An iron catalyst for oxidation of alkyl C-H bonds showing enhanced selectivity for methylenic sites

Proton-triggered chemoselective halogenation of aliphatic C-H bonds with nonheme FeIV-oxo complexes

Halogenation of aliphatic C-H bonds is a chemical transformation performed in nature by mononuclear nonheme iron dependent halogenases. The mechanism involves the formation of an iron(IV)-oxo-chloride species that abstracts the hydrogen atom from the reactive C-H bond to form a carbon-centered radical that selectively reacts with the bound chloride ligand, a process commonly referred to as halide rebound. The factors that determine the halide rebound, as opposed to the reaction with the incipient hydroxide ligand, are not clearly understood and examples of well-defined iron(IV)-oxo-halide compounds competent in C-H halogenation are scarce. In this work we have studied the reactivity of three well-defined iron(IV)-oxo complexes containing variants of the tetradentate 1-(2-pyridylmethyl)-1,4,7-triazacyclononane ligand (Pytacn). Interestingly, these compounds exhibit a change in their chemoselectivity towards the functionalization of C-H bonds under certain conditions: their reaction towards C-H bonds in the presence of a halide anionleads to exclusive oxygenation, while the addition of a superacid results in halogenation. Almost quantitative halogenation of ethylbenzene is observed when using the two systems with more sterically congested ligands and even the chlorination of strong C-H bonds such as those of cyclohexane is performed when a methyl group is present in the sixth position of the pyridine ring of the ligand. Mechanistic studies suggest that both reactions, oxygenation and halogenation, proceed through a common rate determining hydrogen atom transfer step and the presence of the acid dictates the fate of the resulting alkyl radical towards preferential halogenation over oxygenation.

Chemoenzymatic Oxidation of Labdane and Formal Synthesis of Nimbolide

In nature, basic terpene skeletons are produced and subsequently undergo enzymatic or nonenzymatic oxidative transformations, leading to diverse structural variations. To date, thousands of natural products featuring a variety of oxidation patterns have been isolated solely from the labdane family. This work describes a strategy for the comprehensive introduction of oxidation states into the labdane core by employing a combination of enzyme library screening, directed evolution, and sequential chemical oxidation processes. Furthermore, we showcase the functional viability of our chemoenzymatic approach by accomplishing a formal synthesis of nimbolide, highlighting its potential for streamlining the synthesis of complex natural products.

Highly regioselective oxidation of C-H bonds in water using hydrogen peroxide by a cytochrome P450 mimicking iron complex

Cytochrome P450, one of nature's oxidative workhorses, catalyzes the oxidation of C-H bonds in complex biological settings. Extensive research has been conducted over the past five decades to develop a fully functional mimic that activates O2 or H2O2 in water to oxidize strong C-H bonds. We report the first example of a synthetic iron complex that functionally mimics cytochrome P450 in 100% water using H2O2 as the oxidant. This iron complex, in which one methyl group is replaced with a phenyl group in either wing of the macrocycle, oxidized unactivated C-H bonds in small organic molecules with very high selectivity in water (pH 8.5). Several substrates (34 examples) that contained arenes, heteroaromatics, and polar functional groups were oxidized with predictable selectivity and stereoretention with moderate to high yields (50-90%), low catalyst loadings (1-4 mol%) and a small excess of H2O2 (2-3 equiv.) in water. Mechanistic studies indicated the oxoiron(v) to be the active intermediate in water and displayed unprecedented selectivity towards 3° C-H bonds. Under single-turnover conditions, the reactivity of this oxoiron(v) intermediate in water was found to be around 300 fold higher than that in CH3CN, thus implying the role water plays in enzymatic systems.

Carboxylic Acid Directed γ-Lactonization of Unactivated Primary C-H Bonds Catalyzed by Mn Complexes: Application to Stereoselective Natural Product Diversification

Reactions that enable selective functionalization of strong aliphatic C-H bonds open new synthetic paths to rapidly increase molecular complexity and expand chemical space. Particularly valuable are reactions where site-selectivity can be directed toward a specific C-H bond by catalyst control. Herein we describe the catalytic site- and stereoselective γ-lactonization of unactivated primary C-H bonds in carboxylic acid substrates. The system relies on a chiral Mn catalyst that activates aqueous hydrogen peroxide to promote intramolecular lactonization under mild conditions, via carboxylate binding to the metal center. The system exhibits high site-selectivity and enables the oxidation of unactivated primary γ-C-H bonds even in the presence of intrinsically weaker and a priori more reactive secondary and tertiary ones at α- and β-carbons. With substrates bearing nonequivalent γ-C-H bonds, the factors governing site-selectivity have been uncovered. Most remarkably, by manipulating the absolute chirality of the catalyst, γ-lactonization at methyl groups in gem-dimethyl structural units of rigid cyclic and bicyclic carboxylic acids can be achieved with unprecedented levels of diastereoselectivity. Such control has been successfully exploited in the late-stage lactonization of natural products such as camphoric, camphanic, ketopinic, and isoketopinic acids. DFT analysis points toward a rebound type mechanism initiated by intramolecular 1,7-HAT from a primary γ-C-H bond of the bound substrate to a highly reactive Mn^IV-oxyl intermediate, to deliver a carbon radical that rapidly lactonizes through carboxylate transfer. Intramolecular kinetic deuterium isotope effect and ¹⁸O labeling experiments provide strong support to this mechanistic picture.

Electronic control over site-selectivity in hydrogen atom transfer (HAT) based C(sp3)-H functionalization promoted by electrophilic reagents

The direct functionalization of C(sp³)-H bonds represents one of the most investigated approaches to develop new synthetic methodology. Among the available strategies for intermolecular C-H bond functionalization, increasing attention has been devoted to hydrogen atom transfer (HAT) based procedures promoted by radical or radical-like reagents, that offer the opportunity to introduce a large variety of atoms and groups in place of hydrogen under mild conditions. Because of the large number of aliphatic C-H bonds displayed by organic molecules, in these processes control over site-selectivity represents a crucial issue, and the associated factors have been discussed. In this review article, attention will be devoted to the role of electronic effects on C(sp³)-H bond functionalization site-selectivity. Through an analysis of the recent literature, a detailed description of the HAT reagents employed in these processes, the associated mechanistic features and the selectivity patterns observed in the functionalization of substrates of increasing structural complexity will be provided.

Reactivity vs. Selectivity of Biomimetic Catalyst Systems of the Fe(PDP) Family through the Nature and Spin State of the Active Iron-Oxygen Species

Catalytic approaches to late-stage creation of new C-O bonds, especially via oxygenation of particular C-H groups in complex organic molecules, provide challenging tools for the synthesis of biologically active compounds and candidate drugs. In the last decade, significant efforts were invested in designing bioinspired iron based catalyst systems, capable of conducting selective oxidations of organic compounds. The key role of the oxygen-transferring high-valent iron-oxygen species in selective oxygenation is now well established; the next logical step would be gaining insight into the factors governing the oxidation chemo- and stereoselectivity, in relation to the peculiarities of their electronic structure, which would allow introducing the desired level of predictability into those catalytic transformations. In this Personal Account we analyze recent data on the reactivity of bioinspired formally oxoiron(V) catalytically active sites toward organic substrates having C=C and C(sp³ )-H groups. While the majority of reported oxoiron(V) active species are low-spin (S=1/2) complexes, the presence of strong electron-donating groups (NR₁ R₂ ) in the ligand backbone favors the high-spin (S=3/2) ground state. Remarkably, the high-spin perferryl species exhibit higher chemo-, regio-, and stereoselectivity in the oxidations than their low-spin counterparts, thus witnessing the significance of these subtle electronic effects for the selectivity of oxidations conducted by bioinspired catalysts of the Fe(PDP) family.

Site and Enantioselective Aliphatic C-H Oxidation with Bioinspired Chiral Complexes

Selective oxidation of aliphatic C-H bonds stands as an unsolved problem in organic synthesis, with the potential to offer novel paths for preparing molecules of biological interest. The quest for reagents that can perform this class of reactions finds oxygenases and their mechanisms of action as inspiration motifs. Among the numerous families of synthetic catalysts that have been explored, complexes with linear tetraazadentate ligands combining two aliphatic amines and two aromatic amine heterocycles display a structural versatility proven instrumental in the design of C-H oxidation reactions showing site and enantioselectivities, not accessible by conventional oxidants. This manuscript makes a review of recent advances in the field.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Recent Strategies in Non-Heme-Type Metal Complex-Catalyzed Site-, Chemo-, and Enantioselective C–H Oxygenations

C–H bonds are ubiquitous and abundant in organic molecules. If such C–H bonds can be converted into the desired functional groups in a site-, chemo-, diastereo-, and enantio-selective manner, the functionalization of C–H bonds would be an efficient tool for step-, atom- and redox-economic organic synthesis. C–H oxidation, as a typical C–H functionalization, affords hydroxy and carbonyl groups, which are key functional groups in organic synthesis and biological chemistry, directly. Recently, significant developments have been made using non-heme-type transition-metal catalysts. Oxygen functional groups can be introduced to not only simple hydrocarbons but also complex natural products. In this paper, recent developments over the last fourteen years in non-heme-type complex-catalyzed C–H oxidations are reviewed.

1 Introduction

2 Regio- and Chemo-Selective C–H Oxidations

2.1 Tertiary Site-Selective C–H Oxidations

2.2 Secondary Site-Selective C–H Oxidations

2.3 C–H Oxidations of N-Containing Molecules

2.4 C–H Oxidations of Carboxylic Acids

2.5 Chemo- and Site-Selective Methylenic C–H Hydroxylations

3 Enantioselective C–H Oxidations

3.1 Desymmetrizations through C–H Oxidations

3.2 Enantiotopic Methylenic C–H Oxygenations

4 Conclusion

Low-Spin and High-Spin Perferryl Intermediates in Non-Heme Iron Catalyzed Oxidations of Aliphatic C-H Groups

The selectivity patterns of iron catalysts of the Fe(PDP) family in aliphatic C-H oxidation with H₂ O₂ have been studied (PDP=N,N'-bis(pyridine-2-ylmethyl)-2,2'-bipyrrolidine). Cyclohexane, adamantane, 1-bromo-3,7-dimethyloctane, 3,7-dimethyloctyl acetate, (-)-acetoxy-p-menthane, and cis-1,2-dimethylcyclohexane were used as substrates. The studied catalyst systems generate low-spin (S=1/2) oxoiron(V) intermediates or high-spin (S=3/2) oxoiron(V) intermediates, depending on the electron-donating ability of remote substituents at the pyridine rings. The low-spin perferryl intermediates demonstrate lower stability and higher reactivity toward aliphatic C-H groups of cyclohexane than their high-spin congeners, according to the measured self-decay and second-order rate constants k₁ and k₂ . Unexpectedly, there appears to be no uniform correlation between the spin state of the oxoiron(V) intermediates, and the chemo- and regioselectivity of the corresponding catalyst systems in the oxidation of the considered substrates. This contrasts with the asymmetric epoxidations by the same catalyst systems, in which case the epoxidation enantioselectivity increases when passing from the systems featuring the more reactive low-spin perferryl intermediates to those with their less reactive high-spin congeners.

Practical and Selective sp3 C-H Bond Chlorination via Aminium Radicals

The introduction of chlorine atoms into organic molecules is fundamental to the manufacture of industrial chemicals, the elaboration of advanced synthetic intermediates and also the fine-tuning of physicochemical and biological properties of drugs, agrochemicals and polymers. We report here a general and practical photochemical strategy enabling the site-selective chlorination of sp3 C-H bonds. This process exploits the ability of protonated N-chloroamines to serve as aminium radical precursors and also radical chlorinating agents. Upon photochemical initiation, an efficient radical-chain propagation is established allowing the functionalization of a broad range of substrates due to the large number of compatible functionalities. The ability to synergistically maximize both polar and steric effects in the H-atom transfer transition state through appropriate selection of the aminium radical has provided the highest known selectivity in radical sp3 C-H chlorination.

Increasing the steric hindrance around the catalytic core of a self-assembled imine-based non-heme iron catalyst for C-H oxidation

Sterically hindered imine-based non-heme complexes 4 and 5 rapidly self-assemble in acetonitrile at 25 °C, when the corresponding building blocks are added in solution in the proper ratios. Such complexes are investigated as catalysts for the H2O2 oxidation of a series of substrates in order to ascertain the role and the importance of the ligand steric hindrance on the action of the catalytic core 1, previously shown to be an efficient catalyst for aliphatic and aromatic C-H bond oxidation. The study reveals a modest dependence of the output of the oxidation reactions on the presence of bulky substituents in the backbone of the catalyst, both in terms of activity and selectivity. This result supports a previously hypothesized catalytic mechanism, which is based on the hemi-lability of the metal complex. In the active form of the catalyst, one of the pyridine arms temporarily leaves the iron centre, freeing up a lot of room for the access of the substrate.

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.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Activation and Deactivation Strategies Promoted by Medium Effects for Selective Aliphatic C-H Bond Functionalization

Selective functionalization of unactivated aliphatic C-H bonds represents an important goal of modern synthetic chemistry. Differentiating between such bonds in organic molecules with high levels of selectivity remains a crucial issue, and a profound understanding of even the subtlest reactivity trends is needed. Among the methods that have been developed, those based on hydrogen atom transfer (HAT) have attracted considerable interest. Within this framework, medium effects have proved effective in altering the reactivity and site selectivity in synthetically useful C-H functionalization procedures. In this Review, the mechanistic features behind the available strategies are discussed. It is shown that hydrogen bonding and acid-base interactions can promote C-H bond activation or deactivation toward HAT reagents, thereby providing fine-control over the site selectivity and product chemoselectivity as well as useful guidelines for future development and applications.

Enantioselective aliphatic C-H bond oxidation catalyzed by bioinspired complexes

Enantioselective aliphatic C-H bond oxidation simultaneously installs functionality and chirality into hydrocarbon units, converting in a single step readily available, inexpensive and typically inert hydrocarbons into precious building blocks for organic synthesis. The reaction remains however an open problem eager for catalyst development and improvement. Metal complexes reproducing structural and reactivity aspects of oxygenases are emerging as promising homogeneous catalysts for this class of reactions. The present work reviews the current status of field, analyzing the difficulties of the reaction, discussing principles of catalyst design, and critically highlighting the limitations of the current state-of-the-art methodologies.

Selective C(sp3 )-H Aerobic Oxidation Enabled by Decatungstate Photocatalysis in Flow

A mild and selective C(sp3 )-H aerobic oxidation enabled by decatungstate photocatalysis has been developed. The reaction can be significantly improved in a microflow reactor enabling the safe use of oxygen and enhanced irradiation of the reaction mixture. Our method allows for the oxidation of both activated and unactivated C-H bonds (30 examples). The ability to selectively oxidize natural scaffolds, such as (-)-ambroxide, pregnenolone acetate, (+)-sclareolide, and artemisinin, exemplifies the utility of this new method.

Delivering aminopyridine ligands into cancer cells through conjugation to the cell-penetrating peptide BP16

Peptide conjugates incorporating the N-based ligands (Me2)PyTACN or (S,S')-BPBP at the N- or the C-terminus of the cell-penetrating peptide were synthesized (PyTACN-BP16 (), BP16-PyTACN (), BPBP-BP16 (), and BP16-BPBP ()). Metal binding peptides bearing at the N-terminus the ligand, an additional Lys and a β-Ala were also prepared (PyTACN-βAK-BP16 () and BPBP-βAK-BP16 ()). Moreover, taking into account the clathrin-dependent endocytic mechanism of , the enzymatic cleavable tetrapeptide Gly-Phe-Leu-Gly was incorporated between the ligand and the N- or C-terminus of (BPBP-GFLG-BP16 () and BP16-GLFG-BPBP ()). Analysis of the cytotoxicity of all the peptide conjugates showed that: (i) the position of the ligand influenced the IC50 values, (ii) the incorporation of the βAla-Lys dipeptide rendered non active sequences, (iii) peptide conjugates derived from the (S,S')-BPBP ligand were more active than those bearing (Me2)PyTACN, and (iv) the introduction of the cleavable tetrapeptide significantly enhanced the activity of the BPBP conjugates (IC50 of 4.3 to 11.7 μM ( and ) compared to 26.0 to >50 μM (, and )). The most active peptide was BPBP-GFLG-BP16 () (IC50 of 4.3 to 5.0 μM). This high activity was attributed to its high internalization in MCF-7 cells, as shown by flow cytometry, and to the subsequent release of the ligand by the intracellular cleavage of the enzyme-labile spacer, as observed in cathepsin B enzymatic assays. Therefore, these results pave the way for the design of novel peptide conjugates to be used in pro-oxidant anticancer therapies.

Dramatic rate-enhancement of oxygen atom transfer by an iron(iv)-oxo species by equatorial ligand field perturbations

The reactivity and characterization of a novel iron(iv)-oxo species is reported that gives enhanced reactivity as a result of second-coordination sphere perturbations of the ligand system.

Selective C-H Bond Oxidation Catalyzed by the Fe-bTAML Complex: Mechanistic Implications

Nonheme iron complexes bearing tetradentate N-atom-donor ligands with cis labile sites show great promise for chemoselective aliphatic C-H hydroxylation. However, several challenges still limit their widespread application. We report a mechanism-guided development of a peroxidase mimicking iron complex based on the bTAML macrocyclic ligand framework (Fe-bTAML: biuret-modified tetraamido macrocyclic ligand) as a catalyst to perform selective oxidation of unactivated 3° bonds with unprecedented regioselectivity (3°:2° of 110:1 for adamantane oxidation), high stereoretention (99%), and turnover numbers (TONs) up to 300 using mCPBA as the oxidant. Ligand decomposition pathways involving acid-induced demetalation were identified, and this led to the development of more robust and efficient Fe-bTAML complexes that catalyzed chemoselective C-H oxidation. Mechanistic studies, which include correlation of the product formed with the Fe^V(O) reactive intermediates generated during the reaction, indicate that the major pathway involves the cleavage of C-H bonds by Fe^V(O). When these oxidations were performed in the presence of air, the yield of the oxidized product doubled, but the stereoretention remained unchanged. On the basis of ¹⁸O labeling and other mechanistic studies, we propose a mechanism that involves the dual activation of mCPBA and O₂ by Fe-bTAML, leading to formation of the Fe^V(O) intermediate. This high-valent iron oxo remains the active intermediate for most of the reaction, resulting in high regio- and stereoselectivity during product formation.

Computational analysis of site differences in selective aliphatic C–H hydroxylation by nonheme iron–oxo complexes

The CH₃CN-solvent influences selective C–H hydroxylation by nonheme iron complexes due to interactions in the H-bonding formation in H-abstraction.

Direct Selective Oxidative Functionalization of C-H Bonds with H₂O₂: Mn-Aminopyridine Complexes Challenge the Dominance of Non-Heme Fe Catalysts

Non-heme iron(II) complexes are widespread synthetic enzyme models, capable of conducting selective C–H oxidation with H₂O₂ in the presence of carboxylic acid additives. In the last years, structurally similar manganese(II) complexes have been shown to catalyze C–H oxidation with similarly high selectivity, and with much higher efficiency. In this mini-review, recent catalytic and mechanistic data on the selective C–H oxygenations with H₂O₂ in the presence of manganese complexes are overviewed. A distinctive feature of catalyst systems of the type Mn complex/H₂O₂/carboxylic is the existence of two alternative reaction pathways (as found for the oxidation of cumenes), one leading to the formation of alcohol, and the other to ester. The mechanisms of formation of the alcohol and the ester are briefly discussed.

Readily Accessible Bulky Iron Catalysts exhibiting Site Selectivity in the Oxidation of Steroidal Substrates

Bulky iron complexes are described that catalyze the site-selective oxidation of alkyl C-H bonds with hydrogen peroxide under mild conditions. Steric bulk at the iron center is introduced by appending trialkylsilyl groups at the meta-position of the pyridines in tetradentate aminopyridine ligands, and this effect translates into high product yields, an enhanced preferential oxidation of secondary over tertiary C-H bonds, and the ability to perform site-selective oxidation of methylenic sites in terpenoid and steroidal substrates. Unprecedented site selective oxidation at C6 and C12 methylenic sites in steroidal substrates is shown to be governed by the chirality of the catalysts.