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A highly efficient acylative cross-coupling of trialkylboranes with activated amides has been effected at room temperature to give the corresponding alkyl ketones in good to excellent yields by using 1,3-bis(2,6-diisopropyl)phenylimidazolylidene and 3-chloropyridine co-supported palladium chloride, the PEPPSI catalyst, in the presence of K₂CO₃ in methyl tert -butyl ether. The scope and limitations of the protocol were investigated, showing good tolerance of acyl, cyano, and ester functional groups in the amide counterpart while halo group competed via the classical Suzuki coupling. The trialkylboranes generated in situ by hydroboration of olefins with BH₃ or 9-BBN performed similarly to those separately prepared, making this protocol more practical.

We describe the use of acyl imidazoles derived from aliphatic carboxylic acids for the N-heterocyclic carbene/organic photoredox co-catalyzed meta -selective functionalization of electron-rich arenes. Compared to our previous work, a change of the wavelength of the applied LED light from 440 nm to 390 nm promotes this reaction efficiently. [ABSTRACT FROM AUTHOR]

As a significant variant of the Michael reaction, the 1,6‐addition reaction has undergone considerable development over the past decade. This effective strategy enables the synthesis of a variety of novel and potentially bioactive functional molecules. In this review, we summarize the recent progress in NHC‐catalyzed 1,6‐addition reactions, highlighting their efficiency in the rapid synthesis of complex functional molecules. We also provide our perspectives on the future development of this dynamic and highly active research area. [ABSTRACT FROM AUTHOR]

The use of dissolving metal electrodes for the direct electrochemical synthesis of metal complexes has been used widely in the last decade. A major benefit of the electrochemical approach is the minimal by‐products resulting from the synthesis. As such, metal complexes can be produced on‐demand and used directly in catalysis without the need for purification. Furthermore, the electrochemical method enables the production of metal complexes that cannot be synthesized using other methods, including those with base‐sensitive ligands. General principles of the electrochemical method and recent advances in the field are discussed. [ABSTRACT FROM AUTHOR]

The application of cationic Ru(II)‐CNC pincer complexes [Ru(CNC)(CO)(PPh3)Cl]PF6 (1), [Ru(CNC)(PPh3)2Cl]PF6 (2), [Ru(CNC)(η2:η2‐COD)Cl]PF6 (4), and [Ru(CNC)(DMSO)2Cl]PF6 (5) in acceptorless dehydrogenation of alcohols is reported under conventional "oil‐bath" and microwave heating conditions. The effect of different ancillary ligands (CO, PPh3, COD, and DMSO) have been demonstrated during catalysis. Complexes with more π‐acid ligands CO and COD perform better compared with those with PPh3 and DMSO ligands. A plausible mechanism is proposed based on NMR and mass investigation during the catalysis. The catalytic performance matches with the order of trans effect among the four ancillary ligands studied in these complexes. Further, a significant reduction in reaction time, as well as reaction temperature, was observed for catalytic reactions under the microwave‐assisted reaction compared with the oil‐bath heating reaction. [ABSTRACT FROM AUTHOR]

A library of neutral and cationic palladium complexes of cis‐chelating hetero‐dicarbene ligands have been prepared. These ligands contain two different NHC donors allowing for a wider degree of variation, and the impact of the distinct NHC units has been compared using various spectroscopic means. In addition, the catalytic activities of these complexes in the direct C‐H arylation of thiophenes with aryl halides were studied leading to the finding that the neutral dibromido complexes of the type [PdBr2(diNHC)] generally outperformed their cationic [Pd (NCMe)2(diNHC)](OTf)2 counterparts. More importantly, complexes containing more distinct NHC units give rise to superior catalysts due to an amplified "stereoelectronic asymmetry" within the complex. A preliminary photophysical study of some selected thiophene reaction products has been conducted as well. The ability to individually change each NHC donor in such dicarbene ligands allows for a better fine‐tuning of complex properties in search for superior catalysts. [ABSTRACT FROM AUTHOR]

N‐heterocyclic carbene catalysis has been developed as a versatile method for the enantioselective synthesis of complex organic molecules in organic chemistry. Merging of N‐heterocyclic carbene catalysis with transition metal catalysis holds the potential to achieve unprecedented transformations with broad substrate scope and excellent stereoselectivity, which are unfeasible with individual catalyst. Thus, this dual catalysis has attracted increasing attention, and numerous elegant dual catalytic systems have been established. In this review, we summarize the recent achievements of dual NHC/transition metal catalysis, including the reaction design, mechanistic studies and practical applications. [ABSTRACT FROM AUTHOR]

Intermolecular pnictogen bonding (PnB) catalysis has received increased interest in non‐covalent organocatalysis. It has been demonstrated that organic electron‐deficient pnictogen atoms can act as prospective Lewis acids. Here, we present a catalytic approach for the asymmetric synthesis of chiral PIII compounds by combining intramolecular PnB interactions and carbene catalysis. Our design features a pre‐chiral phosphorus molecule bearing two electron‐withdrawing benzoyl groups, resulting in the formation of a σ‐hole at the P atom. X‐ray and non‐covalent interaction (NCI) analysis indicate that the model substrates exhibit intrinsic PnB interaction between the oxygen atom of the formyl group and the phosphorus atom. This induces a conformational locking effect, leading to the crystallization of the phosphorus substrate in a preferred conformation (P212121 chiral group). Under the catalysis of N–heterocyclic carbene, the aldehyde moiety activated by the pnictogen bond selectively reacts with an alcohol to yield the corresponding chiral monoester/phosphorus product with excellent enantioselectivity. This Lewis acidic phosphorus center, aroused by the non‐polarized intramolecular pnictogen bond interaction, assists in conformational and selective regulations, providing unique opportunities for catalysis and beyond. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: We report herein an unprecedented N‐heterocyclic carbene‐catalyzed formal [3 + 3] annulation of ynals with N‐Ts indolin‐3‐ones under the oxidation condition affording the functionalized pyrano[3,2‐b]indol‐2‐ones. The alkynyl acylazoliums via the combination of a carbene with ynals in the presence of oxidate proved to be the important intermediates for the success of this transformation. This method features a broad substrate scope and mild conditions, including axially chiral skeletons with suitable substitutions. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: We report herein an unprecedented N‐heterocyclic carbene‐catalyzed formal [3 + 3] annulation of ynals with N‐Ts indolin‐3‐ones under the oxidation condition affording the functionalized pyrano[3,2‐b]indol‐2‐ones. The alkynyl acylazoliums via the combination of a carbene with ynals in the presence of oxidate proved to be the important intermediates for the success of this transformation. This method features a broad substrate scope and mild conditions, including axially chiral skeletons with suitable substitutions. [ABSTRACT FROM AUTHOR]

[Display omitted] A series of novel ruthenium(ii) complexes with nitron-type N-heterocyclic carbene ligands containing chelating (N -arylcarbamoyl)methyl or carboxymethyl groups have been synthesized. Their evaluation as precatalysts in the hydrogenation of ketones has revealed that the complex containing three N -mesityl groups (one on the carbamoyl- methyl moiety and two on the triazole moiety) is the most efficient one. [ABSTRACT FROM AUTHOR]

Very recently, N-heterocyclic carbenes (NHCs) have found a wide range of applications in the fields of catalysis and nonlinear optics. Herein, we have employed 1,3-bis-(1(S)-benzyl)-4,5-dihydro-imidazol-based carbene as a reference molecule and substituted one H atom from each CH2 of the benzyl groups in both sides by CH3, NH2, and CF3 to study the thermodynamic and opto-electronic properties of NHCs theoretically. It was observed that the enthalpy (H), Gibb's free energy (G), specific heat capacity (Cv), and entropy (S) increase significantly in the presence of the electron-withdrawing groups compared to the electron-donating groups. The IR active in-plane bending vibrations of the CH (NHC) group are shifted to the higher frequency region for the considered substituted molecules compared to the reference carbene. The analysis of the electronic properties shows that the CH3-substituted carbene is more reactive for catalytic activities compared to other NHCs. The calculated nonlinear optical (NLO) properties reveal that the NH2-substituted NHC has the largest hyperpolarizability value whereas the CH3-substituted NHC has the largest dipole moment and polarizability among all, making them potential candidates for the development of NLO materials. [ABSTRACT FROM AUTHOR]

The weak base method in continuous flow is currently one of the most sustainable and attractive approaches towards the synthesis of transition metal N‐heterocyclic carbene (NHC) complexes. Herein we broaden the scope of this method by using it to produce [M(IPr)(OC(H)(CF3)2)] complexes, including copper, silver, and gold. These compounds bear the alkoxide ligand within the coordination sphere of the catalysts, which can open up new ways to benefit from the unique properties offered by HFIP (1,1,1,3,3,3‐hexafluoroisopropanol) in catalysis, while minimizing the needed amount. [ABSTRACT FROM AUTHOR]

The design, inspired by the core‐structure of oxicam has allowed the enantioselective synthesis of benzopyranothiazinone motifs employing N‐heterocyclic carbene (NHC) catalysis. The NHC‐mediated transformation involves the reaction of α,β‐unsaturated aldehydes with suitable substituted benzothiazinone derivatives. This process encompasses the initial formation of α,β‐unsaturated acylazoliums from ynals and concurrent generation of enolates from benzothiazinone. The culmination of these reactions results in an efficient annulation, yielding the desired products with reasonable yields and high stereoselectivities. This study underscores the potential of NHC catalysis in the streamlined synthesis of complex heterocyclic structures. [ABSTRACT FROM AUTHOR]

An electrochemical synthesis of gold(I)‐N‐heterocyclic carbene (Au‐NHC) complexes has been developed. The electrochemical methodology uses only imidazolium salts, gold metal electrodes, and electricity to produce these complexes with hydrogen gas as the only by‐product. This high‐yielding and operationally simple procedure has been used to produce eight mononuclear and three dinuclear Au‐NHC complexes. The electrochemical procedure facilitates a clean reaction with no by‐products. As such, Au‐NHC complexes can be directly transferred to catalytic reactions without work‐up or purification. The Au‐NHC complexes were produced on‐demand and tested as catalysts in a vinylcyclopropanation reaction. All mononuclear Au‐NHC complexes performed similarly to or better than the isolated complexes. [ABSTRACT FROM AUTHOR]

[Display omitted] A new method has been developed for synthesizing nitron- type 1,4-dyaryl-3-arylamino-1,2,4-triazolium salts through a stepwise reaction of N,N '-diarylcarbodiimides with aryl hydrazines, trimethyl orthoformate and Me3SiCl. These salts have been utilized as NHC-proligands to prepare novel (NHC) 2 NiCl 2 and (NHC)NiCpCl (Cp = cyclopentadienyl anion) complexes. Catalytic trials of these complexes uncovered a highly effective catalyst for the Suzuki–Miyaura cross-coupling of non-activated aryl chlorides. [ABSTRACT FROM AUTHOR]

A variety of 5‐indo‐1‐yl pyran‐2‐ones with an N−C axis were prepared via a carbene‐catalyzed oxidative [3+3] annulation of indole‐1‐pyruvate esters and enals. Asymmetric construction of pyran‐2‐one ring from enals ensures the realization of this strategy. The resulting compounds were converted into axially chiral N‐arylindoles with an N−C axis via a cycloaddtion aromatization. [ABSTRACT FROM AUTHOR]

The functionalization of the β-carbon of enals with electrophiles is a signature umpolung reactivity of N-heterocyclic carbene (NHC) derived homoenolates. However, only a limited number of electrophiles are shown to be compatible, with most of them being π-electrophiles. In this study, the successful enantioselective β-alkylation of homoenolates is reported using Csp3 electrophiles through an SN2 strategy. The protocol shows a broad scope regarding alkyl electrophiles, delivering good yields, and excellent enantioselectivities (up to 99% ee). It enables the installation of drug-like structural motifs in either enals or alkylating agents, demonstrating its potential as a valuable tool for late-stage modification. Furthermore, a concise synthetic route is presented to chiral pyrroloindoline-type skeletons. Preliminary mechanistic studies support a direct SN2 mechanism. [ABSTRACT FROM AUTHOR]

A quadruple N‐heterocyclic carbene/cobalt/photoredox/Brønsted base catalysis to realize branch‐selective hydroacylation of styrenes with aromatic and aliphatic aldehydes is demonstrated. This protocol allows access to branched ketones from readily available materials in an atom‐economical manner. The quadruple catalysis can transfer a formyl hydrogen of aldehydes as a hydrogen radical equivalent onto the terminal carbon of an alkene by controlled electron and proton transfers. [ABSTRACT FROM AUTHOR]

The application of N‐heterocyclic carbene (NHC) catalysis under highly diluted oxidative condition to the polycondensation of dialdehydes and diols is herein presented as an alternative, atom‐economical synthetic route to macrocyclic oligoesters (MCOs). The disclosed protocol paves the way to the straightforward access to MCOs, starting from commercial dialdehydes, avoiding the use of toxic diacyl chlorides, commonly employed in traditional MCOs synthetic processes. The method is totally metal‐free, takes place in the green Me‐THF solvent and requires the use of a fully recyclable quinone oxidant. The protocol versatility is confirmed by the employment of fossil‐based and bio‐based monomers such as 2,5‐diformylfuran (DFF), 2,5‐bis(hydroxymethyl)furan (BHMF), and isomannide, synthesizing a series of novel and known synthetically relevant macrocyclic oligoesters, fully characterized by NMR and MALDI‐TOF MS analysis, with product yields (51–86 %) comparable to those obtained by traditional synthetic routes. Finally, to emphasize the synthetic relevance of the target macrocycles, an entropically‐driven ring opening polymerization (ED‐ROP) key study has been performed, optimizing the organocatalyzed synthesis of poly(2,5‐furan‐dimethylene 2,5 furandicarboxylate) (PBHMF) with number‐average molecular weight up to 8200 g mol−1 and 66 % isolated yield. [ABSTRACT FROM AUTHOR]

An efficient protocol for the C−N cross‐coupling of aryl chlorides with (hetero)aryl‐ and alkyl amines under nickel catalysis has been developed. The main advantage of the protocol is the use of a self‐activated Ni/NHC catalytic system generated in situ from readily available bench‐stable air‐tolerant precursors: NiCl2Py2, IPr ⋅ HCl, and sodium tert‐butoxide. A mass spectrometry mechanistic study of the reaction system revealed the dynamics of competitive processes of Ni/NHC active species formation and degradation involving NHC reductive elimination reactions and tert‐butoxide base. Optimization of the NiCl2Py2/IPr ⋅ HCl/tBuONa ratio and the reaction temperature allowed efficient catalysis to be achieved. The developed simple protocol represents an alternative for methods relying on the use of air‐sensitive and unstable Ni(cod)2 or expensive well‐defined Ni/NHC precatalysts. [ABSTRACT FROM AUTHOR]

Conjugated diyne derivatives are important scaffolds in modern organic synthetic chemistry. Using the Glaser reaction involves the coupling of terminal alkynes which can efficiently produce conjugated diyne derivatives, while the use of a stoichiometric amount of copper salts, strong inorganic base, and excess oxidants is generally needed. Developing an environmentally friendly and effective method for the construction of symmetrical 1,3-diynes compounds by Glaser coupling is still highly desirable. In this study, we present an economical method for the production of symmetric diynes starting from various terminal acetylenes in a Glaser reaction. A simple and practical bis-N-heterocyclic carbene ligand has been introduced as efficient ligands for the Cu-catalyzed Glaser reaction. High product yields were obtained at 100 °C for a variety of substrates including aliphatic and aromatic terminal alkynes and differently substituted terminal alkynes including the highly sterically hindered substrate 2-methoxy ethynylbenzene or 2-trifluoromethyl ethynylbenzene and a series of functional groups, such as trifluoromethyl group, ester group, carboxyl group, and nitrile group. The established protocol is carried out in air under base-free condition and is operationally simple. These research work suggest that bis-N-heterocyclic carbene could also an appealing ligand for Glaser reaction and provide a reference for the preparation of symmetric 1,3-diynes in industrial filed. [ABSTRACT FROM AUTHOR]

The ease with which simple starting materials can be transformed into highly functionalized products has made oxidative N‐heterocyclic carbene (NHC) catalysis an area of significant interest. However, the use of stoichiometric amounts of high molecular weight oxidants in most reactions generates an undesired equivalent amount of waste. To address this issue, the use of oxygen as the terminal oxidant in NHC catalysis has been developed. Oxygen is attractive due to its low cost, low molecular weight, and ability to generate water as the sole by‐product. However, molecular oxygen is challenging to use as a reagent in organic synthesis due to its unreactive ground state, which often requires reactions to be run at high temperatures and results in the formation of kinetic side‐products. This review covers the development of aerobic oxidative carbene catalysis, including NHC‐catalyzed reactions with oxygen, strategies for oxygen activation, and selectivity issues under aerobic conditions. [ABSTRACT FROM AUTHOR]

The development of catalytic synthetic approaches towards molecular complexity from simple materials continues to be an ultimate goal in synthetic chemistry. Over the past decades, N‐heterocyclic carbene (NHC) organocatalysis has been extensively investigated to provide opportunities for a vast number of novel chemical transformations. Various activation modes and reactive intermediates enabled by NHC small‐molecule catalysts, such as Breslow intermediates, (homo)enolates, acyl azoliums and their derived unsaturated azoliums exhibit great potential in the construction of complicated skeletons. This personal account will summarize our group's recent work in the exploration of new activation modes of NHC catalysis towards molecular complexity with a focus on the development and applications of NHC to achieve diversity and enantioselectivity in the preparation of functional molecules. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel ruthenium olefin metathesis catalyst (C1-H) with a tridentate hemilabile NHC ligand was compared with its methylated derivative (C1-Me). • The kinetic profile, metathesis activity, and decomposition behavior of C1-H and C1-Me were found to differ significantly. • X-ray structure analysis suggests that this difference is due to the remote torsional strain on the NHC ligand. A ruthenium olefin metathesis catalyst (C1-H) bearing a novel NHC ligand tethered by a labile ethoxy-pyridyl arm was synthesized and characterized. The initiation rate, kinetics, metathesis activity, and decomposition behavior were compared with its methylated derivative (C1-Me), showing that these two catalysts vary profoundly. We demonstrate the considerable influence of remote torsional strain on the catalytic activity and decomposition behavior of this ligand class – a feature that can be exploited for related systems. [ABSTRACT FROM AUTHOR]

In the last decade, major advances have been made in homogeneous gold catalysis. However, AuI/AuIII catalytic cycle remains much less explored due to the reluctance of AuI to undergo oxidative addition and the stability of the AuIII intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N‐heterocyclic carbene) ligands through the development of a new class of L‐shaped heterobidentate ImPy (ImPy=imidazo[1,5‐a]pyridin‐3‐ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ‐donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for AuI/AuIII redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application. [ABSTRACT FROM AUTHOR]

An N‐heterocyclic carbene (NHC) catalyzed asymmetric [3+3] annulation of α‐bromoenals with 5‐aminopyrazoles is described. Using the established methodology, a structurally diverse set of high value dihydropyrazolo[3,4‐b]pyridine‐6‐ones were efficiently constructed in high yields (up to 99%) with excellent enantioselectivities (up to >99%). The easily available starting materials, broad substrate scope, mild reaction conditions, excellent yield and enantioselectivity make this strategy attractive for the asymmetric construction of pyrazolo‐fused pyridone derivatives. [ABSTRACT FROM AUTHOR]

[Display omitted] • Suggested the importance of fluorine migration intermediate in NHC/photoredox catalytic system. • Explained the unique activity of acyl fluoride with NHC compared with other acyl halides. • Elucidated the mechanism of NHC/photoredox/sulfinate triple catalytic system. • Fluorine migration in NHC-catalyzed reactionSpecial reactivity of acyl fluoride with NHCElucidate the mechanism of NHC/photoredox/sulfinate system. Recent advances in N-heterocyclic carbene (NHC)/photoredox catalyzed acylation have underlined the special reactivity of acyl fluoride compared with other acyl halides. Though safer and more convenient, acyl chloride is rarely used with NHC in known references. In a recent work, Studer et al. confirmed that acyl chloride and bromide are inactive in NHC/photoredox acylation, yet reason remains unknown. Herein, computational results showed that [1,2]-fluorine migration intermediate should be responsible for the unique role of acyl fluoride. [ABSTRACT FROM AUTHOR]

The compatibility between gold(I) catalysts and amine transaminases has been explored to transform racemic propargylic alcohols into enantioenriched allylic amines in a straightforward and selective manner. The synthetic approach consists of a gold(I)‐catalysed Meyer‐Schuster rearrangement of a series of 2‐arylpent‐3‐yn‐2‐ols and a subsequent stereoselective enzyme‐catalysed transamination of the resulting α,β‐unsaturated prochiral ketones. The design of cascade processes involving sequential or concurrent approaches has been studied in our search for ideal reaction conditions to produce the desired amines. Thus, the N‐heterocyclic carbene complex [1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]‐[bis(trifluoromethanesulfonyl)‐imide]gold(I) ([Au(IPr)(NTf2)] (A) in aqueous medium was found to be an ideal catalyst, while selective, made‐in‐house and commercial amine transaminases permitted the asymmetric synthesis of both (E)‐4‐arylpent‐3‐en‐2‐amine enantiomers in good isolated yields (53–84%) and excellent stereoselectivities (97 to >99% enantiomeric excess). [ABSTRACT FROM AUTHOR]

Conjugated diyne derivatives are important scaffolds in modern organic synthetic chemistry. Using the Glaser reaction involves the coupling of terminal alkynes which can efficiently produce conjugated diyne derivatives, while the use of a stoichiometric amount of copper salts, strong inorganic base, and excess oxidants is generally needed. Developing an environmentally friendly and effective method for the construction of symmetrical 1,3-diynes compounds by Glaser coupling is still highly desirable. In this study, we present an economical method for the production of symmetric diynes starting from various terminal acetylenes in a Glaser reaction. A simple and practical bis-N-heterocyclic carbene ligand has been introduced as efficient ligands for the Cu-catalyzed Glaser reaction. High product yields were obtained at 100 °C for a variety of substrates including aliphatic and aromatic terminal alkynes and differently substituted terminal alkynes including the highly sterically hindered substrate 2-methoxy ethynylbenzene or 2-trifluoromethyl ethynylbenzene and a series of functional groups, such as trifluoromethyl group, ester group, carboxyl group, and nitrile group. The established protocol is carried out in air under base-free condition and is operationally simple. These research work suggest that bis-N-heterocyclic carbene could also an appealing ligand for Glaser reaction and provide a reference for the preparation of symmetric 1,3-diynes in industrial filed.

A novel N‐heterocyclic carbene (NHC)‐promoted regiodivergent protocol for exo (2‐OH) and endo (5‐OH) isosorbide (IS) imidates synthesis is described. The disclosed strategy allows the straightforward synthesis of monoimidate‐isosorbides (MIIs) endowed with the biologically relevant benzothiazole (BTA) moiety under mild oxidative conditions, starting from easy‐accessible aldimines. Process optimization encompassed the use of stoichiometric diquinone as well as atmospheric oxygen as oxidant species, focusing on reaction dichotomy between oxidative and oxygenative pathways. The reaction scope was investigated on a representative selection of BTA‐containing aldimines and further extended to congeners bearing different biologically pivotal N‐heterocyclic rings (benzoxazole, thiazole and isoxazole), observing good levels of yield (up to 87 %) and regioselectivity (exo/endo up to 10.0; endo/exo up to 7.0). Furthermore, the employment of an immobilized‐NHC under heterogeneous conditions was assessed, showing satisfactory selectivity towards exo‐MIIs (exo/endo=3.8). Overall, the methodology presented provided an unprecedented collection of high value‐added products with potential applications in different fields. [ABSTRACT FROM AUTHOR]

N‐heterocyclic carbene (NHC) catalysis has become a versatile catalysis strategy for building molecules via unique reaction modes. Dual catalytic processes using NHCs in combination with additional modulating agents such as another organocatalyst or transition‐metal catalyst have also been studied. The clear selection of cocatalysts has led to enhanced reactivity, increased yields, and/or improved stereoselectivity. This minireview is to provide readers with a brief overview of the development and applications of NHC and hydrogen‐bond donor for dual catalytic reactions. [ABSTRACT FROM AUTHOR]

In this study, quinoxaline-linked bis(benzimidazolium) salts were synthesized as bis-N-heterocyclic carbene (NHC) precursors. These bis(NHC) precursors were used as multidentate ligands for the construction of bi(hetero)aryls by palladium-catalyzed direct C-H activation process. The in situ prepared palladium complexes by mixtures of the Pd(OAc)2 and the bis(NHC) precursors were used as the catalyst for direct C-H activation of heteroarenes. These catalytic system exhibited a modest catalytic activity in the direct C-H activation of five-membered heteroarenes with a wide range of (hetero)aryl bromides. [ABSTRACT FROM AUTHOR]

In this Concept article we provide a brief overview of the design and preparation of N‐heterocyclic carbenes carrying weakly coordinating anions (WCA‐NHCs). The anionic charge in these ligand systems is located on an exocyclic group, for example, B(C6F5)3, tethered to the backbone of the imidazole ring, thus resembling a weakly coordinating moiety. With the general guiding principle behind the application of WCA‐NHCs being the conversion of otherwise cationic NHC complexes into their overall neutral congeners, numerous transition metal as well as main group element complexes were isolated during the last decade, which are summarized herein. [ABSTRACT FROM AUTHOR]

The feasibility of cooperative catalysis between chiral N‐heterocyclic carbenes and nickel in asymmetric reactions has been demonstrated convincingly. The high efficiency of this catalytic system enables the asymmetric allylic alkylation of isatin‐derived enals with allylic carbonates and [3+3] annulation with racemic vinyl epoxides to provide straightforward access to highly enantioenriched 3,3'‐disubstituted oxindoles. The great practicality of this method in organic synthesis has been showcased by facile product modification and enantioselective synthesis of the key building block to access (−)‐debromoflustramine B. [ABSTRACT FROM AUTHOR]

Two sets of carbohydrate‐NHC hybrid iridium complexes were synthesised in order to combine properties of carbohydrates and triazolylidene (trz) ligands in organometallic catalysis. One set features a direct trz linkage to the anomeric carbohydrate carbon, while the second set is comprised of an ethyl linker between the two functional units. Deprotection of the carbohydrate afforded hybrid complexes that efficiently catalyse the direct hydrogenation of ketones in water. The catalytic activity of the hybrid complexes was influenced by the pH of the aqueous medium and surpassed the activity of carbohydrate‐free or acetyl‐protected analogues (>90 % vs 13 % yield). While no enantiomeric induction was observed for the ethyl‐linked hybrids, a moderate enantiomeric excess (ee) was induced by the directly linked systems. Moreover, these carbohydrate‐trz hybrid complexes displayed mixed inhibitory activity towards a glycosidase from H. orenii that contain a glucose binding site. [ABSTRACT FROM AUTHOR]

A rapid and scalable synthesis of supported NHC−Pd(II)‐pyridine complexes on a benzyloxycalix[8]arene macrocycle is reported. Their subsequent use in Suzuki‐Miyaura benchmark reactions led to the optimisation of the nature of the ligands around the metal centre, delivering two very active precatalysts. They both promote the cross‐coupling of challenging API precursors at low catalytic loadings, when used as insoluble species in EtOH as a green reaction solvent. Moreover, their activity has been shown to be comparable or even superior to that of more conventional homogeneous catalysts. Of main importance, a simple filtration of the insoluble supported catalysts after reaction afforded the lowest Pd contamination values in the target products, in some cases directly approaching the levels authorised by the industrial regulations. Offering furthermore the possibility of easy palladium recovery in the current context of its overconsumption, we are convinced that these calixarene‐supported NHC‐PEPPSI Pd complexes are valuable tools for the fine chemical industry, to prepare metal‐free functionalised biaryl compounds in high yields, ranging from 76 to 94%. [ABSTRACT FROM AUTHOR]

A new catalytic strategy for the one-pot synthesis of N-sulfonylamidines is described. The cationic copper(I) complexes were found to be highly active and efficient under mild conditions in air and in the absence of solvent. A copper acetylide is proposed as key intermediate in this transformation.

N‐heterocyclic carbenes (NHCs) have been the focal point of advanced research on catalysis and bioorganometallic chemistry in the last decade. The field of NHC‐metal mediated catalysis has steadily grown over past years, commencing from the seminal discoveries on various C−C, C−N coupling and acceleration of C−H bond activation reactions. This review presents an overview of the work about the effects of NHC ligand field on metal‐metal contacts in Fe, Ru and Os metal NHC complexes. The influence of these contacts in making the supramolecular assemblies and their consequences on the reactivity patterns is reviewed. Correspondingly, the synergic effects between two metals leading to a unique reactivity pattern are correlated. The suitability and efficiency of various synthetic protocols used to prepare bi‐, tri‐ and multinuclear transition metal NHC complexes where metal‐metal contacts are possible, is discussed. Alternatively, this study on metal‐metal contacts in Fe, Ru and Os metal NHC complexes has great potential in the fields of homogeneous and heterogonous catalysis including the new designs for efficient hydrogen evolution electrocatalysts and oxidation and acylation catalysts. Rational ligand design leading to metal‐metal contacts by the appropriate functionalization of NHCs to investigate their applications in mentioned fields are thus fundamental to our efforts. Apart from their applications, the nature of the metal‐carbene carbon bond, nature of auxiliary ligand(s) and the influence of suitably functionalized NHC ligands led to generate advanced structural motifs have also been emphasized. [ABSTRACT FROM AUTHOR]

Aromatic compounds are ubiquitous in pharmaceutical agents and other functional materials. Therefore, synthetic methods to construct highly functionalized aromatic frameworks have attracted considerable attention, especially when utilizing acyclic precursors instead of pre‐existing benzenoid cores. A plethora of methods exist in the literature for the construction of substituted aromatic products by using transition metal catalysis and base‐promoted benzannulation reactions starting from acyclic precursors. In recent years, organocatalytic approaches such as N‐heterocyclic carbene catalysis have emerged as effective tools for the efficient and regioselective construction of highly functionalized aryl rings under metal‐free and mild reaction conditions. The fundamental objective of this minireview is to highlight the synthetic potential of NHC‐catalyzed benzannulations for the generation of aromatic rings with complex substitution patterns and to discuss these reactions from the perspective of their mechanistic design. [ABSTRACT FROM AUTHOR]

A new methodology for the synthesis of substituted, bicyclic 3‐benzoyl flavanone derivatives by [4+2] annulations of salicylaldehyde with chalcones in the presence of N‐heterocyclic carbene (NHC) as a catalyst, I2 as co‐catalyst and tert‐butyl hydroperoxide (TBHP) as an oxidant in ethanol was developed. The effectiveness of the developed methodology was proved with an array of electron‐deficient to electron‐rich substrates. [ABSTRACT FROM AUTHOR]

Cationic ruthenium pincer complexes, [Ru(CNC)(CO)(PPh3)Cl]X (CNC = 2,6‐bis(1‐methylimidazol‐2‐ylidene)‐pyridine, X = Cl− [1a], PF6− [1b]), [Ru(CNC)(PPh3)2Cl]X (X = Cl− [2a], PF6− [2b]) and [Ru(CNC)(PPh3)2(H)]X (X = Cl− [3a], PF6− [3b]) with triphenylphosphine, CO and halides as coligands have been synthesised and characterised by 1H, 13C, 31P NMR, mass and single‐crystal X‐ray crystallography. The application of Ru complexes in the transfer hydrogenation of a wide range of ketones with 2‐propanol as the hydrogen source is explored. The in situ transformations observed during the synthesis help understand and suggest a plausible mechanism via the hydride complex 3b. All complexes appear to be efficient catalyst precursors for transfer hydrogenation of ketones. [ABSTRACT FROM AUTHOR]

Oxidative N‐heterocyclic carbene catalyst mediated cyclization proved to be an efficient strategy for the construction of various N‐ and O‐ heterocycles. Among several metal‐free methods available for the construction of heterocyclic compounds, oxidative NHC catalysis proved to be an elegant and efficient strategy as several enantioenriched compounds with excellent enantioselectivities can be synthesized in good yields. The most common method of the generation of these heterocycles proceeds via the commonly encountered intermediate in oxidative NHC catalysis (i. e.) acyl azolium intermediate. This intermediate proves to be the active species as it can undergo reaction with suitable substrates leading to the formation of numerous N‐ and O‐ heterocycles. Herein, recent reports on different strategies that enable formal [2+2], [3+2], [3+3], [4+2] or [4+3] annulations between the incoming substrates and acyl azolium species or other active intermediates which can add to suitable substrate leading to various heterocycles were summarized. [ABSTRACT FROM AUTHOR]

Synopsis: Magnesium organometallics on a fluorenyl-tethered NHC in both neutral and anionic form is reported. The anionic version provides an effective kinetic protection as reflected in the heteroleptic MgMe complex and its catalytic hydroamination of primary aminoalkenes. The neutral NHC adduct of Mg{N(SiMe 3) 2 } 2 Br also catalyzes the hydroboration of unsaturated organics. [Display omitted] • Magnesium organometallics supported on a donor-functionalized N -heterocyclic carbene. • Ancillary ligands of effective kinetic protection to alkaline earths. • Homogeneous catalysis like hydroamination and hydroboration. Judicious designing of ancillary ligands with appropriate stetrolelectronics is critical for the chemistries of electropositive alkaline earths that typically make labile coordination to conventional donor sites. We report here a series of magnesium complexes [(L H)Mg(A)Br(thf)] (A = n Bu (1), N(SiMe 3) 2 (2)), [(L)Mg{N(SiMe 3) 2 }] (3), and [(L)MgMe(thf)] (4) supported by a N -heterocyclic carbene L H ([(Flu) H -(CH 2) 2 -NHCDipp]; Dipp = 2,6- i Pr 2 -C 6 H 3) with a fluorenyl sidearm and its monoanionic version L- ([(Flu)-(CH 2) 2 -NHCDipp] -). Complexes 1 , 2 , and 4 are characterized by single crystal X-ray diffractometry. 3 and 4 are additionally analyzed by DFT calculations. The pseudo-bidentate L- appears to be quite an effective ancillary support as evident from the monomeric nature of 4 and the thermal stability of 3 and 4 towards the deleterious Schlenk equilibrium. This is also reflected in 4 ′s catalytic activity in intramolecular hydroamination/cyclization of two primary aminoalkenes as model substrates, for which the presence of a spectator ligand and its ability to thwart the Schlenk equilibrium are critical. It's also the 1st NHC-based main group catalyst for intramolecular hydroamination. Complex 2 also catalyzes the hydroboration of model substrates of carbonyl, ester, amide, and nitrile functions. [ABSTRACT FROM AUTHOR]

The influence of microwave and ultrasonic irradiation on the performance of ammonium-tagged Ru-based catalysts in olefin metathesis transformations in aqueous media was studied. Differences in the catalytic activity in correlation with the nature of the present counter ion and the size of the N-heterocyclic carbene (NHC) ligand were revealed. The presented methodology allows for preparation of a variety of polar and non-polar metathesis products under environmentally friendly conditions.

The present account reviews the most recent noteworthy developments on the synthesis, structure and catalytic applications of Zn‐NHC species, a class of complexes that have attracted attention over the past five to ten years due to their enhanced robustness and hydrolytic stability versus classical Zn organometallics. In particular, thanks to NHC stabilization, access to unprecedented Zn species were recently achieved, including two‐coordinate Zn(II) organocations and thermally stable molecularly well‐defined Zn hydride species, opening the way to effective Zn‐mediated hydro‐silylation/‐boration catalysis of various unsaturated substrates under mild conditions. The potential of NHC−Zn species for the stabilization of unprecedented Zn species and use in various catalytic applications is only emerging and the vast array of readily available NHC structures should promote future developments of the field. [ABSTRACT FROM AUTHOR]