Modeling Rieske dioxygenases: the first example of iron-catalyzed asymmetric cis-dihydroxylation of olefins

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.

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.

Photoredox catalysis harvesting multiple photon or electrochemical energies

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Trading Symmetry for Stereoinduction in Tetradentate, non-C2 -Symmetric Fe(II)-Complexes for Asymmetric Catalysis

A series of stereogenic-at-metal iron complexes comprising a non-C₂ -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.

Differences in chemoselectivity in olefin oxidation by a series of non-porphyrin manganese(IV)-oxo complexes

High valent metal-oxo intermediates are versatile oxidants known to facilitate both oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions in nature. In addition to performing essential yet challenging biological reactions, these intermediates are known for their selectivity in favoring the formation of one oxidation product. To understand the basis for this selectivity, we explore the role of equatorial ligand field perturbations in Mn^IV-oxo complexes on chemoselectivity in cyclohexene oxidation. We also examine reactions of Mn^IV-oxo complexes with cyclohexene-d₁₀, cyclooctene, and styrene. Within this series, the product distribution in olefin oxidation is highly dependent on the coordination environment of the Mn^IV-oxo unit. While Mn^IV-oxo complexes with sterically encumbered, and slightly tilted, MnO units favor CC epoxidation products in cyclohexene oxidation, a less encumbered analogue prefers to cleave allylic C-H bonds, resulting in cyclohexenol and cyclohexenone formation. These conclusions are drawn from GC-MS product analysis of the reaction mixture, changes in the UV-vis absorption spectra, and kinetic analyses. DFT computations establish a trend in thermodynamic properties of the Mn^IV-oxo complexes and their reactivity towards olefin oxidation on the basis of the MnO bond dissociation free energy (BDFE). The most reactive Mn^IV-oxo adduct from this series oxidizes cyclohexene-d₁₀, cyclooctene, and styrene to give corresponding epoxides as the only detected products. Collectively, these results suggest that the chemoselectivity obtained in oxidation of olefins is controlled by both the coordination environment around the MnO unit, which modulates the MnO BDFE, and the BDFEs of the allylic C-H bond of the olefins.

Iron-Catalyzed Highly Enantioselective Addition of Silyl Enol Ethers to α,β-Unsaturated 2-Acyl Imidazoles

A chiral Fe^II(N4) complex (N4 = (R,R)-N,N'-bis(2-isopropylquinolin-8-yl)-1,2-diphenylethane-1,2-diamine) was developed for the asymmetric conjugate addition of silyl enol ethers, including both acyclic ones and cyclohexenone-derived ones, to α,β-unsaturated 2-acyl imidazoles. This Fe^II complex is an effective chiral Lewis acid and was applied in the synthesis of an array of chiral 1,5-dicarbonyl synthons and cyclohexenone derivatives with high yields and enantioselectivities (up to 99% ee).

Electrophotocatalytic Acetoxyhydroxylation of Aryl Olefins

A method for the acetoxyhydroxylation of olefins with syn stereoselectivity under electrophotocatalytic conditions is described. The procedure uses a trisaminocyclopropenium (TAC) ion catalyst with visible light irradiation under a controlled electrochemical potential to convert aryl olefins to the corresponding glycol monoesters with high chemo- and diastereoselectivity. This reaction can be performed in batch or in flow, enabling multigram synthesis of the monoester products.

Effect of the Ligand Backbone on the Reactivity and Mechanistic Paradigm of Non-Heme Iron(IV)-Oxo during Olefin Epoxidation

The oxygen atom transfer (OAT) reactivity of the non-heme [Fe^IV (2PyN2Q)(O)]²⁺ (2) containing the sterically bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has been thoroughly studied with different olefins. The ferryl-oxo complex 2 shows excellent OAT reactivity during epoxidations. The steric encumbrance and electronic effect of the ligand influence the mechanistic shuttle between OAT pathway I and isomerization pathway II (during the reaction stereo pure olefins), resulting in a mixture of cis-trans epoxide products. In contrast, the sterically less hindered and electronically different [Fe^IV (N4Py)(O)]²⁺ (1) provides only cis-stilbene epoxide. A Hammett study suggests the role of dominant inductive electronic along with minor resonance effect during electron transfer from olefin to 2 in the rate-limiting step. Additionally, a computational study supports the involvement of stepwise pathways during olefin epoxidation. The ferryl bend due to the bulkier ligand incorporation leads to destabilization of both d z 2 and d x 2 - y 2 orbitals, leading to a very small quintet-triplet gap and enhanced reactivity for 2 compared to 1. Thus, the present study unveils the role of steric and electronic effects of the ligand towards mechanistic modification during olefin epoxidation.

Catalytic Asymmetric Osmium-Free Dihydroxylation of Alkenes

Asymmetric dihydroxylation of alkenes is one of the cornerstone reactions in organic synthesis, providing a direct entry to optically active vicinal diols, which are not only a subunit in natural products but also versatile building blocks. In recent years, considerable progress in catalytic asymmetric osmium-free dihydroxylation has been achieved. This short review presents a concise summary of the reported methods of catalytic asymmetric osmium-free dihydroxylation.

1 Introduction

2 Iron-Catalyzed Asymmetric syn-Dihydroxylation of Alkenes

3 Manganese-Catalyzed Asymmetric syn-Dihydroxylation of Alkenes

4 Palladium/Gold Bimetallic Nanocluster-Catalyzed Asymmetric syn-Dihydroxylation of Alkenes

5 Enzyme-Catalyzed Asymmetric anti-Dihydroxylation of Alkenes

6 Amine-Catalyzed Asymmetric Formal anti-Dihydroxylation of Enals

7 Diselenide-Catalyzed anti-Dihydroxylation of Alkenes

8 Molybdenum-Catalyzed Asymmetric anti-Dihydroxylation of Allylic­ Alcohols

9 Phase-Transfer-Catalyzed Asymmetric Dihydroxylation of α-Aryl Acrylates

10 Conclusion

Visible light-induced aerobic dioxygenation of α,β-unsaturated amides/alkenes toward selective synthesis of β-oxy alcohols using rose bengal as a photosensitizer

The first visible light-induced aerobic dioxygenation of alkenes for the selective synthesis of β-oxy alcohols was developed using non-toxic rose bengal as a photosensitizer.

A Selective Sulfide Oxidation Catalyzed by Heterogeneous Artificial Metalloenzymes Iron@NikA

Performing a heterogeneous catalysis with proteins is still a challenge. Herein, we demonstrate the importance of cross-linked crystals for sulfoxide oxidation by an artificial enzyme. The biohybrid consists of the insertion of an iron complex into a NikA protein crystal. The heterogeneous catalysts displays a better efficiency-with higher reaction kinetics, a better stability and expand the substrate scope compared to its solution counterpart. Designing crystalline artificial enzymes represents a good alternative to soluble or supported enzymes for the future of synthetic biology.

Iron-Catalyzed Highly Enantioselective cis-Dihydroxylation of Trisubstituted Alkenes with Aqueous H2 O2

Reliable methods for enantioselective cis-dihydroxylation of trisubstituted alkenes are scarce. The iron(II) complex cis-α-[Fe^II (2-Me₂ -BQPN)(OTf)₂ ], which bears a tetradentate N₄ ligand (Me₂ -BQPN=(R,R)-N,N'-dimethyl-N,N'-bis(2-methylquinolin-8-yl)-1,2-diphenylethane-1,2-diamine), was prepared and characterized. With this complex as the catalyst, a broad range of trisubstituted electron-deficient alkenes were efficiently oxidized to chiral cis-diols in yields of up to 98 % and up to 99.9 % ee when using hydrogen peroxide (H₂ O₂ ) as oxidant under mild conditions. Experimental studies (including ¹⁸ O-labeling, ESI-MS, NMR, EPR, and UV/Vis analyses) and DFT calculations were performed to gain mechanistic insight, which suggested possible involvement of a chiral cis-Fe^V (O)₂ reaction intermediate as an active oxidant. This cis-[Fe^II (chiral N₄ ligand)]²⁺ /H₂ O₂ method could be a viable green alternative/complement to the existing OsO₄ -based methods for asymmetric alkene dihydroxylation reactions.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

Manganese Catalysts with Tetradentate N-donor Pyridine-Appended Bipiperidine Ligands for Olefin Epoxidation Reactions: Ligand Electronic Effect and Mechanism

In this work, we described an electron-rich manganese mesoPYBP catalyst, Mn-SR-mesoPYBP(ClO4)2, by introducing electron-donating substituents on the mesoPYBP ligand. We optimized the catalytic performance in olefin epoxidation with H2O2 in the presence of acetic acid. The electron paramagnetic resonance (EPR) and cyclic voltammetry (CV) studies supported that an electronic effect could stabilize the high-valent intermediates in the catalytic cycles of the catalyst, which largely improved the catalytic performance and the reactivity of olefin epoxidation.

Chiral cis-iron(ii) complexes with metal- and ligand-centered chirality for highly regio- and enantioselective alkylation of N-heteroaromatics

Iron-catalyzed highly regio- and enantioselective organic transformations with generality and broad substrate scope have profound applications in modern synthetic chemistry; an example is herein described based on cis-Fe^II complexes having metal- and ligand-centered chirality. The cis-β Fe^II(N₄) complex [Fe^II(L)(OTf)₂] (L = N,N′-bis(2,3-dihydro-1H-cyclopenta-[b]quinoline-5-yl)-N,N′-dimethylcyclohexane-1,2-diamine) is an effective chiral catalyst for highly regio- and enantioselective alkylation of N-heteroaromatics with α,β-unsaturated 2-acyl imidazoles, including asymmetric N1, C2, C3 alkylations of a broad range of indoles (34 examples) and alkylation of pyrroles and anilines (14 examples), all with high product yields (up to 98%), high enantioselectivity (up to >99% ee) and high regioselectivity. DFT calculations revealed that the “chiral-at-metal” cis-β configuration of the iron complex and a secondary π–π interaction are responsible for the high enantioselectivity.

A cis-β Fe^II complex having metal- and ligand-centered chirality catalyzes highly regio- and enantioselective alkylation of indoles (at the N1, C2, or C3 position), pyrroles and anilines with α,β-unsaturated 2-acyl imidazoles (48 examples, up to 99% ee).

Chiral Autoamplification Meets Dynamic Chirality Control to Suggest Nonautocatalytic Chemical Model of Prebiotic Chirality Amplification

Oxidative kinetic resolution of 1-phenylethanol in the presence of manganese complexes, bearing conformationally nonrigid achiral bis-amine-bis-pyridine ligands, in the absence of any exogenous chiral additives, is reported. The only driving force for the chiral discrimination is the small initial enantiomeric imbalance of the scalemic (nonracemic) substrate: the latter dynamically controls the chirality of the catalyst, serving itself as the chiral auxiliary. In effect, the ee of 1-phenylethanol increases monotonously over the reaction course. This dynamic control of catalyst chirality by the substrate has been unprecedented; a consistent kinetic model for this process is presented. The reported catalyzed substrate self-enantioenrichment mechanism is discussed in relation to the problem of prebiotic chirality amplification.

Recent advances in catalytic asymmetric dihydroxylation of olefins

Asymmetric dihydroxylation of olefinic groups is an extremely important synthetic transformation which has been widely utilized to obtain optically pure pharmaceuticals and other fine chemical products. In recent years, catalyst systems for the asymmetric C=C dihydroxylations have attracted significant interest, the major research efforts being focused on designing environmentally benign catalyst systems. This mini-review summarizes recent progress in the field, surveying both novel, less toxic modifications of the classical osmium-based catalysts, and the highly efficient and enantioselective non-osmium catalyst systems.

The bibliography includes 50 references.

Non-Heme FeII Diastereomeric Complexes Bearing a Hexadentate Ligand: Unexpected Consequences for the Spin State and Catalytic Oxidation Properties

The reactivity and selectivity of non-heme Fe^II complexes as oxidation catalysts can be substantially modified by alteration of the ligand backbone or introduction of various substituents. In comparison with the hexadentate ligand N,N,N',N'-tetrakis(pyridin-2-ylmethyl)ethane-1,2-diamine (TPEN), N,N'-bis[1-(pyridin-2-yl)ethyl]-N,N'-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (_2Me L₆ ² ) has a methyl group on two of the four picolyl positions. Fe^II complexation by _2Me L₆ ² yields two diastereomeric complexes with very similar structures, which only differ in the axial/equatorial positions occupied by the methylated pyridyl groups. In solution, these two isomers exhibit different magnetic behaviors. Whereas one isomer exhibits temperature-dependent spin-state conversion between the S=0 and S=2 states, the other is more reluctant towards this spin-state equilibrium and is essentially diamagnetic at room temperature. Their catalytic properties for the oxidation of anisole by H₂ O₂ are very different and correlate with their magnetic properties, which reflect their lability/inertness. These different properties most likely depend on the different steric constraints of the methylated pyridyl groups in the two complexes.

Bioinspired Manganese and Iron Complexes for Enantioselective Oxidation Reactions: Ligand Design, Catalytic Activity, and Beyond

The development of efficient methods for the enantioselective oxidation of organic molecules continues to be an important goal in organic synthesis; in particular, the use of earth-abundant metal catalysts and environmentally friendly oxidants in catalytic asymmetric oxidation reactions has attracted significant interest over the last several decades. In nature, metalloenzymes catalyze a wide range of oxidation reactions by activating dioxygen under mild conditions. Inspired by selective and efficient oxidation reactions catalyzed by metalloenzymes, researchers have developed a number of synthetic model compounds that mimic the functionality of metalloenzymes. Among the reported biomimetic model compounds, tetradentate aminopyridine (N4) ligands have emerged as appealing frameworks because of their easy synthesis and facile diversification, and their complexes with metals such as Fe and Mn have proven to be versatile and powerful catalysts for a variety of (enantioselective) oxidation reactions. In this Account, we describe our efforts on the design of chiral N4 ligands and the use of their manganese and iron complexes in asymmetric oxidation reactions with H₂O₂ as the terminal oxidant, aiming to show general strategies for asymmetric oxidation reactions that can guide the rational design of ligands and relevant metal catalysts. In studies of manganese catalysts, the aryl-substituted (R,R)-mcp [mcp = N,N'-dimethyl-N,N'-bis(pyridine-2-ylmethyl)cyclohexane-1,2-diamine] manganese complexes exhibited high enantioselectivity in the asymmetric epoxidation (AE) of various olefins with H₂O₂ while requiring stoichiometric acetic acid as an additive for the activation of H₂O₂. To address this issue, we established bulkier N4 ligands for this catalytic system in which a catalytic amount of sulfuric acid enables the manganese-complex-catalyzed AE with improved stereocontrol and efficiency. In addition, this system was found to be active for the oxidative kinetic resolution of secondary alcohols. Further exploration of the structure-reactivity relationships has shown that aminobenzimidazole N4 ligands derived from l-proline, in which the conventional pyridine donors are replaced by benzimidazoles, act as promising ligands. These novel C₁-symmetric manganese catalysts showed dramatically improved activities with unprecedented turnover numbers in the AE reactions. Notably, this class of manganese complexes can catalyze the oxidation of the C-H bonds of spirocyclic hydrocarbons and spiroazacyclic compounds in a highly enantioselective manner, providing ready access to chiral spirocyclic β,β'-diketones and spirocyclic alcohols. Remarkably, iron catalysts with these chiral N4 ligands are effective for AE of olefins, enabling rare examples of highly enantioselective syntheses of epoxides by the iron catalysts. Finally, mechanistic studies provide valuable insights into the roles of the carboxylic acid and sulfuric acid in the catalytic oxidation reactions. Thus, the results described in this Account have demonstrated the importance of tunability and compatibility of the ligands for the development of efficient oxidation catalysts with earth-abundant transition metals and environmentally benign oxidants, and we hope that our study will pave the way for the discovery of efficient oxidation catalysis.

cis-Oxoruthenium complexes supported by chiral tetradentate amine (N4) ligands for hydrocarbon oxidations

We report the first examples of ruthenium complexes cis-[(N₄)Ru^IIICl₂]⁺ and cis-[(N₄)Ru^II(OH₂)₂]²⁺ supported by chiral tetradentate amine ligands (N₄), together with a high-valent cis-dioxo complex cis-[(N₄)Ru^VI(O)₂]²⁺ supported by the chiral N₄ ligand mcp (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)Ru^IIICl₂](ClO₄) (1a), cis-[(Me₂mcp)Ru^IIICl₂]ClO₄ (2a) and cis-[(pdp)Ru^IIICl₂](ClO₄) (3a) (Me₂mcp = N,N'-dimethyl-N,N'-bis((6-methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine, pdp = 1,1'-bis(pyridin-2-ylmethyl)-2,2'-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)Ru^III(O₂CCF₃)₂]ClO₄ (1b) and cis-[(pdp)Ru^III(O₃SCF₃)₂]CF₃SO₃ (3c'). Electrochemical analyses showed that the chemically/electrochemically generated cis-[(mcp)Ru^VI(O)₂]²⁺ and cis-[(pdp)Ru^VI(O)₂]²⁺ complexes are strong oxidants with E° = 1.11-1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with D_O-H = 90.1-90.8 kcal mol^-1. The reaction of 1b or its (R,R)-mcp counterpart with excess (NH₄)₂[Ce^IV(NO₃)₆] (CAN) in aqueous medium afforded cis-[(mcp)Ru^VI(O)₂](ClO₄)₂ (1e) or cis-[((R,R)-mcp)Ru^VI(O)₂](ClO₄)₂ (1e*), respectively, a strong oxidant with E(Ru^VI/V) = 0.78 V (vs. Ag/AgNO₃) in acetonitrile solution. Complex 1e oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature, affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by 1e or 1e* in ^t BuOH/H₂O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using 1e*. The cis-[(pdp)Ru^II(OH₂)₂]²⁺ (3c)-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.

Biologically inspired oxidation catalysis using metallopeptides

Metalloenzymes can catalyze the oxidation of hydrocarbons with high efficiency and selectivity. For this reason, they are taken as inspiration for the development of new catalyst. A promising strategy is the combination of metal coordination complexes and peptide chains. The use of metallopeptides in oxidation reactions is discussed.

cis-Dioxorhenium(V/VI) Complexes Supported by Neutral Tetradentate N4 Ligands. Synthesis, Characterization, and Spectroscopy

A series of cis-dioxorhenium(V) complexes containing chiral tetradentate N₄ ligands, including cis-[Re^V(O)₂(pyxn)]⁺ (1; pyxn = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)cyclohexane-1,2-diamine), cis-[Re^V(O)₂(6-Me₂pyxn)]⁺ (cis-2), cis-[Re^V(O)₂(R,R-pdp)]⁺ (3; R,R-pdp = 1,1'-bis((R,R)-2-pyridinylmethyl)-2,2'-bipyrrolidine), cis-[Re^V(O)₂(R,R-6-Me₂pdp)]⁺ (4), and cis-[Re^V(O)₂(bqcn)]⁺ (5; bqcn = N,N'-dimethyl-N,N'-di(quinolin-8-yl)cyclohexane-1,2-diamine), were synthesized. Their structures were established by X-ray crystallography, showing Re-O distances in the range of 1.740(3)-1.769(8) Å and O-Re-O angles of 121.4(2)-124.8(4)°. Their cyclic voltammograms in MeCN (0.1 M [NBu₄]PF₆) display a reversible Re^VI/V couple at E_1/2 = 0.39-0.49 V vs SCE. In aqueous media, three proton-coupled electron transfer reactions corresponding to Re^VI/V, Re^V/III, and Re^III/II couples were observed at pH 1. The Pourbaix diagrams of 1·OTf, 3·OTf, and 5·OTf have been examined. The electronic absorption spectra of the cis-dioxorhenium(V) complexes show three absorption bands at around 800 nm (600-1730 dm³ mol^-1 cm^-1), 580 nm (1700-5580 dm³ mol^-1 cm^-1), and 462-523 nm (3170-6000 dm³ mol^-1 cm^-1). Reaction of 1 with Lewis acids (or protic acids) gave cis-[Re^V(O)(OH)(pyxn)]²⁺ (1·H⁺), in which the Re-O distances are lengthened to 1.788(5) Å. Complex cis-2 resulted from isomerization of trans-2 at elevated temperature. cis-[Re^VI(O)₂(pyxn)](PF₆)₂ (1'·(PF₆)₂) was obtained by constant-potential electrolysis of 1·PF₆ in MeCN (0.1 M [NBu₄]PF₆) at 0.56 V vs SCE; it displays shorter Re-O distances (1.722(4), 1.726(4) Å) and a smaller O-Re-O angle (114.88(18)°) relative to 1 and shows a d-d transition absorption band at 591 nm (ε = 77 dm³ mol^-1 cm^-1). With a driving force of ca. 75 kcal mol^-1, 1' oxidizes hydrocarbons with weak C-H bonds (75.5-76.3 kcal mol^-1) via hydrogen atom abstraction. DFT and TDDFT calculations on the electronic structures and spectroscopic properties of the cis-dioxorhenium(V/VI) complexes were performed.

Oxygen activation by mononuclear nonheme iron dioxygenases involved in the degradation of aromatics

Molecular oxygen is utilized in numerous metabolic pathways fundamental for life. Mononuclear nonheme iron-dependent oxygenase enzymes are well known for their involvement in some of these pathways, activating O₂ so that oxygen atoms can be incorporated into their primary substrates. These reactions often initiate pathways that allow organisms to use stable organic molecules as sources of carbon and energy for growth. From the myriad of reactions in which these enzymes are involved, this perspective recounts the general mechanisms of aromatic dihydroxylation and oxidative ring cleavage, both of which are ubiquitous chemical reactions found in life-sustaining processes. The organic substrate provides all four electrons required for oxygen activation and insertion in the reactions mediated by extradiol and intradiol ring-cleaving catechol dioxygenases. In contrast, two of the electrons are provided by NADH in the cis-dihydroxylation mechanism of Rieske dioxygenases. The catalytic nonheme Fe center, with the aid of active site residues, facilitates these electron transfers to O₂ as key elements of the activation processes. This review discusses some general questions for the catalytic strategies of oxygen activation and insertion into aromatic compounds employed by mononuclear nonheme iron-dependent dioxygenases. These include: (1) how oxygen is activated, (2) whether there are common intermediates before oxygen transfer to the aromatic substrate, and (3) are these key intermediates unique to mononuclear nonheme iron dioxygenases?