Palladium(II)-Catalyzed CH Bond Arylation of Electron-Deficient Arenes at Room Temperature

Palladium-Catalyzed Weak Chelation-Assisted Site-Selective C-H Arylation of N-Aryl Pyridones via 2-fold C-H Activation

Palladium-catalyzed weak chelation-assisted oxidative cross-dehydrogenative coupling of arenes has been accomplished. The use of medicinally important pyridones as the intrinsic directing group, regioselectivity, 2-fold C-H activation, and late-stage modification of bioactive compounds are the important practical features.

1,2,3-Thiadiazole as a Modifiable and Scalable Directing Group for ortho-C-H Functionalization

In the last few decades, directed C-H bond functionalization has had enormous applicability in academia and industry. The development of a novel, readily accessible, and scalable directing group with modifiable ability is highly desirable in C-H functionalization. Herein, we report the 1,2,3-thiadiazole as a modifiable directing group for C-H amidation and alkynylation with dioxazolones, p-toluenesulfonyl azide, and bromoalkynes in high yield. The densely functionalized 1,2,3-thiadiazole products are modified into thioamide, multisubstituted furan, γ-thiapyrone, thiazole, and various alkynyl sulfides through simple and one-step reactions. The competition experiments reveal that the directing ability of 1,2,3-thiadiazole is slightly weaker than pyridine and bidentate amide but stronger than the widely used carboxylate.

Direct Cyclopalladation of Fluorinated Benzyl Amines by Pd3(OAc)6: The Coexistence of Multinuclear Pdn Reaction Pathways Highlights the Importance of Pd Speciation in C-H Bond Activation

Palladacycles are key intermediates in catalytic C-H bond functionalization reactions and important precatalysts for cross-couplings. It is commonly believed that palladacycle formation occurs through the reaction of a substrate bearing a C-H bond ortho to a suitable metal-directing group for interaction with, typically, mononuclear "Pd(OAc)2" species, with cyclopalladation liberating acetic acid as the side product. In this study, we show that N,N-dimethyl-fluoro-benzyl amines, which can be cyclopalladated either ortho or para to fluorine affording two regioisomeric products, can occur by a direct reaction of Pd3(OAc)6, proceeding via higher-order cyclopalladated intermediates. Regioselectivity is altered subtly depending on the ratio of substrate:Pd3(OAc)6 and the solvent used. Our findings are important when considering mechanisms of Pd-mediated reactions involving the intermediacy of palladacycles, of particular relevance in catalytic C-H bond functionalization chemistry.

Palladium-Catalyzed ortho-C-H Alkoxycarbonylation of Aromatic Aldehydes via a Transient Directing Group Strategy

Transient directing groups (TDGs) can be a powerful strategy for directly functionalizing C-H bonds of aldehydes. We report a palladium-catalyzed o-C-H alkoxycarbonylation of benzaldehydes using a catalytic amount of aromatic amine to form a transient imine that plays the role of a monodentate TDG. The reaction conditions were applied to a broad range of aldehydes, and the corresponding 2-formyl benzoates were used as direct precursors for the synthesis of phthalides and 1-isoindolinones.

Hypervalent iodine(iii) promoted C–H/C–H amination/annulation tandem reactions: synthesis of benzimidazoles from simple anilines and aldehydes

A novel hypervalent iodine mediated cascade transformation of anilines and aldehydes to benzimidazoles was developed.

Silver-Free C-H Activation: Strategic Approaches towards Realizing the Full Potential of C-H Activation in Sustainable Organic Synthesis

The activation of carbon-hydrogen bonds is considered as one of the most attractive techniques in synthetic organic chemistry because it bears the potential to shorten synthetic routes as well as to produce complementary product scopes compared to traditional synthetic strategies. However, many current methods employ silver salts as additives, leading to stoichiometric metal waste and thereby preventing the full potential of C-H activation to be exploited. Therefore, the development of silver-free protocols has recently received increasing attention. Mechanistically, silver can serve various roles in C-H activation and thus, avoiding the use of silver requires different approaches based on the role it serves in a given process. In this Review, we present the comparison of silver-based and silver-free methods. Focusing on the strategic approaches to develop silver-free C-H activation, we provide the reader with the means to develop sustainable methods for C-H activation.

Transition-Metal-Catalyzed C–H Arylation Using Organoboron Reagents

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.

1 Introduction

2 Arylation without Directing Group Assistance

2.1 Palladium Catalysis

2.2 Iron Catalysis

2.3 Gold Catalysis

3 Arylation with Directing Group Assistance

3.1 Palladium Catalysis

3.2 Ruthenium Catalysis

3.3 Rhodium Catalysis

3.4 Nickel Catalysis

3.5 Cobalt Catalysis

3.6 Copper Catalysis

4 Conclusion

Dirhodium(II)-Catalyzed C(sp2 )-H Azidation of Benzaldehydes

Multiple steps are needed to achieve the C-H functional of aromatic aldehyde, since the C-H functional reaction usually occurs preferentially at the aldehydic C-H bond over the aryl C-H bond. We report an efficient azidation method mediated by dirhodium(II) catalysts to achieve the direct aryl azidation of aromatic aldehydes avoiding the simultaneous use of protected aldehydes and prefunctionalized arenes. The regioselectivity of this method is similar to those of typical aromatic electrophilic substitution reactions. The resulting azidobenzaldehyde products are versatile building blocks or precursors for the synthesis of many biologically active compounds. The mechanism studies indicate that the one-electron oxidative intermediate Rh₂ ^(II,III) N₃ is responsible for the azide transfer.

Photocatalytic site-selective C-H difluoroalkylation of aromatic aldehydes

The direct photocatalyzed para-selective CAr-H difluoroalkylation of aromatic aldehyde derivatives has been accomplished using a newly explored catalytic system. In addition, when using para-substituted benzaldehydes as substrates, ortho-selective CAr-H difluoroalkylation was also accomplished. It is worth noting that all the above site-selectivity is opposite to traditional Friedel-Crafts reactions of aromatic aldehydes. The preliminary mechanistic investigations indicate that an electrophilic difluoroalkyl radical is involved in the catalytic cycle.

Direct remote δ-C(sp2)–H olefination of β-aryl-substituted aliphatic aldehydes via palladium/enamine co-catalysis

Palladium-catalyzed direct remote δ-C(sp²)–H olefination of β-aryl-substituted aliphatic aldehydes has been achieved using a catalytic amount of secondary amine (n-Bu)₂NH as a co-catalyst.

Relative Strength of Common Directing Groups in Palladium-Catalyzed Aromatic C-H Activation

Efficient functionalization of C-H bonds can be achieved using transition metal catalysts, such as Pd(OAc)2. To better control the regioselectivity in these reactions, some functional groups on the substrate may be used as directing groups, guiding the reactivity to an ortho position. Herein, we describe a methodology to score the relative strength of such directing groups in palladium-catalyzed aromatic C-H activation. The results have been collected into a scale that serves to predict the regioselectivity on molecules with multiple competing directing groups. We demonstrate that this scale yields accurate predictions on over a hundred examples, taken from the literature. In addition to the regioselectivity prediction on complex molecules, the knowledge of the relative strengths of directing groups can also be used to work with new combinations of functionalities, exploring uncharted chemical space.

Chelation-directed remote meta-C-H functionalization of aromatic aldehydes and ketones

We disclose herein the development of a versatile 1,2-diol directing template for palladium-catalyzed remote meta-C-H functionalization of aromatic aldehydes and ketones. In situ-generation of acetals and ketals, as well as removal afterwards, makes the C-H bond functionalization process more straightforward and efficient. This also represents the first example of chelation-directed meta-C-H functionalization of aromatic aldehydes and ketones.

Dioxazolones as masked ester surrogates in the Pd(ii)-catalyzed direct C-H arylation of 6,5-fused heterocycles

A simple and effective Pd(ii)-catalyzed regioselective C(2)-H arylation of 6,5-fused heterocycles with dioxazolones as a masked ester surrogate under mild conditions is reported. The significance of the arylation is highlighted by the new reactivity demonstrated in dioxazolones via proximal C-H activation of the cyclic carbonate of the hydroxamic acid functionality under protic conditions.

Palladium Catalyzed meta-C-H Functionalization of Masked Aromatic Aldehydes

Palladium catalyzed meta-C-H functionalization enabled by transient mediators has the potential to extend the utility of directed ortho-C-H functionalization to remote positions. However, there have been no reports of palladium catalyzed meta-C-H functionalization of aromatic aldehyde derivatives, which are highly versatile intermediates in organic synthesis. Herein we report the development of a directing group that, in the presence of a norbornene derived mediator and an appropriate pyridone ligand, allows palladium catalyzed meta-C-H functionalization of masked aromatic aldehydes. Mechanistic insight regarding the impact of the directing group length on this catalysis is also discussed.

Bi and trinuclear complexes in palladium carboxylate-assisted C-H activation reactions

The role of polynuclear species in C-H activations assisted by palladium carboxylates has not been clear so far. The summary of the key findings covering this issue shows its important role under certain conditions. However, much more effort is necessary for a deeper understanding of the whole issue.

Selective formation of phthalimides from amines, aldehydes and CO by Pd-catalyzed oxidative C–H aminocarbonylation

The Pd/Cu-catalyzed intramolecular C–H bond aminocarbonylation utilizing imines as the amine source has been developed. This transformation provides an efficient and straightforward protocol for the synthesis of phthalimides which widely exist in natural products, pharmaceutical and functional materials. Various functional groups are tolerated and the yields are up to 99%.

Assembly of Diverse Spirocyclic Pyrrolidines via Transient Directing Group Enabled Ortho-C(sp2)-H Alkylation of Benzaldehydes

A diversity-oriented synthesis of useful spirocyclic pyrrolidines was successfully accomplished via late-stage cascade reactions of o-succinimide-substituted benzaldehydes. A catalytic amount of aniline as a transient directing group was efficient for the ruthenium-catalyzed ortho-C(sp²)-H alkylation of benzaldehyde with maleimide. The in situ formed imine overrided a series of other traditional directing groups with excellent site selectivities. More importantly, only 0.5 mol % of ruthenium catalyst was sufficient for a 100 mmol scale-up reaction without column chromatography purification.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Additive-Free Pd-Catalyzed α-Allylation of Imine-Containing Heterocycles

An additive-free Pd-catalyzed α-allylation of different imino-group-ontaining heterocycles is reported. The activation of α-CH pronucleophiles (pKa (DMSO) > 25) occurs without the addition of strong bases or Lewis acids using only the Pd/Xantphos catalyst system. The reaction scope has been studied for various 5- and 6-membered nitrogen-containing heterocycles (yields up to 96%). Mechanistic investigations suggest an initial allylation of the imine-N followed by a Pd-catalyzed formal aza-Claisen rearrangement.

Ruthenium(0)-Catalyzed C-H Arylation of Aromatic Imines under Neutral Conditions: Access to Biaryl Aldehydes

The first ruthenium(0)-catalyzed C-H bond arylation of aromatic imines with arylboronates under neutral conditions is reported. This versatile method provides rapid access to a wide range of biaryl aldehydes that are difficult to assemble using traditional methods with high atom economy. A new hydrogen acceptor for Ru(0) arylation has been identified. This atom-economical strategy has potential for an array of direct applications in Ru(0)-catalyzed C-H bond arylations using removable directing groups. An indole synthesis by a sequential one-pot, multiple C-H activation protocol is reported.

Palladium-catalyzed direct C–H arylation of 3-aryl-2H-benzo[1,2,4]thiadiazine 1,1-dioxides: an efficient strategy to the synthesis of benzothiadiazine-1,1-dioxide derivatives

An efficient strategy to the synthesis of 3-aryl-2H-benzo[1,2,4]thiadiazine 1,1-dioxide derivatives via palladium-catalyzed direct arylation of benzothiadiazine-1,1-dioxides with various aryl iodides is described.

Mild metal-catalyzed C–H activation: examples and concepts

C–H Activation reactions that proceed under mild conditions are more attractive for applications in complex molecule synthesis. Mild C–H transformations reported since 2011 are reviewed and the different concepts and strategies that have enabled their mildness are discussed.

Pd(II)-catalyzed, controllable C-H mono-/diarylation of aryl tetrazoles: concise synthesis of Losartan

A palladium(II)-catalyzed C-H arylation directed by tetrazole, a metabolically stable surrogate for the carboxylic acid group in drug design, has been developed. Excellent mono-/di-selectivity was achieved through adjustment of the protecting site on the tetrazole ring. The synthetic utility of this new transformation was demonstrated in the concise total synthesis of Losartan.

Merging C-H activation and alkene difunctionalization at room temperature: a palladium-catalyzed divergent synthesis of indoles and indolines

A palladium-catalyzed 1,2-carboamination through C-H activation at room temperature is reported for the synthesis of 2-arylindoles, and indolines from readily available, inexpensive aryl ureas and vinyl arenes. The reaction initiates with a urea-directed electrophilic ortho palladation, alkene insertion, and β-hydride elimination sequences to provide the Fujiwara-Moritani arylation product. Subsequently, aza-Wacker cyclization, and β-hydride elimination provide the 2-arylindoles in high yields. Intercepting the common σ-alkyl-Pd intermediate, corresponding indolines are also achieved. The indoline formation is attributed to the generation of stabilized, cationic π-benzyl-Pd species to suppress β-hydride elimination.

Direct oxidative arylation of aryl C - H bonds with aryl boronic acids via pd catalysis directed by the N,N-dimethylaminomethyl group

Biaryl skeletons were directly constructed via palladium-catalyzed ortho-arylation of N,N-dimethyl benzylamine with aryl boronic acids with high efficiency and high regioselectivity under open-flask conditions. The N,N-dimethylaminomethyl group was first applied as a directing group in such an oxidative coupling. Various substrates proved to be efficient coupling partners, furnishing the corresponding ortho-monoarylated or -diarylated arenes in moderate to good yields under mild conditions.

Stoichiometric to catalytic reactivity of the aryl cycloaurated species with arylboronic acids: insight into the mechanism of gold-catalyzed oxidative C(sp2)-H arylation

Based on the well-defined five-membered aryl gold(iii) complexes, [Au(tpy)X2] (3a and 3b) and [AuBr(Ph)(tpy)] (7), as well as the aryl gold(iii) complex [AuCl2(Ph)(tpy)] (8) (tpy = 2-(o-tolyl)pyridine) as reliable models, we present a detailed study of the mechanism for gold(iii)-catalyzed oxidative cross-coupling reactions between cycloaurable arenes and arylboronic acids. Here we report the direct evidence for a mechanistic proposal including arene C-H activation, transmetallation and biaryl reductive elimination. The chelation-assisted C-H activation strategy has been used for the development of the gold(iii)-catalyzed C-H bond arylation of arenes with aryl reagents to forge extended π-conjugated systems.

Mechanism of Pd-catalyzed C(sp3)–H activation of aliphatic amines: an insight from DFT calculations

DFT studies on Pd-catalyzed C(sp³)–H activation of aliphatic amines have been performed using the B3LYP functional. The rate- and regio-determining step of the catalytic cycle is deprotonation of the C_methyl–H bond through a six-membered cyclopalladation transition state.

Palladium-catalyzed regio-selective oxidative C–H bond acylation of azoxybenzenes with alcohols

A novel catalytic system for the ortho-acetyl functionalization of azoxybenzenes via a regio-selective directing-group-assisted strategy was developed.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.