Seeing the cis-Dihydroxylating Intermediate: A Mononuclear Nonheme Iron-Peroxo Complex in cis-Dihydroxylation Reactions Modeling Rieske Dioxygenases

Bio-inspired oxidation catalysis based on proton-coupled electron transfer: Toward efficient and selective oxidation of methane to methanol

In this paper, a trail of my research is described starting from oxidation of alkanes by FeIII-TPA (TPA = tris(2-pyridylmethyl)amine) complexes with alkyl hydroperoxides to Ru-pyridylamine complexes which can be converted to RuIV-oxo complexes in different spin states (S = 1 or 0) through proton-coupled electron-transfer oxidation of the corresponding RuII-aqua complexes, clarifying that those spin states do not affect the reactivity in water. The introduction of strongly donating N-heterocyclic carbene (NHC) moiety allows us to create a RuIII-oxyl complex showing different reactivity from that of RuIV-oxo complexes. Manipulation of second coordination spheres (SCSs) of Ru-TPA complexes is also described, visualizing unique functionality. The introduction of hydrophobic SCS to a FeII-NHC complex enables to catalyze selective oxidation of methane to form methanol in high selectivity in aqueous media based on the "catch-and-release" strategy, which can also allow us to achieve highly selective two-electron oxidation of aromatic compounds.

Debate of Nucleophilic versus Electrophilic Oxidative Aldehyde Deformylation by Mononuclear Nonheme Iron(III)-Peroxo and Iron(IV)-Oxo Complexes

High-valent iron(IV)-oxo species are fleeting intermediates that perform vital reactions in enzymatic catalysis. In contrast, heme and nonheme iron(III)-peroxo intermediates usually act as nucleophiles and are converted to high-valent iron-oxo intermediates for electrophilic oxidation reactions. Herein, we report a study on aldehyde deformylation reactions of 2-phenylpropionaldehyde (2-PPA) and its derivatives by iron(III)-peroxo complexes bearing tetramethylated cyclam (TMC) analogues, including [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3). Reactivity studies by employing deuterated substrates, such as α-[D1]-2-phenylpropionaldehyde and aldehyde-[D]-2-phenylpropionaldehyde, demonstrate that deformylation of 2-PPA by the nonheme iron(III)-peroxo complexes occurs via abstraction of the stronger aldehyde C-H atom, rather than the expected nucleophilic attack or weaker α-C-H atom abstraction reactions. Interestingly, the preference for aldehyde C-H atom abstraction is retained during the deformylation of 2-PPA by iron(IV)-oxo complexes, i.e., [FeIV(O)(13-TMC)]2+ (4) and [FeIV(O)(N4Py)]2+ (5). DFT calculations reproduce the experimental trends in reactivity and reveal that the peroxide O-O bond is cleaved to form an iron(III)-dioxyl species that conducts aldehyde C-H bond abstraction; this chemoselectivity is achieved through stabilizing noncovalent interactions between the oxidants and the aromatic ring of the substrate that positions the aldehyde in close proximity to the FeIII-O2/FeIV═O cores. These new experimental and theoretical findings together with the previous demonstrations of the ability of 1-3 in hydrogen atom transfer, oxygen atom transfer, and cis-dihydroxylation reactions highlight that iron(III)-peroxo cores are not inherently nucleophiles and can have more important functions in chemical and biological oxidation reactions, rather than acting as transient species en route to high-valent metal-oxo intermediates.

Bromide as Noninnocent Ligand to Iron Tames Fenton Chemistry for Chemoselective Nondegrading Oxidation

It has long been the chemistry dogma that the nitrogen-based ligand of iron complexes determines the redox reactivity; tetra- and/or pentadentate nitrogen-based ligand (N-ligand: PDP, porphyrin, N4Py) enables chemo-selective oxidation through high-valent iron species (FeIV/V═O), while bi- and/or tridentate N-ligand leads to the generation of highly reactive oxygen species (ROS) (i.e., hydroxyl radical) via a Fenton chemistry pathway. The effect of inorganic anionic ligands (i.e., halides, pseudohalides, triflate, nitrate, sulfate, etc) of these iron complexes has rarely been examined and overlooked as an "innocent" anion. Herein, we report our discovery that bromide (Br-) is not an innocent ligand to the iron-BPMA complexes [BMPA: bis(2-pyridylmethyl)amine] but a decisive factor for taming the Fenton chemistry (ROS) into a mild [HOBr] oxidant, which allows for chemo- and regioselective oxidation of furans, indoles, and sulfides without noticeable degradation. In contrast to the conventional Fenton chemistry pathway by many tridentate N-ligand iron complexes, our [Fe(BMPA)Br3] mimics haloperoxidases to generate HOBr by oxidation of bromide ion with hydrogen peroxide. The discovery of the bromide effect on iron complexes bridges the gap between Fenton chemistry and haloperoxidase-catalyzed halogenation and might stimulate interest in reinvestigating the "innocent" ligand of iron complexes for discovery of new reactivity and new applications. Additionally, the new catalytic system represents a mild and green oxidation method that might be useful in academia and industry.

Selective Aerobic Peroxidation of Styrene Catalyzed by a Cobalt tert-Butylperoxo Complex

Selective oxidation of styrene to desired products is essential and challenging. In this study, we elucidate a unique pathway for the selective oxidation of styrene to polystyrene peroxo species, catalyzed by the cobalt(III) tert-butylperoxo complex, [CoIII(OO t Bu)(qpy)(NCCH3)]2+ (1), under ambient conditions. Mechanistic investigations, including the structural determination of the diperoxo complex, [CoIII(qpy)(OOCH(Ph)CH2OO t Bu)(NCCH3)]2+ (2), by X-ray analysis and theoretical calculations reveal that the reaction begins with the nucleophilic addition of styrene to the CoIII-OO t Bu moiety in 1. This step is followed by an addition with an O2 molecule, forming a diperoxyl radical (PhCOO•(H)CH2OOtBu), which subsequently rebounds with CoII(qpy) to yield 2. In the presence of excess O2, complex 2 can further react with additional styrene molecules, leading to the formation of cobalt(III) polystyrene peroxo species.

Nonheme iron catalyst mimics heme-dependent haloperoxidase for efficient bromination and oxidation

The [Fe]/H2O2 oxidation system has found wide applications in chemistry and biology. Halogenation with this [Fe]/H2O2 oxidation protocol and halide (X-) in the biological system is well established with the identification of heme-iron-dependent haloperoxidases. However, mimicking such halogenation process is rarely explored for practical use in organic synthesis. Here, we report the development of a nonheme iron catalyst that mimics the heme-iron-dependent haloperoxidases to catalyze the generation of HOBr from H2O2/Br- with high efficiency. We discovered that a tridentate terpyridine (TPY) ligand designed for Fenton chemistry was optimal for FeBr3 to form a stable nonheme iron catalyst [Fe(TPY)Br3], which catalyzed arene bromination, Hunsdiecker-type decarboxylative bromination, bromolactonization, and oxidation of sulfides and thiols. Mechanistic studies revealed that Fenton chemistry ([Fe]/H2O2) might operate to generate hydroxyl radical (HO•), which oxidize bromide ion [Br-] into reactive HOBr. This nonheme iron catalyst represents a biomimetic model for heme-iron-dependent haloperoxidases with potential applications in organic synthesis, drug discovery, and biology.

cis-Dihydroxylation by Synthetic Iron(III)-Peroxo Intermediates and Rieske Dioxygenases: Experimental and Theoretical Approaches Reveal the Key O-O Bond Activation Step

Dioxygen (O2) activation by iron-containing enzymes and biomimetic compounds generates iron-oxygen intermediates, such as iron-superoxo, -peroxo, -hydroperoxo, and -oxo, that mediate oxidative reactions in biological and abiological systems. Among the iron-oxygen intermediates, iron(III)-peroxo species are less frequently implicated as active intermediates in oxidation reactions. In this study, we present the combined experimental and theoretical investigations on cis-dihydroxylation reactions mediated by synthetic mononuclear nonheme iron-peroxo intermediates, demonstrating the importance of supporting ligands and metal centers in activating the peroxo ligand toward the O-O bond homolysis for the cis-dihydroxylation reactions. We found a significant ring size effect of the TMC ligand in [FeIII(O2)(n-TMC)]+ (TMC = tetramethylated tetraazacycloalkane; n = 12, 13, and 14) on the cis-dihydroxylation reactivity order: [FeIII(O2)(12-TMC)]+ > [FeIII(O2)(13-TMC)]+ > [FeIII(O2)(14-TMC)]+. Additionally, we found that only [FeIII(O2)(n-TMC)]+, but not other metal-peroxo complexes such as [MIII(O2)(n-TMC)]+ (M = Mn, Co, and Ni), is reactive for the cis-dihydroxylation of olefins. Using density functional theory (DFT) calculations, we revealed that electron transfer from the Fe dxz orbital to the peroxo σ*(O-O) orbital facilitates the O-O bond homolysis, with the O-O bond cleavage barrier well correlated with the energy gap between the frontier molecular orbitals of dxz and σ*(O-O). Further computational studies showed that the reactivity of the synthetic [FeIII(O2)(12-TMC)]+ complex is comparable to that of Rieske dioxygenases in cis-dihydroxylation, providing compelling evidence of the potential involvement of Fe(III)-peroxo species in Rieske dioxygenases. Thus, the present results significantly advance our understanding of the cis-dihydroxylation mechanisms by Rieske dioxygenases and synthetic nonheme iron-peroxo models.

Mechanistic Insights into Oxidation of Benzaldehyde by Co-Peroxo Complexes

Transition metal-peroxide complexes play a crucial role as intermediates in oxidation reactions. To unravel the mechanism of benzaldehyde oxidation by the Co-peroxo complex, we conducted density functional theory (DFT) calculations. The identified competing mechanisms include nucleophilic attack and hydrogen atom transfer (HAT). The nucleophilic attack pathway involves Co-O cleavage and nucleophilic attack, leading to the formation of the benzoate product. And the HAT pathway comprises O-O cleavage and HAT, ultimately resulting in the benzoate product. DFT calculations revealed that the formation of the end-on Co-superoxo complex 2 through Co-O cleavage, starting from the side-on Co-peroxo complex 1, is much more favorable than the formation of the two-terminal oxyl-radical intermediate 3 through O-O cleavage. Compared with the nucleophilic attack of benzaldehyde by 2, the abstraction of a hydrogen atom from benzaldehyde by 3 requires higher energy. The nature of the nucleophilicity of 2 and 3 accounts for the reactivity of the reaction.

Photo-reduction facilitated stachydrine oxidative N-demethylation reaction: A case study of Rieske non-heme iron oxygenase Stc2 from Sinorhizobium meliloti

Rieske-type non-heme iron oxygenases (ROs) are an important family of non-heme iron enzymes. They catalyze a diverse range of transformations in secondary metabolite biosynthesis and xenobiotic bioremediation. ROs typically shuttle electrons from NAD(P)H to the oxygenase component via reductase component(s). This chapter describes our recent biochemical characterization of stachydrine demethylase Stc2 from Sinorhizobium meliloti. In this work, the eosin Y/sodium sulfite pair serves as the photoreduction system to replace the NAD(P)H-reductase system. We describe Stc2 protein purification and quality control details as well as a flow-chemistry to separate the photo-reduction half-reaction and the oxidation half-reaction. Our study demonstrates that the eosin Y/sodium sulfite photo-reduction pair is a NAD(P)H-reductase surrogate for Stc2-catalysis in a flow-chemistry setting. Experimental protocols used in this light-driven Stc2 catalysis are likely to be applicable as a photo-reduction system for other redox enzymes.

Dearomative syn-Dihydroxylation of Naphthalenes with a Biomimetic Iron Catalyst

Arenes are interesting feedstocks for organic synthesis because of their natural abundance. However, the stability conferred by aromaticity severely limits their reactivity, mostly to reactions where aromaticity is retained. Methods for oxidative dearomatization of unactivated arenes are exceedingly rare but particularly valuable because the introduction of Csp3-O bonds transforms the flat aromatic ring in 3D skeletons and confers the oxygenated molecules with a very rich chemistry suitable for diversification. Mimicking the activity of naphthalene dioxygenase (NDO), a non-heme iron-dependent bacterial enzyme, herein we describe the catalytic syn-dihydroxylation of naphthalenes with hydrogen peroxide, employing a sterically encumbered and exceedingly reactive yet chemoselective iron catalyst. The high electrophilicity of hypervalent iron oxo species is devised as a key to enabling overcoming the aromatically promoted kinetic stability. Interestingly, the first dihydroxylation of the arene renders a reactive olefinic site ready for further dihydroxylation. Sequential bis-dihydroxylation of a broad range of naphthalenes provides valuable tetrahydroxylated products in preparative yields, amenable for rapid diversification.

Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N-tetramethylated cyclam analogues (n-TMC), [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n-TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O2 δ bond increase gradually as the ring size of the n-TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1-3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3; the [FeIII(O2)(n-TMC)]+ complex with a smaller n-TMC ring size, 12-TMC, is much more reactive than that with a larger n-TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n-TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.

A high-spin alkylperoxo-iron(iii) complex with cis-anionic ligands: implications for the superoxide reductase mechanism

The N3O macrocycle of the 12-TMCO ligand stabilizes a high spin (S = 5/2) [FeIII(12-TMCO)(OOtBu)Cl]+ (3-Cl) species in the reaction of [FeII(12-TMCO)(OTf)2] (1-(OTf)2) with tert-butylhydroperoxide (tBuOOH) in the presence of tetraethylammonium chloride (NEt4Cl) in acetonitrile at -20 °C. In the absence of NEt4Cl the oxo-iron(iv) complex 2 [FeIV(12-TMCO)(O)(CH3CN)]2+ is formed, which can be further converted to 3-Cl by adding NEt4Cl and tBuOOH. The role of the cis-chloride ligand in the stabilization of the FeIII-OOtBu moiety can be extended to other anions including the thiolate ligand relevant to the enzyme superoxide reductase (SOR). The present study underlines the importance of subtle electronic changes and secondary interactions in the stability of the biologically relevant metal-dioxygen intermediates. It also provides some rationale for the dramatically different outcomes of the chemistry of iron(iii)peroxy intermediates formed in the catalytic cycles of SOR (Fe-O cleavage) and cytochrome P450 (O-O bond lysis) in similar N4S coordination environments.

Mononuclear Non-Heme Manganese-Catalyzed Enantioselective cis-Dihydroxylation of Alkenes Modeling Rieske Dioxygenases

The practical catalytic enantioselective cis-dihydroxylation of olefins that utilize earth-abundant first-row transition metal catalysts under environmentally friendly conditions is an important yet challenging task. Inspired by the cis-dihydroxylation reactions catalyzed by Rieske dioxygenases and non-heme iron models, we report the biologically inspired cis-dihydroxylation catalysis that employs an inexpensive and readily available mononuclear non-heme manganese complex bearing a tetradentate nitrogen-donor ligand and aqueous hydrogen peroxide (H2O2) and potassium peroxymonosulfate (KHSO5) as terminal oxidants. A wide range of olefins are efficiently oxidized to enantioenriched cis-diols in practically useful yields with excellent cis-dihydroxylation selectivity and enantioselectivity (up to 99% ee). Mechanistic studies, such as isotopically 18O-labeled water experiments, and density functional theory (DFT) calculations support that a manganese(V)-oxo-hydroxo (HO-MnV═O) species, which is formed via the water-assisted heterolytic O-O bond cleavage of putative manganese(III)-hydroperoxide and manganese(III)-peroxysulfate precursors, is the active oxidant that effects the cis-dihydroxylation of olefins; this is reminiscent of the frequently postulated iron(V)-oxo-hydroxo (HO-FeV═O) species in the catalytic arene and alkene cis-dihydroxylation reactions by Rieske dioxygenases and synthetic non-heme iron models. Further, DFT calculations for the mechanism of the HO-MnV═O-mediated enantioselective cis-dihydroxylation of olefins reveal that the first oxo attack step controls the enantioselectivity, which exhibits a high preference for cis-dihydroxylation over epoxidation. In this study, we are able to replicate both the catalytic function and the key chemical principles of Rieske dioxygenases in mononuclear non-heme manganese-catalyzed enantioselective cis-dihydroxylation of olefins.