Palladium-Catalyzed Dehydrogenative Cross-Couplings of Benzazoles with Azoles

Mono and Dinuclear Palladium Pincer Complexes of NNSe Ligand as a Catalyst for Decarboxylative Direct C-H Heteroarylation of (Hetero)arenes

This report describes the synthesis of a new NNSe pincer ligand and its mono- and dinuclear palladium(II) pincer complexes. In the absence of a base, a dinuclear palladium pincer complex (C1) was isolated, while in the presence of Et3 N base a mononuclear palladium pincer complex (C2) was obtained. The new ligand and complexes were characterized using techniques like 1 H, 13 C{1 H} nuclear magnetic resonance (NMR), fourier transform infrared (FTIR), high-resolution mass spectrometry (HRMS), ultraviolet-visible (UV-Visible), and cyclic voltammetry. Both the complexes showed pincer coordination mode with a distorted square planar geometry. The complex C1 has two pincer ligands attached through a Pd-Pd bond in a dinuclear pincer fashion. The air and moisture-insensitive, thermally robust palladium pincer complexes were used as the catalyst for decarboxylative direct C-H heteroarylation of (hetero)arenes. Among the complexes, dinuclear pincer complex C1 showed better catalytic activity. A variety of (hetero)arenes were successfully activated (43-87 % yield) using only 2.5 mol % of catalyst loading under mild reaction conditions. The PPh3 and Hg poisoning experiments suggested a homogeneous nature of catalysis. A plausible reaction pathway was proposed for the dinuclear palladium pincer complex catalyzed decarboxylative C-H bond activation reaction of (hetero)arenes.

N-Iodosuccinimide-Mediated Synthesis of Benzo-Fused Bisimidazoles Enabled by a One-Pot Tandem Reaction of Fluorinated Propargyl Amidines

A N-iodosuccinimide (NIS)-mediated divergent and efficient tandem reaction between fluorinated propargyl amidines and aromatic o-diamines without any metal catalyst and additive under mild reaction conditions was developed for the synthesis of benzo-fused bisimidazoles in moderate to excellent yields. Preliminary mechanistic studies suggested that this reaction proceeded by an intermediate of secondary amine derived from 5-iodomethyl imidazole, and NIS played another role of oxidation reagent to promote the formation of a benzimidazole motif.

Dehydrogenative Syntheses of Biazoles via a "Pre-Join" Approach

The structural motif of biazoles is the predominant substructure of many natural products, pharmaceuticals, and organic materials. Considerable efforts have focused on synthesizing these compounds; however, a limited number of processes have been reported for the efficient formation of biazoles. Herein, we report a "pre-join" approach for the dehydrogenative synthesis of biazoles, which are challenging to prepare using conventional methods. A bench-stable and easily synthesized pyrazine-based group is critical for this transformation. This strategy enables the homocoupling of biazoles and the heterocoupling of two different azoles. Due to the broad substrate scope, this strategy exhibits potential for use in other fields, such as medicine, materials, and natural product chemistry.

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.

RhIII -Catalyzed C6-Selective Oxidative C-H/C-H Crosscoupling of 2-Pyridones with Thiophenes

A rhodium(III)-catalyzed C6-selective dehydrogenative cross-coupling of 2-pyridones with thiophenes was developed for the synthesis of 6-thiophenyl pyridin-2(1H)-one derivatives. In this reaction, the excellent site selectivity was controlled by the 2-pyridyl directing group on the nitrogen of the pyridone ring. Control experiments indicated that the N-pyridyl was essential for the transformation. To the best of our knowledge, this procedure is the first successful example of the direct C6 heteroarylation of 2-pyridones with electron-rich thiophene derivatives. 4-Pyridone was also used as substrate to generate the corresponding C2 heteroarylated product. Moreover, this pyridyl directing group was readily removable to generate the biheteroaryl structures with a free N-H group.

Accessing C2-Functionalized 1,3-(Benz)azoles through Transition Metal-Catalyzed C-H Activation

The skeletal presence of 1,3-azoles in a variety of bioactive natural products, pharmacophores, and organic materials demands the derivatization of such heteroarenes regioselectively. Plenty of cross-coupling as well as cyclocondensation reactions have been performed to build up these skeletons but remained commercially unrealizable. A couple of severe drawbacks are faced by these traditional protocols that require a more straightforward strategy to obviate them. Transition metal-catalyzed C-H functionalization has emerged as a superior alternative in that context. 1,3-Azoles and their benzo counterparts have been extensively functionalized exploiting both noble and earth-abundant transition metals. Lately, C-2 functionalization have gained much traction due to the ease of attaining high regioselectivity and installation of synthetically manipulative functionalities. This critical review presents a bird's eye view of all major C-2 functionalization of (benz)azoles catalyzed by a diverse set of metals performed over the past 15 years.

Pd/Cu-Catalyzed C-H/C-H Cross Coupling of (Hetero)Arenes with Azoles through Arylsulfonium Intermediates

A highly efficient method for the selective formal C-H/C-H cross-coupling of azoles and (hetero)arenes was established through arylsulfonium intermediates under transition-metal catalysis, which produced a variety of 2-(hetero)aryl azoles in good to excellent yields. Advantages of the reaction included mildness, a good functional group tolerance, a wide range of substrates, a high regio- and chemoselectivity, one-pot procedures, and the late-stage functionalization of complex molecules without the use of oxidants, offering a promising strategy for the transition-metal-catalyzed C-H arylation of azoles.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Construction of 2-(2-Arylphenyl)azoles via Cobalt-Catalyzed C-H/C-H Cross-Coupling Reactions and Evaluation of Their Antifungal Activity

Although compounds with a 2-(2-arylphenyl) benzoxazole motif are biologically important, there are only a few methods for synthesizing them. Herein, we report an efficient method for synthesis of such compounds by means of cobalt-catalyzed C-H/C-H cross-coupling reactions. This method has a broad substrate scope and good tolerance for sensitive functional groups. In addition, we demonstrate that introducing a heteroarene moiety to biphenyl compounds enhanced their antifungal activity.

Synthesis of imidazopyridine-fused indoles via one-pot sequential Knoevenagel condensation and cross dehydrogenative coupling

A simple and efficient strategy for the synthesis of imidazopyridine-fused indoles has been developed that involves one-pot sequential Knoevenagel condensation of readily available active methylene azoles with N-substituted-1H-indole-3-carboxaldehydes or N-substituted-1H-indole-2-carboxaldehydes followed by palladium-catalyzed intramolecular cross dehydrogenative coupling reaction. A series of 36 derivatives was prepared by using this strategy. The products were obtained in moderate to excellent (32-94%) yields and showed broad substrate scope with tolerance of various functional groups and was amiable for gram scale preparation without problems.

The decarboxylative C–H heteroarylation of azoles catalysed by nickel catalysts to access unsymmetrical biheteroaryls

The direct decarboxylative C–H heteroarylation of azoles with heteroaryl carboxylic acids through nickel catalysts has been developed.

Recent developments in palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C–Z (Z = B, Si, Sn, S, N, C, H) bonds in bi(hetero)aryl synthesis

This review article describes the palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C_(Ar)–Z (Z = B, Si, Sn, S, N, C, H) bonds for facile bi(hetero)aryl synthesis in the past 10 years.

Palladium-catalyzed oxidative C–H/C–H cross-coupling of imidazopyridines with azoles

A palladium-catalyzed oxidative C–H/C–H cross-coupling of imidazopyridines with azoles using air as the oxidant has been developed.

Oxidant-Controlled C-sp2/sp3-H Cross-Dehydrogenative Coupling of N-Heterocycles with Benzylamines

Oxidant controlled ionic liquid mediated cross-dehydrogenative coupling (CDC) of benzylamines with N-heterocycles having sp² or sp³ carbon resulted in the formation of C-benzoylated or alkenylated products. Benzoylation of N-heterocycles occurs via (NH₄)₂S₂O₈ catalyzed benzoyl radical formation. An oxidative alkenylation of N-heterocycles having C-sp³ carbon (2-methylaza-arenes) occurs via deamination of benzylamine followed by C-sp³-H bond activation in high stereoselectivity. Both benzoylation and alkenylation protocols are metal-free, green, simple, efficient, and tolerate a wide variety of functional groups.

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.

Density Functional and Kinetic Monte Carlo Study of Cu-Catalyzed Cross-Dehydrogenative Coupling Reaction of Thiazoles with THF

Cu-catalyzed cross-dehydrogenative coupling (CDC) reaction of thiazoles with THF has been studied with the density functional theory method and kinetic Monte Carlo (kMC) simulations. Our results show that the previously proposed concerted metalation-deprotonation mechanism is unfavorable. On the basis of the DFT calculation and kMC simulation results, a new mechanism is proposed. In the favorable mechanism, the Cu(II) catalyst first combines with the thiazoles, forming an organocopper species that then binds to the THF radical. The rate-limiting step, C-C bond formation, is realized through an intramolecular structural rearrangement. The Cu catalyst works as a matchmaker to render the C-C bond formation. Kinetic Monte Carlo simulations demonstrate that one should be careful with the conclusions drawn simply from the calculated barriers.

Diversified syntheses of multifunctionalized thiazole derivatives via regioselective and programmed C-H activation

The sequential construction of diversified multifunctionalized thiazole derivatives through Pd-catalyzed regioselective C-H alkenylation has been accomplished. This versatile approach provides the diversified thiazole derivatives featuring orthogonal substitution patterns at the C-2, C-4 and C-5 positions from mono-substituted (2- or 4-substituted) thiazole derivatives or even more challenging simple thiazole.

Cu(i)-Catalyzed oxidative homo-coupling of thiazoline-4-carboxylates: synthesis of 4,4′-bithiazoline derivatives

Cu(i)-Catalyzed oxidative homo-coupling of thiazoline-4-carboxylates with good functional group tolerance has been developed.

Synthesis of esters via sp3 C–H functionalisation

This review describes various methods for the synthesis of eleven classes of esters involving sp³ C–H functionalisation.

Cobalt-promoted dimerization of aminoquinoline benzamides

A method for aminoquinoline-directed, cobalt-promoted dimerization of benzamides has been developed. Reactions proceed in ethanol solvent in the presence of Mn(OAc)2 cocatalyst and Na2CO3 base and use oxygen as a terminal oxidant. Bromo, iodo, nitro, ether, and ester moieties are compatible with the reaction conditions. Cross-coupling of electronically dissimilar aminoquinoline benzamides proceeds with modest yields and selectivities.