Metal-catalyzed C H activation/functionalization: The fundamentals

Recent Advances on Copper-Catalyzed C–C Bond Formation via C–H Functionalization

Reactions that form C–C bonds are at the heart of many important transformations, both in industry and in academia. From the myriad of catalytic approaches to achieve such transformations, those relying on C–H functionalization are gaining increasing interest due to their inherent sustainable nature. In this short review, we showcase the most recent advances in the field of C–C bond formation via C–H functionalization, but focusing only on those methodologies relying on copper catalysts. This coinage metal has gained increased popularity in recent years, not only because it is cheaper and more abundant than precious metals, but also thanks to its rich and versatile chemistry.

1 Introduction

2 Cross-Dehydrogenative Coupling under Thermal Conditions

2.1 C(sp3)–C(sp3) Bond Formation

2.2 C(sp3)–C(sp2) Bond Formation

2.3 C(sp2)–C(sp2) Bond Formation

2.4 C(sp3)–C(sp) Bond Formation

3 Cross-Dehydrogenative Coupling under Photochemical Conditions

3.1 C(sp3)–C(sp3) Bond Formation

3.2 C(sp3)–C(sp2) and C(sp3)–C(sp) Bond Formation

4 Conclusion and Perspective

Ruthenium-catalyzed, site-selective C-H activation: access to C5-substituted azaflavanone

A site-selective ruthenium-catalyzed keto group assisted C–H bond activation of 2-aryl tetrahydroquinoline (azaflavanone) derivatives has been achieved with a variety of alkenes for the first time. A wide range of substrates was utilized for the synthesis of a wide variety of alkenylated azaflavanones. This simple and efficient protocol provides the C5-substituted azaflavanone derivatives in high yields with a broad range of functional group tolerance. Further, the C5-alkenylated products were converted into substituted 2-aryl quinoline derivatives in good yields.

A site-selective ruthenium-catalyzed keto group assisted C–H bond activation of 2-aryl tetrahydroquinoline (azaflavanone) derivatives has been achieved with a variety of alkenes for the first time.

Catalytic C-H to C-M (M = Al, Mg) bond transformations with heterometallic complexes

C-H functionalisation is one of the cornerstones of modern catalysis and remains a topic of contemporary interest due its high efficiency and atom-economy. Among these reactions, C-H borylation, that is the transformation of C-H to C-B bonds, has experienced a fast development because of the wide utility of organoboron reagents as synthetic intermediates. The mechanistic background is now well-understood and the role of transition metal boryl or σ-borane intermediates in this transformation is well documented. This mini-review focuses on efforts made by our group, and others, to establish palladium- and calcium-catalysed methods for C-H metalation employing heavier main group elements (M = Al, Mg). These are new catalytic reactions first accomplished in our group that we have termed C-H alumination and magnesiation respectively. Unusual heterometallic complexes have been identified as key on-cycle intermediates and their unique reactivity is discussed in the context of new catalytic pathways for C-H functionalisation. Hence, this mini-review summarises the recent progress in the area of C-H metalation reactions as well as the new opportunities that may arise from this concept.

In silico rationalisation of selectivity and reactivity in Pd-catalysed C-H activation reactions

A computational approach has been developed to automatically generate and analyse the structures of the intermediates of palladium-catalysed carbon-hydrogen (C-H) activation reactions as well as to predict the final products. Implemented as a high-performance computing cluster tool, it has been shown to correctly choose the mechanism and rationalise regioselectivity of chosen examples from open literature reports. The developed methodology is capable of predicting reactivity of various substrates by differentiation between two major mechanisms - proton abstraction and electrophilic aromatic substitution. An attempt has been made to predict new C-H activation reactions. This methodology can also be used for the automated reaction planning, as well as a starting point for microkinetic modelling.

"On Water" Palladium Catalyzed Direct Arylation of 1H-Indazole and 1H-7-Azaindazole

The C3 direct arylation of 1H-indazole and 1H-7-azaindazole has been a significant challenge due to the lack of the reactivity at this position. In this paper, we describe a mild and an efficient synthesis of new series of C3-aryled 1H-indazoles and C3-aryled 1H-7-azaindazoles via a C3 direct arylation using water as solvent. On water, PPh₃ was effective as a ligand along with a lower charge of the catalyst Pd(OAc)₂ (5 mol%) at 100 °C, leading to C3-aryled 1H-indazoles or C3-aryled 1H-7-azaindazoles in moderate to good yields.

New Trends in Enantioselective Cross-Dehydrogenative Coupling

The development of cross-dehydrogenative coupling in recent years has simplified the synthesis of many materials, as a result of facile C–H activation, which, together with its greater atom economy and environmental friendliness, has made an impact on modern organic chemistry. Indeed, many C–C and C–X (X = N, O, P, S, B, or Si) coupling reactions can now be performed directly between two C–H bonds or a C–H and an X–H bond, simply by adding catalytic amounts of a metal salt to a mixture of the two and an oxidant to accept the two hydrogen atoms released. Chiral organocatalysts or chiral ligands have been joined to promote enantioselective processes, resulting in the development of efficient reaction cascades that provide products in high yields and high levels of asymmetric induction through cooperative catalysis. In recent years, photochemical oxidation and electrochemistry have widened even more the scope of cross-dehydrogenative coupling (CDC). In this review, we summarized the recent literature in this subject, hoping that it will inspire many new synthetic strategies.

Metal-free oxidative C(sp3)–H functionalization: a facile route to quinoline formaldehydes from methyl-azaheteroarenes

A facile protocol for the synthesis of quinoline formaldehydes via direct oxidative C–H bonds functionalization of methyl-azaheteroarenes in the presence of I2–DMSO has been described. This method is metal-free and easy to operate. This reaction provided a convenient route for the preparation of a range of important quinoline formaldehydes.

Palladium-Catalyzed [3+2] and [2+2+2] Annulations of 4-Iodo-2-quinolones with Activated Alkynes through Selective C-H Activation

The palladium-catalyzed reaction of 4-iodo-2-quinolones with activated alkynes was investigated. Cyclopenta[de]quinoline-2(1 H)-ones and/or phenanthridine-6(5 H)-ones were obtained through [3+2] annulation involving aromatic C-H activation or [2+2+2] annulation involving vinylic C-H activation, respectively. Reasonable mechanisms for the formation of these annulation products have been proposed based on density functional theory calculations.

The oxidation of cyclo-olefin by the S = 2 ground-state complex [FeIV(O)(TQA)(NCMe)]2

Density functional theory calculation is used to investigate the oxidation of cyclo-olefin (cyclobutene, cyclopentene, cyclohexene, cycloheptene, and cyclo-octene) by the complex [Fe^IV(O)(TQA)(NCMe)]²⁺, which has S = 2 ground state, and the effect of electronic factors and steric hindrance on reaction barriers. Our results suggest that the oxo-iron(IV) complex can oxidise C-H and C = C bonds via a single-state mechanism, and two different ways of electron transport exist. The energy barriers initially decrease with increasing substrate size, and the trend then reverses. Comparison of the energy barrier in different systems reveals that except for the reaction between [Fe^IV(O)(TQA)(NCMe)]²⁺ and cycloheptene, oxo-iron(IV) complexes prefer epoxidation to hydroxylation. However, the hydroxylated product is more stable than the corresponding epoxidated product. This result indicates that the products of epoxidation tend to decompose first. The energy barrier of hydroxylation and epoxidation originates from the balance of orbital interaction and Pauli repulsion from the equatorial ligand and protons on the approaching substrate. In this regard, we calculate the weak interaction between two fragments (oxo-iron complex and substrates) using the independent gradient model and drawn the corresponding 3D isosurface representations of reactants.

Iron-Catalysed C(sp2)-H Borylation Enabled by Carboxylate Activation

Arene C(sp²)-H bond borylation reactions provide rapid and efficient routes to synthetically versatile boronic esters. While iridium catalysts are well established for this reaction, the discovery and development of methods using Earth-abundant alternatives is limited to just a few examples. Applying an in situ catalyst activation method using air-stable and easily handed reagents, the iron-catalysed C(sp²)-H borylation reactions of furans and thiophenes under blue light irradiation have been developed. Key reaction intermediates have been prepared and characterised, and suggest two mechanistic pathways are in action involving both C-H metallation and the formation of an iron boryl species.

Complete Benzene Oxidation over Mono and Bimetallic Pd—Au Catalysts on Alumina-Supported Y-Doped Ceria

The protection of environment and human health stimulates intensive research for abatement of volatile organic compounds (VOCs) in the atmosphere. Complete catalytic oxidation is an efficient, environmentally friendly and economically feasible method for elimination of VOCs. This study aims to design high performing and cost-effective catalytic formulations by exploration of appropriate and economically viable supports. Alumina-supported ceria (30 wt.%) and Y2O3 (1 wt.%)-doped ceria were prepared by mechanical mixing and were used as support of mono Au (2 wt.%) and Pd (1 wt.%) and bimetallic Pd-Au catalysts. The characterization by textural measurements, X-ray powder diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), EPR (electron paramagnetic resonance) and temperature-programmed reduction (TPR) was carried out in order to clarify the relationship between catalyst composition, textural, structural and surface properties, reducibility and catalytic performance for complete benzene oxidation. Among all studied catalysts, Pd-based catalysts exhibited the best combustion activity. In particular, monometallic Pd on alumina supported Y-doped ceria attained 100% of complete benzene conversion at 180 °C. These catalytic materials have potential to meet stringent emission regulations in an economical and effective way.

Hydrophobic zeolite modification for in situ peroxide formation in methane oxidation to methanol

Selective partial oxidation of methane to methanol suffers from low efficiency. Here, we report a heterogeneous catalyst system for enhanced methanol productivity in methane oxidation by in situ generated hydrogen peroxide at mild temperature (70°C). The catalyst was synthesized by fixation of AuPd alloy nanoparticles within aluminosilicate zeolite crystals, followed by modification of the external surface of the zeolite with organosilanes. The silanes appear to allow diffusion of hydrogen, oxygen, and methane to the catalyst active sites, while confining the generated peroxide there to enhance its reaction probability. At 17.3% conversion of methane, methanol selectivity reached 92%, corresponding to methanol productivity up to 91.6 millimoles per gram of AuPd per hour.

Regioselective C–H hydroxylation of n-alkanes using Shilov-type Pt catalysis in perfluorinated micro-emulsions

In this work, the potential of combining Shilov-type Pt^II and micellar catalysis to realize the challenging terminal C–H hydroxylation of saturated n-alkanes using water as the reaction medium is demonstrated.

Remote functional group directed C–H activation by an Ir(iii) phenanthroline complex

The regioselectivity of the C–H activation of 1-butanol and 1-methoxybutane by an iridium(iii) phenanthroline complex was studied in the gas phase and revealed activation at gamma and delta carbons.

The regioselective coupling of 2-arylquinazolinone C–H with aldehydes and benzyl alcohols under oxidative conditions

Palladium catalyzed direct and regioselective cross dehydrogenative coupling (CDC) of 2-arylquinazoline-4-one endowed with a quinazolinone nucleus as an inherent directing group with aldehyde and oxidative coupling with benzyl alcohol was developed.

Photocatalyzed allylic derivatization reactions

Catalytic allylation reactions are important methodologies to produce fine chemicals and synthetic building blocks. This review discloses state-of-the-art photocatalyzed allylation methodologies, their reaction mechanisms, and synthetic applications.

Convenient synthesis of quinoline-fused triazolo-azepine/oxepine derivatives through Pd-catalyzed C–H functionalisation of triazoles

The convenient synthesis of a novel polycyclic fused system comprising three different heterocycles, viz., quinolines, azepines/oxepines and triazoles is presented in high yields.

Calcium-Catalyzed Arene C-H Bond Activation by Low-Valent AlI

The low‐valent ß‐diketiminate complex (^DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(^DIPPBDI)CaH]₂ at 20 °C led to (^DIPPBDI)Al(Ph)H (^DIPPBDI=CH[C(CH₃)N‐DIPP]₂, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C−H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (^DIPPBDI)Al(m‐tolyl)H with I₂ gave m‐tolyl iodide, H₂ and (^DIPPBDI)AlI₂ which was recycled to (^DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (^DIPPBDI)Al(H)M(^DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (^DIPPBDI)Al(H)Ca(^DIPPBDI). This is in equilibrium with the hydride bridged complex (^DIPPBDI)Al(μ‐H)Ca(^DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C−H bond activation.

Mild C-H functionalization of alkanes catalyzed by bioinspired copper(ii) cores

Three new copper(ii) coordination compounds formulated as [Cu(H_1.5bdea)₂](hba)·2H₂O (1), [Cu₂(μ-Hbdea)₂(aca)₂]·4H₂O (2), and [Cu₂(μ-Hbdea)₂(μ-bdca)]_n (3) were generated by aqueous medium self-assembly synthesis from Cu(NO₃)₂, N-butyldiethanolamine (H₂bdea) as a main N,O-chelating building block and different carboxylic acids [4-hydroxybenzoic (Hhba), 9-anthracenecarboxylic (Haca), or 4,4'-biphenyldicarboxylic (H₂bdca) acid] as supporting carboxylate ligands. The structures of products range from discrete mono- (1) or dicopper(ii) (2) cores to a 1D coordination polymer (3), and widen a family of copper(ii) coordination compounds derived from H₂bdea. The obtained compounds were applied as bioinspired homogeneous catalysts for the mild C-H functionalization of saturated hydrocarbons (cyclic and linear C₅-C₈ alkanes). Two model catalytic reactions were explored, namely the oxidation of hydrocarbons with H₂O₂ to a mixture of alcohols and ketones, and the carboxylation of alkanes with CO/S₂O₈^2- to carboxylic acids. Both processes proceed under mild conditions with a high efficiency and the effects of different parameters (e.g., reaction time and presence of acid promoter, amount of catalyst and solvent composition, substrate scope and selectivity features) were studied and discussed in detail. In particular, an interesting promoting effect of water was unveiled in the oxidation of cyclohexane that is especially remarkable in the reaction catalyzed by 3, thus allowing a potential use of diluted, in situ generated solutions of hydrogen peroxide. Moreover, the obtained values of product yields (up to 41% based on alkane substrate) are very high when dealing with the C-H functionalization of saturated hydrocarbons and the mild conditions of these catalytic reactions (50-60 °C, H₂O/CH₃CN medium). This study thus contributes to an important field of alkane functionalization and provides a notable example of new Cu-based catalytic systems that can be easily generated by self-assembly from simple and low-cost chemicals.

Artificial Metalloenzymes: Challenges and Opportunities

Artificial metalloenzymes (ArMs) result from the incorporation of an abiotic metal cofactor within a protein scaffold. From the earliest techniques of transition metals adsorbed on silk fibers, the field of ArMs has expanded dramatically over the past 60 years to encompass a range of reaction classes and inspired approaches: Assembly of the ArMs has taken multiple forms with both covalent and supramolecular anchoring strategies, while the scaffolds have been intuitively selected and evolved, repurposed, or designed in silico. Herein, we discuss some of the most prominent recent examples of ArMs to highlight the challenges and opportunities presented by the field.

PtII -Catalyzed Hydroxylation of Terminal Aliphatic C(sp3 )-H Bonds with Molecular Oxygen

The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C-H bonds remains a formidable challenge, due to difficulties in replacing Pt^IV with a more economically viable oxidant, particularly O₂ . We report the potential of employing FeCl₂ as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O₂ in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O₂ in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt⁰ was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N₂ in the gaseous atmosphere. Finally, stability tests revealed the involvement of Pt^II and FeCl₂ in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C-H bonds with O₂ is presented.

Recent Advances in Oxidative R1-H/R2-H Cross-Coupling with Hydrogen Evolution via Photo-/Electrochemistry

Photo-/electrochemical catalyzed oxidative R¹-H/R²-H cross-coupling with hydrogen evolution has become an increasingly important issue for molecular synthesis. The dream of construction of C-C/C-X bonds from readily available C-H/X-H with release of H₂ can be facilely achieved without external chemical oxidants, providing a greener model for chemical bond formation. Given the great influence of these reactions in organic chemistry, we give a summary of the state of the art in oxidative R¹-H/R²-H cross-coupling with hydrogen evolution via photo/electrochemistry, and we hope this review will stimulate the development of a greener synthetic strategy in the near future.

Investigations of the generality of quaternary ammonium salts as alkylating agents in direct C-H alkylation reactions: solid alternatives for gaseous olefins

C-H alkylation reactions using short chain olefins as alkylating agents could be operationally simplified on the lab scale by using quaternary ammonium salts as precursors for these gaseous reagents: Hofmann elimination delivers in situ the desired alkenes with the advantage that the alkene concentration in the liquid phase is high. In case a catalytic system did not tolerate the conditions for Hofmann elimination, a very simple spatial separation of both reactions, Hofmann elimination and direct alkylation, was achieved to circumvent possible side reactions or catalyst deactivation. Additionally, the truly catalytically active species of a rhodium(i) mediated alkylation reaction could be identified by using this approach.

Palladium-Catalyzed Chemoselective Activation of sp3 vs sp2 C-H Bonds: Oxidative Coupling To Form Quaternary Centers

The oxidative activation of alkyl C-H bonds vs arene C-H bonds with Pd(OAc)₂ has been found to be generalizable to a number of nucleophilic substrates allowing the formation of a range of hindered quaternary centers. The substrates share a common mechanistic path wherein Pd(II) initiates an oxidative dimerization. The resultant dimer modifies the palladium catalyst to favor activation of alkyl C-H bonds in contrast to the trends typically observed via a concerted metalation deprotonation mechanism. Notably, insertion occurs at the terminus of the alkyl arene for hindered substrates. Two different oxidant systems were discovered that turn over the process. Parameters have been identified that predict, which substrates are productive in this reaction.

Multiple Aromatic C-H Bond Activations by an Unsaturated Dirhenium Carbonyl Complex

Reactions of Re₂(CO)₈(μ-C₆H₅)(μ-H), 1, with naphthalene and anthracene have yielded the first multiply-CH activated arene products through the reductive elimination of benzene from 1 and multiple oxidative-additions of the dirhenium octacarbonyl grouping to these polycyclic aromatic compounds under very mild conditions. In addition, compound 1 was found to react with itself to yield the bis-Re₂-metalated C₆H₄ bridged compound Re₂(CO)₈(μ-H)(μ-1,μ-3-C₆H₄)Re₂(CO)₈(μ-H), 3. Reaction of 1 with naphthalene yielded two doubly CH activated isomers, Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-3,4-C₁₀H₆)Re₂(CO)₈(μ-H), 4, 41% yield, and Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-5,6-C₁₀H₆)Re₂(CO)₈(μ-H), 5, via the mono CH activated intermediate Re₂(CO)₈(μ-η²-C₁₀H₇)(μ-H), 2. Compound 4 contains two Re₂(CO)₈(μ-H) groups on one C₆ ring formed by CH activations at the 2- and 4-positions. Compound 5 contains two Re₂(CO)₈(μ-H) groups; one formed by CH activation at the 2-position on one C₆ ring and the other formed by CH activation at the 6-position (or centrosymmetrically related 2'-position) on the second C₆ ring. The Re₂(CO)₈(μ-H) groups are coordinated to the C₆ rings by binuclear σ + π coordination to two adjacent carbon atoms in the rings. Compound 1 reacts with anthracene to yield the mono-CH activated compound Re₂(CO)₈(μ-η²-1,2-C₁₄H₉)(μ-H), 6, and two doubly CH activated compounds, Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-3,4-C₁₄H₈)Re₂(CO)₈(μ-H), 7, and Re₂(CO)₈(μ-H)(μ-η²-1,2-μ-η²-5,6-C₁₄H₈)Re₂(CO)₈(μ-H), 8. Compounds 7 and 8 are isomers that are structurally similar to 4 and 5. Compounds 7 and 8 can also be obtained in good yields from the reaction of 6 with 1. In the presence of a 5/1 ratio of 1/anthracene, a small amount (5% yield) of the tetra-substituted anthracene product [Re₂(CO)₈(μ-H)]₄(μ-η²-1,2-μ-η²-3,4-μ-η²-5,6-μ-η²-7,8-C₁₄H₆), 9, was formed. Compound 9 contains four σ + π coordinated Re₂(CO)₈(μ-H) groups formed by oxidative additions of the CH bonds of anthracene to the Re₂(CO)₈ groups at the 2, 4, 6, and 8 positions of the three ring system. Molecular orbital calculations have been performed for all new compounds in order to develop an understanding of the bonding of the ring systems to the Re₂(CO)₈(μ-H) groups. All new compounds were characterized by single-crystal X-ray diffraction analyses.

Visible light-mediated organophotocatalyzed C-H bond functionalization reactions

Over the last decade, a variety of methodologies for the direct functionalization of C-H bonds have been developed. Among others, visible light photoredox reactions have recently emerged as one of the most efficient and highly selective processes for the direct introduction of a functionality into organic molecules. Easy reaction setups, as well as mild reaction conditions, make this approach superior to other methodologies applying transition metals or strong oxidants, in terms of both costs and substrate and functional group tolerance. In this review, the recent developments in organophotocatalyzed C-H bond functionalization reactions are presented.

Ru(ii)-Catalysed synthesis of (1H)-isothiochromenes by oxidative coupling of benzylthioethers with internal alkynes

(1H)-Isothiochromenes have been prepared by Ru-catalysed oxidative coupling of benzylthioethers with internal alkynes.