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Singlet carbenes are not always isolable and often even elude direct detection. When they escape observation, their formation can sometimes be evidenced by in situ trapping experiments. However, is carbene‐like reactivity genuine evidence of carbene formation? Herein, using the first example of a spectroscopically characterized cyclic (amino)(aryl)carbene (CAArC), we cast doubt on the most common carbene trapping reactions as sufficient proof of carbene formation. [ABSTRACT FROM AUTHOR]

Multimetallic NHC (N-heterocyclic carbene) complexes comprising platinum(II) centers are rare. Four new dinuclear platinum(II) complexes possessing a combination of NHC and Py/4-Cl(Py) ligands are presented. Both classical and mesoionic carbene (MIC) ligands have been employed for the complexation. All complexes have been characterized using standard spectroscopic tools. The coordination of both classical NHC and pyridine ligands around the platinum(II) center in complex [2] has been established by using single crystal XRD. [ABSTRACT FROM AUTHOR]

This work presents the synthesis and characterization of mono‐ (1) and di‐nuclear (2) imidazolium salts stabilized by [AuPPh3]+ fragments. The presence of the gold moiety induces a significant decrease in the carbenic proton's acidic character (high pKa). This high stability is consistent with the pronounced instability of the conjugate bases obtained through deprotonation using strong bases. The formation of carbene species is accompanied by the identification of a 1,2‐rearrangement process in which a preference for the C‐bound species over the N‐bound species is observed. Experimental techniques such as NMR, single‐crystal X‐ray diffraction analysis, mass spectrometry, and computational calculations are employed to investigate the reactivity exhibited by the imidazolium salts. Additionally, the electrochemical properties of the two imidazolium salts were also investigated. This study reveals that both species 1 and 2 display two cathodic peaks which are related to two electro‐chemical irreversible reduction events. The results obtained from both experimental and theoretical studies of this system reveal a strong tendency of the [AuPPh3]+ fragment to stabilize imidazolium salts. Additionally, they demonstrate the preference of this fragment for C‐bound species over N‐bound ones, with the former proving to be highly unstable even under severe conditions of air and moisture exclusion. [ABSTRACT FROM AUTHOR]

This article explores the generation of alkylidene carbenes through tetrazole fragmentation and their applications in organic synthesis. It discusses different methods for generating alkylidene carbenes, such as the loss of N2 from 1-diazoalkenes and the fragmentation of tetrazole intermediates. The text also discusses the synthesis of heteroatom-substituted alkynes (HATs) and heterocyclic alkynes (HATs) using various methods and their potential applications in the synthesis of biologically active natural products. The article highlights the versatility and efficiency of the cyclopropane (CP) method for compound synthesis and provides detailed examples and reactions to illustrate its effectiveness. [Extracted from the article]

Two N-alkyloxy-N′-phenylimidazolium proligands and the corresponding platinum(II) cyclometalated N-alkyloxyimidazol-2-ylidene complexes with β-diketonate auxiliary ligands, [(CNOHC^C*)Pt(L∩L)] (L∩L = acetyacetonate (acac) or 1,3-bis(2,4,6-trimethylphenyl)-propane-1,3-dionato (mesacac)) were synthesized and fully characterized. In addition, a Au(I) monocarbene complex was synthesized, isolated and characterized. Solid-state structures of two cyclometalated platinum(II) NOHC complexes and the Au(I) NOHC complex were obtained providing structural proof. [ABSTRACT FROM AUTHOR]

Complexes of borane with six‐membered N‐heterocyclic carbenes derived from readily accessible 1,3,5‐triazinium salts are described. Similar to the widely used imidazolium‐type borane complex, the obtained complexes behave as agents for the hydrogen atom transfer in the Giese reaction and as sources of the boryl radicals in addition to imines. The amine nitrogen of these complexes does not undergo alkylation, but it can bind with another borane molecule. [ABSTRACT FROM AUTHOR]

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N‐triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon‐atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α‐ or γ‐selective C−H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3‐containing N‐heterocyclic frameworks, such as 2,5‐dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate‐ or catalyst‐controlled chemo‐ and regioselectivity as well as the reaction mechanism. [ABSTRACT FROM AUTHOR]

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N‐triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon‐atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α‐ or γ‐selective C−H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3‐containing N‐heterocyclic frameworks, such as 2,5‐dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate‐ or catalyst‐controlled chemo‐ and regioselectivity as well as the reaction mechanism. [ABSTRACT FROM AUTHOR]

This article discusses the development of chiral bifunctional NHC-guanidine ligands for asymmetric hydrogenation. The researchers have shown that these ligands can control chemoselectivity in catalytic reactions, allowing for the reduction of esters to alcohols. They have also synthesized chiral bifunctional ligand precursors using commercially available chiral building blocks. These ligands have been tested in the asymmetric copper(I)-catalyzed hydrogenation of acetophenone, demonstrating the potential for stereoinduction. The results suggest that these ligands could have broader applications in asymmetric catalysis. [Extracted from the article]

This article discusses the use of photocatalysis in [2,3]-sigmatropic rearrangement reactions of diazo compounds. Metal catalysts are commonly used to access the carbene fragment necessary for these reactions, but visible-light-mediated transformations have gained interest for their green and sustainable synthesis potential. The authors explore the use of heavy atoms and organosulfur compounds as catalysts in these reactions, and optimize the conditions using ruthenium catalysts. The photocatalytic rearrangement reactions are found to be efficient and yield moderate to high yields of the desired products. [Extracted from the article]

The chemoselective N-H insertion of unsymmetrical diamines into carbene is a longstanding challenge. A simple copper-catalyzed strategy for synthesizing C-substituted piperazinones is described, employing easily accessible diazo compounds and 1,2-diamines. The reaction proceeded via chemo-selective carbene insertion at the comparatively less nucleophilic amine, followed by instantaneous cyclization. The protocol was further extended to access NH-free piperazinone, and the synthesis of a Mianserin derivative. [ABSTRACT FROM AUTHOR]

A series of mixed N‐heterocylic carbene (NHC) palladium complexes bearing C^N bidentate ligands derived from benzylamine, phenethylamine, and biphenylamine were prepared and fully characterized by NMR spectroscopy, ESI‐MS spectrometry, and X‐ray diffraction analyses. The complexes were potent in inhibiting proliferation of HepG2 and Caco2 cancer cells, and a SAR (structure–activity relationship) study revealed C^N ligands with stronger electron donation and moderate steric hindrance are more cytotoxic. DNA binding study through UV–vis absorption titration indicated that such complexes bound stronger to CT‐DNA molecules with higher binding constants. Additional molecular docking of the best performer suggested a groove binding mode through hydrogen bonding and π‐π stacking interactions. The apoptosis induced by the two best performers of α,α‐dimethylbenzylamine and S‐α‐methylbenzylamine ligated complexes was further analyzed by morphology change, cell cycle distribution, ROS production, and mitochondrial membrane potential studies. [ABSTRACT FROM AUTHOR]

Acyclic diaminocarbenes (ADCs)–Metal complex having strong donor ability and thermal stability led to extensive usability across every area of inorganic and organometallic chemistry. The unique properties of acyclic diaminocarbenes (ADCs) provide certain advantages over other carbene ligands and have the potential to make a great impact in catalysis. Further, the straightforward synthesis of M–ADCs (metal bound acyclic diaminocarbenes) complexes via metal-mediated reaction provides a wide range of well-defined metal carbene catalysts, which might inspire more researchers to devise unsymmetrically substituted, chiral, and novel acyclic carbene compounds. Although the above synthetic route is limited to a few late transition metals, but have great opportunities to expand the scope of this method. The application of M–ADCs complexes as a catalyst for several organic transformation reactions such as various cross-coupling reactions and asymmetric synthesis like hydroarylation, hydroazidation, hydroamination, cyclization and addition reactions which have shown comparable or even higher activities than the analogous M–NHCs based on all the reports presented. Recent findings of donor ability of several ADC ligands would be useful in fine-tuning the electronic properties, and then a catalyst with a certain combination of donicity and steric requirement could open new doors in catalytic reactivity. Thus, the objective of this review is to assess the recent growths that have been made in designing novel and chiral ADCs ligands and synthesizing ADCs–Metal complexes and to highlight catalytic activities of metal acyclic diaminocarbene complexes for cross-coupling reactions. [ABSTRACT FROM AUTHOR]

A new mode of reactivity of N‐sulfonylhydrazones towards alkynes is described: ketone‐derived N‐tosylhydrazones (NTHs) bearing α‐H atoms behave as [NN] synthons in their reactions with alkoxy alkynyl Fischer carbene complexes (FCCs) and only the two N atoms are incorporated into the newly formed pyrazole ring. The scope of the reaction, leading to 5‐methoxy‐N‐alkenyl pyrazoles, proved to be wide regarding both partners; thus, a variety of aryl and primary, secondary, and tertiary alkyl substituents are tolerated within the FCC, while aryl alkyl NTHs, symmetric and asymmetric dialkyl NTHs and NTHs derived of cyclic ketones can be also employed. In addition, the reaction can be performed at large scale and applied for late‐stage diversification of natural products. A reasonable reaction mechanism is also proposed, which is supported by deuteration experiments and DFT calculations. [ABSTRACT FROM AUTHOR]

The reaction of the amine‐stabilized alane (NMe3) ⋅ AlH31 with the backbone‐saturated N‐heterocyclic carbene (NHC) SIDipp (SIDipp=1,3‐bis‐{2,6‐di‐iso‐propyl‐phenyl}‐imidazolidin‐2‐ylidene) at 0 °C yielded the NHC alane adduct (SIDipp) ⋅ AlH32. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe3) ⋅ AlH(RER‐SIDippH2) 3 ⋅ (NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {=1,3,4,5‐tetramethyl‐imidazolin‐2‐ylidene}, IiPrMe {=1,3‐di‐iso‐propyl‐4,5‐dimethyl‐imidazolin‐2‐ylidene}, and IiPr {=1,3‐di‐iso‐propyl‐imidazolin‐2‐ylidene}) afforded the NHC‐stabilized RER‐products (NHC) ⋅ AlH(RER‐SIDippH2) 3 ⋅ (NHC) (NHC=IMeMe, IiPrMe, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (=1,3‐bis‐{2,6‐di‐iso‐propyl‐phenyl}‐imidazolin‐2‐ylidene), SIDipp and ItBu (=1,3‐di‐tert‐butyl‐imidazolin‐2‐ylidene). The compounds 3 ⋅ (NHC) were also obtained starting from (SIDipp) ⋅ AlH32 and NHC at room temperature. Heating solutions of (SIDipp) ⋅ AlH32 without additional base to 95 °C resulted in decarbonization of the NHC and substitution of the carbene carbon atom with aluminum hydride under loss of ethene. Subsequent dimerization afforded cis‐[AlH{μ‐N(Dipp)CH2CH2N(Dipp)}]24_dimer. Heating solutions of the NHC‐ligated aluminum alkyls (SIDipp) ⋅ AlR32R (R=Me, Et) to 145 °C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR25R (R=Me, Et). [ABSTRACT FROM AUTHOR]

We report here a formal insertion of a metal carbene complex in a single carbon‐carbon bond. This behavior occurs with the participation of an α‐imino gold carbene complex, generated from benzofused triazapentalenes or, in a one‐pot procedure, from their 1‐propargyl‐1H‐benzotriazole precursors, and gold‐catalytically activated ynamides. As the result, 3H‐indole derivatives were obtained, with formation of a quaternary center. A computational analysis carried out on the reaction mechanism indicates that the bulkiness of the gold ligand plays a key role forcing a conformation, which includes aromatic interactions, that favors the approximation of the carbenic carbon to the reactive site. In addition, a negative crossover experiment rules out a cationic arene migration pathway. [ABSTRACT FROM AUTHOR]

Transition metal‐catalyzed carbene transfer reactions have a century‐old history in organic chemistry and are a primary method for the synthesis of cyclopropanes. Much of the work in this field has focused on the use of diazo compounds and related precursors, which can transfer a carbene fragment to a catalyst with concomitant loss of a stable byproduct. Despite the utility of this approach, there are persistent limitations in the scope of viable carbenes, most notably those lacking stabilizing substituents. By coupling carbene transfer chemistry with two‐electron redox cycles, it is possible to expand the available starting materials that can be used as carbene precursors. In this Minireview, we discuss emerging catalytic reductive cyclopropanation reactions using either gem‐dihaloalkanes or carbonyl compounds. This strategy is inspired by classic stoichiometric transformations, such as the Simmons–Smith cyclopropanation and the Clemmensen reduction, but instead entails the formation of a catalytically generated transition metal carbene or carbenoid. We also present recent efforts to generate carbenes directly from methylene (CR2H2) groups via a formal 1,1‐dehydrogenation. These reactions are currently restricted to substrates containing electron‐withdrawing substituents, which serve to facilitate deprotonation and subsequent oxidation of the anion. [ABSTRACT FROM AUTHOR]

Hydrolytically stable PdII and PtII complexes supported by acyclic diaminocarbene ligands represent a novel class of structural organometallic anticancer agents exhibiting nanomolar antiproliferative activity in a panel of cancer cell lines (IC50 0.07–0.81 μM) and up to 300‐fold selectivity for cancer cells over normal primary fibroblasts. The lead drug candidate was 300 times more potent than cisplatin in vitro and showed higher efficacy in reducing the growth of aggressive MDA‐MB‐231 xenograft tumors in mice. [ABSTRACT FROM AUTHOR]

This article presents a new experimental protocol for benchmarking the reactivity of chiral catalysts in asymmetric catalysis. The protocol involves comparing the performance of a benchmarking catalyst to that of a structurally dissimilar catalyst in a single competition experiment. The authors validated this approach using N-heterocyclic carbenes (NHCs) as organocatalysts and phosphoramidites as chiral ligands for transition-metal catalysis. The results demonstrate the usefulness of this protocol in understanding the relationship between catalyst structure and reactivity in asymmetric catalysis. The article also pays tribute to Keith Fagnou, a respected colleague. [Extracted from the article]

Borylation of a tungsten‐bound N2 ligand and halide abstraction provides access to a cationic complex with an unprecedented linear NNBR ligand. This complex undergoes [3+2] cycloaddition with azides, and an unexpected chain‐extension reaction with an iminoborane, leading to a complex with a five‐atom B/N chain. These two [NNBR]‐containing complexes, inorganic analogues of E. O. Fischer's alkynylcarbynes, are very rare examples of molecules containing all‐inorganic chains of sp‐hybridized atoms. [ABSTRACT FROM AUTHOR]

The insertion of unsaturated organic molecules into a P−P bond of oligophosphorus compounds has the potential to expand the organophosphorus and organometallic chemistry and to make it more variable with the selective formation of desirable products. This article reviews the insertion reaction for oligophosphorus compounds and focuses in particular on non‐metals, unsaturated organic substrates, carbenes and carbene analogues as starting reagents. [ABSTRACT FROM AUTHOR]

N‐Heterocyclic carbene (NHC) analogues of Wittig hydrocarbon, [(NHC)(Stil)(NHC)] (3a‐c) (NHC = SIPr (1a) = C[N(Dipp)CH2]2, Dipp = 2,6‐iPr2C6H3; IPr (1b) = C[N(Dipp)CH]2; Me‐IPr (1c) = C[N(Dipp)CMe]2 and Stil = C6H4CHCHC6H4) have been reported as crystalline solids. 3a‐c are prepared by two‐electron reductions of the corresponding bis‐1,3‐imidazoli(ni)um bromides [(NHC)(Stil)NHC)](Br)2 (2a‐c) with KC8 in >94 % yields. 2a‐c are accessible by the nickel catalyzed direct C−C coupling of NHCs (1a‐c) with (E)‐4,4'‐dibromostilbene. One‐electron oxidation of 3a,b yields the corresponding radical cations [(NHC)(Stil)NHC)]B(C6F5)44a,b. All compounds have been characterized by UV‐Vis/NMR/EPR spectroscopy as well as 2a, 3a, and 3b by single crystal X‐ray diffraction. The electronic structures of representative systems have been analyzed by quantum chemical calculations. [ABSTRACT FROM AUTHOR]

An extended class of organic multi‐redox systems was derived from bicyclic(alkyl)amino carbenes (BICAACs). The highly‐conjugated system undergoes a total of 4 redox events spanning a 1.8 V redox range. These organic compounds exhibited four different stable redox states (dication, radical cation, neutral and radical anion), and all of them were characterized either by single crystal X‐ray study and/or various spectroscopic studies. Three of the four redox states are stable to air and moisture. The availability of stable multiple redox states demonstrated promise towards their efficacy in the symmetric H‐cell charge/discharge cycling. Among various redox states, the dication/neutral state works efficiently and continuously for 1500 cycles in 2e− charge/discharge process outside glovebox in commercially available DMF with minimum capacity loss (retaining nearly 90 % Coulombic efficiency). Surprisingly, the efficiency of the redox cycle was retained even if the system was exposed to air for 30 days when it slowly regenerated to the initial deep blue radical cation, and it exhibited another 100 charge/discharge cycles with a minimal capacity loss. Such a stable H‐cell cycling ability is not well known among organic molecule‐based systems. [ABSTRACT FROM AUTHOR]

N‐Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo‐ and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules. [ABSTRACT FROM AUTHOR]

Herein, we present an approach for catalytic orthogonal glycosylation utilizing earth‐abundant copper carbenes. This method operates under mild conditions and employs readily accessible starting materials, including benchtop stable enynal‐derived glycosyl donors, synthesized at the gram scale. The reaction accommodates a variety of glycosyl acceptors, including primary, secondary, and tertiary alcohols. The enynal‐derived copper carbenes exhibit remarkable reactivity and selectivity, allowing for the formation of glycosidic linkages with different protecting groups and stereochemical patterns. This approach provides access to both 1,2‐cis‐ and ‐trans‐glycosidic linkages. The product stereoselectivity is independent of the anomeric configuration of the glycosyl donor, which also has orthogonal reactivity to widely used alkynes and thioglycoside donors. An iterative synthesis of a trisaccharide further demonstrates the application of this orthogonal reactivity. [ABSTRACT FROM AUTHOR]

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics. [ABSTRACT FROM AUTHOR]

A series of metallophthalocyanine (PcM) complexes with axial N-heterocyclic carbene ligands (NHC; 1,3-diisopropylimidazol-2-ylidene (DIP) and 1,3-dimethylbenzimidazol-2-ylidene (DMB)) were prepared and structurally characterized. PcCoII(DIP), PcZnII(DIP), and PcZnII(DMB) are five-coordinate complexes with mild dome-type Pc-ring distortions, while PcFeII(DIP)2 is six-coordinate and has a very large ruffle-type ring-distortion with respect to typical PcM(L)2 systems. The distortion is induced by the highly steric axial DIP ligands. The distortions were quantified and classified by their bond lengths and torsion angles, and according to the normal-coordinate structural decomposition (NSD) analysis. Upon ligation of the NHC, the insoluble PcM materials were solublized in common organic solvents, with typical UV-visible Q-band maxima observable between 658 and 677 nm; the increased solubility is rationalized in terms of the reduced solid-state aggregation of the complexes, attributable to the axial ligation. [ABSTRACT FROM AUTHOR]

In this study, we provide a theoretical explanation for the experimentally observed decrease in the organocatalytic activity of N-aryl imidazolylidenes methylated at the C4/5-H positions in the benzoin condensation of aromatic aldehydes. A comparative quantum chemical study of energy profiles for the NHC-mediated benzoin condensation of furfural has revealed a high energy barrier to the formation of the IPrMe-based furanic Breslow intermediate that can be attributed to the negative steric interactions between the imidazole backbone methyl groups and N-aryl substituents. [ABSTRACT FROM AUTHOR]

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]-playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands. [ABSTRACT FROM AUTHOR]

The development of stereoselective olefination using sulfur ylidederived vinyl carbenes with diazo esters and acetals is reported. Both reactions proceed through nucleophilic addition to electrophiles at the γ-position of an in situ-generated 2-alkoxy furan intermediate. The synthetic utility of the developed method is demonstrated by the total synthesis of rubrolide E. Detailed mechanistic investigations and quantum chemical calculations provide insight into the reaction mechanism. [ABSTRACT FROM AUTHOR]

N-heterocyclic carbenes (NHCs) are a versatile class of ligands that have been widely applied in transition metal catalysis. However, NHCs are soft bases, and as such, complexes of NHCs with hard acids—such as many first-row transition metals—are typically sensitive to air and moisture, hence requiring multistep preparations. In this work, two examples of rare copper (II)-NHC complexes were prepared directly from 1,3-bis(pyridylmethyl)imidazolium chloride and 1,3-bis(2-pyridylmethyl)benzimidazolium chloride in protic conditions with no effort to exclude air or moisture. X-ray crystallographic analysis showed that both species display distorted square pyramidal geometry. A probe reaction demonstrated both complexes displayed poor catalytic activity for Chan–Evans–Lam coupling reactions. [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]

In the past few decades, N-heterocyclic carbenes (NHCs) have opened the new field of organocatalysis in synthetic organic chemistry. This review highlights the dramatic progress in the field of NHC-catalyzed C–O bond formation based on the activation of aldehyde C(sp2)–H bonds. The oxidative and redox transformations for the synthesis of various molecules with structural diversity and complexity are summarized. Furthermore, new methods and strategies for NHC catalysis are emerging continuously; thus, cooperative catalysis with Brønsted acid, hydrogen-bonding catalyst, transition-metal catalyst, and photocatalyst are also described. [ABSTRACT FROM AUTHOR]

The alkylation of substituted 5,6-dihydro[1,2,4]triazolo[3,4-a]isoquinolines with benzylic chlorides afforded quaternary 5,6-dihydro[1,2,4]triazolo[3,4-a]isoquinolinium salts. The reaction of the latter with PdCl2 and pyridine derivatives gave PEPPSI complexes. The cytotoxic activity of the new complexes was evaluated against the HEK293, A549, and HCT116 cell lines. [ABSTRACT FROM AUTHOR]

An efficient method for the synthesis of 1-substituted 5-aryl-1,2,4-triazoles by selective C—H arylation of 1-substituted 1,2,4-triazoles with non-activated aryl chlorides and aryl bromides under the catalysis by a palladium complex with a N-heterocyclic carbene ligand IPr*OMe using 1,1,2,2-tetraphenylethane-1,2-diol (TPEDO) as a catalyst activator is reported. [ABSTRACT FROM AUTHOR]

New 3-arylamino-1,2,4-triazolium salts containing an N-arylcarbamoylmethyl group at the N(1) atom of the 1,2,4-triazole ring were synthesized. They can be used as precursors of N-heterocyclic carbenes (NHCs) capable of coordinating to metals via not only the C(5) atom of the heterocycle but also the atom of the N-carbamoylmethyl group. The selective palladation of the obtained triazolium salts with palladium chloride in pyridine afforded new Pd/NHC complexes, which exhibited enhanced stability against the decomposition accompanied by the Pd—C(NHC) bond cleavage in the presence of strong bases. The enhanced stability of these complexes against bases is attributed to the following two factors: the ionization of the NHC ligand and the formation of chelate complexes. The ionization of the NHC ligand occurs via the deprotonation of the arylamino NH group bound to the triazole ring. The chelate complexes are formed as a result of the deprotonation of the hemilabile N-arylcarbamoylmethyl group and its coordination to the palladium atom giving rise to a Pd—N bond. The synthesized Pd/NHC complexes were found to be highly efficient catalysts for the Suzuki—Miyaura reaction between aryl bromides and arylboronic acids. [ABSTRACT FROM AUTHOR]

The reaction of readily available N,N′-diaryl-4-chloromethylimidazolium chlorides with thiols afforded 4-(alkyl)arylsulfanylmethyl-N,N′-diarylimidazolium salts. The oxidation of these salts with hydrogen peroxide catalyzed by ammonium paramolybdate gave the previously unknown 4-(alkyl)arylsulfonylmethyl-N,N′-diarylimidazolium salts. The synthesized imidazolium salts can be used as sources of N-heterocyclic carbenes (NHCs). The conditions were found for the selective palladation of these compounds involving the C(2) atom of the imidazole ring. Palladium complexes with NHC ligands functionalized with RSCH2 and RSO2CH2 groups were synthesized. The potential applicability of these complexes as catalysts for cross-coupling reactions was demonstrated. [ABSTRACT FROM AUTHOR]

Alkylation of N‐substituted imidazoles ImR' (R'=2,4,6‐trimethylphenyl, 2,6‐diisopropylphenyl, 1‐adamantyl) with Mn(I) methylenephosphonium complexes [Cp(CO)2Mn(η2‐P,C‐R2P=C(H)Ph)](BF4) (PR2=PPh2, PCy2, trans‐PC(H)PhCH2CH2C (H)Ph) followed by photochemical demetallation afforded a series of bidentate NHC‐phosphine ligand precursors [R2PC(H)PhImR'](BF4) in moderate to good yield. The same strategy was successfully applied to N‐functionalized imidazoles ImL (L=2‐pyridyl, CH2SMe, CH2ImMe) to afford selectively NHC core pincer pre‐ligands featuring phosphine/thioether, phosphine/NHC and phosphine/pyridine side arms. For PCy2 derivatives, free NHCs in both bidentate and pincer series generated by deprotonation of the corresponding cationic precursors were shown to be persistent at room temperature. [ABSTRACT FROM AUTHOR]

The tetrel-bonded complexes formed between the selected boron-substituted carbenes and TH3F (T = C, Si, Ge, Sn, Pb) have been explored by using the high-level quantum chemical method. The binding sites of TH3F in the complexes are generally consistent with the positions of VS,min points of the carbenes, and the -Eint values of the complexes exhibit a general increasing tendency with the increase of the -VS,min values of the carbenes for a given TH3F. The complexes can be divided into the type-I and type-II groups. The type-I complexes are characterized by the 3c(B-C-B)→BD*(T-F) and BD(B-C)→BD*(T-F) orbital interactions, and the type-II complexes are characterized by the LP(C)→BD*(T-F) orbital interactions. The type-II complexes have more negative Eint values than the type-I complexes. The -Eint values of the type-I complexes exhibit a roughly increasing tendency with the increase of the T atomic number, and the -Eint values go in the order SnH3F > PbH3F > GeH3F > SiH3F > CH3F for most type-I complexes, which is consistent with the order of VS,max values of TH3F. Unlike type-I complexes, the -Eint values of the type-II complexes exhibit a zigzag tendency with the increase of the T atomic number, and the -Eint values go in the order SiH3F > SnH3F > GeH3F > PbH3F > CH3F for type-II complexes. The AIM and SAPT analysis has been employed to explore the nature of the intermolecular interactions in these complexes. [ABSTRACT FROM AUTHOR]

(NHC)→E coordination interactions were known where NHC is an N‐heterocyclic carbene, and E is a main group element (B, C, N, Si, P). Recently, it was suggested that compounds with (NHC)→S coordination chemistry are also possible. This work reports quantum chemical analysis and synthesis of (NHC)→S‐R(+) complexes in which benzothiazol‐2‐ylidene acts as a ligand. A Density functional study established that (NHC)→S interaction can best be described as a coordination interaction. Synthetic efforts were made, initially, to generate divalent sulfur compounds containing benzothiazole substituents. N‐alkylation of the heterocyclic ring in these sulfides using methyl triflate led to the generation of the desired products with (NHC)→S coordination chemistry, which involves the in situ generation of NHC ring ligands. The observed changes in the 13C NMR spectra, before and after methylation, confirmed the change in the electronic character of the C−S bond from a covalent character to a coordination character. [ABSTRACT FROM AUTHOR]

The synthesis of ruthenium-complexes with cyclic (amino)(barrelene)carbenes (namely CABCs) as ligands is reported. Isolated in moderate to good yields, these new complexes showed impressive thermal stability at 110 °C over several days. Good catalytic performances were demonstrated in various ring-closing metathesis (RCM), macrocyclic–RCM, ring-closing enyne metathesis (RCEYM), cross-metathesis (CM), and ring-opening cross metathesis (ROCM) reactions. [ABSTRACT FROM AUTHOR]

Carbenes with a σ0π2 singlet ground state are rare and little is known about their chemistry. Here, we study the potential formation of such carbenes by removal of two electrons from carbones/donor-substituted allenes. The desired electron configuration becomes favorable in the case of bis-diiminium substitution (CAAC motif). [ABSTRACT FROM AUTHOR]

Efficiently generating C1 building blocks from environmentally friendly carbon sources, such as through photocatalytic CO2 reduction, is essential for fostering a sustainable circular economy. The pursuit of mild catalytic activation methods has yielded powerful catalysts that can be synergistically employed alongside various reaction media to enhance overall performance. Herein, we elucidate the influence of diverse imidazolium‐based ionic liquids as additives for visible‐light‐driven CO2 reduction with ruthenium(II)‐ and rhenium(I)‐bipyridine complexes. Our investigation reveals that incorporating ionic liquids into traditional solvents at concentrations below 10 % can markedly boost CO production while suppressing H2 generation. The best results were obtained for the highly basic ionic liquid [C2mim][OAc], resulting in a substantial rise in CO formation from 0.3 μmol/h to 5.4 μmol/h and an increase in turnover number from 3 to 59. This study underscores the cooperative influence of imidazolium‐based ionic liquids on CO2 photoreduction while circumventing their use as primary solvents, thus offering a promising avenue for sustainable chemical synthesis. [ABSTRACT FROM AUTHOR]

We report the room temperature dimerization of carbon monoxide mediated by C4/C5‐vicinal anionic dicarbenes Li(ADC) (ADC = ArC{(Dipp)NC}2; Dipp = 2,6‐iPr2C6H3; Ar = Ph, DMP (4‐Me2NC6H4), Bp (4‐PhC6H4)) to yield (E)‐ethene‐1,2‐bis(olate) (i.e. −O−C=C−O− = COen) bridged mesoionic carbene (iMIC) lithium compounds COen‐[(iMIC)Li]2 (COen‐[iMIC]2 = [ArC{(Dipp)NC}2(CO)]2) in quantitative yields. COen‐[(iMIC)Li]2 are highly colored stable solids, exhibit a strikingly small HOMO–LUMO energy gap, and readily undergo 2e‐oxidations with selenium, CuCl (or CuCl2), and AgCl to afford the dinuclear compounds COon‐[(iMIC)E]2 (E = Se, CuCl, AgCl) featuring a 1,2‐dione bridged neutral bis‐iMIC (i.e. COon‐[iMIC]2 = [ArC{(Dipp)NC}2(C=O)]2). COen‐[(iMIC)Li]2 undergo redox‐neutral salt metathesis reactions with LiAlH4 and (Et2O)2BeBr2 and afford COen‐[(iMIC)AlH2]2 and COen‐[(iMIC)BeBr]2, in which the dianionic COen‐moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X‐ray diffraction. Stereoelectronic properties of COon‐[iMIC]2 are quantified by experimental and theoretical methods. [ABSTRACT FROM AUTHOR]

Phosphorus-bridged rigid, bent bis(N-heterocyclic) carbenes have not been reported, so far, despite having structural features that could make them interesting ligands in coordination and main group element chemistry. In previous reports, we had demonstrated that tuning of σ3- and σ4-phosphorus environments in planarised bis(NHCs) affects electronic properties and can provide additional coordination sites. Herein, we report on first examples of synthesis and conversion of 1,4-diphosphabarrelene-related compounds into rigid bent, doubly P-bridged bis(NHCs). The formation of main group element adducts with substrates from group 13, 14 and 15 illustrates opportunities to access novel scaffolds and to create nonplanar branching points. DFT calculations reveal the new bis(NHCs) to be good candidates as novel soft/hard ligands with up to four coordination sites. The synthesis of a dinuclear Fe(CO)4 complex is demonstrated. The thermal retro-[4 + 2] cycloaddition was theoretically and experimentally explored for a variety of ionic and zwitterionic 1,4-diphosphabarrelenes, and the generation and trapping of a dinuclear Fe(0) bis(NHC) complex with a tricyclic 1σ2,4 σ2-diphosphinine scaffold is presented. [ABSTRACT FROM AUTHOR]

Polyfluorinated substituents often enhance effectiveness, improve the stability within metabolic processes, and boost the lipophilicity of biologically active compounds. However, methods for their introduction into aliphatic carbon chains remain very limited. A potentially general route to integrate the fluorinated scaffolds into organic molecules involves insertion of fluorine‐containing carbenes into C(sp3)−H bonds. The electron‐withdrawing characteristics of perfluoroalkyl groups enhances the reactivity of these carbenes which should enable the functionalization of unactivated C(sp3)−H bonds. Curiously, it appears that use of perfluoroalkyl‐containing carbenes in alkane C−H functionalization is exceedingly rare. This concept describes photolysis, enzymatic catalysis, and transition metal catalysis as three primary approaches to C(sp3)−H functionalization by trifluoromethylcarbene and its homologues. [ABSTRACT FROM AUTHOR]

Azetidines, being four‐membered N‐heterocycles, possess significant potential in contemporary medicinal chemistry owing to their favorable pharmacokinetic properties. Regrettably, the incorporation of functionalized azetidines into pharmaceutical lead structures has been impeded by the absence of efficient synthetic methods for their synthesis. In this study, a Rh‐catalyzed one‐carbon ring expansion of aziridines with vinyl‐N‐triftosylhydrazones is presented, which facilitates the synthesis of high value‐added 2‐alkenyl azetidine products. This research represents the first example of ring expansion of aziridines enabled by vinyl carbenes. Additionally, a one‐pot two‐step protocol, initiated from cinnamaldehyde, was successfully achieved, offering a step‐economical and facile approach for the synthesis of these compounds. The pivotal aspect of this successful transformation lies in the in situ formation of an alkenyl aziridinium ylide intermediate. Experimental investigations, coupled with computational studies, suggest that a diradical pathway is involved in the reaction mechanism. [ABSTRACT FROM AUTHOR]

Azetidines, being four‐membered N‐heterocycles, possess significant potential in contemporary medicinal chemistry owing to their favorable pharmacokinetic properties. Regrettably, the incorporation of functionalized azetidines into pharmaceutical lead structures has been impeded by the absence of efficient synthetic methods for their synthesis. In this study, a Rh‐catalyzed one‐carbon ring expansion of aziridines with vinyl‐N‐triftosylhydrazones is presented, which facilitates the synthesis of high value‐added 2‐alkenyl azetidine products. This research represents the first example of ring expansion of aziridines enabled by vinyl carbenes. Additionally, a one‐pot two‐step protocol, initiated from cinnamaldehyde, was successfully achieved, offering a step‐economical and facile approach for the synthesis of these compounds. The pivotal aspect of this successful transformation lies in the in situ formation of an alkenyl aziridinium ylide intermediate. Experimental investigations, coupled with computational studies, suggest that a diradical pathway is involved in the reaction mechanism. [ABSTRACT FROM AUTHOR]

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R−Pn. Phosphanylidene‐ and arsanylidenephosphoranes (R−Pn(PMe3); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R−P(PMe3) and R’−As(PMe3) gives access to arsaphosphenes of the type RP=AsR’. Three examples are presented and the properties and reactivity of Mes*P=AsDipTer (1) (Mes*=2,4,6‐tBu3‐C6H2; DipTer=2,6‐(2,6‐iPr2C6H3)2‐C6H3) were studied in detail. Solid state 31P NMR spectroscopy revealed a large 31P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe4 (IMe4=:C(MeNCMe)2) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe4) and DipTerAs(IMe4), which is additionally shown for diarsenes. [ABSTRACT FROM AUTHOR]

Given the prevalence of heterocyclic scaffolds in drug‐related molecules, converting these highly modular heterocyclic scaffolds into structural diversified and dearomatized analogs is an ideal strategy for improving their physicochemical and pharmacokinetic properties. Here, we described an efficient method for silver carbene‐mediated dearomative N−N bond cleavage leading to skeletal hopping between indazole and 1,2‐dihydroquinazoline via a highly selective single‐carbon insertion procedure. Using this methodology, a series of dihydroquinazoline analogues with diarylmethylene‐substituted quaternary carbon centers were constructed with excellent yields and good functional group compatibility, which was further illustrated by the late‐stage diversification of important pharmaceutically active ingredients. DFT calculations indicated that the silver catalyst not only induces the formation of the silver carbene, but also activates the diazahexatriene intermediate, which plays a crucial role in the formation of the C−N bond. [ABSTRACT FROM AUTHOR]