; C095210:tetralin; D010100:Oxygen