Recent advances in the synthesis of (hetero)aryl-substituted heteroarenes via transition metal-catalysed direct (hetero)arylation of heteroarene C–H bonds with aryl halides or pseudohalides, diaryliodonium salts, and potassium aryltrifluoroborates

Unconventional approaches for synthesis of 2-substituted benzothiazoles

Benzothiazoles are among the most important heterocycles in pharmaceuticals, natural products, and functional materials. Hence, developing direct and sustainable routes by replacing hazardous/toxic thiol reagents with nontoxic reagents for constructing 2-substituted benzothiazoles is of utmost importance. In this review, we have highlighted many synthetic methods for the synthesis of 2-substituted benzothiazoles from different starting materials under transition metal catalysis or metal-free systems.

Transition Metal-Driven Selectivity in Direct C-H Arylation of Imidazo[2,1-b]Thiazole

A selective direct arylation of the different Csp2-H bonds of imidazo[2,1-b]thiazole with (hetero) aryl halides can be achieved simply by switching from a palladium catalyst system to the use of stoichiometric amounts of copper. The observed selectivity, also rationalized by DFT calculations, can be explained by a change in the mechanistic pathways between electrophilic palladation and base-promoted C-H metalation.

Plausible PEPPSI catalysts for direct C-H functionalization of five-membered heterocyclic bioactive motifs: synthesis, spectral, X-ray crystallographic characterizations and catalytic activity

In this study, a series of benzimidazolium salts were synthesized as asymmetric N-heterocyclic carbene (NHC) precursors. Nine novel palladium complexes with the general formula [PdX2(NHC)(pyridine)] were synthesized using benzimidazolium salts in the PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) theme. All synthesized Pd(ii) complexes are stable. The synthesized compounds were thoroughly characterized by respective spectroscopic techniques, such as 1HNMR, 13C NMR, FTIR spectroscopy, X-ray crystallography and elemental analysis. The geometric structure of the palladium N-heterocyclic carbene has been optimized in the framework of density functional theory (DFT) using the B3LYP-D3 dispersion functional with LANL2DZ as a basis set. The on/off mechanism of pyridine assisted Pd-NHC complexes made them the best C-H functionalized catalysts for regioselective C-5 arylated products. Five membered heterocyclic compounds such as 2-acetyl furan, furfuryl acetate 2-acetylthiophene and N-methylpyrrole-2-carboxaldehyde were treated with numerous aryl bromides and arylchlorides under optimal catalytic reaction conditions. Interestingly, all the prepared catalysts possessed essential structural features that facilitated the formation of desired coupling products in quantitative yield with excellent selectivity. The arylation reaction of bromoacetophenone was highly catalytically active with only 1 mol% catalyst loading at 150 °C for 2 hours. To check the efficiency of the synthesized complexes, three different five member heterocyclic substrates (2-acetylfuran, 2-acetylthiophen, 2-propylthaizole) were tested with a number of aryl bromides bearing both electron-donating and electron-withdrawing groups on para position. The data in Tables 2-4. Indicated that electron-donating groups on the para position of aryl halide decreased the catalytic conversion while electron-withdrawing groups increased the catalytic conversion this was due to the high nucleophilicity of the electron-donating substituents.

Direct Arylation Synthesis of Small Molecular Acceptors for Organic Solar Cells

In recent years, small molecular acceptors (SMAs) have extensively promoted the progress of organic solar cells (OSCs). The facile tuning of chemical structures affords SMAs excellent tunability of their absorption and energy levels, and it gives SMA-based OSCs slight energy loss, enabling OSCs to achieve high power conversion efficiencies (e.g., >18%). However, SMAs always suffer complicated chemical structures requiring multiple-step synthesis and cumbersome purification, which is unfavorable to the large-scale production of SMAs and OSC devices for industrialization. Direct arylation coupling reaction via aromatic C-H bonds activation allows for the synthesis of SMAs under mild conditions, and it simultaneously reduces synthetic steps, synthetic difficulty, and toxic by-products. This review provides an overview of the progress of SMA synthesis through direct arylation and summarizes the typical reaction conditions to highlight the field's challenges. Significantly, the impacts of direct arylation conditions on reaction activity and reaction yield of the different reactants' structures are discussed and highlighted. This review gives a comprehensive view of preparing SMAs by direct arylation reactions to cause attention to the facile and low-cost synthesis of photovoltaic materials for OSCs.

Catalyst-Controlled Nickel-Catalyzed Intramolecular endo-Selective C-H Cyclization of Benzimidazoles with Alkenes

Compared with the widely explored exo-selective C-H cyclization, transition metal-catalyzed endo-selective C-H cyclization of benzimidazoles with alkenes has been a formidable challenge. Previous efforts mainly rely on substrate-controlled methods, rendering the product complexity restricted. Herein we report a catalyst-controlled method to facilitate endo-cyclization, in which a bulky N-heterocyclic carbene ligand and ^tBuOK base-enabled Ni-Al bimetallic catalyst prove critical to the endo selectivity.

Ligandless Palladium-Catalyzed Direct C-5 Arylation of Azoles Promoted by Benzoic Acid in Anisole

The palladium-catalyzed direct arylation of azoles with (hetero)aryl halides is nowadays one of the most versatile and efficient procedures for the selective synthesis of heterobiaryls. Although this procedure is, due to its characteristics, also of great interest in the industrial field, the wide use of a reaction medium such as DMF or DMA, two polar aprotic solvents coded as dangerous according to environmental, health, safety (EHS) parameters, strongly limits its actual use. In contrast, the use of aromatic solvents as the reaction medium for direct arylations, although some of them show good EHS values, is poorly reported, probably due to their low solvent power against reagents and their potential involvement in undesired side reactions. In this paper we report an unprecedented selective C-5 arylation procedure involving anisole as an EHS green reaction solvent. In addition, the beneficial role of benzoic acid as an additive was also highlighted, a role that had never been previously described.

Palladium-Catalysed Intermolecular Direct C–H Bond Arylation of Heteroarenes with Reagents Alternative to Aryl Halides: Current State of the Art

Abstract:

Abstract: This unprecedented review with 322 references provides a critical up-to-date picture of the Pd-catalysed intermolecular direct C–H bond arylation of heteroarenes with arylating reagents alternative to aryl halides that include aryl sulfonates (aryl triflates, tosylates, mesylates, and imidazole-1-sulfonates), diaryliodonium salts, [(diacetoxy)iodo]arenes, arenediazonium salts, 1-aryltriazenes, arylhydrazines and N’-arylhydrazides, arenesulfonyl chlorides, sodium arenesulfinates, arenesulfinic acids, and arenesulfonohydrazides. Particular attention has been paid to summarise the preparation of the various arylating reagents and to highlight the practicality, versatility, and limitations of the various developed arylation protocols, also comparing their results with those achieved in analogous Pd-catalysed arylation reactions involving the use of aryl halides as electrophiles. Mechanistic proposals have also been briefly summarised and discussed. However, data concerning Pd-catalysed direct C–H bond arylations involving the C–H bonds of aryl substituents of the examined heteroarene derivatives have not been taken into account.

Towards novel tacrine analogues: Pd(dppf)Cl2·CH2Cl2 catalyzed improved synthesis, in silico docking and hepatotoxicity studies

A plethora of 6-(hetero)aryl C–C and C–N bonded tacrine analogues has been made accessible by employing palladium mediated (Suzuki–Miyaura, Heck, Sonogashira, Stille and Buchwald) cross-coupling reactions, starting from either halogenated or borylated residues. The successful use of Pd(dppf)Cl₂·CH₂Cl₂ as a common catalytic system in realizing all these otherwise challenging transformations is the highlight of our optimized protocols. The analogues thus synthesized allow the available chemical space around the C-6 of this biologically relevant tacrine core to be explored. The in silico docking studies of the synthesized compounds were carried out against the acetylcholinesterase (AChE) enzyme. The hepatotoxicity studies of these compounds were done against complexes of CYP1A2 and CYP3A4 proteins with known inhibitors like 7,8-benzoflavone and ketoconazole, respectively.

24 synthesized compounds by various cross-coupling reactions on 6-bromo tacrine. Molecular docking and toxicity prediction studies were also performed.

Recent advances in rhodium-catalysed cross-dehydrogenative-coupling between two C(sp2)-H bonds

Rhodium-catalysed cross-dehydrogenative coupling (CDC) has received considerable attention in recent years. This modern technology has been considered as an attractive synthetic tool for selective C−C bond formation due to (1)...

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.

Aromatic Metamorphosis of Thiophenes by Means of Desulfurative Dilithiation

A new mode of aromatic metamorphosis has been developed, which allows thiophenes and their benzo-fused derivatives to be converted to a variety of exotic heteroles. This transformation involves 1) the efficient generation of key 1,4-dianions by means of desulfurative dilithiation with lithium powder and 2) the subsequent trapping of the dianions with heteroatom electrophiles in a one-pot manner. Via the desulfurative dilithiation, the sulfur atoms of thiophenes are replaced also with a carbon-carbon double bond or a 1,2-phenylene for the construction of benzene rings.

Effective Tools for the Metal-Catalyzed Regiodivergent Direct Arylations of (Hetero)arenes

The direct functionalization of two different C-H bonds of the same organic molecule using different procedures - also called regiodivergent C-H bond functionalization - currently represents an important research topic in organic chemistry, as it demonstrates the versatility of C-H bond functionalization methodology. Over the last decade, the number of tools to control such regiodivergent C-H bond functionalizations has increased significantly. In this account, we will present the various tools that allowed us to arylate different positions of various (hetero)arenes, via a C-H bond functionalization, using palladium or ruthenium catalysis.

An Updated Review on the Synthesis and Antibacterial Activity of Molecular Hybrids and Conjugates Bearing Imidazole Moiety

The rapid growth of serious infections caused by antibiotic resistant bacteria, especially the nosocomial ESKAPE pathogens, has been acknowledged by Governments and scientists and is one of the world's major health problems. Various strategies have been and are currently investigated and developed to reduce and/or delay the bacterial resistance. One of these strategies regards the design and development of antimicrobial hybrids and conjugates. This unprecedented critical review, in which our continuing interest in the synthesis and evaluation of the bioactivity of imidazole derivatives is testified, aims to summarise and comment on the results obtained from the end of the 1900s until February 2020 in studies conducted by numerous international research groups on the synthesis and evaluation of the antibacterial properties of imidazole-based molecular hybrids and conjugates in which the pharmacophoric constituents of these compounds are directly covalently linked or connected through a linker or spacer. In this review, significant attention was paid to summarise the strategies used to overcome the antibiotic resistance of pathogens whose infections are difficult to treat with conventional antibiotics. However, it does not include literature data on the synthesis and evaluation of the bioactivity of hybrids and conjugates in which an imidazole moiety is fused with a carbo- or heterocyclic subunit.

C–H functionalization of quinazolinones by transition metal catalysis

Quinazolinone and its derivatives are an important class of heterocyclic scaffolds in pharmaceuticals and natural products. This review provides the recent research advances in the transition metal catalyzed selective C–H bond functionalization of quinazolinone.

Direct Vicinal Difunctionalization of Thiophenes Enabled by the Palladium/Norbornene Cooperative Catalysis

Herein we report a direct vicinal difunctionalization of thiophenes via the palladium/norbornene (Pd/NBE) cooperative catalysis. A series of mono- and disubstituted thiophenes can be difunctionalized site-selectively and regioselectively at the C4 and C5 positions in good yields, enabled by an arsine ligand and a unique amide-based NBE. The synthetic utility has been shown in derivatizations of complex bioactive compounds and an open-flask gram-scale preparation. Preliminary results have been obtained in the difunctionalization of furans and a direct C4-selective arylation of 2-substituted thiophenes.

Regioselective Pd-catalyzed direct C1- and C2-arylations of lilolidine for the access to 5,6-dihydropyrrolo[3,2,1-ij]quinoline derivatives

The Pd-catalyzed C–H bond functionalization of lilolidine was investigated. The use of a palladium-diphosphine catalyst associated to acetate bases in DMA was found to promote the regioselective arylation at α-position of the nitrogen atom of lilolidine with a wide variety of aryl bromides. From these α-arylated lilolidines, a second arylation at the β-position gives the access to α,β-diarylated lilolidines containing two different aryl groups. The one pot access to α,β-diarylated lilolidines with two identical aryl groups is also possible by using a larger amount of aryl bromide. The synthesis of 5,6-dihydrodibenzo[a,c]pyrido[3,2,1-jk]carbazoles from lilolidine via three successive direct arylations is also described. Therefore, this methodology provides a straightforward access to several lilolidine derivatives from commercially available compounds via one, two or three C–H bond functionalization steps allowing to tune their biological properties.

Rhodium(III)-Catalyzed Regioselective C(sp2)-H Functionalization of 7-Arylpyrazolo[1,5-a]pyrimidines with Dioxazolones as Amidating Agents

Rh(III)-catalyzed C-H functionalization of 7-arylpyrazolo[1,5-a]pyrimidines was developed wherein the pyrazolo[1,5-a]pyrimidine moiety is reported for the first time to direct the C-H bond activation. Various 7-arylpyrazolo[1,5-a]pyrimidines underwent smooth C-H amidation with alkyl-, aryl-, and heteroaryl-substituted dioxazolones to afford the products in moderate to good yields. Mechanistic studies suggest that a six-membered rhodacycle intermediate involving N1 might play a key role in the regioselective catalytic cycle.

Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2-N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the "aromatic metamorphosis" of indoles into 1,2-benzazaborins via the insertion of boron into the C2-N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

Cross-Coupling and Related Reactions: Connecting Past Success to the Development of New Reactions for the Future

Cross-coupling reactions, which were discovered almost 50 years ago, are widely used in both industry and academia. Even though cross-coupling reactions now represent mature technology, there is still a significant amount of research in this area that aims to improve the scope of these reactions, develop more efficient catalysts, and make reactions more practical. In this tutorial, a brief background to cross-coupling reactions is provided, and then the major advances in cross-coupling research over the last 20 years are described. These include the development of improved ligands and precatalysts for cross-coupling and the extension of cross-coupling reactions to a much wider range of electrophiles. For example, cross-coupling reactions are now common with sp3-hybridized electrophiles as well as ester, amide, ether, and aziridine substrates. For many of these more modern substrates, traditional palladium-based catalysts are less efficient than systems based on first-row transition metals such as nickel. Conventional cross-coupling reactions have also inspired the development of a range of related reactions, such as cross-electrophile and decarboxylative couplings as well as couplings based on metallaphotoredox chemistry. The development of these new reactions is probably at the same stage as traditional cross-coupling reactions 30 years ago, and this tutorial highlights how many of the same strategies used to improve cross-coupling reactions may also be applicable to making the new reactions more practical.

Copper-catalyzed synthesis of pyrazolo[5,1-a]isoquinoline derivatives from 2-gem-dipyrazolylvinylbromobenzenes

An efficient copper-catalyzed synthesis of pyrazolo[5,1-a]isoquinolines from 2-gem-dipyrazolylvinylbromobenzenes under mild conditions has been developed.