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N-heterocyclic carbenes (NHCs) are effective organocatalysts, which are widely used in various cycloaddition reactions. Among them, the [3 + 3] cycloaddition reaction type can be used for the synthesis of six-membered ring, which is also one of the most important methods for the formation of carbon–carbon bonds in organic chemistry. In this paper, a possible reaction pathway for the stereoselective [3 + 3] cycloaddition of bromoenal and β-tetralone have been investigated at the M06-2X/6–31G(d,p)/IEFPCM(THF) level. Our suggested mechanism includes binding of the NHC to bromoenal, 1,2-proton transfer, debromination, 1,3-proton shift, Michael addition, protonation, deprotonation and cyclisation, and NHC regeneration. The stereochemistry of the reaction is determined by the Michael addition step, and the favourable pathway generates the R-configurational dihydropyranone. Furthermore, through the use of global reactivity indexes (GRIs), we are able to demonstrate that the nucleophilicity of the bromoenal is increased by the NHC. The mechanistic insights gained in this work should be helpful in the rational design of potential catalysts for analogous reactions. [ABSTRACT FROM AUTHOR]

Multidentate bis‐NHCs ligands with various spacers are expected to combine the advantages of coordination chemistry and catalysis. These offer opportunities to construct various organometallic frameworks involving transition metals and main group elements. Therefore, developing a general procedure for synthesizing a variety of carbene salt precursors using a convenient technique is key in this context. The extended protocol of a solvent‐free approach for synthesizing various bridged bis‐imidazolium carbene salts, including tris and tetrakis‐imidazolium precursors, is reported here. This method can be performed in the laboratory, leading to high yields (80–95 %) and isolated as analytically pure, multigram, bench‐stable compounds. [ABSTRACT FROM AUTHOR]

We report the synthesis, characterisation and reactivity of the stable imidazol-2-ylidene EtIDip (EtIPr), {EtCN(Dip)}2C:, Dip = 2,6-iPr2C6H3, as a chemically robust alternative to IDip (IPr), {HCN(Dip)}2C:. The N-heterocyclic carbene EtIDip could be further converted to the oxidised species [EtIDipCl]Cl, EtIDipF2, EtIDipO, and EtIDipSe, and the group 13 element complexes EtIDipEX3, with E = B, X = Br; E = Al, X = I; E = Ga, X = I; E = Al, X = H. The properties of the EtIDip and IDip ligands are compared and the molecular structures of (DipNCEt)2, [EtIDipH]Cl, [EtIDipH]I, EtIDip, [EtIDipCl]Cl, EtIDipF2, EtIDipO, EtIDipBBr3, EtIDipAlI3, EtIDipGaI3, and EtIDipAlH3 have been determined. [ABSTRACT FROM AUTHOR]

To study the effect of a dye on the photoluminescence (PL) properties of metal complexes, a series of gold(I) complexes were synthesized, containing a 7‐amino‐4‐methylcoumarin luminophore. The complexes are comprised of a coumarin moiety featuring different ancillary ligands, specifically N‐heterocyclic carbenes, triphenylphosphine, and diphenyl‐2‐pyridylphosphine. The synthesized gold(I) complexes are luminescent both in solution and the solid state at room temperature and 77 K. Complexes of different nuclearity, i.e., mono‐, di‐ and trinuclear compounds were synthesized. A clear trend between the nuclearity and the quantum yields can be seen. The coumarin dye not only improves the PL properties, but also enhances the luminescence of trinuclear clusters, which are otherwise known to be weak emitters in solution. The optical absorption properties were investigated in detail by quantum chemical calculations. [ABSTRACT FROM AUTHOR]

In this study, we discuss the synthesis and structural characterization of three N‐heterocyclic carbene isothiocyanate zwitterionic ligands, 1,3‐bis(2,6‐diisopropylphenyl)‐2‐N‐phenyl‐thiocarbamoyl imidazolium (ImDipp‐CSNPh, L1), 1,3‐bis(tert‐butyl)‐2‐N‐phenylthiocarbamoyl imidazolium (ImtBu‐CSNPh, L2) and 1,3‐bis(2,4,6‐trimethylphenyl)‐2‐N‐phenylthiocarbamoyl imidazolium (ImMes‐CSNPh, L3) and their cadmium complexes, [(ImDipp‐CSNPh)2CdCl][CdCl3] (1) and [(ImtBu‐CSNPh)Cd(μ‐Cl)Cl]2 (2). Both bench‐stable cadmium complexes were synthesized by directly reacting the corresponding NHC‐CSNPh adducts with anhydrous cadmium chloride. The solid‐state structures of zwitterionic NHC‐isothiocyanate betaine adducts (L1‐L3) and cadmium complexes (1 and 2) were investigated with the assistance of single crystal X‐ray diffraction analysis. A steric modification of N‐substituent on NHC governs a unique structural change from betaine ImDipp‐CSNPh coordinated cationic cadmium complex to neutral dimeric cadmium complex with ImtBu‐CSNPh. [ABSTRACT FROM AUTHOR]

An efficient N-heterocyclic carbene (NHC)-catalyzed asymmetric conjugate addition reaction to afford synthetically challenging benzofuranone derivatives having vicinal all-carbon quaternary and tertiary stereocenters is presented. The reaction operates solely through noncovalent interaction between a newly designed NHC and the substrates, providing access to a series of functionalized benzofuranones in good yields and with high ee values. The protocol applies to preparative-scale synthesis. A catalytic cycle involving a noncovalent substrate–NHC interaction is implicated in the process, based on a mechanistic control study. [ABSTRACT FROM AUTHOR]

Unusual open‐chain tetradentate NHC‐phosphine pre‐ligand [Ph2PCH(Ph)ImCH2ImC(H)PhPPh2](BF4)2 was prepared in a good yield by metal‐mediated approach starting from Mn(I) complex [Cp(CO)2Mn(η2‐Ph2P=C(H)Ph)](BF4) and bis(imidazole) derivative ImCH2Im. Its reaction with Pd(OAc)2 afforded dicationic Pd(II) complex of bis(NHC)bis(phosphine) ligand adopting κ4PĈĈP coordination mode as a mixture of two diastereomers. The deprotonation of the C(H)Ph ligand bridge in the latter using KN(SiMe3)2 as base provided stable complex exhibiting unconventional phosphonium ylide moiety, whereas all attempts to detect the corresponding bis(ylide) derivative failed. [ABSTRACT FROM AUTHOR]

Herein, we describe the confinement of a N‐Heterocyclic Carbene (NHC) organocatalyst in the cavity of cyclodextrins (CDs). These confined organocatalysts allow the formylation of amines through CO2 hydrosilylation. The presence of the cavity of the CDs leads to substrate‐selectivity between amines in competition reactions. The use of the smallest α‐CD induces the best selectivities but also increased reactivity compared to the bigger β‐CD. A careful study conducted by NMR and DFT revealed that in α‐CD, the complexed CO2 interacts with the cavity through hydrogen bonds. These H‐bonds destabilize the NHC–CO2 adduct and are accountable for the higher reactivity observed using α‐CD. [ABSTRACT FROM AUTHOR]

Novel 1,2,4-triazolium salts, 4-aryl-3-arylamino-1-(pyridin-2-yl)-1,2,4-triazolium bromides, were synthesized. These triazolium salts can be used as precursors of base-ionizable N-heterocyclic carbene (NHC) ligands capable of deprotonating the NH-acidic arylamino group. New Pd/NHC complexes, in which NHC acts as a bidentate C,N-donor chelating ligand, were prepared by metallation of the synthesized triazolium salts with palladium acetate. It was found that the obtained complexes have enhanced stability in strongly basic media and may be of interest as potential catalysts for cross-coupling reactions. [ABSTRACT FROM AUTHOR]

Diradicals based on a meta‐quinodimethane (m‐QDM) scaffold generally have a triplet ground state and are rather scarce. Herein, m‐QDM‐based non‐Kekulé diradicals [3,3′‐(NHC)2BP] (3‐NHC) (NHC = SIPr = C{N(Dipp)CH2}2; IPr = C{N(Dipp)CH}2, Me‐IPr = C{N(Dipp)CMe}2; Dipp = 2,6‐iPr2C6H3; BP = 1,1′‐C6H4C6H4) featuring N‐heterocyclic carbene (NHC) pendants are reported as crystalline solids. The EPR spectra of 3‐NHC show both allowed (Δms = 1) and forbidden (Δms = 2; 'half‐field') transitions characteristic for triplet diradicals. Variable temperature EPR studies however reveal a singlet ground state for 3‐SIPr. Consistent with the EPR spectra, calculations predict a remarkably small singlet‐triplet energy gap (ΔEST ≤ 0.26 kcal/mol) for the 3‐NHC compounds. The calculated singlet diradical character for the ground states of the 3‐NHC compounds amounts to ~99 %. [ABSTRACT FROM AUTHOR]

We report the catalytic synthesis of 3‐hydroxy‐2‐butanon (acetoin) from acetaldehyde as a key step in the synthesis of C4‐molecules from ethanol. Facile C−C bond formation at the α‐carbon of the C2 building block is achieved using an N‐heterocyclic carbene (NHC) catalyst. The immobilization of the catalyst on a Merrifield's peptide resin and its spectroscopic characterisation using solid‐state Nuclear Magnetic Resonance (NMR) is described herein. The immobilization of the NHC catalyst allows for process intensification steps and the reported catalytic system was subjected to batch recycling as well as continuous flow experiments. The robustness of the catalytic system was shown over a maximum of 10 h time‐on‐stream. Overall, high selectivity S>90 % was observed. The observed deactivation of the catalyst with increasing time‐on‐stream is explained by ex‐situ1H solution‐state, as well as 13C and 15N solid‐state NMR spectra allowing us to develop a deeper understanding of the underlying decomposition mechanism of the catalyst. [ABSTRACT FROM AUTHOR]

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon–fluorine bonds. Simultaneously, sterically‐hindered N‐heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition‐metal‐catalysis. Herein, we report the first class of sterically‐hindered, fluorinated N‐heterocyclic carbenes. These ligands feature variable fluorine substitution at the N‐aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one‐pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron‐donating and π‐accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon–fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers. [ABSTRACT FROM AUTHOR]

A stereoselective hydroxyallylation reaction of cyclopropenes with cyclopropanols is achieved under zinc‐mediated conditions, affording densely functionalized cyclopropanes with excellent diastereocontrol over three contiguous stereocenters within and outside the cyclopropane ring. A racemic variant of the reaction is synergistically promoted by catalytic N‐heterocyclic carbene (NHC) and organic base, whereas chiral amino alcohol‐derived bifunctional NHC enables a catalytic enantioselective variant. The reaction likely involves the generation of enolized zinc homoenolate via ring‐opening of zinc cyclopropoxide and enolization of the resulting homoenolate, followed by its addition to the cyclopropene as a prochiral allylzinc nucleophile. Our mechanistic investigations highlighted the transient nature of enolized homoenolate, which, once generated from thermodynamically predominant cyclopropoxide, immediately proceeds to allylzincation with cyclopropene. The NHC not only promotes the rate‐determining generation of enolized homoenolate but also engages in the allylzincation process. The resulting cyclopropylzinc species undergoes partial in situ protonation while partially remaining intact, thereby leaving an opportunity for trapping with an external electrophile. [ABSTRACT FROM AUTHOR]

An unprecedented chemodivergent strategy for parallel kinetic resolution (PKR) is disclosed through which two planar chiral products bearing different structures were simultaneously afforded with opposite stereoselectivities. Two achiral esters are activated by one single chiral N‐heterocyclic carbene (NHC) catalyst to react with the different enantiomers of the racemic imine substrate in a parallel fashion. Two products bearing distinct structures and opposite stereoselectivities are respectively afforded from the same reaction system in good to excellent yields, enantio‐ and diastereoselectivities. Control experiments and kinetic studies are carried out to probe the kinetic and dynamic properties during the reaction progress. The planar chiral pyridine and lactam products show interesting applications in both asymmetric synthesis and pesticide development. [ABSTRACT FROM AUTHOR]

N‐Heterocyclic olefins (NHOs), possessing highly polarizable and remarkably electron‐rich double bonds, have been effectively utilized as exceptional anchors for surface modifications. Herein, the adsorption, orientation, and electronic properties of NHOs on a metal surface are investigated. On Cu(111), the sterically low‐demanding IMe‐NHO is compared to its analogous IMe‐NHC counterpart. High‐resolution electron energy‐loss spectroscopy (HREELS) measurements show for both molecules a flat‐lying ring adsorption configuration. While the NHC adopts a dimer configuration including a Cu adatom, the NHO chemisorbs over a C–Cu bond perpendicular to the surface. This distinct difference leads for the IMe‐NHOs to have a higher thermal stability on the surface. Moreover, IMe‐NHOs introduce a higher net electron transfer to the surface compared to the IMe‐NHCs, which results in a stronger effect on the work function. These results highlight the role of NHOs in surface science as they extend the functionalization capabilities of NHCs into stronger electronic modification. [ABSTRACT FROM AUTHOR]

A facile and practical one‐pot synthesis of hydrobenzoin has been developed from the readily available aromatic aldehydes, under NHC catalysis. Basically, the present research elucidates a novel, sustainable approach for the synthesis of hydrobenzoin, capitalizing on the catalytic prowess of N‐heterocyclic carbene (NHC) and utilizing sodium borohydride (NaBH4) as an effective reducing partner in this one‐pot conversion. By harnessing the unique reactivity of NHCs, this methodology enables the efficient formation of C−C bonds, leading to the streamlined synthesis of hydrobenzoin with high yields while tolerating various functional groups. Furthermore, this catalytic system offers notable advantages in terms of atom economy, mild reaction conditions, and operational simplicity, making it a promising strategy for sustainable chemical synthesis. The detailed mechanistic insights provided herein shed light on the underlying catalytic pathways, facilitating a deeper understanding of the reaction mechanism. Overall, this study underscores the significant potential of NHC catalysis as a versatile tool for the sustainable synthesis of valuable organic compounds, thereby contributing to the advancement of green chemistry principles and the pursuit of environmentally benign chemical processes. [ABSTRACT FROM AUTHOR]

The umpolung of aldimines using N‐heterocyclic carbenes (NHCs) is less explored compared to the established polarity reversal of aldehydes. Described herein is an NHC‐catalyzed imine umpolung /6π‐electrocyclization cascade, which leads to the atom‐ and pot‐economic synthesis of biologically important dihydrochromeno indoles. For the first time, the nucleophilic aza‐Breslow intermediates have been intercepted with unactivated alkynes. Preliminary mechanistic and DFT studies shed light on the role of the phenolic −OH moiety in promoting the addition of the aza‐Breslow intermediate to the unactivated alkyne via an intramolecular proton transfer in a stepwise manner. DFT studies also support the regioselectivity preference for the 5‐exo‐dig cyclization pathway, leading to the exclusive formation of the indole products. Moreover, a comparison of Gibbs free energies provides insight into a thermodynamically preferred 6π‐electrocyclization over a competing oxa‐Michael pathway. Further, this strategy is applied to the formal synthesis of a Hepatitis C Virus (HCV) NS5A inhibitor in a step‐economical method. [ABSTRACT FROM AUTHOR]

Manganese catalysis has attracted significant attention in the last few years. In comparison with the extensively studied Mn complexes bearing phosphine‐based pincer ligands, catalysis with Mn containing N‐heterocyclic carbenes (NHC) is poorly developed. In this work, new manganese(I) complexes bearing an NHC ligand with picolyl wingtip substituents (an N,C,N ligand) have been synthesized and fully characterized. The structure of fac‐[Mn(CO)3(κ2N,C,N)Br] and fac‐[Mn(CO)3(κ3N,C,N)](OTf) complexes with a bidentate and tridentate coordination of the N,C,N ligand, respectively, has been elucidated by single crystal X‐ray diffraction studies. These Mn complexes were applied as catalysts for the visible light‐induced hydrosilylation of ketones. The mild protocol operated efficiently at room temperature under visible light irradiation (blue LED), enabling the conversion of a wide variety of aromatic ketones to their respective alcohols in high yields. Evidence for radical intermediates and the participation of Mn‐H species has been demonstrated by a series of trapping experiments. [ABSTRACT FROM AUTHOR]

Invasive fungal diseases affect more than two million people worldwide. The increasing incidence of invasive fungal infections is the result of many factors, including an increase in the resistance to current drugs. As such, there is an urgent need to obtain new drugs that are efficient, selective, and able to overcome existing resistance mechanisms. Candida yeasts are responsible for more than 70% of all nosocomial invasive fungal diseases. In this work, we describe the synthesis of nickel (II)(NHC) complexes based on xanthines, by direct metalation of xanthinium salts with nickelocene NiCp2to yield [NiCpI(NHC)] complexes. For methyl caffeine, a biscarbene complex [NiCp(NHC)2]+is also formed, resulting from carbene dissociation from the corresponding monocarbene. [NiCpI(NHC)] complexes are active as antifungals for Candida yeasts and show toxicity for human cells (HeLa) that is dependent on the substitution of N7 of the xanthine moiety. The biscarbene complex 5[NiCp(NHC)2]+is highly selective for Candida glabrata and shows very low toxicity for human cells, being a promising candidate for selective treatment of C. glabrata infections. [ABSTRACT FROM AUTHOR]

Tetrahydropyridazines constitute an important structural motif found in numerous natural products and pharmaceutical compounds. Herein, we report an aminoacylation reaction of alkenes that enables the synthesis of 1,4,5,6‐tetrahydropyridazines through cooperative N‐heterocyclic carbene (NHC) and photoredox catalysis. This approach involves the 6‐endo‐trig cyclization of N‐centered hydrazonyl radicals, generated via single‐electron oxidation of hydrazones, followed by a radical–radical coupling step. The mild process tolerates a wide range of common functional groups and affords a variety of tetrahydropyridazines in moderate to high yields. Preliminary investigations using chiral NHC catalysts demonstrate the potential of this protocol for asymmetric radical reactions. [ABSTRACT FROM AUTHOR]

The 2,7‐fluorenone‐linked bis(6‐imidazo[1,5‐a]pyridinium) salt H2‐1(PF6)2 reacts with Ag2O in CH3CN to yield the [2]catenane [Ag4(1)4](PF6)4. The [2]catenane rearranges in DMF to yield two metallamacrocycles [Ag2(1)2](PF6)2. 2,7‐Fluorenone‐bridged bis‐(imidazolium) salts H2‐L(PF6)2 (L=2 a, 2 b) react with Ag2O in CH3CN to yield metallamacrocycles [Ag2(L)2](PF6)2 with interplanar distances between the fluorenone rings too small for [2]catenane formation. Intra‐ and intermolecular π⋅⋅⋅π interactions between the fluorenone groups were observed by X‐ray crystallography. The strongly kinked 2,7‐fluorenone bridged bis(5‐imidazo[1,5‐a]pyridinium) salt H2‐4(PF6)2 reacts with Ag2O to yield [Ag2(4)(CN)](PF6), while the tetranuclear assembly [Ag4(4)2(CO3)](PF6)2 was obtained in the presence of K2CO3. [ABSTRACT FROM AUTHOR]

A new approach to the alkylation of azoles with alkenes catalyzed by the nickel complexes with N-heterocyclic carbenes (Ni/NHC) was developed. The catalytically active Ni/NHC complexes are formed in situ from the air-stable precursors: the nickel chloride complex with pyridine (NiCl2Py2), imidazolium salt IMes•HCl (IMes is 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) as the carbene source, and sodium tert-butoxide (ButONa). The advantages of the developed approach are availability of the components of the catalytic system, no necessity to apply special agents for the reduction of NiII to Ni0, and simplification of the synthesis procedure. [ABSTRACT FROM AUTHOR]

Organic‐hybrid particle‐based materials are increasingly important in (opto)electronics, sensing, and catalysis due to their printability and stretchability as well as their potential for unique synergistic functional effects. However, these functional properties are often limited due to poor electronic coupling between the organic shell and the nanoparticle. N‐heterocyclic carbenes (NHCs) belong to the most promising anchors to achieve electronic delocalization across the interface, as they form robust and highly conductive bonds with metals and offer a plethora of functionalization possibilities. Despite the outstanding potential of the conductive NHC‐metal bond, synthetic challenges have so far limited its application to the improvement of colloidal stabilities, disregarding the potential of the conductive anchor. Here, NHC anchors are used to modify redox‐active gold nanoparticles (AuNPs) with conjugated triphenylamines (TPA). The resulting AuNPs exhibit excellent thermal and redox stability benefiting from the robust NHC‐gold bond. As electrochromic materials, the hybrid materials show pronounced color changes from red to dark green, a highly stable cycling stability (1000 cycles), and a fast response speed (5.6 s/2.1 s). Furthermore, TPA‐NHC@AuNP exhibits an ionization potential of 5.3 eV and a distinct out‐of‐plane conductivity, making them a promising candidate for application as hole transport layers in optoelectronic devices. [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]

Luminescent organic radicals have garnered increasing attention owing to their versatile applications in sensing, imaging, and organic light-emitting diodes (OLEDs), attributed to their unique emission properties originating from the doublet spin state. However, the natural narrow bandgap of organic free radicals typically limits their emission to the long-wavelength region. Designing luminescent organic radicals with short-wavelength emission remains a significant challenge. Herein, a series of carbon-centered radicals with short-wavelength emission (383–476 nm) by combining N-heterocyclic carbenes with various polycyclic aromatic hydrocarbons (PAHs) (2-naphthyl, 2aI and 2bI; 2-phenanthryl, 2aII and 2bII; 2-anthryl, 2aIII and 2bIII; 3-phenanthryl, 2aIV and 2bIV). Theoretical calculations reveal that the introduction of PAHs significantly increases the ΔED2-D1 in 2aI–III and 2bI–III compared to that in phenyl-derived radical congeners. Consequently, the internal transition from D2 to D1 is impeded, leading to a high yield of D2 emission and a suppressed Kasha's rule, thereby overcoming the limitations imposed by their narrow bandgap. For 2aIV and 2bIV, despite a moderately large ΔED2-D1 value, the ΔED3-D1 value exceeds 1 eV, indicating that their emission likely originates from the D3 state. Furthermore, we utilized 2aIII and 2bIII as emissive materials in OLEDs, resulting in blue emissions with external quantum efficiencies of 7.5% and 6.5%, respectively. [ABSTRACT FROM AUTHOR]

2‐tetrelavinylidenes (C=EH2; E=Si, Ge) are according to quantum chemical studies the least stable isomers on the [E,C,2H] potential energy hypersurface isomerizing easily via the trans‐bent tetrelaacetylenes HE≡CH to the thermodynamically most stable 1‐tetrelavinylidenes (E=CH2). Consequently, experimental studies on 2‐tetrelavinylidenes (C=ER2) and their derivatives are lacking. Herein we report experimental and theoretical studies of the first N‐heterocyclic carbene (NHC) supported 2‐silavinylidene (NHC)C=SiBr(Tbb) (1‐Si: NHC=C[N(Dipp)CH]2, Dipp=2,6‐diisopropylphenyl, Tbb=2,6‐bis[bis(trimethylsilyl)methyl]‐4‐tert‐butylphenyl) and the isovalent 2‐germavinylidenes (NHC)C=GeBr(R) (1‐Ge, 1‐GeMind: R=Tbb, Mind (1,1,3,3,5,5,7,7‐octamethyl‐s‐hydrindacene‐4‐yl)). The NHC‐supported 2‐tetrelavinylidenes were obtained selectively from the 1,2‐dibromoditetrelenes (E)‐(R)BrE=EBr(R) using the diazoolefin (NHC)CN2 as vinylidene transfer reagent. 1‐E (E=Si, Ge) have a planar vinylidene core, a bent‐dicoordinated vinylidene carbon atom (CVNL), a very short E=CVNL bond and an almost orthogonal orientation of the NHC five‐membered ring to the vinylidene core. Quantum chemical analysis of the electronic structures of 1‐E suggest a significantly bent 1‐tetrelaallene and tetrelyne character. NMR studies shed light into the dynamics of 1‐E involving NHC‐rotation around the CVNL−CNHC bond with a low activation barrier. Furthermore, the synthetic potential of 1‐E is demonstrated by the synthesis and full characterization of the unprecedented NHC‐supported bromogermynes BrGe=C(EBr2Tbb)(NHC) (2‐SiGe: E=Si; 2‐GeGe: E=Ge). [ABSTRACT FROM AUTHOR]

A formal [4+2] annulation reaction of phthalaldehydes and 3‐ylideneoxindoles through a tandem process of a Stetter–aldol reaction was accomplished by the application N‐heterocyclic carbenes as effective catalysts. Under mild conditions, this unprecedented cascade reaction readily occurs in good yield, enabling straightforward access to functionalized tetrahydronaphthalene‐fused spirooxindoles. For the first time, the feasibility of an asymmetric Stetter–aldol reaction has also been briefly explored by employing chiral organic bases to promote the asymmetric transformation. [ABSTRACT FROM AUTHOR]

Intramolecular benzoin condensation, which is a powerful and useful method for the formation of carbon−carbon bonds from aldehydes, is also an advantageous method for the preparation of a wide variety of α‐hydroxy ketones. The α‐Hydroxy carbonyl groups are an important structure for natural products, industrial, and pharmaceutical materials. In this study, it was aimed to synthesize new bulky α‐hydroxy ketones that can be used both as pharmaceutical materials and can lead to the synthesis of new organocatalysts. An effective method was applied for the synthesis of bulky hydroxy ketones 2 a–l using intramolecular benzoin condensation catalyzed by N‐heterocyclic carbene (I) from 4‐arenoxybenzaldehyde 1 a–f or 4‐thioetherbenzaldehyde 1 g–l derivatives and 12 new bulky α‐hydroxy ketone compounds 2 a–l were synthesized in 70–87 % isolated yield. The synthesized compounds were characterized by spectroscopic and chromatographic methods such as FT‐IR, NMR, LC–MS, and CHN analysis. Additionally, compound 2 h selected as a model substrate was converted into 1,2 diol 3 h, 1,2‐diketone 4 h, and 1,2‐keto acetate 5 h derivatives, and structural analyses were performed. Moreover, chiral hydroxy ketone 2 h was obtained with 100 % ee due to the enzymatic hydrolysis reaction catalyzed by Lipase from Candida rugosa (CRL). [ABSTRACT FROM AUTHOR]

We report the stepwise synthesis of a novel series of heterometallic gold(I)/palladium(II) complexes supported by heteroditopic imidazol‐2‐ylidene/triazol‐5‐ylidene ligands. All new compounds were characterized by means of 1H and 13C NMR spectroscopy, melting points, and elemental analyses. The versatility of the ligand platforms, additionally allows for the preparation of cationic chelate biscarbenic palladium(II) complexes through full deprotonation of the mixed azolium precursors. Characterization studies were completed by the determination of the molecular structures of some synthetic intermediates. The new biscarbenic palladium(II) complexes were tested as catalysts in the cross‐coupling of boronic acid to a variety of chloroarenes and the hydrosilylation of terminal alkynes. Both catalytic processes provide good to excellent yields under low catalyst loadings. [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]

The syntheses of the neutral molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes of the general formula [Mo(NAr)(CHCMe2Ph)(NHC)XY] (Ar=2‐tBu‐C6H4, 2‐CF3‐C6H4, 2,6‐Me2‐C6H3, 2,6‐Cl2‐C6H3, adamantyl; X, Y=OTf, OC(CF3)3, OCH(CF3)2, OC6F5, SC6F5, 2,5‐bis(pentafluorophenyl)phen‐1‐yl) bearing electron‐withdrawing NHCs (1,3‐dimethyl‐4,5‐dichloroimidazol‐2‐ylidene (IMeCl2), 1,3,4‐triphenyl‐1,2,4‐triazol‐5‐ylidene (TPT)) are reported. Complementary, the corresponding cationic molybdenum imido alkylidene NHC complexes of the general formula [Mo(NAr)(CHCMe2R)(NHC)X+][B(ArF)4−/Al(OC(CF3)3)4−] (R=Me, Ph; B(ArF)4−=tetrakis (3,5‐bis(trifluoromethyl)phenyl)borate) have been prepared. Aiming at tungsten oxo complexes, reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)2] with [1,3‐dimethyl‐4,5‐dichloroimidazol‐2‐ylidene⋅AgI] (IMeCl2⋅AgI) followed by the addition of lithium terphenoxide yields [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)2]. For comparison, [W(O)Cl(CHCMe2Ph)(IMes)(OSi(OtBu)3)] was prepared via reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)(IMes)] with KOSi(OtBu)3. [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)(Et2O)+][B(ArF)4−] (DPPO=2,6‐diphenylphenoxide) became accessible via reaction of [W(O)(DPPO)2(CHCMe2Ph)(IMeCl2)] with anilinium B(ArF)4−. The structural peculiarities of selected complexes are reported. Benchmark ring‐closing metathesis and homometathesis reactions revealed that the neutral complexes bearing weakly coordinating NHCs such as IMeCl2 and TPT possessed only moderate activity, which could, however, be improved by preparing the corresponding cationic metal alkylidene complexes. [ABSTRACT FROM AUTHOR]

We communicate a versatile and user-friendly Cu-based catalytic method that allows for the selective hydrogenation of enoates and alkynes. The introduced protocol is free from any ex ante modifications of the used Cu(I) precursors by air-sensitive phosphines or elaborate N-heterocyclic carbene ligands. The conjugate hydrogenation of enoates and the (selective) reduction of C≡C bonds is achieved through a [Cu(CH3CN)4]+/tert-butoxide pair whereby we describe a delicate influence of the base cation on the chemoselectivity of the respective transformation. In case of the ester-to-alcohol reduction, a combination of simple CuI and NaOtBu proved to be successful. Deuteration experiments are included to address certain mechanistic aspects of the introduced catalytic system. All requisite chemicals are readily obtainable through commercial channels and the catalyst assembly is set up on the bench without the need for special lab-technical precautions.

Abstract N‐Heterocyclic olefins (NHOs), possessing highly polarizable and remarkably electron‐rich double bonds, have been effectively utilized as exceptional anchors for surface modifications. Herein, the adsorption, orientation, and electronic properties of NHOs on a metal surface are investigated. On Cu(111), the sterically low‐demanding IMe‐NHO is compared to its analogous IMe‐NHC counterpart. High‐resolution electron energy‐loss spectroscopy (HREELS) measurements show for both molecules a flat‐lying ring adsorption configuration. While the NHC adopts a dimer configuration including a Cu adatom, the NHO chemisorbs over a C–Cu bond perpendicular to the surface. This distinct difference leads for the IMe‐NHOs to have a higher thermal stability on the surface. Moreover, IMe‐NHOs introduce a higher net electron transfer to the surface compared to the IMe‐NHCs, which results in a stronger effect on the work function. These results highlight the role of NHOs in surface science as they extend the functionalization capabilities of NHCs into stronger electronic modification.

Synthesis of organometallic compounds has had an enormous impact on medicine. In this context, gold complexes are gaining much interest since the discovery of the cytotoxic effect of cisplatin. On the other hand, in the last two decades, the mechanochemical synthetic approaches have been developed considerably demonstrating that they could also be a powerful tool enabling environmentally benign and sustainable synthesis of metal complexes. The present work focuses on mechanochemical synthesis of precursors and gold–NHC complexes of type NHCAuCl and [AubisNHC]+. The mechanochemical approach has been studied to afford four substituted imidazoles, eight imidazolium salts and six NHCAuCl and one [AubisNHC]+. Substituted imidazoles were obtained with yields varying between 29–99%. Five imidazolium salts bearing aliphatic carbon atoms were obtained, with yields from 46–81%. It is important to notice that the reaction can follow the aging process giving rise to imidazolium salts in very good yields. Concerning the gold(I) complexes, for the first time, six mono NHC complexes of type NHCAuCl have been synthesized, five of them with yields varying between 41–83%. Finally, compound 19 [AubisNHC]+ has been obtained not only by transmetallation, but most gratifyingly through direct metalation in 73% yield.

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]

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 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]

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]

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]

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 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]