Aromatic C−H Hydroxylation Reactions with Hydrogen Peroxide Catalyzed by Bulky Manganese Complexes

A density functional theory analysis of the C-H activation reactivity of iron(IV)-oxo complexes with an 'O' substituted tetramethylcyclam macrocycle

In this article, we present a meticulous computational study to foresee the effect of an oxygen-rich macrocycle on the reactivity for C-H activation. For this study, a widely studied nonheme Fe(IV)O molecule with a TMC (1,4,8,11-tetramethyl 1,4,8,11-tetraazacyclotetradecane) macrocycle that is equatorially attached to four nitrogen atoms (designated as N4) and acetonitrile as an axial ligand has been taken into account. For the goal of hetero-substitution, step-by-step replacement of the N4 framework with O atoms, i.e., N4, N3O1, N2O2, N1O3, and O4 systems, has been considered, and dihydroanthracene (DHA) has been used as the substrate. In order to neutralise the system and prevent the self-interaction error in DFT, triflate counterions have also been included in the calculations. The study of the energetics of these C-H bond activation reactions and the potential energy surfaces mapped therefore reveal that the initial hydrogen abstraction, which is the rate-determining step, follows the two-state reactivity (TSR) pattern, which means that the originally excited quintet state falls lower in the transition state and the product. The reaction follows the hydrogen atom transfer (HAT) mechanism, as indicated by the spin density studies. The results revealed a fascinating reactivity order, in which the reactivity increases with the enrichment of the oxygen atom in the equatorial position, namely the order follows N4 < N3O1 < N2O2 < N1O3 < O4. The impacts of oxygen substitution on quantum mechanical tunneling and the H/D kinetic isotope effect have also been investigated. When analysing the causes of this reactivity pattern, a number of variables have been identified, including the reactant-like transition structure, spin density distribution, distortion energy, and energies of the electron acceptor orbital, i.e., the energy of the LUMO (σ*z2), which validate the obtained outcome. Our results also show very good agreement with earlier combined experimental and theoretical studies considering TMC and TMCO-type complexes. The DFT predictions reported here will undoubtedly encourage experimental research in this biomimetic field, as they provide an alternative with higher reactivity in which heteroatoms can be substituted for the traditional nitrogen atom.

DFT Mechanistic Investigation into Ni(II)-Catalyzed Hydroxylation of Benzene to Phenol by H2O2

Introduction of oxygen into aromatic C-H bonds is intriguing from both fundamental and practical perspectives. Although the 3d metal-catalyzed hydroxylation of arenes by H2O2 has been developed by several prominent researchers, a definitive mechanism for these crucial transformations remains elusive. Herein, density functional theory calculations were used to shed light on the mechanism of the established hydroxylation reaction of benzene with H2O2, catalyzed by [NiII(tepa)]2+ (tepa = tris[2-(pyridin-2-yl)ethyl]amine). Dinickel(III) bis(μ-oxo) species have been proposed as the key intermediate responsible for the benzene hydroxylation reaction. Our findings indicate that while the dinickel dioxygen species can be generated as a stable structure, it cannot serve as an active catalyst in this transformation. The calculations allowed us to unveil an unprecedented mechanism composed of six main steps as follows: (i) deprotonation of coordinated H2O2, (ii) oxidative addition, (iii) water elimination, (iv) benzene addition, (v) ketone generation, and (vi) tautomerization and regeneration of the active catalyst. Addition of benzene to oxygen, which occurs via a radical mechanism, turns out to be the rate-determining step in the overall reaction. This study demonstrates the critical role of Ni-oxyl species in such transformations, highlighting how the unpaired spin density value on oxygen and positive charges on the Ni-O• complex affect the activation barrier for benzene addition.

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.

Bioinspired Non-Heme Mn Catalysts for Regio- and Stereoselective Oxyfunctionalizations with H2 O2

In recent years, metalloenzymes-mediated highly selective oxidations of organic substrates under mild conditions have been inspiration for developing synthetic bioinspired catalyst systems, capable of conducting such processes in the laboratory (and, in the future, in industry), relying on easy-to-handle and environmentally benign oxidants such as H2 O2 . To date, non-heme manganese complexes with chiral bis-amino-bis-pyridylmethyl and structurally related ligands are considered as possessing the highest synthetic potential, having demonstrated the ability to mediate a variety of chemo- and stereoselective oxidative transformations, such as epoxidations, C(sp3 )-H hydroxylations and ketonizations, oxidative desymmetrizations, kinetic resolutions, etc. Furthermore, in the past few years non-heme Mn based catalysts have become the major platform for studies focused on getting insight into the molecular mechanisms of oxidant activation and (stereo)selective oxygen transfer, testing non-traditional hydroperoxide oxidants, engineering catalytic sites with enzyme-like substrate recognition-based selectivity, exploration of catalytic regioselectivity trends in the oxidation of biologically active substrates of natural origin. This contribution summarizes the progress in manganese catalyzed C-H oxygenative transformations of organic substrates, achieved essentially in the past 5 years (late 2018-2023).

Reaction landscape of a mononuclear MnIII-hydroxo complex with hydrogen peroxide

Peroxomanganese species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. However, many of these intermediates have yet to be observed. Here, we report the formation of a series of intermediates, each generated from the reaction of the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq2Me)]+ with hydrogen peroxide under slightly different conditions. By changing the acidity of the reaction mixture and/or the quantity of hydrogen peroxide added, we are able to control which intermediate forms. Using a combination of electronic absorption, 1H NMR, EPR, and X-ray absorption spectroscopies, as well as density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations, we formulate these intermediates as the bis(μ-oxo)dimanganese(III,IV) complex [MnIIIMnIV(μ-O)2(dpaq2Me)2]+, the MnIII-hydroperoxo complex [MnIII(OOH)(dpaq2Me)]+, and the MnIII-peroxo complex [MnIII(O2)(dpaq2Me)]. The formation of the MnIII-hydroperoxo species from the reaction of a MnIII-hydroxo complex with hydrogen peroxide mimics an elementary reaction proposed for many synthetic manganese catalysts that activate hydrogen peroxide.

Enzymatic Nitrogen Incorporation Using Hydroxylamine

Hydroxylamine-derived reagents have enabled versatile nitrene transfer reactions for introducing nitrogen-containing functionalities in small-molecule catalysis, as well as biocatalysis. These reagents, however, result in a poor atom economy and stoichiometric organic waste. Activating hydroxylamine (NH2OH) for nitrene transfer offers a low-cost and sustainable route to amine synthesis, since water is the sole byproduct. Despite its presence in nature, hydroxylamine is not known to be used for enzymatic nitrogen incorporation in biosynthesis. Here, we report an engineered heme enzyme that can utilize hydroxylammonium chloride, an inexpensive commodity chemical, for nitrene transfer. Directed evolution of Pyrobaculum arsenaticum protoglobin generated efficient enzymes for benzylic C-H primary amination and styrene aminohydroxylation. Mechanistic studies supported a stepwise radical pathway involving rate-limiting hydrogen atom transfer. This unprecedented activity is a useful addition to the "nitrene transferase" repertoire and hints at possible future discovery of natural enzymes that use hydroxylamine for amination chemistry.

C-H Bonds as Functional Groups: Simultaneous Generation of Multiple Stereocenters by Enantioselective Hydroxylation at Unactivated Tertiary C-H Bonds

Enantioselective C-H oxidation is a standing chemical challenge foreseen as a powerful tool to transform readily available organic molecules into precious oxygenated building blocks. Here, we describe a catalytic enantioselective hydroxylation of tertiary C-H bonds in cyclohexane scaffolds with H2O2, an evolved manganese catalyst that provides structural complementary to the substrate similarly to the lock-and-key recognition operating in enzymatic active sites. Theoretical calculations unveil that enantioselectivity is governed by the precise fitting of the substrate scaffold into the catalytic site, through a network of complementary weak non-covalent interactions. Stereoretentive C(sp3)-H hydroxylation results in a single-step generation of multiple stereogenic centers (up to 4) that can be orthogonally manipulated by conventional methods providing rapid access, from a single precursor to a variety of chiral scaffolds.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

Semi-Rational Design of L-Isoleucine Dioxygenase Generated Its Activity for Aromatic Amino Acid Hydroxylation

Fe (II)-and 2-ketoglutarate-dependent dioxygenases (Fe (II)/α-KG DOs) have been applied to catalyze hydroxylation of amino acids. However, the Fe (II)/α-KG DOs that have been developed and characterized are not sufficient. L-isoleucine dioxygenase (IDO) is an Fe (II)/α-KG DO that specifically catalyzes the formation of 4-hydroxyisoleucine (4-HIL) from L-isoleucine (L-Ile) and exhibits a substrate specificity toward L-aliphatic amino acids. To expand the substrate spectrum of IDO toward aromatic amino acids, in this study, we analyzed the regularity of the substrate spectrum of IDO using molecular dynamics (MD) simulation and found that the distance between Fe2+, C2 of α-KG and amino acid chain's C4 may be critical for regulating the substrate specificity of the enzyme. The mutation sites (Y143, S153 and R227) were also subjected to single point saturation mutations based on polarity pockets and residue free energy contributions. It was found that Y143D, Y143I and S153A mutants exhibited catalytic L-phenylalanine activity, while Y143I, S153A, S153Q and S153Y exhibited catalytic L-homophenylalanine activity. Consequently, this study extended the substrate spectrum of IDO with aromatic amino acids and enhanced its application property.

Bio-Inspired Iron Pentadentate Complexes as Dioxygen Activators in the Oxidation of Cyclohexene and Limonene

The use of dioxygen as an oxidant in fine chemicals production is an emerging problem in chemistry for environmental and economical reasons. In acetonitrile, the [(N4Py)Fe^II]²⁺ complex, [N4Py-N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine] in the presence of the substrate activates dioxygen for the oxygenation of cyclohexene and limonene. Cyclohexane is oxidized mainly to 2-cyclohexen-1-one, and 2-cyclohexen-1-ol, cyclohexene oxide is formed in much smaller amounts. Limonene gives as the main products limonene oxide, carvone, and carveol. Perillaldehyde and perillyl alcohol are also present in the products but to a lesser extent. The investigated system is twice as efficient as the [(bpy)₂Fe^II]²⁺/O₂/cyclohexene system and comparable to the [(bpy)₂Mn^II]²⁺/O₂/limonene system. Using cyclic voltammetry, it has been shown that, when the catalyst, dioxgen, and substrate are present simultaneously in the reaction mixture, the iron(IV) oxo adduct [(N4Py)Fe^IV=O]²⁺ is formed, which is the oxidative species. This observation is supported by DFT calculations.

Bio-inspired Cu(II) amido-quinoline complexes as catalysts for aromatic C-H bond hydroxylation

Cu(II) complexes supported by tetradentate amido-quinoline acyclic ligands (L1 & L2) have been synthesized, characterized, and employed as catalysts for aromatic C-H hydroxylation using H₂O₂ as an oxidant in the absence of an external base with a high selectivity of around 90% for phenols via the non-radical pathway (TON ≥720). The KIE value, various spectroscopic studies and DFT calculation supported the involvement of Cu(II)-OOH species.

Ligand-Enabled C-H Hydroxylation with Aqueous H2O2 at Room Temperature

With the large number of Pd(II)-catalyzed C-H activation reactions of native substrates developed in the past decade, the development of catalysts to enable the use of green oxidants under safe and practical conditions has become an increasingly important challenge. Notably, the compatibility of Pd(II) catalysts with sustainable aqueous H2O2 has been a long-standing challenge in catalysis including Wacker-type oxidations. We report herein a bifunctional bidentate carboxyl-pyridone (CarboxPyridone) ligand that enables room-temperature Pd-catalyzed C-H hydroxylation of a broad range of benzoic and phenylacetic acids with an industry-compatible oxidant, aqueous hydrogen peroxide (35% H2O2). The scalability of this methodology is demonstrated by a 1000 mmol scale reaction of ibuprofen (206 g) using only a 1 mol % Pd catalyst loading. The utility of this protocol is further illustrated through derivatization of the products and synthesis of polyfluorinated natural product coumestan and pterocarpene from phenol intermediates prepared using this methodology.

Role of "S" Substitution on C-H Activation Reactivity of Iron(IV)-Oxo Cyclam Complexes: a Computational Investigation

A comprehensive density functional theory (DFT) investigation has been presented in this article to address the role of equatorial sulfur ligation in C-H activation. A non-heme iron-oxo compound with four nitrogen atoms constituting the equatorially connected macrocyclic framework (represented as N₄) [Fe(IV)═O(THC)(CH₃CN)]²⁺(THC = 1,4,8,11-tetrahydro1,4,8,11-tetraazacyclotetradecane) has been considered as the base compound. Other complexes have been anticipated by the sequential replacement of this nitrogen by sulfur, that is, N₄, N₃S₁, N₂S₂, N₁S₃, and S₄. Counterions, as always, have been considered to avoid the self-interaction error in DFT. Generally, the anti-conformers (with respect to equatorial N-H and Fe═O) turned out to be the most stable. It was found that with the enrichment of the equatorial sulfur atom, reactivity increases successively, that is, we get the trend N₄ < N₃S₁ < N₂S₂ < N₁S₃ < S_4. Our investigations have also verified the available experimental results where it has been reported that N₂S₂ is more reactive than N₄ in their mixed conformation. In search of insights into this typical pattern of reactivity, the interplay of several factors has been recognized, such as the distortion energy which decreases for the transition states with the addition of sulfur; the spin density on the oxygen atom which increases implying that the radical character of abstractor increases on sulfur ligation; the energy of the electron acceptor orbital (the lowest unoccupied molecular orbital (σ_z²*)) which decreases continuously with the sulfur substitution; and the triplet-quintet oxidant energy gap which decreases consistently with S enrichment in the equatorial position. The computational predictions reported here, if further validated by experiments, will definitely encourage the synthesis of sulfur-ligated bio-inspired complexes instead of the ones constituting nitrogen exclusively.

C(sp3)-H oxidation and chlorination catalysed by a bioinspired pincer iron(III) complex

A pincer iron(III) catalyst for the oxidation and chlorination of C(sp³)-H bonds was developed. Oxidation of a diagnostic substrate cis-decalin implies that a long-lived carbon-centred radical is involved. Mechanistic studies suggest that an Fe-oxo species could be responsible for the rate-determining C-H activation step. This report expands the scope of non-heme catalysts for C-H functionalisation.

Chemoselective Oxyfunctionalization of Functionalized Benzylic Compounds with a Manganese Catalyst

Whilst allowing for easy access to synthetically versatile motifs and for modification of bioactive molecules, the chemoselective benzylic oxidation reactions of functionalized alkyl arenes remain challenging. Reported in this study is a new non-heme Mn catalyst stabilized by a bipiperidine-based tetradentate ligand, which enables methylene oxidation of benzylic compounds by H2 O2 , showing high activity and excellent chemoselectivity under mild conditions. The protocol tolerates an unprecedentedly wide range of functional groups, including carboxylic acid and derivatives, ketone, cyano, azide, acetate, sulfonate, alkyne, amino acid, and amine units, thus providing a low-cost, more sustainable and robust pathway for the facile synthesis of ketones, increase of complexity of organic molecules, and late-stage modification of drugs.

para-Selective hydroxylation of alkyl aryl ethers

para-Selective hydroxylation of alkyl aryl ethers is established, which proceeds with a ruthenium(II) catalyst, hypervalent iodine(III) and trifluoroacetic anhydride via a radical mechanism. This protocol tolerates a wide scope of substrates and provides a facile and efficient method for preparing clinical drugs monobenzone and pramocaine on a gram scale.

Exploration of highly electron-rich manganese complexes in enantioselective oxidation catalysis; a focus on enantioselective benzylic oxidation

The development of highly electron-rich manganese complexes for enantioselective benzylic oxidation (and asymmetric epoxidation) is described, to provide chiral benzylic alcohols and epoxides in good yields and enantioselectivites.