Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

Transition Metal Complexes Containing Selenium Ligands for Catalytic Reduction, Oxidation, and Hydrofunctionalization Reactions

Transition metal-mediated catalytic reduction, oxidation, and hydrofunctionalization reactions are important organic reactions and are considered highly atom-economical. Owing to their unique properties, selenium ligated numerous transition metals-based complexes have been reported for diverse catalytic applications. This review presents the synthesis of various selenium-supported transition metal complexes and their catalytic applications in reduction, oxidation, and hydrofunctionalization reactions. Furthermore, we compare the catalytic activity of various organoselenium ligand-containing transition metal complexes and the replacement of selenium with other chalcogen elements.

H8-BINOL-Derived Chiral η6-Benzene Ligands: New Opportunities for the Ruthenium-Catalyzed Asymmetric C-H Activation

Given the tremendous success of (p-cymene)RuII-catalyzed C-H activation over the past 20 years, the community has long been aware that the development of chiral η6-benzene (Ben) ligands should be a potent strategy for achieving the attractive but incredibly underdeveloped ruthenium(II)-catalyzed asymmetric C-H activation. However, it has rarely been achieved due to the severe difficulty in developing proper chiral Ben ligands. In particular, designing chiral Ben ligands by connecting a benzene fragment to a chiral framework including benzene rings remained an unsolved challenge until this effort. Here we present a novel class of axially chiral Ben ligands derived from readily available (S)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol ((S)-H8-BINOL) in 4-8 steps. Notably, when coordinated with ruthenium, such chiral Ben ligand containing three benzene rings only forms one of the three possible isomeric BenRuII complexes. The related chiral BenRuII catalysts could effectively catalyze the asymmetric C-H activation of N-sulfonyl ketimines with alkynes, affording a range of chiral spirocyclic sultams in up to 99 % yield with up to >99 % ee. These catalysts are expected to find broad applications in future.

Development of ruthenium complexes with S-donor ligands for application in synthesis, catalytic acceptorless alcohol dehydrogenation and crossed-aldol condensation

The reaction of [Ru(dmso)4Cl2] with a potassium salt of four xanthate (RO-C(S)S-; R = Me, Et, iPr and tBu) ligands (depicted as Ln; n = 1-4) in hot methanol afforded a group of mixed-ligand complexes of type [Ru(Ln)2(dmso)2]. The crystal structures of all the four complexes have been determined, which show that the xanthate ligands are bound to the metal center forming four-membered chelates and dmso is coordinated through sulfur and they are mutually cis. The relative thermodynamic stability of this cis and the other possible trans-isomers of these complexes has been assessed with the help of DFT calculations, which have revealed that the cis-isomer is the more stable isomer. The coordinated dmso in the [Ru(Ln)2(dmso)2] complexes could be easily displaced by chelating bidentate ligands (depicted as L') to furnish complexes of type [Ru(Ln)2(L')], as demonstrated through isolation of two such complexes, viz. [Ru(L3)2(bpy)] and [Ru(L2)2(phen)] (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline). The crystal structure of [Ru(L3)2(bpy)] has been determined and the structure of [Ru(L2)2(phen)] has been optimized by the DFT method. The electronic spectra of the four [Ru(Ln)2(dmso)2] complexes and the two derivatives ([Ru(Ln)2(L')]; n = 3, L' = bpy; n = 2, L' = phen), recorded in dichloromethane solutions, show intense absorptions spanning the visible and ultraviolet regions, which have been analyzed by the TDDFT method. The [Ru(Ln)2(dmso)2] complexes are found to serve as efficient catalyst precursors for the acceptorless dehydrogenation of 2-propanol followed by crossed-aldol condensation with substituted benzaldehydes (and related aldehydes), using tert-butoxide as the co-catalyst, producing dibenzylideneacetone derivatives in good yields.

Recent Advances in the Nickel-Catalyzed Alkylation of C-H Bonds

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources.

Asymmetric Ruthenium-Catalyzed C-H Activation by a Versatile Chiral-Amide-Directing Strategy

A versatile and readily available chiral amide directing group has been developed for the ruthenium(II)-catalyzed asymmetric C-H activation. Asymmetric C-H activation of the related chiral benzamides with various olefins, aldehydes and propargylic alcohols has been accomplished with high stereoselectivities, affording a series of chiral products including 3,4-dihydroisocoumarins (up to 96 % ee), isocoumarins (up to 92 % ee), phthalides (up to 99 % ee), chiral bicyclo[2.2.1]heptanes (>20 : 1 dr), 4-alkylidene-3,4-dihydroisocoumarins (up to 97 % ee) and allenes (>20 : 1 dr). Importantly, our methodologies enabled concise syntheses of many biologically active compounds and natural products (e.g., Montroumarin, Cyclosporone E, Cyclosporone Q, Concentricolide, Chuangxinol, and Eleutherol).

Synthesis of N-Fused Polycyclic Indole Derivatives via Ru(II)-Catalyzed C-H Bond Activation and Intramolecular Hydroarylation

A new synthesis of N-fused tetracyclic indole derivatives and their related polycyclic analogues has been developed based on ruthenium(II)-catalyzed C-H activation and intramolecular hydroarylation. A series of polycyclic indoles with a 3-formyl group have been prepared in good to high yields. Various aliphatic and aromatic amines have been studied to form a transient directing group with the aldehyde for the catalytic process. A significant impact of the structures of the aromatic amines was identified, and 1-naphthylamine was shown to enable the catalytic process. DFT computations were performed to gain further insight into the role of the transient directing groups.

2-Hydroxypyridine-based Ligands as Promoter in Ruthenium(II) Catalyzed C-H Bond Activation/Arylation Reactions

A class of 2-hydroxypyridine based ligands are explored to achieve enhanced catalytic activity for ortho-C-H bond activation/arylation reaction over [(η6 -p-cymene)RuCl2 ]2 catalyst in water. Extensive studies using a series of substituted 2-hydroxypyridine based ligands (L1-L6) inferred that 5-trifluoromethyl-2-hydroxypyridine (L6) exhibited favorable effects to enhance the catalytic activity of Ru(II) catalyst for ortho C-H bond arylation of 2-phenylpyridine by 8 folds compared to those performed without ligands. The (η6 -p-cymene)Ru - L6 system also exhibited enhanced catalytic activity for ortho C-H bond arylation of 2-phenylpyridine using a variety of aryl halides. NMR and mass investigations inferred the presence of several ligand coordinated Ru(II) species, suggesting the involvement of these species in C-H bond activation reaction. Further in concurrence with the experimental findings, the density functional theory (DFT) calculations also evidenced the prominent role of 2-hydroxypyridine based ligands in Ru(II) catalyzed C-H bond arylation of 2-phenylpyridine with lower energy barrier for the C-H activation step.

Ru(II)-Catalyzed Weakly Coordinating Carbonyl-Assisted Dialkynylation of (Hetero)Aryl Ketones

Functionalized aryl(heteroaryl) ketones are present in many natural products as key structural components and serve as basic synthetic building blocks for various organic transformation reactions. Therefore, the development of an effective and sustainable route for making these classes of compounds remains challenging yet highly desirable. Herein, we report a simple and efficient catalytic system for dialkynylation of aromatic/heteroaromatic ketones via a double C-H bond activation in the presence of less expensive ruthenium(II)-salt as a catalyst using the weakly and native carbonyl group as the desired directing group. The developed protocol is highly compatible, tolerant, and sustainable toward various functional groups. The synthetic utility of the developed protocol has been demonstrated through the scale-up synthesis and functional group transformation. Control experiments support the involvement of the base-assisted internal electrophilic substitution (BIES) reaction pathway.

Enantioselective C-H Bond Functionalization Involving Arene Ruthenium(II) Catalysis

The p-Cymene ruthenium(II) complex is one of the most widely used catalysts in C-H activation. However, enantioselective C-H activation promoted by arene ruthenium(II) complexes has not been realized until recently. The revealed strategies include intramolecular nitrene C-H insertion, the use of chiral transient directing groups, chiral carboxylic acid, relay catalysis, and chiral arene ligands. In this minireview, these advances are summarized and discussed in the hope of spurring further developments.

Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition

The [Ru(PPh3)2Cl-piperidine(4-aminomethyl)] complex (mono-Ru) was synthesized from [Ru(PPh3)3Cl2] and 4-(aminomethyl)piperidine, whereas the [(PPh3)PdCl(Shiff-pip)] complex (mono-Pd) was obtained by reacting [Pd(PPh3)2Cl2] with its respective Schiff base ligand, both at a 1:1 molar ratio. The heterobimetallic [RuCl2(PPh3)2](μ-Schiff)Pd(PPh3)Cl] complex (Ru/Pd) was synthesized via a one-pot, three-component reaction of mono-Ru, [(Pd(PPh3)2Cl2] and salicylaldehyde. All complexes were fully characterized by FTIR, UV-Vis, and NMR spectroscopy, as well as elemental analysis, MALDI-TOF mass spectrometry, cyclic voltammetry, and computational studies. Ru/Pd was able to polymerize norbornene (NBE) by two different mechanisms: ROMP and vinyl polymerization. The Ru fragment was active for ROMP of NBE, reaching yields of 68 and 31% for mono-Ru and Ru/Pd, respectively, when the [NBE]/[Ru] = 3000 molar ratio and 5 μL EDA addition were employed at 50 °C. The poly(norbornene) (polyNBE) obtained presented an order of magnitude of 104 g mol−1 and Ð values between 1.48 and 1.79. For the vinyl polymerization of NBE, the Pd fragment was active using MAO as a cocatalyst, reaching a yield of 47.0% for Ru/Pd and quantitative yields for mono-Pd when [Al]/[Pd] = 2500 and [NBE]/[Pd] = 20,000 molar ratios were employed, both at 60 °C.

Ru(II)-catalyzed external auxiliary-free primary amide-directed inverse Sonogashira reaction on (hetero)arylamides

Herein, we report ruthenium(II)-catalyzed weakly coordinating primary amide-assisted ortho-di-alkynylation of (hetero)arylamides via double C-H bond activation in the presence of bromo-alkynes as coupling partners. The attractive features of the developed strategy lie in the usage of an inexpensive ruthenium(II)-salt, external auxiliary-free directing group and simple reaction conditions, along with a broad substrate scope, high reaction yields and scale-up synthesis.

Chiral Arene Ligand as Stereocontroller for Asymmetric C-H Activation

Development of chiral ligands is the most fundamental task in metal-catalyzed asymmetric synthesis. In the last 60 years, various kinds of ligands have been sophisticatedly developed. However, it remains a long-standing challenge to develop practically useful chiral η⁶ -arene ligands, thereby seriously hampering the asymmetric synthesis promoted by arene-metal catalysts. Herein, we report the design and synthesis of a class of readily tunable, C₂ -symmetric chiral arene ligands derived from [2.2]paracyclophane. Its ruthenium(II) complexes have been prepared and successfully applied in the enantioselective C-H activation to afford a series of axially chiral isoquinolones (up to 99 % yield and 96 % ee). This study not only lays chemists' longstanding doubts about whether it is possible to use chiral arene ligands to stereocontrol ruthenium(II)-catalyzed asymmetric C-H activation, but also opens up a new avenue to achieve asymmetric C-H activation.

meta-Allylation of Arenes via Ruthenium-Catalyzed Cross-Dehydrogenative Coupling

A successful example of oxidative meta-dehydrogenative allylation of arenes with alkenes has been developed using Ru(PPh₃)₃Cl₂ as a catalyst and DTBP as an oxidant. In the allylation process, pyrimidines, pyrazoles, and purines, found widely in nucleosides, were effective auxiliary groups. Gram-scale experiments took place smoothly under optimized conditions. Mechanistic studies indicated that ruthenium-catalyzed meta-dehydrogenative allylation was a free-radical process. The allylation process developed herein provides an efficient and practical strategy to prepare versatile meta-allylated arenes.

Regio-Selective C3- and N-Alkylation of Indolines in Water under Air Using Alcohols

We disclosed a regio-selective C-H and N-H bond functionalization of indolines using alcohols in water via tandem dehydrogenation of N-heterocycles and alcohols. A diverse range of N- and C3-alkylated indolines/indoles were accessed utilizing a new cooperative iridium catalyst. The practical applicability of this methodology was demonstrated by the preparative-scale synthesis and synthesis of a psychoactive drug, N,N-dimethyltryptamine. A catalytic cycle is proposed based on several kinetic experiments, series of control experiments and density functional theory calculations.

Pd and photoredox dual catalysis assisted decarboxylative ortho-benzoylation of N-phenyl-7-azaindoles

Directing group assisted decarboxylative ortho-benzoylation of N-aryl-7-azaindoles with α-keto acids has been achieved by synergistic visible light promoted photoredox and palladium catalysis. The approach tenders rapid entry to aryl ketone...

Recent Advances in Transition-Metal-Catalyzed Selective C–H Alkoxycarbonyldifluoromethylation Reactions of Aromatic Substrates

Fluorine is well-known as a very special element. Approximately 30% of agrochemicals and 20% of all drugs contain fluorine; most of those compounds have unique functions in biochemistry, pharmacy, and bioscience and those containing alkoxycarbonyldifluoromethyl functional groups often have irreplaceable roles. Therefore, the selective introduction of alkoxycarbonyldifluoromethylated functional groups into various aromatic substrates has significant practical application. This review describes recent advances in selective alkoxycarbonyldifluoromethylation of aromatic substrates by using different catalytic strategies (cyclometalated ruthenium complex, transient regulating and visible-light-induced strategies).

1 Introduction

2 para-C–H Alkoxycarbonyldifluoromethylation of Aromatic Derivatives

2.1 Ruthenium Catalysis

2.2 Palladium Catalysis

2.3 Visible-Light Catalysis

2.4 Iron Catalysis

3 meta-C–H Alkoxycarbonyldifluoromethylation of Aromatic Derivatives

3.1 Ruthenium Catalysis

3.2 Palladium Catalysis

4 The Influence of Transition Metals and Directing Groups on Site Selectivity of Alkoxycarbonyldifluoromethylation

4.1 The Influence of Directing Groups on the Site Selectivity of Alkoxycarbonyldifluoromethylation

4.2 The Influence of Transition Metals on the Site Selectivity of Alkoxycarbonyldifluoromethylation

5 Conclusions

Ru(II)-Catalyzed Controlled Cross-Dehydrogenative Coupling of Benzamides with Activated Olefins via Weakly Coordinating Primary Amides

Ru(II)-catalyzed regioselective ortho-alkenylation of primary benzamides with activated olefins has been realized over the competitive cyclized products. This reaction overall proceeds via a cross-dehydrogenative coupling (CDC) reaction using a simple and weakly coordinating primary amide group in the presence of an inexpensive Ru(II) salt and allows the controlled introduction of olefin motifs at the ortho-position of benzamides. The key to the success of this strategy depends on fine-tuning the reaction conditions. The developed protocol has demonstrated excellent regio/diastereoselectivity and a good functional group tolerance with wide substrate scope and obviates the requirement of external auxiliaries as well as the costly metal catalyst. Detailed mechanistic studies indicate the involvement of the base-assisted internal electrophilic-type substitution (BIES) step in the reaction mechanism.

Arene-Osmium(II) Complexes in Homogeneous Catalysis

Although the application of arene-osmium(II) complexes in homogeneous catalysis has been much less studied than that of their ruthenium analogues, different works have shown that, in some instances, a comparable or even superior effectiveness can be achieved with this particular class of compounds. This review article focuses on the catalytic applications of arene-osmium(II) complexes. Among others, transfer hydrogenation, hydrogenation, oxidation, and nitrile hydration reactions, as well as different C-C bond forming processes, are comprehensively discussed.

Electricity-induced micro-flow C–H/N–H alkyne annulation: a greener approach to access heteroaromatic compounds

Fast access to structurally diverse heteroaromatic compounds is demonstrated in an integrated continuous-flow manner without employing any expensive oxidant.

Highlights of the Most Cited Papers in the Section “Catalysis in Organic and Polymer Chemistry”

The aim of this editorial is to highlight the most cited papers published during 2018–2019 in the section “Catalysis in Organic and Polymer Chemistry” of the journal Catalysts [...]

Ruthenium-Catalyzed C(sp2)-H Bond Bisallylation with Imidazopyridines as Directing Groups

A Ru(II)-catalyzed bisallylation of imidazopyridines with vinylcyclopropanes or vinyl cyclic carbonate has been successfully realized. Notably, pharmacophore imidazopyridine was utilized as an intrinsic directing group, which gave access to value-added bisallylated products in high yields via double tandem C-H and C-C/C-O activation. The current methodology was featured with broad substrate scope, good functional group compatibility, and operational simplicity.

Iron-Catalyzed Highly para-Selective Difluoromethylation of Arenes

Direct functionalization of a C-H bond at either the meta or para position by only changing the catalyst system poses a significant challenge. We herein report the [Fe(TPP)Cl]-enabled, selective, C-H difluoromethylation of arenes using BrCF₂CO₂Et as the difluoromethylation source, which successfully altered the selectivity from the meta to the para position. A preliminary mechanistic study revealed the iron porphyrin complex not only activated the aromatic ring but also induced para selectivity due to the influence of ligand sterics.

Unmasking Arene Ruthenium Building Blocks

We have, like many others, contributed to the development and to the popularity of arene ruthenium assemblies. From early on, our research was driven by applications, mainly biological (therapeutic, drug delivery, DNA interactions, photodynamic therapy, imaging). For nearly 15 years, we have focused on the use of arene ruthenium building block as a tool to construct added-value objects. In this account, we want to give the basic reasons behind our choice, and uncover our most successful examples, with an emphasis on the foreseen applications.

Meta-Dehydrogenative Alkylation of Arenes with Ethers, Ketones, and Esters Catalyzed by Ruthenium

A meta-dehydrogenative alkylation of arenes with cyclic ethers, ketones, and esters catalyzed by ruthenium is achieved in the presence of a di-tert-butyl peroxide (DTBP) oxidant. Interestingly, when quinoline and isoquinoline are employed as the directing group, or a chain ether as alkylation reagent, the system produces Minisci reaction products. Mechanistic study indicates that meta-dehydrogenative alkylation is a radical process initiated by DTBP with the assistance of a C_Ar-Ru bond ortho/para-directing effect.

Ruthenium-Catalyzed Electrochemical Synthesis of Indolines through Dehydrogenative [3 + 2] Annulation with H2 Evolution

A dehydrogenative [3 + 2] annulation reaction of aniline derivatives and alkenes has been developed via the ruthenium-electron catalytic systems for the synthesis of versatile indolines. Electricity is used as a sustainable oxidant to regenerate the active Ru(II) catalyst and promote H₂ evolution. This protocol is ecofriendly and easy to handle as it uses a simple undivided cell in mild conditions without the employment of metal oxidants.

Cp*Co(III)-Catalyzed C-H Hydroarylation of Alkynes and Alkenes and Beyond: A Versatile Synthetic Tool

The use of earth-abundant first-row transition metals, such as cobalt, in C-H activation reactions for the construction and functionalization of a wide variety of structures has become a central topic in synthetic chemistry over the last few years. In this context, the emergence of cobalt catalysts bearing pentamethylcyclopentadienyl ligands (Cp*) has had a major impact on the development of synthetic methodologies. Cp*Co(III) complexes have been proven to possess unique reactivity compared, for example, to their Rh(III) counterparts, obtaining improved chemo- or regioselectivities, as well as yielding new reactivities. This perspective is focused on recent advances on the alkylation and alkenylation reactions of (hetero)arenes with alkenes and alkynes under Cp*Co(III) catalysis.

Catalytic addition of C-H bonds across C-C unsaturated systems promoted by iridium(i) and its group IX congeners

Transition metal-catalyzed hydrocarbonations of unsaturated substrates have emerged as powerful synthetic tools for increasing molecular complexity in an atom-economical manner. Although this field was traditionally dominated by low valent rhodium and ruthenium catalysts, in recent years, there have been many reports based on the use of iridium complexes. In many cases, these reactions have a different course from those of their rhodium homologs, and even allow performing otherwise inviable transformations. In this review we aim to provide an informative journey, from the early pioneering examples in the field, most of them based on other metals than iridium, to the most recent transformations catalyzed by designed Ir(i) complexes. The review is organized by the type of C-H bond that is activated (with C sp², sp or sp³), as well as by the C-C unsaturated partner that is used as a hydrocarbonation partner (alkyne, allene or alkene). Importantly, we discuss the mechanistic foundations of the methods highlighting the differences from those previously proposed for processes catalyzed by related metals, particularly those of the same group (Co and Rh).

Cp*Co(III)-Catalyzed Ketone-Directed ortho-C-H Activation for the Synthesis of Indene Derivatives

A weakly coordinating, carbonyl-assisted C-H activation of aromatic systems with α,β-unsaturated ketone and subsequent aldol condensation has been developed using a Cp*Co(CO)I₂ catalyst. The developed method is the first example of indene synthesis by cobalt-catalyzed C-H activation. In addition, the reaction requires mild reaction conditions and easily accessible starting materials, and it shows excellent functional group compatibility.

Conformationally Driven Ru(II)-Catalyzed Multiple ortho-C–H Bond Activation in Diphenylpyrazine Derivatives in Water: Where Is the Limit?

Ru(II)/carboxylate/PPh3 catalyst system enabled the preparation of highly conjugated pyrazine derivatives in water under microwave irradiation. Both nitrogen atoms efficiently dictated cleavage of the ortho-C–H bonds in both benzene rings of 2,3-diphenylpyrazine substrates through chelation assistance. In conformationally more flexible diphenyldihydropyrazine 1, the arylation of four ortho-C–H bonds was possible, while in the aromatic analog 2, the triarylation was the limit.

Synthesis of 3-alkenylindoles through regioselective C-H alkenylation of indoles by a ruthenium nanocatalyst

3-Alkenylindoles are biologically and medicinally very important compounds, and their syntheses have received considerable attention. Herein, we report the synthesis of 3-alkenylindoles via a regioselective alkenylation of indoles, catalysed by a ruthenium nanocatalyst (RuNC). The reaction tolerates several electron-withdrawing and electron-donating groups on the indole moiety. Additionally, a "robustness screen" has also been employed to demonstrate the tolerance of several functional groups relevant to medicinal chemistry. With respect to the Ru nanocatalyst, it has been demonstrated that it is recoverable and recyclable up to four cycles. Also, the catalyst acts through a heterogeneous mechanism, which has been proven by various techniques, such as ICPMS and three-phase tests. The nature of the Ru nanocatalyst surface has also been thoroughly examined by various techniques, and it has been found that the oxides on the surface are responsible for the high catalytic efficiency of the Ru nanocatalyst.