Highly efficient, regioselective, and stereospecific oxidation of aliphatic C-H groups with H2O2, catalyzed by aminopyridine manganese complexes

Intramolecular C-H Oxidation in Iron(V)-oxo-carboxylato Species Relevant in the γ-Lactonization of Alkyl Carboxylic Acids

High-valent oxoiron species have been invoked as oxidizing agents in a variety of iron-dependent oxygenases. Taking inspiration from nature, selected nonheme iron complexes have been developed as catalysts to elicit C-H oxidation through the mediation of putative oxoiron(V) species, akin to those proposed for Rieske oxygenases. The addition of carboxylic acids in these iron-catalyzed C-H oxidations has proved highly beneficial in terms of product yields and selectivities, suggesting the direct involvement of iron(V)-oxo-carboxylato species. When the carboxylic acid functionality is present in the alkane substrate, it acts as a directing group, enabling the selective intramolecular γ-C-H hydroxylation that eventually affords γ-lactones. While this mechanistic frame is solidly supported by previous mechanistic studies, direct spectroscopic detection of the key iron(V)-oxo-carboxylato intermediate and its competence for engaging in the selective γ-C-H oxidation leading to lactonization have not been accomplished. In this work, we generate a series of well-defined iron(V)-oxo-carboxylato species (2c-2f) differing in the nature of the bound carboxylate ligand. Species 2c-2f are characterized by a set of spectroscopic techniques, including UV-vis spectroscopy, cold-spray ionization mass spectrometry (CSI-MS), and, in selected cases, EPR and Mössbauer spectroscopies. We demonstrate that 2c-2f undergo site-selective γ-lactonization of the carboxylate ligand in a stereoretentive manner, thus unequivocally identifying metal-oxo-carboxylato species as the powerful yet selective C-H cleaving species in catalytic γ-lactonization reactions of carboxylic acids. Reactivity experiments confirm that the intramolecular formation of γ-lactones is in competition with the intermolecular oxidation of external alkanes and olefins. Finally, mechanistic studies, together with DFT calculations, support a mechanism involving a site-selective C-H cleavage in the γ-position of the carboxylate ligand by the oxo moiety, followed by a fast carboxylate rebound, eventually leading to the selective formation of γ-lactones.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Catalyst and Medium Control over Rebound Pathways in Manganese-Catalyzed Methylenic C-H Bond Oxidation

The C(sp3)-H bond oxygenation of a variety of cyclopropane containing hydrocarbons with hydrogen peroxide catalyzed by manganese complexes containing aminopyridine tetradentate ligands was carried out. Oxidations were performed in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 2,2,2-trifluoroethanol (TFE) using different manganese catalysts and carboxylic acid co-ligands, where steric and electronic properties were systematically modified. Functionalization selectively occurs at the most activated C-H bonds that are α- to cyclopropane, providing access to carboxylate or 2,2,2-trifluoroethanolate transfer products, with no competition, in favorable cases, from the generally dominant hydroxylation reaction. The formation of mixtures of unrearranged and rearranged esters (oxidation in HFIP in the presence of a carboxylic acid) and ethers (oxidation in TFE) with full control over diastereoselectivity was observed, confirming the involvement of delocalized cationic intermediates in these transformations. Despite such a complex mechanistic scenario, by fine-tuning of catalyst and carboxylic acid sterics and electronics and leveraging on the relative contribution of cationic pathways to the reaction mechanism, control over product chemoselectivity could be systematically achieved. Taken together, the results reported herein provide powerful catalytic tools to rationally manipulate ligand transfer pathways in C-H oxidations of cyclopropane containing hydrocarbons, delivering novel products in good yields and, in some cases, outstanding selectivities, expanding the available toolbox for the development of synthetically useful C-H functionalization procedures.

Development of Catalytic Site-Selective C-H Oxidation

Direct C-H bond oxygenation is a strong and useful tool for the construction of oxygen functional groups. After Chen and White's pioneering works, various non-heme-type iron and manganese complexes were introduced, leading to strong development in this area. However, for this method to become a truly useful tool for synthetic organic chemistry, it is necessary to make further efforts to improve site-selectivity, and catalyst durability. Recently, we found that non-heme-type ruthenium complex cis-1 presents efficient catalysis in C(sp3 )-H oxygenation under acidic conditions. cis-1-catalysed C-H oxygenation can oxidize various substrates including highly complex natural compounds using hypervalent iodine reagents as a terminal oxidant. Moreover, the catalyst system can use almost stoichiometric water molecules as the oxygen source through reversible hydrolysis of PhI(OCOR)2 . It is a strong tool for producing isotopic-oxygen-labelled compounds. Moreover, the environmentally friendly hydrogen peroxide can be used as a terminal oxidant under acidic conditions.

Site-selective methylene C-H oxidation of an alkyl diamine enabled by supramolecular recognition using a bioinspired manganese catalyst

Site-selective oxidation of aliphatic C-H bonds is a powerful synthetic tool because it enables rapid build-up of product complexity and diversity from simple precursors. Besides the poor reactivity of alkyl C-H bonds, the main challenge in this reaction consists in differentiating between the multiple similar sites present in most organic molecules. Herein, a manganese oxidation catalyst equipped with two 18-benzo-6-crown ether receptors has been employed in the oxidation of the long chain tetradecane-1,14-diamine. 1H-NMR studies evidence simultaneous binding of the two protonated amine moieties to the crown ether receptors. This recognition has been used to pursue site-selective oxidation of a methylenic site, using hydrogen peroxide as oxidant in the presence of carboxylic acids as co-ligands. Excellent site-selectivity towards the central methylenic sites (C6 and C7) is observed, overcoming selectivity parameters derived from polar deactivation by simple amine protonation and selectivity observed in the oxidation of related monoprotonated amines.

A preparative small-molecule mimic of liver CYP450 enzymes in the aliphatic C-H oxidation of carbocyclic N-heterocycles

An emerging trend in small-molecule pharmaceuticals, generally composed of nitrogen heterocycles (N-heterocycles), is the incorporation of aliphatic fragments. Derivatization of the aliphatic fragments to improve drug properties or identify metabolites often requires lengthy de novo syntheses. Cytochrome P450 (CYP450) enzymes are capable of direct site- and chemo-selective oxidation of a broad range of substrates but are not preparative. A chemoinformatic analysis underscored limited structural diversity of N-heterocyclic substrates oxidized using chemical methods relative to pharmaceutical chemical space. Here, we describe a preparative chemical method for direct aliphatic oxidation that tolerates a wide range of nitrogen functionality (chemoselective) and matches the site of oxidation (site-selective) of liver CYP450 enzymes. Commercial small-molecule catalyst Mn(CF3-PDP) selectively effects direct methylene oxidation in compounds bearing 25 distinct heterocycles including 14 out of 27 of the most frequent N-heterocycles found in U.S. Food and Drug Administration (FDA)-approved drugs. Mn(CF3-PDP) oxidations of carbocyclic bioisostere drug candidates (for example, HCV NS5B and COX-2 inhibitors including valdecoxib and celecoxib derivatives) and precursors of antipsychotic drugs blonanserin, buspirone, and tiospirone and the fungicide penconazole are demonstrated to match the major site of aliphatic metabolism obtained with liver microsomes. Oxidations are demonstrated at low Mn(CF3-PDP) loadings (2.5 to 5 mol%) on gram scales of substrate to furnish preparative amounts of oxidized products. A chemoinformatic analysis supports that Mn(CF3-PDP) significantly expands the pharmaceutical chemical space accessible to small-molecule C-H oxidation catalysis.

A Predictably Selective Palladium-Catalyzed Aliphatic C-H Oxygenation

Direct oxygenation of nonactivated aliphatic C(sp³)-H groups with peroxycarboxylic acids in the presence of palladium tris(pyridylmethyl)amine complex (0.6 mol %) is reported, providing the corresponding hydroxylated derivatives in up to 94% yields. The oxidation of 3° C-H groups occurs stereospecifically, with the catalyst system demonstrating extremely high sensitivity to electronic effects (adamantane oxidation: 3°:2° up to >300). This suggests potential applications for the 3°-regioselective oxidative functionalization of complex molecules of natural origin.

Manganese-Catalyzed Regioselective C-H Lactonization and Hydroxylation of Fatty Acids with H2O2

Herein, we report the direct selective C-H lactonization of fatty acids (C5-C16), catalyzed by manganese(II) complexes bearing bis-amino-bis-pyridine ligands. The catalyst system uses the environmentally benign hydrogen peroxide as oxidant and exhibits high efficiency (100-200 TON), providing under optimized conditions γ-lactones in 60-90% yields. Remarkably, by changing the reaction conditions, the oxidation of hexanoic acid can be diverted toward formation of δ-caprolactone in up to 67% yield. Furthermore, the possibility of obtaining (ω-1)-hydroxy derivatives from linear C7-C10 acids in up to 48% yields has been demonstrated.

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.

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.

Advancements in multifunctional manganese complexes for catalytic hydrogen transfer reactions

Catalytic hydrogen transfer reactions have enormous academic and industrial applications for the production of diverse molecular scaffolds. Over the past few decades, precious late transition-metal catalysts were employed for these reactions. The early transition metals have recently gained much attention due to their lower cost, less toxicity, and overall sustainability. In this regard, manganese, which is the third most abundant transition metal in the Earth's crust, has emerged as a viable alternative. However, the key to the success of such manganese-based complexes lies in the multifunctional ligand design and choice of appropriate ancillary ligands, which helps them mimic and, even in some cases, supersede noble metals' activities. The metal-ligand bifunctionality, achieved via deprotonation of the acidic C-H or N-H bonds, is one of the powerful strategies employed for this purpose. Alongside, the ligand hemilability in which a weakly chelating group tunes in between the coordinated and uncoordinated stages could effectively stabilize the reactive intermediates, thereby facilitating substrate activation and catalysis. Redox non-innocent ligands acting as an electron sink, thereby helping the metal center in steps gaining or losing electrons, and non-classical metal-ligand cooperativity has also played a significant role in the ligand design for manganese catalysis. The strategies were not only employed for the chemoselective hydrogenation of different reducible functionalities but also for the C-X (X = C/N) coupling reactions via HT and downstream cascade processes. This article features multifunctional ligand-based manganese complexes, highlighting the importance of ligand design and choice of ancillary ligands for achieving the desired catalytic activity and selectivity for HT reactions. We have also discussed the detailed reaction pathways for metal complexes involving bifunctionality, hemilability, redox activity, and indirect metal-ligand cooperativity. The synthetic utilization of those complexes in different organic transformations has also been detailed.

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

Novel Tdp1 Inhibitors Based on Adamantane Connected with Monoterpene Moieties via Heterocyclic Fragments

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) is a promising target for anticancer therapy due to its ability to counter the effects topoisomerase 1 (Top1) poison, such as topotecan, thus, decreasing their efficacy. Compounds containing adamantane and monoterpenoid residues connected via 1,2,4-triazole or 1,3,4-thiadiazole linkers were synthesized and tested against Tdp1. All the derivatives exhibited inhibition at low micromolar or nanomolar concentrations with the most potent inhibitors having IC₅₀ values in the 0.35–0.57 µM range. The cytotoxicity was determined in the HeLa, HCT-116 and SW837 cancer cell lines; moderate CC₅₀ (µM) values were seen from the mid-teens to no effect at 100 µM. Furthermore, citral derivative 20c, α-pinene-derived compounds 20f, 20g and 25c, and the citronellic acid derivative 25b were found to have a sensitizing effect in conjunction with topotecan in the HeLa cervical cancer and colon adenocarcinoma HCT-116 cell lines. The ligands are predicted to bind in the catalytic pocket of Tdp1 and have favorable physicochemical properties for further development as a potential adjunct therapy with Top1 poisons.

Electrocatalytic Water Oxidation with α-[Fe(mcp)(OTf)2] and Analogues

The complex α-[Fe(mcp)(OTf)₂] (mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex α-[Fe(mcp)(OTf)₂] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp–iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE ∼ 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET).

Generation and Reactivity of Amidyl Radicals: Manganese-Mediated Atom-Transfer Reaction

A simple and efficient protocol to generate amidyl radicals from amine functionalities through a manganese-mediated atom-transfer reaction has been developed. This approach employs an earth-abundant and inexpensive manganese complex, Mn₂ (CO)₁₀ , as the catalyst and visible light as the energy input. Using this strategy, site-selective chlorination of unactivated C(sp³ )-H bonds of aliphatic amines and intramolecular/intermolecular chloroaminations of unactivated alkenes were readily realized under mild reaction conditions, thus providing efficient access to a range of synthetically valuable alkyl chlorides, chlorinated pyrrolidines, and vicinal chloroamine derivatives. These practical reactions exhibit a broad substrate scope and tolerate a wide array of functional groups, and complex molecules including various marketed drug derivatives.

Chemoselective Tertiary C-H Hydroxylation for Late-Stage Functionalization with Mn(PDP)/Chloroacetic Acid Catalysis

Aromatic and heterocyclic functionality are ubiquitous in pharmaceuticals. Herein, we disclose a new Mn(PDP)catalyst system using chloroacetic acid additive capable of chemoselectively oxidizing remote tertiary C(sp ³)-H bonds in the presence of a broad range of aromatic and heterocyclic moieties. Although catalyst loadings can be lowered to 0.1 mol% under a Mn(PDP)/acetic acid system for aromatic and non-basic nitrogen heterocycle substrates, the Mn(PDP)/chloroacetic acid system generally affords 10-15% higher isolated yields on these substrates and is uniquely effective for remote C(sp ³)-H hydroxylations in substrates housing basic nitrogen heterocycles. The demonstrated ability to perform Mn(PDP)/chloroacetic acid C(sp ³)-H oxidations in pharmaceutically relevant complex molecules on multi-gram scales will facilitate drug discovery processes via late-stage functionalization.

High-Spin Mn(V)-Oxo Intermediate in Nonheme Manganese Complex-Catalyzed Alkane Hydroxylation Reaction: Experimental and Theoretical Approach

Mononuclear nonheme manganese complexes are highly efficient catalysts in the catalytic oxidation of hydrocarbons by hydrogen peroxide in the presence of carboxylic acids. Although high-valent Mn(V)-oxo complexes have been proposed as the active oxidants that afford high regio-, stereo-, and enantioselectivities in the catalytic oxidation reactions, the importance of the spin state (e.g., S = 0 or 1) of the proposed Mn(V)-oxo species is an area that requires further study. In the present study, we have theoretically demonstrated that a mononuclear nonheme Mn(V)-oxo species with an S = 1 ground spin state is the active oxidant that effects the stereo- and enantioselective alkane hydroxylation reaction; it is noted that synthetic octahedral Mn(V)-oxo complexes, characterized spectroscopically and/or structurally, possess an S = 0 spin state and are sluggish oxidants. In an experimental approach, we have investigated the catalytic hydroxylation of alkanes by a mononuclear nonheme Mn(II) complex, [(S-PMB)Mn^II]²⁺, and H₂O₂ in the presence of carboxylic acids; alcohol is the major product with high stereo- and enantioselectivities. A synthetic Mn(IV)-oxo complex, [(S-PMB)Mn^IV(O)]²⁺, is inactive in C-H bond activation reactions, ruling out the Mn(IV)-oxo species as an active oxidant. DFT calculations have shown that a Mn(V)-oxo species with an S = 1 spin state, [(S-PMB)Mn^V(O)(OAc)]²⁺, is highly reactive and capable of oxygenating the C-H bond via oxygen rebound mechanism; we propose that the triplet spin state of the Mn(V)-oxo species results from the consequence of breaking the equatorial symmetry due to the binding of an equatorial oxygen from an acetate ligand. Thus, the present study reports that, different from the previously reported S = 0 Mn(V)-oxo species, Mn(V)-oxo species with a triplet ground spin state are highly reactive oxidants that are responsible for the regio-, stereo-, and enantioselectivities in the catalytic hydroxylation of alkanes by mononuclear nonheme manganese complexes and terminal oxidants.

Enzyme-like substrate-selectivity in C–H oxidation enabled by recognition

Substrate-selective C–H oxidation: supramolecular recognition enhances the reactivity of the bound substrate and enables its substrate-selective hydroxylation.

Chemoselective methylene oxidation in aromatic molecules

Despite significant progress in the development of site-selective aliphatic C-H oxidations over the past decade, the ability to oxidize strong methylene C-H bonds in the presence of more oxidatively labile aromatic functionalities remains a major unsolved problem. Such chemoselective reactivity is highly desirable for enabling late-stage oxidative derivatizations of pharmaceuticals and medicinally important natural products that often contain such functionality. Here, we report a simple manganese small-molecule catalyst Mn(CF₃-PDP) system that achieves such chemoselectivity via an unexpected synergy of catalyst design and acid additive. Preparative remote methylene oxidation is obtained in 50 aromatic compounds housing medicinally relevant halogen, oxygen, heterocyclic and biaryl moieties. Late-stage methylene oxidation is demonstrated on four drug scaffolds, including the ethinylestradiol scaffold where other non-directed C-H oxidants that tolerate aromatic groups effect oxidation at only activated tertiary benzylic sites. Rapid generation of a known metabolite (piragliatin) from an advanced intermediate is demonstrated.

Catalytic oxidation of alcohols with novel non-heme N4-tetradentate manganese(ii) complexes

We report the preparation and characterisation of a series of novel non-heme N4-tetradentate Mn(OTf)2 complexes of the type, [(L)MnOTf2], where L = R,R and S,S enantiomers of BPMCN, its 6-methyl and 6-bromo derivatives as well as the novel ligand BMIMCN (BPMCN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-(R,R/S,S)-1,2-diaminocyclohexane, BMIMCN = N,N'-dimethyl-N,N'-bis(1-methyl-2-imidazolemethyl)-(R,R/S,S)-1,2-diaminocyclohexane). Solid state structural analysis of the BMIMCN-ligated Mn-triflate complexes (R,R-C4 and S,S-C4) revealed opposite helicity but identical metal site accessibility. This feature was exploited in the catalytic oxidation of primary and secondary alcohols, with hydrogen peroxide as oxidant and acetic acid as co-catalyst. Complexes R,R-C4 and S,S-C4 displayed the highest activity in benzyl alcohol oxidation, attributed to the electron-donating property of the BMIMCN ligand. Complex S,S-C4, displayed high activity for a variety of primary alcohol substrates, but the reaction suffered from reduced selectivity and side-reactions due to the presence of acetic acid. In contrast, secondary alcohol substrates could be oxidised to the corresponding ketone products in excellent isolated yields under mild reaction conditions and short reaction times.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

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.

Aliphatic C-H Bond Oxidation with Hydrogen Peroxide Catalyzed by Manganese Complexes: Directing Selectivity through Torsional Effects

Substituted N-cyclohexyl amides undergo aliphatic C-H bond oxidation with H₂O₂ catalyzed by manganese complexes. The reactions are directed by torsional effects leading to site-selective oxidation of cis-1,4-, trans-1,3-, and cis-1,2-cyclohexanediamides. The corresponding diastereoisomers are unreactive under the same conditions. Competitive oxidation of cis- trans mixtures of 4-substituted N-cyclohexylamides leads to quantitative conversion of the cis-isomers, allowing isolation and successive conversion of the trans-isomers into densely functionalized oxidation products with excellent site selectivity and good enantioselectivity.

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.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

In recent years, osmium complexes have found applications not only in thecis-hydroxylation of olefins but also very efficient in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides.

Recent advances in manganese-catalysed C–H activation: scope and mechanism

Manganese catalysed C–H activation has emerged as a promising green alternative to transition metal mediated processes.

Chemoselective Aliphatic C-H Bond Oxidation Enabled by Polarity Reversal

Methods for selective oxidation of aliphatic C-H bonds are called on to revolutionize organic synthesis by providing novel and more efficient paths. Realization of this goal requires the discovery of mechanisms that can alter in a predictable manner the innate reactivity of these bonds. Ideally, these mechanisms need to make oxidation of aliphatic C-H bonds, which are recognized as relatively inert, compatible with the presence of electron rich functional groups that are highly susceptible to oxidation. Furthermore, predictable modification of the relative reactivity of different C-H bonds within a molecule would enable rapid diversification of the resulting oxidation products. Herein we show that by engaging in hydrogen bonding, fluorinated alcohols exert a polarity reversal on electron rich functional groups, directing iron and manganese catalyzed oxidation toward a priori stronger and unactivated C-H bonds. As a result, selective hydroxylation of methylenic sites in hydrocarbons and remote aliphatic C-H oxidation of otherwise sensitive alcohol, ether, amide, and amine substrates is achieved employing aqueous hydrogen peroxide as oxidant. Oxidations occur in a predictable manner, with outstanding levels of product chemoselectivity, preserving the first-formed hydroxylation product, thus representing an extremely valuable tool for synthetic planning and development.