Enzyme Models DOI: 10.1002/anie.200802346 Hydrogen Atom Abstraction and Hydride Transfer Reactions by Iron(IV)–Oxo Porphyrins** Yu Jin Jeong, Yaeun Kang, Ah-Rim Han, Yong-Min Lee, Hiroaki Kotani, Shunichi Fukuzumi,* and Wonwoo Nam* High-valent iron(IV)–oxo species are frequently invoked as the key oxidizing intermediates in the catalytic cycles of heme and nonheme iron enzymes. [1–3] In heme iron enzymes, high- valent iron(IV)–oxo species are proposed as reactive inter- mediates in dioxygen activation and oxygen-atom transfer reactions, such as iron(IV)–oxo porphyrin p-radical cations, referred to as compound I, and iron(IV)–oxo porphyrins, referred to as compound II. [1] Whereas the reactivity of compound I has been well investigated with synthetic iron porphyrins in various oxidation reactions, [1,2c,4] compound II has been rarely used as an oxidant because of its low oxidizing power in nature. [5] Very recently, we have observed significant progress in understanding the reactivities of mononuclear nonheme iron(IV)–oxo complexes, which are analogues to compoun- d II in heme enzymes, in a variety of oxidation reactions, [2,6–8] including the activation of C ÀH bonds of alkanes and alkyl aromatics. [7] As part of our efforts to elucidate the reactivities and mechanisms of iron(IV)–oxo complexes of heme and nonheme ligands in oxidation reactions, we performed hydrogen-atom abstraction (H-atom abstraction) and hy- dride-transfer reactions with iron(IV)–oxo porphyrins gen- erated in situ (Scheme 1). Herein, we report the first exam- ples of C À H bond activation of alkyl aromatics and hydride transfer of dihydronicotinamide adenine dinucleotide (NADH) analogues by iron(IV)–oxo porphyrin complexes. The nature of the active oxidant(s), such as an iron(IV)–oxo porphyrin versus an iron(IV)–oxo porphyrin p-radical cation, in the C ÀH bond activation and hydride-transfer reactions is also discussed. Iron(IV)–oxo porphyrin complexes [Fe IV (O)(tpfpp)] (1; tpfpp = meso-tetrakis(pentafluorophenyl)porphinato di- anion), [Fe IV (O)(tdfpp)] (2 ; tdfpp = meso-tetrakis(2,6- difluorophenyl)porphinato dianion), and [Fe IV (O)(tdcpp)] (3 ; tdcpp = meso-tetrakis(2,6-dichlorophenyl)porphinato di- anion) (Scheme 1a) were prepared by treating the iron(III) porphyrin chlorides with meta-chloroperbenzoic acid (m- CPBA) in the presence of a small amount of H 2 O in a solvent mixture of CH 3 CN and CH 2 Cl 2 (9:1) at 15 8C. [5] Subsequently, the iron(IV)–oxo complexes were used in the oxidation of alkyl aromatics with weak C ÀH bonds, such as xanthene (75.5 kcal mol À1 ), 9,10-dihydroanthracene (DHA) (77 kcal mol À1 ), 1,4-cyclohexadiene (CHD) (78 kcal mol À1 ), and fluo- rene (80 kcal mol À1 ) (Scheme 1 b). [9] Upon addition of sub- strates to a reaction solution of 1, the iron(IV)–oxo porphyrin complex reverted back to the starting iron(III) porphyrin complex, showing isosbestic points at 472, 526, and 566 nm in the UV/Vis spectrum (Figure 1a). Product analysis of the reaction solutions revealed that xanthone (40 Æ 8 % based on the amount of 1 formed), anthracene (42 Æ 7%), benzene (41 Æ 7%), and 9-fluorenone (26 Æ 5%) were yielded as major products in the reactions of xanthene, DHA, CHD, and fluorene, respectively [see Eq. (1) for the oxidation of DHA by 1]. Fitting the kinetic data for the pseudo-first-order decay of 1 allowed us to determine k obs values, and plotting Scheme 1. Iron(IV)–oxo porphyrins and substrates used in this study. [*] Dr. H. Kotani, Prof. Dr. S. Fukuzumi Department of Material and Life Science Graduate School of Engineering, Osaka University SORST (Japan) Science and Technology Agency (JST) Suita, Osaka 565-0871 (Japan) Fax: (+ 81) 6-6879-7370 E-mail: fukuzumi@chem.eng.osaka-u.ac.jp Y. J. Jeong, [+] Y. Kang, [+] A.-R. Han, Dr. Y.-M. Lee, Prof.Dr. W. Nam Department of Chemistry and Nano Science Ewha Womans University, Seoul 120-750 (Korea) Fax: (+ 82) 2-3277-4441 E-mail: wwnam@ewha.ac.kr [ + ] These authors contributed equally to this work. [**] The research at EWU was supported by KOSEF/MOST through the CRI Program, Korea, and the research at OU was supported by a Grant-in-Aid (No. 19205019) from the Ministry of Education, Culture, Sports, Science and Technology (Japan). Angewandte Chemie 7321 Angew. Chem. Int. Ed. 2008, 47, 7321–7324 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim