(NHC)CuI (NHC = N-Heterocyclic Carbene) Complexes as Efficient Catalysts for the Reduction of Carbonyl Compounds

Three water-soluble copper(II) N-heterocyclic carbene complexes: toward copper-catalyzed ketone reduction under sustainable conditions

A series of tridentate copper(II) N-heterocyclic carbene (NHC) complexes with imidazole, benzimidazole, and 5,6-dimethylbenzimidazole azole rings were synthesized and comprehensively characterized via X-ray crystallography, ESI-MS, cyclic voltammetry, and UV-Vis and EPR spectroscopic studies. These complexes were then utilized for the optimization of ketone reduction under sustainable conditions using 2-acetylpyridine and phenylsilane. The relationships between product formation, temperature, reaction time, and catalyst loading for the hydrogenation reactions are covered in detail. Reduction of eighteen different aliphatic, cyclic, and aromatic ketones were demonstrated, which were compatible to produce the corresponding products in moderate to good yields. These systems were used to develop related DNA-hybrid catalytic systems, but only supported weak enantioselectivity. Further thermodynamic experiments showed Cu-NHC complexes did not demonstrate specific binding to DNA, which is consistent with their limited selectivity.

The structures and reactivity of NHC-supported copper(i) triphenylgermyls

Deprotonation of triphenyl germane with NHC-supported copper alkoxides afforded four novel (NHC)CuGePh3 complexes. Of these, (IPr)CuGePh3 (IPr = :C{N(2,6-iPr2C6H3)CH}2) was selected for further investigation. Analysis by EDA-NOCV indicates it to be a germyl nucleophile and its σ-bond metathesis reaction with a range of p-block halides confirmed it to be a convenient source of [Ph3Ge]-. The Cu-Ge bond of (IPr)CuGePh3 underwent π-bond insertions with tBuNCS, CS2, and PhNCO to furnish a series of germyl substituted carboxylate derivatives, (IPr)CuXC(Y)GePh3 (X = S, NPh; Y = S, NtBu, O), which were structurally characterised. (IPr)CuGePh3 inserted phenyl acetylene, providing both the Markovnikov and anti-Markovnikov products. The (NHC)CuGePh3 compounds were validated as catalytic intermediates; addition of 10 mol% of NHC-copper(i) alkoxide to a mixture of triphenyl germane and a tin(iv) alkoxide resulted in a tin/germanium cross coupling with concomitant formation of alcohol. Moreover, a catalytic hydrogermylation of Michael acceptors was developed with Ph3GeH adding to 7 activated alkenes in good conversions and yields in the presence of 10 mol% of NHC-copper(i) alkoxide. In all cases, this reaction provided the β-germylated substrate implicating nucleophilicity at germanium.

Application of N-heterocyclic carbene-Cu(I) complexes as catalysts in organic synthesis: a review

N-Heterocyclic carbenes (NHCs) are a special type of carbenes in which the carbene carbon atom is part of the nitrogen heterocyclic ring. Due to the simplicity of their synthesis and the modularity of their stereoelectronic properties, NHCs have unquestionably emerged as one of the most fascinating and well-known species in chemical science. The remarkable stability of NHCs can be attributed to both kinetic as well as thermodynamic effects caused by its structural features. NHCs constitute a well-established class of new ligands in organometallic chemistry. Although initially NHCs were regarded as pure σ-donor ligands, later experimental and theoretical studies established the presence of a significant back donation from the d-orbital of the metal to the π* orbital of the NHC. Over the last two decades, NHC-metal complexes have been extensively used as efficient catalysts in different types of organic reactions. Of these, NHC-Cu(I) complexes found prominence for various reasons, such as ease of preparation, possibility of structural diversity, low cost, and versatile applications. This article overviews applications of NHC-Cu(I) complexes as catalysts in organic synthesis over the last 12 years, which include hydrosilylation reactions, conjugate addition, [3 + 2] cycloaddition, A3 reaction, boration and hydroboration, N-H and C(sp2)-H carboxylation, C(sp2)-H alkenylation and allylation, C(sp2)-H arylation, C(sp2)-H amidation, and C(sp2)-H thiolation. Preceding the section of applications, a brief description of the structure of NHCs, nature of NHC-metal bond, and methods of preparation of NHC-Cu complexes is provided.

Reactions of noble-metal oxides in ionic liquids near room temperature

The reaction of Ag₂O, Au₂O₃, and HgO with CuCl, CuI, AgCl, AgI, AuCl, and AuI in ionic liquids ([EMIm]Cl, [BMIm]Cl) near room temperature (20-80 °C) is evaluated and results in the new compounds (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃], [(C₈H₁₄N₂)₂Hg][AgCl₃], and [EMIm][Ag₂I₂Cl]. Thereof, (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃], and [(C₈H₁₄N₂)₂Hg][AgCl₃] are NHC complexes (NHC: N-heterocyclic carbene) with M-C bonds (M: Cu, Ag, Au, Hg). Whereas (C₈H₁₄N₂)CuCl and (C₈H₁₄N₂)AgI crystallize as single molecules, (C₆H₁₀N₂)AuCl is dimerized via aurophilic interactions. [(C₈H₁₄N₂)₂Hg][CuCl₃] and [(C₈H₁₄N₂)₂Hg][AgCl₃] exhibit Hg atoms with two Hg-C bonds. Moreover, (C₈H₁₄N₂)AgI shows intense green fluorescence at room temperature with a quantum yield of 44%, whereas all other compounds do not show any emission at room temperature. Finally, [EMIm][Ag₂I₂Cl] is not an NHC compound but contains _∞ ¹[AgI_1/2I_2/4Cl_1/2]^- chains with infinite d¹⁰-d¹⁰ interaction of the silver atoms. The title compounds are characterized by single-crystal structure analysis, infrared spectroscopy, thermogravimetry, and fluorescence spectroscopy.

Monomeric and Polymeric Mesoionic N-Heterocyclic Carbene-Tethered Silver Nanoparticles: Synthesis, Stability, and Catalytic Activity

In recent years, N-heterocyclic carbenes (NHCs) have garnered significant attention as promising alternatives to thiols to stabilize metallic nanoparticles and planar surfaces. While most studies thus far have focused on NHC-functionalized gold nanoparticles (AuNPs), as an ideal platform to investigate the role of NHCs in stabilizing such nanoparticles, their ability to protect more unstable coinage metal nanoparticles, such as silver nanoparticles (AgNPs), has been largely overlooked. This is despite the fact that AgNPs possess a much more sensitive optical response that, upon their enhanced stability, can broaden their scope of application in various fields, including nanomedicine and catalysis. In this study, the synthesis and use of monomeric and polymeric mesoionic NHC-Ag(I) complexes as precursors to mono- and multidentate NHC-tethered AgNPs are reported. The polymeric analog was obtained by first synthesizing a polymer, containing 1,2,3-triazole repeat units, employing the copper-catalyzed alkyne-azide cycloaddition click polymerization of monomers containing diazide- and dialkyne functional groups. Subsequent quaternization of the triazole moieties and Ag insertion yielded the target NHC-Ag-containing polymer. Using this polymer as well as its monomeric analog as substrates, AgNPs with either catenated networks of NHCs or monomeric NHCs were fabricated by their reduction using borane-tert-butylamine complex. Our stability studies demonstrate that while monomeric NHCs impart some degree of stability to AgNPs, particularly at elevated temperatures in aqueous as well as organic medium, their polymeric analogs further enhance their stability in acidic environment (pH = 2) and against glutathione (3 mM), as an example of a biologically relevant thiol, in aqueous media. To highlight the application of these NHC-functionalized AgNPs in catalysis, we explore the aqueous phase reduction of methyl orange and 4-nitrophenol.

A Review on The Medicinal And Industrial Applications of N-Containing Heterocycles

Nitrogen-containing heterocycles constitute an important division of organic chemistry. The structural and functional diversity in nitrogen-containing heterocyclic compounds emanates from the presence and nature of the heteroatom that optimizes the compound for a specific application. Nitrogen heterocycles have been found to mimic various endogenous metabolites and natural products, highlighting their pivotal role in current drug design. Their applications are manifold and are predominantly used as pharmaceuticals, corrosion inhibitors, polymers, agrochemicals, dyes, developers, etc. Additionally, their catalytic behavior has rendered these compounds notable precursors in synthesizing various important organic compounds. The rate at which nitrogen heterocycles are synthesized explains this organic chemistry domain's vitality and usefulness. The present review article focuses on nitrogen-containing heterocycles as a versatile scaffold for current applications of organic chemistry.

Helicenic N-heterocyclic carbene copper(I) complex displaying circularly polarized blue fluorescence

Enantiopure copper(I) chloride complexes bearing a monodentate N-(carbo[6]helicenyl)-NHC ligand have been prepared and characterized experimentally and computationally. Their high stability enables the stereochemistry to be probed by X-ray crystallography and NMR spectroscopy. The resolved enantiomeric complexes emit circularly polarized blue fluorescence with g_lum ∼1.3 × 10^-3 in solution. The photophysical and chiroptical properties of these systems, with their helicene-centred origin, are similar to those of the organic helicene-benzimidazole precursor proligand, although the reverse axial chirality configuration is preferentially observed for the complex compared to the ligand.

Reactions of Mesityl Azide with Ferrocene‐Based N‐Heterocyclic Germylenes, Stannylenes and Plumbylenes, Including PPh 2 ‐Functionalised Congeners

The reactivity of ferrocene‐based N‐heterocyclic tetrylenes [{Fe(η⁵−C₅H₄−NSitBuMe₂)₂}E] (E=Ge, Sn, Pb) towards mesityl azide (MesN₃) is compared with that of PPh₂‐functionalised congeners exhibiting two possible reaction sites, namely the E^II and P^III atom. For E=Ge and Sn the reaction occurs at the E^II atom, leading to the formation of N₂ and an E^IV=NMes unit. The germanimines are sufficiently stable for isolation. The stannanimines furnish follow‐up products, either by [2+3] cycloaddition with MesN₃ or, in the PPh₂‐substituted case, by NMes transfer from the Sn^IV to the P^III atom. Whereas [{Fe(η⁵−C₅H₄−NSitBuMe₂)₂}Pb] and other diaminoplumbylenes studied are inert even under forcing conditions, the PPh₂‐substituted congener forms an addition product with MesN₃, thus showing a behaviour similar to that of frustrated Lewis pairs. The germylenes of this study afford copper(I) complexes with CuCl, including the first structurally characterised linear dicoordinate halogenido complex [CuX(L)] with a heavier tetrylene ligand L.

Synthesis of MAuAg (M = Ni, Pd, or Pt) and NiAuCu Heterotrimetallic Complexes Ligated by a Tritopic Carbanionic N-Heterocyclic Carbene

Heterotrimetallic complexes (NiAuAg, PdAuAg, PtAuAg, and NiAuCu) containing a tritopic N-heterocyclic carbene (NHC) have been synthesized for the first time through the deprotonation and metalation of heterodimetallic complexes and were structurally characterized by single-crystal X-ray diffraction. The carbene character of the donor groups in the tritopic NHC complexes was established on the basis of structural and NMR analyses.

Heterobimetallic Coinage Metal-Ruthenium Complexes Supported by Anionic N-Heterocyclic Carbenes

The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating borate moiety (WCA‐NHC, WCA=B(C₆F₅)₃, NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were used for the preparation of silver(I) or copper(I) WCA‐NHC complexes. While the reactions in THF with AgCl or CuCl afforded anionic mono‐ and dicarbene complexes with solvated lithium counterions [Li(THF)_n]⁺ (n=3, 4), the reactions in toluene proceeded with elimination of LiCl and formation of the neutral phosphine and arene complexes [(WCA‐NHC)M(PPh₃)] and [(WCA‐NHC)M(η²‐toluene)] (M=Ag, Cu). The latter were used for the preparation of chlorido‐ and iodido‐bridged heterobimetallic Ag/Ru and Cu/Ru complexes [(WCA‐NHC)M(μ‐X)₂Ru(PPh₃)(η⁶‐p‐cymene)] (M=Ag, Cu, X=Cl; M=Ag, X=I). Surprisingly, these complexes resisted the elimination of CuCl, AgCl, or AgI, precluding WCA‐NHC transmetalation.

Computational Study of Triphosphine-Ligated Cu(I) Catalysts for Hydrogenation of CO2 to Formate

The catalyzed hydrogenation of CO₂ to formate via a triphosphine-ligated Cu(I) was studied computationally at the density functional theory level in the presence of a self-consistent reaction field. Of the four functionals benchmarked, M06 was generally in the best agreement with the available experimentally estimated values. Two bases, DBU and TBD, were studied in the context of two proposed mechanisms in the MeCN solvent. Activation of H₂ was explored by using LCu(DBU)⁺ to form LCuH. Dissociation of a ligand arm results in higher barriers to form the key hydride complex, LCuH. The preferred mechanism passes through a transition state, where the H₂ has one H atom interacting with the copper center and the other H atom interacting with the N atom of the base, similar to H₂ insertion into a frustrated Lewis pair. There is no significant difference between the choice of a base, DBU or TBD, with respect to the proposed mechanisms. We propose that the experimentally observed differences between DBU and TBD reactivities for this mechanism are due to off-pathway changes.

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry

Fluoride abstraction from different types of transition metal fluoride complexes [Ln MF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C2 F5 )3 PF2 to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2 F5 )3 PF3 ]- ) is reported. (C2 F5 )3 PF2 reacted with trans-[Ni(iPr2 Im)2 (ArF )F] (iPr2 Im=1,3-diisopropylimidazolin-2-ylidene; ArF =C6 F5 , 1 a; 4-CF3 -C6 F4 , 1 b; 4-C6 F5 -C6 F4 , 1 c) through fluoride transfer to form the complex salts trans-[Ni(iPr2 Im)2 (solv)(ArF )]FAP (2 a-c[solv]; solv=Et2 O, CH2 Cl2 , THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh3 , solvent coordination was suppressed and the complexes trans-[Ni(iPr2 Im)2 (PPh3 )(C6 F5 )]FAP (trans-2 a[PPh3 ]) and cis-[Ni(iPr2 Im)2 (Dipp2 Im)(C6 F5 )]FAP (cis-2 a[Dipp2 Im]) (Dipp2 Im=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were isolated. Fluoride abstraction from [(Dipp2 Im)CuF] (3) in CH2 Cl2 or 1,2-difluorobenzene led to the isolation of [{(Dipp2 Im)Cu}2 ]2+ 2 FAP- (4). Subsequent reaction of 4 with PPh3 and different carbenes resulted in the complexes [(Dipp2 Im)Cu(LB)]FAP (5 a-e, LB=Lewis base). In the presence of C6 Me6 , fluoride transfer afforded [(Dipp2 Im)Cu(C6 Me6 )]FAP (5 f), which serves as a source of [(Dipp2 Im)Cu)]+ . Fluoride abstraction of [Cp2 TiF2 ] (7) resulted in the formation of dinuclear [FCp2 Ti(μ-F)TiCp2 F]FAP (8) (Cp=η5 -C5 H5 ) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.

Planar-chiral ferrocene-based triazolylidene copper complexes: synthesis, characterization, and catalysis in asymmetric borylation of α,β-unsaturated ester

1,2,3-Triazol-5-ylidenes have recently attracted considerable attention as versatile ligands because of their strong electron-donating properties and structural diversities. While some efforts have been devoted to the development of chiral triazolylidene-metal complexes, there is no example achieving asymmetric induction by base-metal complexes with triazolylidene ligands. Herein, we synthesized planar-chiral ferrocene-based triazolylidene copper complexes, which enabled the asymmetric borylation of methyl cinnamate with bis(pinacolato)diboron with good enantioselectivity.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

The synthesis and full characterization of new air-stable AgI and CuI complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag2 O afforded a collection of AgI complexes, and their first use as ligand transfer reagents led to novel isostructural CuI or AuI complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new CuI complexes. The solid-state structures feature large N-CNHC -N angles (118-128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%Vbur ) values span easily in the 50-57 % range, and that one of (9-Dipp)CuBr complex (%Vbur =57.5) overcomes to other known erNHC-metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76-93 % product at the 0.5-2.5 mol % catalyst loading, proving the stability of CuI erNHC complexes at elevated temperatures (100 °C).

Incorporation of coinage metal–NHC complexes into heptaphosphide clusters

A Me₃Si-protected P₇ cage reacts with N-heterocyclic-carbene complexes of coinage metals to yield a mononuclear Cu(i) complex featuring a Cu(η⁴-P₇) core and a trinuclear Au(i) complex with linearly coordinated metal ions attached to the P₇ cluster.

Chiral Bicyclic NHC/Cu Complexes for Catalytic Asymmetric Borylation of α,β-Unsaturated Esters

The potential of using chiral bicyclic NHC ligands that exhibit modularity was investigated in the Cu-catalyzed asymmetric borylation reaction of α,β-unsaturated esters. After screening for ligands and optimization of the reaction conditions, the corresponding products were afforded with good enantioselectivities (up to 85% ee).

Well-defined nickel(II) tetrazole-saccharinate complex as homogeneous catalyst on the reduction of aldehydes: scope and reaction mechanism

A new (tetrazole-saccharin)nickel complex is shown to be a valuable catalyst for the hydrosilative reduction of aldehydes under microwave radiation at low temperatures. With typical 1 mol% content of the catalyst (microwave power range of 5–15 W) most reactions are complete within 30 min. The Ni(II)-catalyzed reduction of aldehydes, with a useful scope, was established for the first time by using this catalyst, and is competitive with the most effective transition-metal catalysts known for such transformation. The catalyst reveals tolerance to different functional groups, is air and moisture stable, and is readily prepared in straightforward synthetic steps. Supported by experimental data and DFT calculations, a plausible reaction mechanism involving the new catalytic system is outlined.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

New anionic rhodium complexes as catalysts for the reduction of acetophenone and its derivatives

New anionic rhodium(iii) complexes, obtained by a simple reaction of RhCl3 with organic chlorides (derivatives of imidazole and pyridine), have been employed as catalysts for hydrosilylation (reduction) of acetophenone derivatives. The reactions, in which 1,1,1,3,5,5,5-heptamethyltrisiloxane was a reducing agent, proceeded in a biphasic system because the above complexes are insoluble in the reaction medium. Thereby easy isolation of the complexes from post-reaction mixtures was possible after reaction completion. This is the first example of the application of rhodium complexes of this type as catalysts for ketone reduction. The complexes have shown high activity and enabled obtaining the hydrosilylation product in a very short time and in the range of low concentrations (0.1 mol%). By using FT-IR in situ analysis that enables measuring product concentrations in real time, a comparison has been made of the catalytic activity for hydrosilylation of acetophenone and methoxyacetophenone isomers shown by four rhodium complexes ([C+][RhCl4 -]) differing in cations and the most effective catalyst for this process has been distinguished.

Ag–NHC anchored on silica: an efficient ultra-low loading catalyst for regioselective 1,2,3-triazole synthesis

A silica-supported silver complex, Ag–NHC@SiO₂, was prepared by an anchoring coordination technique, which was successfully employed for the click reaction under mild reaction conditions.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Carbonyl and ester C–O bond hydrosilylation using κ4-diimine nickel catalysts

(^Ph2PPrDI)Ni chemoselectively catalyzes α-allyl ester C–O bond hydrosilylation to prepare silyl esters with turnover frequencies of up to 990 h⁻¹.

Water-soluble transition metal complexes of ruthenium(ii), osmium(ii), rhodium(iii) and iridium(iii) with chelating N-heterocyclic carbene ligands in hydrogenation and transfer hydrogenation catalysis

The synthesis of novel Ru(ii), Os(ii), Rh(iii) and Ir(iii) mono-N-heterocyclic carbene (NHC) complexes and their application in hydrogenation catalysis is reported.

Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes

Metal carbenes are often proposed as reactive intermediates in the late transition metal-catalysed transformations of alkynes. Owing to their high reactivity, however, isolation and structural characterization of metal carbene intermediates in these transformations has remained unknown. Herein, we report the isolation of two acyclic gold and copper carbene intermediates in either Au(I)- or Cu(I)-catalysed cyclization of N-alkynyl formamidines through five-exo-dig cyclization. X-ray diffraction, ¹³C NMR spectra data and computational analyses provide evidence for the formation of a gold carbene intermediate with a carbocation-like electronic character. Using the intrinsic bond orbital (IBO) approach, we also evaluate the π-stabilizing effects of organic substituents at the carbene carbon atom in the gold carbene intermediate. Another rare six-membered copper carbene complex is also obtained through 6-endo-dig cyclization. These metal carbenes have proven reactive toward oxidation. The metal-promoted cyclization of N-alkynyl formamidine provides a facile approach to synthesize metal carbene species.

Metal carbenes are typically highly reactive and unstable species, proposed as intermediates in numerous catalytic processes. Here the authors report the isolation and characterization of gold and copper carbene intermediates involved in catalytic cyclization reactions.

Syntheses of transition metal methoxysiloxides

The paper describes three methods for the preparation of methoxysiloxide complexes, a rare class of complexes of relevance to room temperature vulcanization (RTV) of polysiloxanes.

Cross C–S coupling reaction catalyzed by copper(i) N-heterocyclic carbene complexes

Copper(i) N-heterocyclic carbenes catalyze the cross C–S coupling reaction with good activities towards aryl halides. A plausible catalytic cycle is proposed, with the support of DFT calculation.