New N-heterocyclic Carbene Research Articles

N-Heterocyclic carbene: thiazolylidene–Cu(I) complexes: microwave-assisted synthesis and use as catalyst in A3 reaction

A simple method involving microwave-assisted reaction of 3-benzylthiazolium/benzothiazolium bromides with cuprous oxide has been developed for the synthesis of four new N-heterocyclic carbene (NHC): thiazolylidene/benzothiazolylidene–Cu(I) complexes. Structures of the complexes could be established on the basis of the IR, 1H, and 13C NMR studies and elemental analysis. A representative NHC:benzothiazolylidene–Cu(I) complex was used as catalyst for the microwave-assisted A3 reaction of phenylacetylene with a secondary amine and aldehyde to obtain new substituted propargylamines in excellent yields. A theoretical investigation of the model A3 reaction at the DFT level reveals initial coordination of the alkyne with the NHC–Cu(I) complex followed by its attack on the iminium ion generated from the reaction of secondary amine with aldehyde. In the presence of the catalyst, the activation free energy barrier (ΔG#) is lowered by ca. 3 kcal mol−1.

Development of a Graphene Oxide-Supported N-Heterocyclic Carbene Copper(I) Complex as a Heterogeneous Catalyst for the Selective N-Monoalkylation of Amines

A new N-heterocyclic carbene (NHC) copper(I) complex supported on graphene oxide (GO-NHC-Cu) was synthesised and thoroughly characterised by various instrumental techniques such as FT-IR, FT-Raman, PXRD, XPS, FESEM, EDX, HRTEM, TGA and ICP-OES. The catalytic activity of the supported complex was explored in the N-alkylation of anilines with alcohols under solvent-free and aerobic conditions to afford monoalkylated products in good to excellent yields (20 products, 83–96%). All products were isolated and characterised by 1H and 13C{1H} NMR spectroscopy. The catalyst was recuperated from the reaction mixture by simple filtration and reused for up to five successive cycles with insignificant loss in the catalytic activity. The control experiments showed that the reaction proceeded in aerobic conditions. The green chemistry metrics for the reaction were found to be fairly close to the ideal values: carbon efficiency (95.9%), E-factor (0.15), atom economy (92.14%), process mass intensity (1.15) and reaction mass efficiency (86.80%). The air stability, selectivity, recyclability of the catalyst, and the high yields of the products under solvent-free conditions are some of the salient features of the reported methodology.

Biological Activity of NHC-Gold-Alkynyl Complexes Derived from 3-Hydroxyflavones.

In this paper we describe the synthesis of new N-heterocyclic carbene (NHC) gold(I) derivatives with flavone-derived ligands with a propargyl ether group. The compounds were screened for their antimicrobial and anticancer activities, showing greater activity against bacteria than against colon cancer cells (Caco-2). Complexes [Au(L2b)(IMe)] (1b) and [Au(L2b)(IPr)] (2b) were found to be active against both Gram-positive and Gram-negative strains. The mechanism of action of 1b was evaluated by measurement of thioredoxin reductase (TrxR) and dihydrofolate reductase (DHFR) activity, besides scanning electron microscopy (SEM). Inhibition of the enzyme thioredoxin reductase is not observed in either Escherichia Coli or Caco-2 cells; however, DHFR activity is compromised after incubation of E. coli cells with complex 1b. Moreover, loss of structural integrity and change in bacterial shape is observed in the images obtained from scanning electron microscopy (SEM) after treatment E. coli cells with complex 1b.

Cover Feature: Role of Anionic Backbone in NHC‐Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition (Chem. Eur. J. 16/2022)

A series of new N-heterocyclic carbene (NHC) based Cu(I) and Ag(I) complexes were synthesized, analyzed, and evaluated for their potential usage in atomic layer deposition (ALD) of coinage metal thin films. Employing the same set of diketonate-type ligands for both metals did result in thermally stable Cu(I) complexes suitable for ALD, while the Ag(I) complexes exhibit an inherently unstable nature and different bonding features. More information can be found in the Research Article by Anjana Devi et al. (DOI: 10.1002/chem.202103798).

Role of Anionic Backbone in NHC-Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition.

Cu and Ag precursors that are volatile, reactive, and thermally stable are currently of high interest for their application in atomic‐layer deposition (ALD) of thin metal films. In pursuit of new precursors for coinage metals, namely Cu and Ag, a series of new N‐heterocyclic carbene (NHC)‐based CuI and AgI complexes were synthesized. Modifications in the substitution pattern of diketonate‐based anionic backbones led to five monomeric Cu complexes and four closely related Ag complexes with the general formula [M( tBuNHC)(R)] (M=Cu, Ag; tBuNHC=1,3‐di‐tert‐butyl‐imidazolin‐2‐ylidene; R=diketonate). Thermal analysis indicated that most of the Cu complexes are thermally stable and volatile compared to the more fragile Ag analogs. One of the promising Cu precursors was evaluated for the ALD of nanoparticulate Cu metal deposits by using hydroquinone as the reducing agent at appreciably low deposition temperatures (145–160 °C). This study highlights the considerable impact of the employed ligand sphere on the structural and thermal properties of metal complexes that are relevant for vapor‐phase processing of thin films.

A simple synthetic entryway into (N‐heterocyclic carbene)gold‐steroidyl complexes and their anticancer activity

A straightforward synthetic route to new N‐heterocyclic carbene (NHC)‐gold‐steroidyl complexes is reported. The desired complexes were obtained using a weak base (such as K2CO3) through a concerted‐metallation‐deprotonation (CMD) reaction mechanism occurring between [Au(NHC)Cl] and ethisterone as a model steroid‐based alkyne. Most complexes displayed good cytotoxicity against a panel of cancer cell lines with IC50 values in the low micromolar range. Cellular uptake of the most active complex 2a into MCF‐7 breast cancer cells was facilitated by the coordinated ethisterone ligand.

Well-Defined NHC-Ni Complexes as Catalysts: Preparation, Structures and Mechanistic Studies in Cross-Coupling Reactions.

Developmental studies are ongoing to discover a way to utilise new N-heterocyclic carbene (NHC)-Ni complexes as catalysts. Using a bulky NHC ligand, it is possible to synthesise an NHC/phosphine-mixed heteroleptic Ni(II) complex, which can serve as an excellent catalyst for various cross-coupling reactions. During the study of the reaction mechanisms using these Ni complexes, NHC-Ni(I) complexes were accidentally discovered, and it was observed that they exhibit excellent catalytic activity for cross-coupling reactions. The possibility of the presence of NHC-Ni(I) intermediates in these catalytic reaction pathways has been experimentally demonstrated. Depending on the type of reaction, dinuclear Ni(I) and mononuclear Ni(I) complexes can function as intermediates. The results of the investigation of each reaction mechanism are summarised, and the prospects are described.

Antimicrobial activity, inhibition of biofilm formation, and molecular docking study of novel Ag-NHC complexes

In this study, we reported the synthesis of a new series of Silver(I)-N-heterocyclic carbene complexes which were obtained from the corresponding new N-heterocyclic carbene (NHC) precursors. All new compounds were characterized by elemental analysis, FT-IR, LC-MS, 1H NMR, and 13C{1H} NMR spectroscopy. These new N-heterocyclic carbene precursors and Ag(I)-NHC complexes were evaluated for their antibacterial, antifungal, and antibiofilm activities. The biological activities of synthesized products were compared with standard drugs. All compounds have moderate antibacterial and antibiofilm activities. In particular, while compound 2a was found to be the best inhibitor of E. coli biofilms, while compound 2d inhibited C. albicans biofilm at the highest rate. All the compounds were also analyzed by molecular docking methods with the certain target molecules.

Regiodivergent Isosorbide Acylation by Oxidative N-Heterocyclic Carbene Catalysis in Batch and Continuous Flow

A new N-heterocyclic carbene (NHC)-catalyzed strategy for the regioselective monoesterification of isosorbide (IS) at either the endo (5-OH) or exo (2-OH) position is described. Site-selective acylation proceeds under oxidative conditions in the presence of a quinone oxidant using aldehydes as mild acylating agents. Experimental evidences suggest a role of the stereoelectronic features of the acyl azolium salt intermediate in determining the selectivity of the acylation process. The solvent effect was also investigated, considering conventional and sustainable solvents. Aromatic aldehydes, including bio-based furfural and 5-hydroxymethyl furfural, together with α,β-unsaturated aldehydes proved to be effective reaction partners affording monoacyl-isosorbides with satisfactory levels of regioselectivity (exo/endo: 5.3–3.5; endo/exo: 5.3–3.3). Additionally, the exo-selective triazolium salt promoter was successfully transferred into the heterogeneous phase and applied to continuous-flow catalysis. In particular, the polystyrene-supported version of the selected NHC showed a catalytic activity comparable to that of the homogeneous counterpart in terms of both conversion efficiency (turnover number = 108) and regioselectivity (exo/endo up to 5.3). Also, the corresponding packed-bed mesoreactor was operated with long-term stability (ca. 110 h on stream) to produce 2-benzoyl-IS (1.32 mmol h–1 mmolcat–1), which is the key intermediate in the synthesis of a commercial active pharmaceutical ingredient, namely, the vasodilator isosorbide-5-mononitrate.

Synthesis of N-heterocyclic carbene-Pd(II)-5-phenyloxazole complexes and initial studies of their catalytic activity toward the Buchwald-Hartwig amination of aryl chlorides

Three new N-heterocyclic carbene (NHC)-Pd(II) complexes using 5-phenyloxazole as the ancillary ligand have been obtained in moderate to good yields by a one-pot reaction of the corresponding imidazolium salts, palladium chloride and 5-phenyloxazole under mild conditions. Initial studies showed that one of the complexes is an efficient catalyst for the Buchwald-Hartwig amination of aryl chlorides with various secondary and primary amines under the varied catalyst loading of 0.01-0.05 mol%, thus it will enrich the chemistry of NHCs and give an alternative catalyst for the coupling of challenging while cost-low aryl chlorides.

Cu(I)–N-heterocyclic carbene-catalyzed base free C–N bond formation of arylboronic acids with amines and azoles

Abstract A new N-heterocyclic carbene (NHC) precursor of imidazolium chloride and its corresponding Cu(I)–NHC complex 1 was synthesized. The complex 1 was found to be a highly effective catalyst for Chan-Evans-Lam coupling of arylboronic acid with amines and azoles (including imidazole, pyrazole and triazole), without addition of base at room temperature. Various substituents on three substrates can be tolerated, giving the desired coupling products in good to excellent yields (62–94%). The method is practical and offers an alternative to the corresponding copper-catalyzed Chan-Evans-Lam process for the construction of C–N bonds.

Modular Synthesis of New Bicyclic Carbene Precursors

: A series of new N-heterocyclic carbene (NHC) precursors, containing bicyclic pyrrolo[1,2- c]imidazole framework, were prepared from N-(tert-butoxycarbonyl)-L-proline (1-Boc-L-proline). The sequential attachment of nitrogen nucleophiles and subsequent ring closure gave the desired bicyclic NHC precursors in good yields. The structures of these new NHC precursors were determined on the basis of spectroscopic techniques, including NMR and MS.

Bulky-Yet-Flexible Carbene Ligands and Their Use in Palladium Cross-Coupling

In recent years, several classes of new N-heterocyclic carbene (NHC) ligands were developed around the concept of “flexible steric bulk”. The steric hindrance of these ligands brings stability to the active species, while ligand flexibility still allows for the approach of the substrate. In this review, the synthesis of several types of new classes, such as IBiox, cyclic alkyl amino carbenes (CAAC), ITent, and IPr* are discussed, as well as how they move the state-of-the-art in palladium catalyzed cross-coupling forward.

An N-Heterocyclic Carbene Based Silver Precursor for Plasma-Enhanced Spatial Atomic Layer Deposition of Silver Thin Films at Atmospheric Pressure.

A new N-heterocyclic carbene (NHC)-based silver amide compound, 1,3-di-tert-butyl-imidazolin-2-ylidene silver(I) 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide [(NHC)Ag(hmds)] was synthesized and analyzed by single-crystal X-ray diffraction, 1 H and 13 C NMR spectroscopy, as well as EI mass spectrometry, and subsequently evaluated for its thermal characteristics. This new halogen- and phosphine-free Ag atomic layer deposition (ALD) precursor was tested successfully for silver thin film growth in atmospheric pressure plasma enhanced spatial (APP-ALD). High-purity conductive Ag thin films with a low sheet resistance of 0.9 Ω/sq (resistivity: 10-5 Ωcm) were deposited at 100 °C and characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, optical transmittance, and Rutherford back-scattering techniques. The carbene-based Ag precursor and the new APP-ALD process are significant developments in the field of precursor chemistry as well as metal ALD processing.

The synthesis and characterization of 1-(Allyl)-3-(2-methylbenzyl)benzimidazolium chloride: FT-IR, NMR, and DFT computational investigation

A new N-heterocyclic carbene (NHC) salt was synthesized by the reaction of 1-allyl benzimidazole with 2-methyl benzyl chloride. The structure of N-heterocyclic carbene salt was determined by elemental analysis, FT-IR, 1H NMR and 13C NMR spectroscopy and supported by the computational spectral analyses. For this purpose, the stable conformational structures obtained by Potential Energy Surface (PES) scan at B3LYP/6-31G (d,p) level of the theory have been used to elucidate the spectroscopic studies of the NHC salt as well as to search the electronic properties being important to predict the chemical reactivity behavior and nonlinearity properties of the NHC salt. FMO amplitudes and MEP diagrams have been used to show the chemically active sites of each stable conformer.

N-Heterocyclic carbene-Pd(II)-PPh3 complexes as a new highly efficient catalyst system for the Sonogashira cross-coupling reaction: Synthesis, characterization and biological activities

A novel series of N-heterocyclic carbene-phosphine palladium(II) complexes has been synthesized and fully characterized by IR, 1H NMR, 13C NMR, and 31P NMR spectroscopies, and elemental analysis. The new N-heterocyclic carbene (NHC)-phosphine palladium(II) complexes 3a–h have been easily prepared by the reaction of the corresponding PEPPSI (pyridine-enhanced precatalyst preparation stabilization and initiation) complexes 2a–h and triphenylphosphine in dichloromethane in high yields. These complexes were applied as catalyst precursors which efficiently catalyzed Sonogashira reactions between aryl bromides and phenylacetylene to afford the corresponding products in good yields. The bulky NHC-Pd-PPh3 complexes 3 were tested against Gram-positive and Gram-negative bacteria to study their biological activity. All the complexes exhibit antibacterial against these organisms. Investigation of the anti-acetylcholinesterase activity of the studied complexes showed that compounds 3a and 3b exhibited moderate activity at 100 μg mL−1 and product 3b is the most active.

Co Nanoparticles Encapsulated in N-Doped Carbon Nanosheets: Enhancing Oxygen Reduction Catalysis without Metal-Nitrogen Bonding.

It is known that introducing metal nanoparticles (e.g., Fe and Co) into N-doped carbons can enhance the activity of N-doped carbons toward the oxygen reduction reaction (ORR). However, introducing metals into N-doped carbons inevitably causes the formation of multiple active sites. Thus, it is challenging to identify the active sites and unravel mechanisms responsible for enhanced ORR activity. Herein, by developing a new N-heterocyclic carbene (NHC)-Co complex as the nitrogen- and metal-containing precursor, we report the synthesis of N-doped carbon nanosheets embedded with Co nanoparticles as highly active ORR catalysts without direct metal-nitrogen bonding. Electrochemical measurements and X-ray absorption spectroscopy indicate that the carbon-nitrogen sites surrounding Co nanoparticles are responsible for the observed ORR activity and stability. Density functional theory calculations further reveal that Co nanoparticles could facilitate the protonation of O2 and thus promote the ORR activity. These results provide new prospects in the rational design and synthesis of heteroatom-doped carbon materials as non-precious-metal catalysts for various electrochemical reactions.

Synthesis, Structure and Catalytic Activity of NHC-Ag(I) Carboxylate Complexes.

A general synthetic route was used to prepare 15 new N-heterocyclic carbene (NHC)-Ag(I) complexes bearing anionic carboxylate ligands [Ag(NHC)(O2 CR)], including a homologous series of complexes of sterically flexible ITent ligands, which permit a systematic spectroscopic and theoretical study of the structural and electronic features of these compounds. The complexes displayed a significant ligand-accelerated effect in the intramolecular cyclisation of propargylic amides to oxazolidines. The substrate scope is highly complementary to that previously achieved by NHC-Au and pyridyl-Ag(I) complexes.

Correction: Synthesis, crystal structures, and in vitro anticancer properties of new N-heterocyclic carbene (NHC) silver(i)- and gold(i)/(iii)-complexes: a rare example of silver(i)–NHC complex involved in redox transmetallation

Correction for ‘Synthesis, crystal structures, and in vitro anticancer properties of new N-heterocyclic carbene (NHC) silver(i)- and gold(i)/(iii)-complexes: a rare example of silver(i)–NHC complex involved in redox transmetallation’ by Rosenani A. Haque et al., RSC Adv., 2016, 6, 60407–60421.

Improvements and Applications of the Transition Metal‐Free Asymmetric Allylic Alkylation using Grignard Reagents and Magnesium Alanates

AbstractTwo new N‐heterocyclic carbene (NHC) ligands have been synthesized and employed in the transition metal‐free asymmetric allylic alkylation (AAA) mediated by Grignard reagents and magnesium alanates. The employment of these ligands showed high yields and improved regio‐ and enantioselectivity in the formation of tertiary and quaternary stereocenters. Moreover, the low catalyst loading (up to 0.3 mol%) and high scalability (up to 10 mmol) of this improved methodology provide a convenient access to biologically active compounds and synthetically valuable intermediates.magnified image