Experimental and Theoretical Evidence for Nonheme Iron(III) Alkylperoxo Species as Sluggish Oxidants in Oxygenation Reactions

Seo, Mi Sook; Kamachi, Takashi; Kouno, Tomohisa; Murata, Koji; Park, Mi Joo; Yoshizawa, Kazunari; Nam, Wonwoo

doi:10.1002/anie.200604219

Cited by 70 publications

(40 citation statements)

References 38 publications

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“…Based on spectroscopic and reactivity data, we conclude that the active species is generated by fragmentation of the alkylperoxoiron(IV) unit (Scheme 3). Indeed, this scenario is consistent with previous studies where Fe III - and Cu II -alkylperoxo complexes were found to be inert but gave rise to very reactive intermediates via O-O bond homolysis 48–50…”

Section: Discussionsupporting

confidence: 92%

Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study

Fiedler

Que

2009

Inorg. Chem.

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In a previous paper (Jensen et al., J. Am. Chem. Soc. 2005, 127, 10512), we reported the synthesis of the turquoise-colored intermediate [Fe IV (β-BPMCN)(OO t Bu)(OH)] 2+ (Tq; BPMCN = N,N′-bis (2-pyridylmethyl)-N,N′-dimethyl-trans-1,2-diaminocyclohexane). The structure of Tq is unprecedented, as it represents the only synthetic example to date of a non-heme Fe IV complex with both alkylperoxo and hydroxide ligands. Given the significance of similar high-valent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at −70 °C, a temperature at which it is stable, and found that it is capable of activating weak X-H bonds (X = C, O) with bond dissociation energies ≤~80 kcal/mol. The Fe IV -OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at −45 °C, Tq decays at a rate that is independent of substrate identity and concentration, forming a species capable of oxidizing substrates with stronger C-H bonds. Parallel reactivity studies were also conducted with the related oxoiron(IV) complexes [Fe IV (β-BPMCN)(O)(X)] 2+ (3-X; X = pyridine or nitrile), thereby permitting a direct comparison of the reactivity of Fe IV centers with oxo and hydroxide ligands. We found that the H-atom abstracting ability of the Fe IV =O species greatly exceeds that of the Fe IV -OH species, generally by greater than 100-fold. Examination of the electronic structures of Tq and 3-X with density functional theory (DFT) provides a rationale for their differing reactivities.

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Section: Discussionsupporting

confidence: 92%

Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study

Fiedler

Que

2009

Inorg. Chem.

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“…Similarly, Nam and coworkers have found [(tpa)Fe III (OOtBu)] 2+ to be unreactive with oxidizable substrates. 46 The decay of 2a and 2b was also found to be insensitive to O 2 , consistent with the complete oxidation of 1 . Upon either warming or prolonged standing in solution over the course of a few hours at −15 °C (Figures 3, and S-5 – S-7), 2a and 2b each convert to unstable EPR-silent products.…”

supporting

confidence: 56%

Structural and Spectroscopic Characterization of Metastable Thiolate-Ligated Manganese(III)–Alkylperoxo Species

2011

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Metastable Mn-peroxo species are proposed to form as key intermediates in biological oxidation reactions involving O2 and C–H bond activation. The majority of these have yet to be spectroscopically characterized, and their inherent instability, in most cases, precludes structural characterization. Cysteinate-ligated metal-peroxos have been shown to form as reactive intermediates in both heme and non-heme iron enzymes. Herein we report the only examples of isolable Mn(III)-alkylperoxo species, and the first two examples of structurally characterized synthetic thiolate-ligated metal-peroxos. Spectroscopic data, including electronic absorption and IR spectra, and ESI mass spectra for 16O vs 18O-labelled metastable Mn(III)-OOR (R= tBu, Cm) are discussed, as well as preliminary reactivity.

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“…A simplified mechanistic picture for ROOH is outlined in Scheme (R=H, t Bu) 11a. 48, 49 It seems to be generally accepted that the reaction of an iron complex with ROOH first yields an iron–peroxo species [Fe III OOR] ( 11 ), which has been described as not oxidizing organic substrates and is only a precursor of the actual oxidants 50…”

Section: Resultsmentioning

confidence: 99%

Iron(II) α‐Aminopyridine Complexes and Their Catalytic Activity in Oxidation Reactions: A Comparative Study of Activity and Ligand Decomposition

et al. 2012

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New well‐defined FeII complexes bearing bi‐ and tridentate α‐aminopyridine ligands were synthesized, and their catalytic activity in the oxidation of hydrocarbons and alcohols utilizing peroxide oxidants was investigated. The tridendate bis(picolyl)amine ligand 6 and its benzylated analogue 7 were converted into complexes [FeII(6)2]OTf2 (96 %, X‐ray; OTf=CF3SO3−) and [FeII(7)2]OTf2 (90 %). The bidentate aminopyridine ligand 8 was converted into [FeII(8)2(OTf)2] (93 %, X‐ray). The new complexes are catalytically active in the oxidation of secondary alcohols and benzylic methylene groups to the corresponding ketones, of toluene to benzaldehyde, and of cyclohexene to cyclohexene oxide (3 mol % catalyst, tBuOOH (4 equiv), RT, 2–6 h, 28 to 85 % yield of isolated product). The catalytic oxidation of cyclohexane with ROOH (R=H, tBu) to an alcohol/ketone mixture with low ratio revealed that these oxidations follow largely a radical mechanism, except when [FeII(6)2]OTf2 was employed and H2O2 was added slowly. Together with known bi‐ and tetradendate iron complexes, a comparative study showed slight reactivity differences for the newly prepared complexes, with the highest observed for [FeII(6)2]OTf2 and [FeII(7)2]OTf2. The reaction of the new complexes with peroxides was followed over time by UV/Visible spectroscopy; this revealed a fast reaction between the two reactants within minutes. Ligand‐decomposition pathways were investigated, and revealed that the NCH2 units of the complexes are rapidly oxidized to the corresponding amides NCO. The iron complex [FeII(6)2]OTf2 showed no decrease in catalytic activity and a moderate decrease in selectivity when first subjected to oxidative conditions similar to those employed in catalysis. Thus, oxidative ligand deterioration had a marginal effect on the catalytic activity of the iron complex [FeII(6)2]OTf2.

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Experimental and Theoretical Evidence for Nonheme Iron(III) Alkylperoxo Species as Sluggish Oxidants in Oxygenation Reactions

Cited by 70 publications

References 38 publications

Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study

Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study

Structural and Spectroscopic Characterization of Metastable Thiolate-Ligated Manganese(III)–Alkylperoxo Species

Iron(II) α‐Aminopyridine Complexes and Their Catalytic Activity in Oxidation Reactions: A Comparative Study of Activity and Ligand Decomposition

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