Direct C-arylation Research Articles

Recent advances in sustainable N-heterocyclic carbene-Pd(II)-pyridine (PEPPSI) catalysts: A review

Various catalysts in homogeneous or heterogeneous catalysis deploy unconventional reaction pathways by lowering the activation energy (AE) barrier, controlling the selectivity, and creating environmental impact, thereby bringing about economic viability. Hence, the study of these methodologies is of immense interest. To develop a new chemistry, there is much scope for the invention of brilliant candidates that could effectively catalyze diverse reaction methodologies. The palladium-catalyzed reactions motivate interesting applications on various organic transformations under mild reaction conditions. Although phosphorous designed ligands or catalysts have been used, despite their expensiveness, sensitivity and other properties, there is the necessity of developing even better cross-coupling ligands or catalysts such as N-heterocyclic carbene (NHC)-based palladium complexes. These palladium-NHCs (Pd–NHC) are novel and universal nucleophilic entities that have come into light as the most successful class of catalysts in organometallic chemistry. In the same class, a specific category of palladium-NHCs such as palladium-pyridine enhanced pre-catalyst preparation by the stabilization initiation (palladium-PEPPSI) complexes, are emerging as versatile alternatives to phosphine containing palladium complexes for various cross-coupling reactions due to their excellent catalytic activity. Further to mention that NHCs are recently extensively used as ancillary ligands in organometallic chemistry, which includes industrial-related catalytic transformations due to strong σ-donors to metal centres. Apart from this, many NHC-metal complexes are the fascinating consideration in material science as probable metallo-pharmaceuticals. The current review offers a brief exploration of palladium-PEPPSI complexes over the past few years. Further, the synthesis of a variety of these types of catalysts, their applications in Suzuki-Miyaura, Buchwald-Hartwig, Sonogashira, Negishi couplings direct C2-arylation, O–C(O) cleavage, α-arylation/alkylation of carbonyl compounds and trans-amidation reactions via cross-coupling methodologies, which are covered. Additionally, reported recent developments on reusable heterogeneous PdPEPPSI complexes and their catalytic applications are being covered. Finally, the chiral Pd complexes and their asymmetric transformations are discussed.

Luminescent Crystalline Carbon- and Nitrogen-Centered Organic Radicals Based on N-Heterocyclic Carbene-Triphenylamine Hybrids.

Developing luminescent radicals with tunable emission is a challenging task due to the limitation of alternative skeletons. Herein, a series of carbene-triphenylamine hybrids were prepared by the direct C2-arylation of N-heterocyclic carbenes with 4-bromo-N,N-bis(4-methoxyphenyl)aniline. These hybrids showed multiple redox-active properties and could be converted to carbon-centered luminescent radicals with blue-to-cyan emissions (λmax : 436-486 nm) or nitrogen-centered luminescent radicals with orange emissions (λmax : 590-623 nm) through chemical reduction or oxidation, respectively. The radical species were characterized by electron paramagnetic resonance spectroscopy, ultraviolet-visible spectroscopy, and single-crystal X-ray diffractometry analysis. Notably, the corresponding nitrogen-centered radicals exhibited good stability in atmospheric air, and their thermal decomposition temperatures were determined to be above 200 °C. In addition, spectral and theoretical calculations indicate that all radicals exhibit anti-Kasha emissions.

Palladium-catalyzed direct C5-arylation or C4,C5-diarylation of 2-alkylisothiazol-3-ones

The regioselectivity of the Pd-catalyzed direct arylation of unsubstituted 2-alkylisothiazol-3(2H)-ones was investigated. Conditions for the regioselective palladium-catalyzed direct C5-arylation of 2-alkylisothiazol-3-ones using aryl bromides as the coupling partners are reported. This procedure tolerates a wide variety of substituents such as nitro, nitrile, ester, chloro, fluoro, trifluoromethyl, trifluoromethoxy, difluoromethoxy at para-, meta- and also ortho-positions on the aryl bromide. Both methyl- and octyl-substituents at 2-position of alkylisothiazol-3-ones are tolerated. Moreover, at a more elevated temperature in the presence of a larger excess of the aryl bromide, the access to the C4,C5-diarylated alkylisothiazol-3-ones is also possible, revealing that the C4-position of isothiazol-3(2H)-ones is reactive for direct arylation when the C5-position is blocked. Therefore, this method provides a one pot access to a wide variety of isothiazolinone derivatives allowing to modify easily their biological properties.

Cesium-Fluoride-Promoted Synthesis of Stable Organocesium Reagents and Their Ambident Reactivities with Arynes.

Described are the diverse reactivities of novel, stable, ambident thio-organocesium reagents (bearing electron withdrawing groups) against benzynes. Reactions at reflux temperature predominantly led to the generation of various functionalized stable sulfonium ylides and at 40 °C the same reaction underwent direct c-arylation. Furthermore, lack of internal hydrogen on the cesium reagent helped to produce different ortho-bifunctional arynes in both the reactions. Interestingly, depending on the reactivities of substrates, the one-pot tri-component procedure generated either ylides or σ-bond insertion products.

Substituted N-heterocyclic carbene PEPPSI-type palladium complexes with different N-coordinated ligands: Involvement in the direct C[sbnd]H bond activation of heteroarenes derivatives with aryl bromide and their antimicrobial, anti-inflammatory and antioxidant activities

In this study new benzimidazolium salt as N-heterocyclic carbene precursors and their related new PEPPSI-Pd(II)–N-heterocyclic carbene (NHC) complexes 3a-e were prepared and caracterized. The structures of all compounds have been characterized by 1H NMR, 13C NMR, and IR spectroscopy, as well as elemental analysis techniques and Direct analysis in real time (DART)–time-of-flight mass spectrometry (TOF-MS), which support the proposed structures. Further confirmations of structural details were provided by a single-crystal X-ray. A single crystal of PEPPSI-Pd(II)–N-heterocyclic carbene (NHC) complex 3c shows that the coordination geometry around Pd slightly distorted square-planar geometry. Next, the PEPPSI-Pd(II)–N-heterocyclic carbene (NHC) complexes 3a-e were used as catalysts in the direct C5-arylation of 2-acetyl furan and 2-acetylthiophene with various aryl bromides. These complexes exhibited moderate to high catalytic activities and gave CH activation selectively at the C5-position. Further, PEPPSI-Pd(II)–N-heterocyclic carbene (NHC) complexes 3a-e have been evaluated for their potential antibacterial properties against a panel of bacterial strains namely, Micrococcus luteus, Listeria monocytogenes, Salmonella Typhimurium, Staphylococcus aureus, candida albican and pseudomonas aeruginosa. In addition; these new PEPPSI-Pd(II)–N-heterocyclic carbene (NHC) complexes 3a-e were investigated for anti-oxidant activities by super oxide radical; DPPH (2,2-Diphenyl-1-picrylhydrazyl); and hydroxyl radical scavenging assays; in which most of them displayed significant antioxidant activities. Furthermore; PEPPSI-Pd(II)–N-heterocyclic carbene (NHC) complexes 3a-e were evaluated for anti-inflammatory activity by indirect haemolytic and lipoxygenase inhibition assays and revealed good activity.

Regiocontrolled palladium-catalyzed direct C2-arylation of a difluorobenzo[d]imidazole

Conditions for the regioselective palladium-catalyzed direct arylation of a 6,7-difluorobenzo[d]imidazole using aryl bromides as the coupling partners are described. The site selectivity of the arylation was found to be in favor of the C2-carbon of the difluorobenzo[d]imidazole; whereas the difluoro-substituted ring remained untouched, even in the presence of an excess of aryl bromide. This method tolerates a variety of substituents at para-, meta- and ortho-positions on the aryl bromide and also N-containing heteroaryl bromides.

N-Heterocyclic carbene-palladium-PEPPSI complexes and their catalytic activity in the direct C–H bond activation of heteroarene derivatives with aryl bromides: synthesis, and antimicrobial and antioxidant activities

The Pd–NHC–PEPPSI complexes were used as catalysts in the direct C5-arylation of 2-acetyl furan, 2-acetylthiophene, and 2n-propylthiazole with different aryl bromides and were evaluated for their potential antibacterial and antioxidant properties.

Direct C2-arylation of N-acyl pyrroles with aryl halides under palladium catalysis

C2-arylation of N-acyl pyrroles with aryl halides is developed for the first time using Pd(PPh3)4 as a catalyst in combination with Ag2CO3 under air, which allowed the application of a good compatibility catalytic system. This protocol provides a straightforward method for the preparation of valuable arylated pyrroles in moderate to good yields under the standard conditions with good substrate tolerance. Interestingly, while N-benzoyl pyrroles reacted well, the use of substrates with a thiophene or furan ring indicated that the thiophene and furan rings are more reactive than pyrrole for the present catalytic system.

Synthesis, characterization and catalytic activity of PEPPSI-type palladium–NHC complexes

Eight benzimidazolium salts (2a–h) with two nitrogen atoms substituted by various alkyl groups have been synthesized in high yields. The benzimidazolium salts were readily converted into the corresponding PEPPSI-type palladium–NHC complexes (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization, and initiation) (3a–h). The structures of all compounds were characterized by 1H NMR, 13C NMR, and IR spectroscopy as well as by elemental analysis techniques, which support the proposed structures. The catalytic activity of the PEPPSI-type palladium–NHC complexes was evaluated with respect to the direct C5-arylation of 2-substituted heteroaryl derivatives (thiophene, furan and thiazole) with various aryl bromides. This arylation occurs efficiently and selectively at the C5-position of the 2-substituted thiophene, furan and thiazole derivatives.

Regioselective Direct C2 Arylation of Indole, Benzothiophene and Benzofuran: Utilization of Reusable Pd NPs and NHC-Pd@MNPs Catalyst for C–H Activation Reaction

A regioselective C2 arylation of indoles, benzothiophene and benzofuran without directing group has been accomplished using economically cheap Pd NPs and NHC-Pd@MNPs catalyst. The reusable catalyst is efficiently employed to access C2 arylated heterocycles in good to excellent yield. The reusability of the catalyst is studied up to five cycles and a gram-scale synthesis has been achieved. The reaction mechanism is well supported by control experiments and literature precedents.

The first used butylene linked bis(N-heterocyclic carbene)-palladium-PEPPSI complexes in the direct arylation of furan and pyrrole

We reported the synthesis and characterization of butylene linked bis-benzimidazole based N-heterocyclic carbene (NHC) precursors and their PEPPSI-type bimetallic Pd-complexes, and we implemented its activities in the direct C5-arylation of 2-acetylfuran and 1-methylpyrrole-2-carboxaldehyde with various aryl bromides. The catalytic activities of the synthesized palladium-complexes were found to be very high. These complexes were fully characterized by different techniques such as 1H NMR, 13C NMR, FT-IR spectroscopy and elemental analysis.

Stereoelectronic Properties of 1,2,4-Triazole-Derived N-heterocyclic Carbenes - A Theoretical Study

Stereoelectronic Properties of 1,2,4-Triazole-Derived N-heterocyclic Carbenes - A Theoretical Study

Direct arylation of heteroarenes by PEPPSI-type palladium–NHC complexes and representative quantum chemical calculations for the compound which the structure was determined by X-ray crystallography

In this study, we synthesized five new PEPPSI-type Pd-NHC complexes with high yields around 78–83%. The structures of all complexes were characterized by FT-IR, 1H NMR, and 13C{1H} NMR spectroscopies. Further, the structure of 2c was determined by X-ray crystallography. The single-crystal structure of 2c shows that coordination geometry around Pd is distorted square planar. The Pd-NHC complexes were efficient catalysts for the direct C5-arylation of 2-n-butylthiazole, 2-n-butylfuran, and 2-n-butylthiophene with various aryl-bromides and showed high catalytic activity for arylation reaction using only 1 mol% catalyst loading at 130 °C for 1 h. The conversions for substrates containing electron-withdrawing groups were higher than for substituents containing electron-donating groups. In order to evaluate the catalytic activity of the complexes, representative electronic and spectroscopic simulations by DFT method have been performed for 2c. NBO analysis has revealed that the strongest interaction contributing to the lowering of the molecular stabilization energy for 2c is predicted as π(C11-N27) → π*(N26-C28) with E(2) = 73.93 kcal mol−1. In addition, FMO analysis has implied that 2c is more chemically reactive because of its lower energy gap value. According to the other quantum chemical parameters, 2c is softer, better electrophile and has more charge-transfer capability than 1c.

Regiocontrolled palladium-catalyzed direct C2-arylations of Methoxalen using benzenesulfonyl chlorides and C2,C3-diarylations using aryl bromides as the aryl sources

Conditions for the regioselective palladium-catalyzed direct C2-arylation and C2,C3-diarylation of Methoxalen are described. The use of benzenesulfonyl chlorides as the coupling partner allowed control of the regioselectivity of the arylation in favor of the C2 carbon of Methoxalen. This method tolerates a variety of substituents on the benzenesulfonyl chloride derivative. Notably, 2-, 3-, and 4-bromobenzenesulfonyl chlorides and 4-iodobenzenesulfonyl chloride were coupled with Methoxalen without cleavage of the C-Br or C-I bonds. Conversely, the use of an excess amount of aryl bromides as the aryl source provides access to C2,C3-diarlyated Methoxalenes.

Hydroxyl directed C-arylation: synthesis of 3-hydroxyflavones and 2-phenyl-3-hydroxy pyran-4-ones under transition-metal free conditions.

An efficient, transition-metal free and direct C-arylation of 3-hydroxychromone moieties in the presence of a base, air as an oxidant and arylhydrazines as arylating agents to furnish highly biologically active flavonols or 3-hydroxyflavones has been developed. We have further extended our methodology for the C-arylation of the 5-hydroxy pyran-4-one moiety. The role of the free hydroxyl group towards C-arylation has been delineated.

Metal-free direct C-arylation of 1,3-dicarbonyl compounds and ethyl cyanoacetate: a platform to access diverse arrays of meta-functionalized phenols

A base mediated, highly convenient strategy for the direct C-arylation of 1,3-dicarbonyls and cyanoacetate with various phenol derivatives as aryl partners is presented. The present work excels in forming a C-C bond at the meta-position of the phenols, which is traditionally challenging to functionalize. This protocol further leads the way to have scalable, straightforward access to phenol assimilated heterocycles which have powerful applications both in synthetic chemistry and medicinal research.

Synthesis of N-heterocyclic carbene-palladium-PEPPSI complexes and their catalytic activity in the direct C-H bond activation

A series of benzimidazolium salts having their two nitrogen atoms substituted by bulky groups have been synthesized. The benzimidazolium salts were readily converted into the corresponding palladium-NHC-PEPPSI complexes with general formula [PdBr2(NHC) (Py)], (NHC = N-heterocyclic carbene; PEPPSI = pyridine-enhanced precatalyst preparation, stabilisation, and initiation). The structures of all new compounds were characterized by NMR, IR spectroscopy and microanalysis techniques, which support the proposed structures. The molecular structure of complex 2g was determined by single-crystal X-ray diffraction study. Next, the palladium-NHC-PEPPSI complexes were used as catalysts in the direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde by aryl halides. These complexes exhibited moderate to high catalytic activities and gave C-H activation selectively at the C5-position of 1-methylpyrrole-2-carboxaldehyde. Both electron-donating and electron-withdrawing substituents were well tolerated with catalytic systems based on these complexes, even non-activated aryl chlorides such as chlorobenzene or 4-chlorotoluene were coupled with pyrrole in moderate yields.

Direct C2-arylation of quinoline N-oxides by boronic esters; a molecular approach on the efficient metal-free method in C–C cross-coupling reactions

Kinetic and thermodynamic aspects of the cross-coupling reaction of quinoline N-oxides by various boronic esters in the gas and solvent phases were investigated, theoretically. Two types of boronic esters were considered for the reaction. There are various substitutes in types 1 and 2 which bonded to oxygen and boron atoms, respectively. For investigation of the chemical kinetic aspects of both types, different characters of the rate-determining step (RDS) were considered. Electron density of the involved bonds at the TS, natural charge density of the atoms and donor–accepter orbital interactions are the criteria which were used for the RDS justification of type 1. Nucleus-independent chemical shifts index was applied for description of type 2 behavior in cross-coupling reactions. Theoretical evaluation reveals that a large charge density on the oxygen atom of the O–B bond leads to a higher ability of proton abstraction which reduces ∆G ≠ values in type 1 boronic esters. Furthermore, a large aromaticity character in the quinoline ring at the TS in type 2 boronic esters causes a decrease in the ∆G ≠ value. Finally, based on the obtained results, the ability of proton abstraction of type 1 and the aromaticity character of type 2 boronic esters are the main factors in the energy barrier of the studied reactions.

Synthesis of Pyrrolotriazoloisoquinoline Frameworks by Intramolecular Cu-Mediated or Free Radical Arylation of Triazoles.

The cyclization of (2-bromophenyl)pyrrolyl-1,2,4-triazoles via copper-mediated intramolecular direct C-arylation of 1,2,4-triazoles was first accomplished under triazole-NHC control to give unknown fused heterocyclic skeletons, pyrrolo[3,2-c][1,2,4]triazolo[5,1-a] or [3,4-a]isoquinolines. The primary products underwent a triazole ring opening under the basic arylation conditions, providing N-(1H-pyrrolo[3,2-c]isoquinolin-5-yl)cyanamides. The formation of the cyanamides from isomeric pyrrolo[3,2-c][1,2,4]triazolo[3,4-a]isoquinolines involves, besides the triazole ring opening, the unusual migration of the cyano group. Cyanamides can be easily reduced to 1H-pyrrolo[3,2-c]isoquinolin-5-amines, the first NH2-substituted derivatives of 1H-pyrrolo[3,2-c]isoquinoline. An insight into the mechanism of the triazole ring cleavage was achieved by performing a DFT study at the B3LYP/6-31G+(d,p) level. Free radical cyclization of (2-bromophenyl)pyrrolyl-1,2,4-triazoles with TTMSS/AIBN under neutral conditions allows obtaining pyrrolo[3,2-c][1,2,4]triazolo[5,1-a] and [3,4-a]isoquinolines, as well two more new heterocyclic systems, pyrrolo[3,4-c][1,2,4]triazolo[5,1-a] and [3,4-a]isoquinolines, in good yields without triazole ring cleavage. The developed cyclizations provide a concise, atom-economical route to novel fluorescent fused polyheterocycles containing pyrrole and 1,2,4-triazole moieties.

Palladium-catalyzed direct C2-arylation of free (N H) indoles via norbornene-mediated regioselective C H activation

A palladium-catalyzed direct C2-arylation reaction of free (NH) indoles has been developed. This reaction relies on a norbornene-mediated CH activation process on the indole ring, which features high regioselectivity and excellent functional group tolerance.