Ferrocenes bearing planar chirality have been demonstrated to be highly efficient ligands or catalysts in asymmetric catalysis. In view of their atom and step economies, direct asymmetric C—H bond functionalization is the most concise and powerful method for the construction of planar chiral ferrocenes compared with traditional approaches. This review summarizes recent progress on the development of novel methods to synthesize planar chiral compounds via transition- metal (Cu-, Pd-, Ir-, Rh-, Au-, Pt-) catalyzed asymmetric C—H bond functionalization. Preparation of a variety of new planar chiral ferrocene-based catalysts and ligands by utilizing these methods and their application in catalytic asymmetric reactions are also discussed.

Progress in the synthesis and application of the cyclopalladated derivatives of ferrocene with a donor nitrogen atom in the directing group is surveyed including the planar chirality and enantioselective catalysis of organic reactions and rearrangements. Transannular palladation has been found giving achiral 1,1'-disubstituted ferrocenes of ansa-structure. Cyclopalladated ferrocenes have been widely used as catalysts in the cross-coupling reactions (Suzuki, Heck, aza-Claisen, etc. )

An intramolecular dinuclear zinc complex was used in asymmetric Friedel-Crafts alkylation of pyrrole with a wide range of chalcone derivatives. This dinuclear zinc complex was prepared in situ by reacting the chiral ligand (S,S)-1 with 2 equiv. of ZnEt₂. A series of β-pyrrole-substituted dihydrochalcones were formed mostly in excellent yields (up to 99%) and excellent enantioselectivities (up to >99% ee) by using 15 mol% catalyst loading under mild conditions. A possible mechanism was proposed to explain the origin of the asymmetric induction.

A high efficient method for the synthesis of 1,3-diynes derivatives which employed terminal alkynes as the substrates and copper(I) oxide as the catalyst was developed. This method possessed the character of base-free and mild reaction conditions. The reaction mechanism was also studied. Furthermore, this reaction could be magnified to gram scale and the catalyst of copper(I) oxide could be recycled.

A simple and practical visible-light-induced protocol has been developed for the construction of 3-arylquinoxa-lin-2(1H)-ones via Eosin Y-catalyzed direct C-H 3-arylation of quinoxalin-2(H)-ones with aryl diazonium salts at room temperature in air. The present reaction provides a cost-effective and operationally straightforward approach to the target products in moderate to good yields, and does not require any metal reagents, bases, acids, and strong oxidants.

Direct synthesis of the hydroxymethylated arene derivatives via ruthenium(Ⅲ)-catalyzed nitrogen atom directed C-H activation is described. The reaction proceeds smoothly at room temperature and generates the corresponding products in moderate to excellent yields. Meanwhile, it has a broad substrate scope and opens up an attractive avenue for the application of direct hydroxymethylation in the synthesis of biologically active compounds.

Cu(Ⅱ)-promoted C-H nitration of arenes has been disclosed with the aid of N,O-bidentate directing group. The protocol was operationally simple by using NaNO₂as the nitration source. Various amide substrates were tolerated in the reaction system, which establishes opportunities for developing simple and facile methods, and enriches the strategies to access aromatic nitro derivatives.

Phenylalanine derivatives are widely found in a broad range of pharmaceutical molecules and exhibit significant biological activity. Direct C-H bond functionalization of phenylalanine derivatives to offer an efficient approach to construct C-I, C(sp²)-C(sp²), C(sp²)-C(sp), C(sp²)-C(sp³), C-N, C-B and C-O bonds is described. The transformation involves ortho-C-H bond iodination and the following intramolecular amination, ortho-alkylation, ortho-arylation, ortho-alkenylation, ortho-alkynylation, ortho-acylation, ortho-acetoxylation, ortho-amination, ortho-boronation, meta-arylation and meta-alky-lation.

A practical ruthenium-catalyzed C(sp²)-H alkenylation and alkylation of 1-benzyl-1H-pyrazole with alkene or a,β-unsaturated ketones under the assistance of pyrazole group were developed. This method has the advantages of high selectivity, providing only the ortho-alkenylation or ortho-alkylation products in high yields. This protocol provides an efficient and new method for the functionalization of 1-benzyl-1H-pyrazoles.

An one-pot iron-catalyzed oxidative formylation of 2-methylquinolines to produce 2-quinoline carboxaldehydes under microwave irradiation has been achieved by employing O₂ as the oxygen donor. The reaction was general for the substrates with a wide range of functional groups, providing a yield of 48%~80%. The preliminary mechanistic studies revealed that the reaction underwent a radical pathway. Advantages of this method include the easy operation, short reaction time and good selectivity.

The cross dehydrogenation coupling reaction realizes direct coupling of two different X-H bonds to form a new chemical bond, thus featuring excellent step and atom economy. The traditional cross dehydrogenation coupling reaction was usually catalyzed by transition metal, which had the problems of expensive catalyst and serious pollution. Visible light-induced electron transfer is also an effective way to realize direct functionalization of X-H bonds. Visible-light-induced cross dehydrogenation coupling reaction under transition metal-free conditions is widely concerned due to the advantages of cleanliness, safety as well as high step and atom economy. Classified by the type of bonding, the research progress of these reactions is reviewed, and their future outlook is also discussed.

The C-H activation assisted by directing groups is an important measure for selective C-H activation at specific positions, and it is also one of the hot spots in the research field of organic chemistry. As an efficient directing group, carboxylic acid has the advantages of low cost, low toxicity, easy modification and using as traceless directing groups. Recent development on C-H functionalization directed by carboxyl group according to different coupling modes is summarized, and the representative reaction mechanism is briefly described. Existing problems with a brief outlook in this field are also presented.

Carbazole and its derivatives are widely used in the field of medicine and photoelectric materials. A kind of stable cyclic hypervalent iodine reagents containing carbazole group was developed, which belong to benziodoxole compounds. In the presence of Cu(I) catalyst, these reagents reacted with aromatic substrates to give N-aryl carbazole derivatives. The reaction conditions are mild and suitable for a variety of electron-rich arenes. And a radical mechanism was proposed.

A gold(I)-catalyzed 1,2-silyl migration on 1,8-di-substituted naphthalene rings was realized by synergistic effect of Lewis acid and water. High yields were achieved under relatively mild conditions.

The indoles motifs are widely found in the nature. One of the efficient strategy to access the indole derivatives is through the direct C-H functionalization of indole framework itself under transition-metal catalysis. Herein, the research advances on the transition-metal-catalyzed C-H arylation of indoles are reviewed.

A series of bis(alkoxo)palladium complexes (2 mol%) based on pyridine-containing alcohol ligand were tested for Fujiwara-Moritani reaction of thiophenes/furans with various olefins. The desired products were isolated in moderate to excellent yields under mild conditions. A possible concerted metalation-deprotonation (CMD) pathway for this transformation was proved by control experiments and ESI(+)-MS analysis.

Alkaline-earth-metal compounds have been widely concerned due to its abundant reserve and the low-cost. In recent years, alkaline-earth-metal catalysis has achieved great progress in dehydrocoupling, hydroboration, hydrophosphination, hydroamination, hydrosilylation reactions experimentally and therotically. These types of reaction and catalytic mechanism, leading to indentify the role of alkaline-earth-metal in hydrogenation and dehydrogenation reaction are summarized. These reactions, in which the metal-hydride act as active species, generally undergo the reaction pathway involving the cleavage and formation of the metal-hydride covalent bond. The reaction features and mechanisms are generally recognized accrossing to the classification and discussion of these reactions, which would provide guidance for further development of alkaline-earth-metal catalysis.

Transition metal-catalyzed direct C-H bond functionalization strategy has aroused wide concerns due to its high atom-economy and step-economy. Imine synthons exhibit diverse reactivities and are commonly employed to assemble nitrogen-containing complex molecules through direct C-H functionalization strategies, their reaction types and mechanisms are summarized in this paper.

Aromatic nitro compounds are of great importance as chemical raw materials and organic synthesis intermediates. Traditional electrophilic nitration is difficult to achieve regioselective nitrification. In recent years, transition-metal-catalyzed C-H bond activation has developed rapidly. Most functional groups can be introduced into specific sites of aromatics through the chelation of transition metals with directing groups. Directing-group assisted C-H nitration, which is catalyzed by palladium, rhodium, ruthenium and other transition-metal, has made important progress due to its less byproducts, good regioselectivity and environmental protection. According to different transition metal catalysts, the research progress on transition-metal-catalyzed directing-group assisted C-H nitration is summarized, and the limitations of the research field and prospects for future develop-ment are presented.

Maleimide, a common motif in a variety of natural alkaloids, has been extensively investigated due to its noteworthy biological activities and optical properties. Additionally, it can be transformed into many important heterocyclic frameworks such as succinimides, pyrrolidines, and 2-pyrrolidones. Thus, a great deal of attention has been focused on the development of new synthetic routes to access polyfunctionalized maleimides. In this article, the recent research progress in functionalization of double bond is reviewed based on maleimides according to Michael addition, oxidative coupling and cycloaddition reaction.

A copper(Ⅱ)-catalyzed aerobic oxidative coupling of aromatic sulfonyl hydrazides with amines for the synthesis of aromatic sulfonamides was described. In contrast to previously described methods, this reaction employs copper/O₂ as the catalytic system, and generates N₂ as the only byproduct, which provides an environmentally benign synthetic route for aromatic sulfonamides.

A practical method for the oxidation of aromatic side chains was established for the preparation of aromatic aldehydes and ketones. Using NiCl₂ as the catalyst, substituted toluenes were oxidized with Na₂S₂O₈ at the benzylic site for the synthesis of the corresponding aldehydes in the yield of 22%~79%. Ethylbenzene analogs were oxidized more easily to obtain the corresponding ketones with 64%~84% yields. The oxidation of benzyl alcohol analogs was completed to acquire the corresponding carbonyl compounds in shorter time with better selectivity and yields. The method has the advantages such as the mild reaction conditions, no requirement for precious metals or additional promoter, and good selectivity.

α-Amino acids are the units of proteins, which not only widely occur in many biological important compounds and natural products, but also are useful as organic catalysts or ligands for asymmetric synthesis. Among them, glycines are particularly useful building blocks in organic synthesis. Direct C(sp³)-H bond functionalization of glycine derivatives provided an attractive synthesis strategy for the construction of a variety of α-substituted α-amino acids. The recent progress in the α-C(sp³)-H bond activation of glycine derivatives, with various reagents to form carbon-carbon and carbon-heteroatom bond, and oxidative coupling/cyclization reaction involving glycine derivatives is reviewed.

The C (sp³)-H adjacent to heteroatoms can be readily functionalized to C-C, C-N, C-O bonds etc. via cascade[1, n]-hydride transfer/cyclization, which shows high potency to construct 5-membered, 6-membered and all carbon rings. This intriguing cascade process can be employed to synthesize common skeletons of significant natural products and pharmaceutical molecules. Chiral amines, Lewis acids and Brønsted acids have been successfully utilized to catalyze the asymmetric cascade reaction.

A metal-free, operationally simple synthetic protocol for the buildup of unsymmetric 1, 1'-biaryl-2, 2'-diamine compounds was reported. This reaction was performed by the treatment of 2-naphthols with aryl hydrazines under no transition metal catalyst and no ligand conditions, which has the advantages of simple operation and one-step synthesis.

In 1967, Fujiwara and Moritani reported a type of cross coupling reaction where an aromatic C-H bond is directly coupled to an olefinic C-H bond, generating a new C-C bond in the first time. This reaction is performed in the presence of a transition metal, and typically palladium. In the next few decades, palladium-catalyzed oxidative coupling reactions have become an important method in organic synthesis, and a lot of achievements in scientific research have emerged in the relevant areas. The progress in oxidative coupling reaction of olefins, alkynes and aromatic hydrocarbons catalyzed by palladium is reviewed.

C-H bonds are widely existed in almost all the organic compounds. Transition-metal-catalyzed C-H functionalizations usually have high reaction efficiency and high atom-economy. However, traditional strategies based on such transition-metal catalyzed C-H activations generally result in poor selectivities, because C-H bonds in one molecule facilely display similar reactivity. It is difficult to be utilized in preparation of natural products, pharmaceuticals and biomolecules. However, directing group can induce the metal to activate proximal C-H bonds via cyclometallated intermediates, improve the regioselectivity of the transformations. Therefore, it is extremely significant to deveplop auxiliary-induced C-H bonds activations. The research progress of directing group-induced C-H activation reactions and mechanisms for recent ten years are sumarized.

The functional switch of a C—H activation directing group to a pharmacophore is introduced and analyzed, and the value of the pharmacophore and the application of C—H activation are exemplified. It is concluded that many pharmacophores, such as N-containing heteroaromatic, nitrile, carboxylic acid, amide and sulfonamide groups, are ideal directing groups for C—H activation enabling the subsequent stages of drug synthesis, and showing that there is a correlation between a directing group and a pharmacophore. The late-stage functionalization will greatly simplify and effectively improve the possibility of discovering new drugs and potentially shortening the overall synthesis. The latest breakthroughs of C—H activation and application in the drug discovery process are reviewed as case studies, providing several industrial examples of using a pharmacophore as directing group for drug synthesis. It is believed that this development will promote a more rapid and green drug synthesis.

Carbon-hydrogen bonds are the most extensive and basic chemical bonds existed in organic compounds. Electrochemical functionalization and direct conversion of aromatic C—H bonds is a green, sustainable, and atomically economical transformation pathway, which avoids the pre-functionalization of reactants. The anodic electrooxidation of aromatics allows the formation of C—X (X=C, N, O, S) bonds and the preparation of fused aromatic rings without the use of oxidants. Certain C—H activation reactions with chemoselectivity and regioselectivity can also be achieved by the optimization of electrode materials, electrolytes, and solvents. Vourious reactions focusing on the electrochemical functionalizations of C—H bonds in aromatic compounds are mainly reviewed.

Imines are very important class of compounds and have been widely utilized in fine chemicals, pharmaceuticals and chemical industry. The C=N double bond in imine is an important nitrogen source in different types of reactions due to its high reactive activity. Due to its high atom economy, catalytic direct coupling of alcohols and amines to imines based on metal catalysts has attracted much attention and maken great progress in recent years. In this paper, the advances in direct coupling of alcohols and amines to imines catalyzed by metal catalysts are reviewed.

In the presence of 20 mol% K₂S₂O₈, a variety of 3-aryltellurylindoles were obtained in high yields via the telluration of indole dervatives with diarylditellurides catalyzed by 20 mol% I₂. This process tolerates a wide spectrum of different indole derivatives and diarylditellurides. Other advantages include mild reaction conditions, high yields and atom economy, and an efficient route to 3-aryltellurylindoles is afforded.

Heterocyclic compounds are widely applied in medicine, fine chemical engineering and the related industrial fields. Consequently, the development of efficient strategies for heterocycle constructions continues to be of great appeal in organic synthesis. In recent years, the I₂-catalyzed direct C-H bond functionalizations have emerged as one of the most efficient synthetic protocols to construct diverse heterocycles. In this review, the recent advances in I₂-catalyzed C-H bond functionalizations by the types of C-C and C-N/O/S bond formations are summarized, and an outlook of this research filed is given.

The C-H bond activation has become one of the hot fields of organic chemistry in recent years due to its atom economy and step simplicity. In the conventional C-H bond activations, an equivalent amount of oxidants is usually added to regenerate the catalyst and allow the catalytic cycle proceed smoothly. N-Phenoxyacetamide, as a novel reaction substrate containing an oxidizing directing group, can effectively avoid the use of an equivalent external oxidants. Thus, the C-H activation could be conducted under redox neutral conditions. In this paper, the latest research progress of N-phenoxyacetamides in the field of organic synthesis, especially C-H bond activations is reviewed, and the mechanism of the reaction is discussed.

Copper-catalyzed Ullmann-type C-N coupling reaction is one of the most effective methods to construct the C-N bonds. Water, as green and clean solvent, has been successfully employed in various organic reactions. According to the structure of ligands, the progress of ligand-assisted copper-catalyzed C-N cross-coupling reaction in aqueous media or pure water is summarized comprehensively, and the ligands include diamines, hydrazides, phenanthrolines, carbohydrates, pyridine-N-oxides, quinolines, oximes and salen. In addition, ligand-free copper-catalyzed C-N coupling reactions in aquous media or pure water are also reviewed.

MIL-101 was synthesized by hydrothermal method, and Pd/MIL-101 catalyst was prepared by supporting palladium nanoparticles on metal-organic frameworks MIL-101. The microstructure and features of the catalyst were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), inductive coupled plasma (ICP), high resolution transmission electron microscopy (HRTEM) and N₂ adsorption, respectively. The experimental results reveal that the Pd nanoparticles on Pd/MIL-101 range from 1.5 nm to 2.5 nm, and the content of Pd nanoparticles is 1.5%. The catalytic experiments show that Pd/MIL-101 has a high catalytic activity for the C₂ arylation of benzofuran. For less active aryl bromides, the reaction gave moderate yields. Moreover, the catalyst could be recycled many times. After 5 runs of catalysis, the catalyst can still maintain high reactivity. This strategy provides a simple, effective method for the synthesis of benzofuran derivatives.

The chiral triarylmethane frameworks are featured in many biologically important molecules. As a result, the synthesis of chiral triarylmethanes has received tremendous attention from the chemists. Herein, we reported the chiral phosphoric acid catalyzed dehydrative arylation of 3-indolylmethanols with tryptophols, leading to the efficient synthesis of a series of structurally diversified chiral bisindolyl-substituted triarylmethanes in moderate to good yields (up to 80% yield) with acceptable enantioselectivities (up to 88% ee). The chiral phosphoric acid played an important role not only in the dehydration of 3-indolylmethanols, but also in the control of enantioselectivity via hydrogen-bonding and ion-pairing interactions. The only byproduct was water, indicating that this catalytic asymmetric dehydrative arylation reaction was environment-friendly and in accordance with the requirements of green chemistry. In addition, the mild reaction condition and wide substrate scope of the reaction have successfully demonstrated the great potential of organocatalysis in the chiral triarylmethanes.

Construction of C-N bond is one of the most important areas in synthetic organic chemistry and medical chemistry. Many synthetic methods for the construction of C-N bond have been reported over the past few years. Copper-catalyzed Chan-Lam reactions represent one of the most powerful and straightforward tools to construct C-N bonds. In this paper, the recent progress in Chan-Lam coupling of N-compounds with (hetero) aryl boronates to construct C-N bond based on the reaction mechanism, reaction system, the scope of substrates, etc., is reviewed.

Recent years the great progress in transition metal-catalyzed direct aromatic C—H oxidation has been witnessed, which has been utilized in the preparation of various phenolic compounds. These transformations employ inter alia palladium, copper, ruthenium, iridium, etc. as the transition metal catalysts, and hypervalent iodine, persulfate, or oxygen as the oxidants. There have been several reviews in which the C—H oxidations with specific transition metal or oxidant was discussed. This review focuses specifically on transition metal-catalyzed aromatic C—H oxidations with ortho-, meta-, or para-selectivity, and rationalizes the possible generation mechanism of regio-selectivities, which might be controlled by the directing group via chelation-assistance, the ligand, or intrinsic properties of the substrate. The discussion section indicated the existing problems of transition metal-catalyzed aromatic C—H oxidations, as well as the possible limiting factors for the development and application of this strategy.

Due to the lower configuration stability of vinylarenes, arsing from the relatively lower rotational barriers, their catalytic asymmetric synthesis remains a daunting task. Oxindoles moiety are privileged framework of natural products and building blocks of bioactive molecules as well as pharmaceuticals. The asymmetric synthesis of axial chiral vinylarenes fearturing oxindole moiety via sequential carbopalladation/C-H olefination from readily available materials with palladium catalysis has been developed. (4R,5R)-(-)-2,2-Dimethyl-α,α,α',α'-tetraphenyl-1,3-dioxolane-4,5-dimethanol (TADDOL)-derived phosphoramidite gave products with good yield and moderate ee value. The erosion of optical purity was not observed even after heating the product for 10 h at 110℃, which indicates the excellent stability of the chiral axial.