Regioselective Direct C-3 Arylation of Imidazo[1,2-a]pyridines with Aryl Tosylates and Mesylates Promoted by Palladium–Phosphine Complexes

Synthesis and site selective C-H functionalization of imidazo-[1,2-a]pyridines

Imidazo[1,2-a]pyridine has attracted much interest in drug development because of its potent medicinal properties, therefore the discovery of novel methods for its synthesis and functionalization continues to be an exciting area of research. Although transition metal catalysis has fuelled the most significant developments, extremely beneficial metal-free approaches have also been identified. Even though pertinent reviews focused on imidazo[1,2-a]pyridine synthesis, properties (physicochemical and medicinal), and functionalization at the C3 position have been published, none of these reviews has focused on the outcomes obtained in the field of global ring functionalization. We wish here to describe a brief synthesis and an overview of all the functionalization reactions at each carbon atom, viz, C2, C3, C5, C6, C7 and C8 of this scaffold, divided into sections based on site-selectivity and the type of functionalization methods used.

Facile Assembly of Modular-Type Phosphines for Tackling Modern Arylation Processes

This Account presents an overview of a promising collection of phosphine ligands simply made from the modular Fischer indolization process and their applications in modern arylation processes. Using one easily accessible 2-arylindole scaffold, three major phosphino-moiety-positioned ligand series can be readily generated. We have attempted to explore challenging electrophilic and nucleophilic partners for the coupling reaction using the modular ligand tool. For the electrophilic partner study, CM-phos-type ligands, where the phosphino group is located at the 2-arene position of 2-arylindole, allow the successful cross-coupling of aryl mesylates. The CM-phos ligand forms a palladacycle before entering the cross-coupling catalytic cycle. For the nucleophilic partner investigation, the indole C3-positioned phosphines show the first accomplishment of Pd-catalyzed organotitanium nucleophile arylation. Indeed, the aryl-titanium nucleophile undergoes cross-coupling more efficiently than does the organoboron coupling partner in particular cases. Moreover, in the indole C3-positioned phosphine series, the -PPh₂-containing ligands perform better in the highly sterically hindered cross-coupling of aryl chlorides than do ligands containing the -PCy₂ moiety. The catalyst loading can even be reduced to 0.2 mol % Pd for tetra-ortho-substituted biaryl synthesis. This finding offers a new perspective on the next-generation design of phosphine ligands in which the sterically bulky and electron-rich -PR₂ group (R = alkyl) may not be necessary for the cross-coupling of aryl chlorides. In general, we hypothesize that a good balance of steric and electronic properties for entertaining the oxidative addition and reductive elimination steps is crucial to the success of the reaction. For the steric factor, the highly sterically congested -PR₂ group normally favors the reductive elimination, yet we conjecture that this sterically bulky group would serve as an obstacle for the incoming aryl halides. For the electronic factor, the electron rich -PR₂ group is believed to support the oxidative cleavage of the C(Ar)-Cl bond by donating more electron density to the corresponding σ* orbital. Nevertheless, the high electron richness of the -PR₂ group may disfavor the reductive elimination electronically. Overall, an appropriate balance of both electron density and steric bulkiness is suggested to allow the sterically hindered cross-coupling to proceed smoothly. We have found that the -PPh₂-containing ligand is a good starting point for this investigation. The formation of aromatic carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds from aryl chlorides was successfully realized using our proprietary phosphines.In addition to the indole-core-bearing ligand skeleton, we also explored the relevant imidazolyl and carbazolyl phosphines for their unique applications. Interestingly, the carbazolyl ligand, having more flexible C-N axial chirality, displays particular interchangeable Pd-N and Pd-arene coordination, which facilitates both oxidative addition and reductive elimination processes. Moreover, this C-N axially chiral ligand allows the successful asymmetric Suzuki-Miyaura coupling for attaining the most sterically hindered tetra-ortho-substituted biaryls with excellent enantioselectivity. The rationale behind these scientifically interesting findings is presented in detail.

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.

Visible-light-promoted direct C3-trifluoromethylation and perfluoroalkylation of imidazopyridines

An efficient method for direct trifluoromethylation and perfluoroalkylation at C3 of imidazopyridines through visible light-promoted C-H bond functionalization was developed. Under the irradiation of a blue LED, a series of C3-perfluoroalkyl-substituted imidazopyridines were synthesized from the corresponding imidazopyridines and perfluoroalkyl iodides in moderate to good yields at room temperature. It should be mentioned that this reaction proceeded in the absence of any transition-metal catalyst, oxidant and photocatalyst.

Metal-free oxidative decarbonylative halogenation of fused imidazoles

An efficient strategy has been developed for the deformylative halogenation of carbaldehyde imidazo-fused heterocycles in the presence of TBHP controlled by temperature.

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

A comprehensive account of recent advances in the synthesis of imidazopyridines, assisted through transition-metal-catalyzed multicomponent reactions, C-H activation/functionalization and coupling reactions are highlighted in this review article. The basic illustration of this review comprises of schemes with concise account of explanatory text. The schemes depict the reaction conditions along with a quick look into the mechanism involved to render a deep understanding of the catalytic role. At some instances optimizations of certain features have been illustrated through tables, i.e., selectivity of catalyst, loading of the catalyst and percentage yield with different substrates. Each of the reported examples has been rigorously analyzed for reacting substrates, reaction conditions and transition metals used as the catalyst. This review will be helpful to the chemists in understanding the challenges associated with the reported methods as well as the future possibilities, both in the choice of substrates and catalysts. This review would be quite appealing to a wider range of organic chemists in academia and industrial R&D sectors working in the field of heterocyclic syntheses. In a nutshell, this review will be a guiding torch to envisage: (i) the role of various transition metals in the domain dedicated towards method development and (ii) for the modifications needed thereof in the R&D sector.

Catalyst free, C-3 functionalization of imidazo[1,2-a]pyridines to rapidly access new chemical space for drug discovery efforts

Multicomponent reactions (MCRs) are robust tools for the rapid synthesis of complex, small molecule libraries for use in drug discovery and development. By utilizing MCR chemistry, we developed a protocol to functionalize the C-3 position of imidazo[1,2-a]pyridine through a three component, decarboxylation reaction involving imidazo[1,2-a]pyridine, glyoxalic acid, and boronic acid.

Synthesis of Thiophene-Based π-Conjugated Oligomers via Ligand-Enabled Pd-Catalyzed Suzuki-Miyaura Coupling of Haloterthienyls

A ligand-enabled Pd-catalyzed Suzuki-Miyaura coupling of haloterthienyls for the synthesis of various thiophene-based π-conjugated oligomers including quinquethiophenes is demonstrated. An indolyl phosphine ligand plays an important role in this transformation. Thiopheneboronic acids were well applied, which might open up a window for the application of thiopheneboronic acids in the synthesis of thiophene-based π-conjugated oligomers in materials chemistry.

A General Palladium-Phosphine Complex To Explore Aryl Tosylates in the N-Arylation of Amines: Scope and Limitations

The scope and limitations of the monoselective N-arylation of various amines by using aryl and hetaryl tosylates are presented. The air-stable and easily accessible Pd(OAc)₂ /CM-phos {CM-phos=2-[2-(dicyclohexylphosphino)phenyl]-1-methyl-1H-indole}catalyst system was able to deal with a wide range of aryl tosylate substrates as well as amine nucleophiles, including primary and secondary cyclic/acyclic aliphatic amines and anilines. NH-Bearing heterocycles such as indole, carbazole, pyrrole, 10-phenothiazine, and 10-phenoxazine were shown to be feasible coupling partners under this catalytic system. The described reaction conditions tolerate a wide range of functional groups and allow an array of aromatic amines as well as unsymmetrical amine products to be easily accessed from the various phenolic derivatives. Interestingly, this catalyst system even offers the opportunity to perform the reaction in water medium. We also report the intermolecular coupling of optically active α-central chiral amines with aryl tosylates without erosion of the enantiomeric purity.

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.

Metal-free C–H arylation of imidazoheterocycles with aryl hydrazines

Metal-free C–H arylation of imidazo[1,2-a]pyridines at C-3 position with arylhydrazines in presence of DBU has been developed at room temperature under ambient air.

A Palladium-Catalyzed α-Arylation of Oxindoles with Aryl Tosylates

A palladium-catalyzed monoselective C3 arylation of 2-oxindoles with aryl tosylates is described. With the Pd/CM-phos catalyst system, the corresponding 3-arylated oxindoles can be obtained in good to excellent yields (≤97%). The reaction conditions are mild (using 0.5 mol % Pd in general and KF as a base), and functional groups such as methyl ester, NH amido, and enolizable keto moieties are found to be compatible.

Visible-light-mediated C3-azolylation of imidazo[1,2-a]pyridines with 2-bromoazoles

3-Heterocycle-substituted imidazo[1,2-a]pyridines were produced by a visible-light-mediated coupling reaction of imidazo[1,2-a]pyridines with 2-bromoazoles.

N-Heterocyclic carbene–palladium(ii)–1-methylimidazole complex catalyzed direct C–H bond arylation of imidazo[1,2-a]pyridines with aryl chlorides

NHC–Pd(ii) complex catalyzed C–H bond arylation of imidazo[1,2-a]pyridines with aryl chlorides was reported.

Exploiting Aryl Mesylates and Tosylates in Catalytic Mono-α-arylation of Aryl- and Heteroarylketones

The first general palladium catalyst for the catalytic mono-α-arylation of aryl- and heteroarylketones with aryl mesylates and tosylates is described. The newly developed indolyl-derived phosphine ligand L7 has been identified to promote this reaction efficiently. The key to success is attributed to the enhanced steric congestion of the catalyst and effective oxidative addition of the C(Ar)-OMs bond. In the presence of Pd(OAc)2 (0.25-2.5 mol %) and L7, selective monoarylations are achieved with ample reaction scope and product yields up to 95%. Importantly, we demonstrated the applicability of this protocol with the modification of biological phenolic compounds, rendering it amenable for functionalization of phenolic (pro)drugs.

An unprecedented Pd-catalyzed decarboxylative coupling reaction of aromatic carboxylic acids in aqueous medium under air: synthesis of 3-aryl-imidazo[1,2-a]pyridines from aryl chlorides

An efficient and practical protocol for palladium-catalyzed decarboxylative arylation of imidazo[1,2-a]pyridine-3-carboxylic acids with aryl chlorides has been developed. Note that the reaction could proceed smoothly without an additive in aqueous medium under an ambient atmosphere, and the addition of H2O could effectively promote the decarboxylative arylation. Particularly noteworthy is that these results represent the first examples of Pd-catalyzed decarboxylative coupling reactions of (hetero) aromatic carboxylic acids in aqueous medium under air, and the first successful examples of the synthesis of 3-aryl-imidazo[1,2-a]pyridines using cheap, diverse aryl chlorides and heteroaryl chlorides as the starting materials.

Oxidative organophotoredox catalysis: a regioselective synthesis of 2-nitro substituted imidazopyridines and 3-substituted indoles, initiated by visible light

Intramolecular C–N heterocyclization and C–C bond formation under visible light irradiation at room temperature was accomplished with a metal-free photoredox catalyst.

Cu(i)-catalyzed double C–H amination: synthesis of 2-iodo-imidazo[1,2-a]pyridines

An iodine/CuI mediated double oxidative C–H amination reaction has been developed for the synthesis of 2-iodo-imidazo[1,2-a]pyridines, which can serve as active pharmaceutical ingredients (API) of marketed drugs like saripidem and nicopidem.

Oxidative coupling between C(sp2)–H and C(sp3)–H bonds of indoles and cyclic ethers/cycloalkanes

Cross-dehydrogenative-coupling (CDC) between C–H/C–H bonds of indoles and cyclic ethers/cycloalkanes is made viable through a simple transition-metal-free pathway.

Ligand-free Pd-catalysed decarboxylative arylation of imidazo[1,2-a]pyridine-3-carboxylic acids with aryl bromides

A facile ligand-free method for Pd(OAc)₂ catalysed decarboxylative arylation of imidazo[1,2-a]pyridine-3-carboxylic acids with hetero(aryl) bromides has been developed.