N-Heterocyclic Carbenes Catalyzed Phospho-Aldol Reaction of Aldehydes

An efficient phospho-aldol reaction of aldehydes catalyzed by N-heterocyclic carbenes （NHCs） has been developed. With 10 mol% stable NHC 1,3-bis（2,6-diisopropylphenyl）imidazol-2-ylidene, various aldehydes reacted with dialkylphosphites smoothly to provide a-hydroxy phosphonates in 59%--99% yield. In this process, NHC was assumed to function as a carbon-centered bronsted base.

An Ultralong Low-Temperature Phosphorescent Pentagonal-Prismatic Organometallic Cylinder Featuring Pentaphenylpyrrole-N-Heterocyclic Carbenes

A pentagonal-prismatic cylinder[Ag5(L)2](PF6)5 obtained from the pentaphenylpyrrole-bridged pentaimidazolium salt H5-L(PF6)5 and AgI ions was determined by X-ray diffraction analyses.The target organometallic cylinder not only exhibited enhanced fluorescence emission in dilute solution at room temperature but also showed an improved phosphorescence ratio compared with the free precursor and maintained a long lifetime(1.39 s)in the solid state at 77 K.Furthermore,the experimental results and DFT calculations confirmed that the formation of the organometallic cylinder promoted intersystem crossing.Meanwhile,the frontier orbitals of[Ag5(L)2](PF6)5 showed the main contribution of building block PPP as the luminescence source of[Ag5(L)2](PF6)5 by a modest heavy-atom effect.These results provide a strategy for constructing enhanced phosphorescent emission and long lifetime organometallic supramolecular phosphorescent materials.

Stereoselective Ring-opening Polymerization of rac-Lactide by Bulky Chiral and Achiral N-heterocyclic Carbenes

Despite the extraordinary success has been achieved in metal catalyst-promoted stereoselective ring-opening polymerization（ROP） of rac-lactide（rac-LA）, well-controlled stereoselective rac-LA ROP by organic catalyst still remains a scientific challenge. Here we report our investigations into organocatalytic stereoselective ROP of rac-LA by utilizing novel bulky chiral and achiral N-heterocyclic carbenes（NHC）, 1,3-bis-（1′-naphthylethyl）imidazolin-2-ylidene. The effect of polymerization conditions（e.g. solvent, temperature, alcohol initiator） on ROP behavior by these bulky NHCs has been fully studied, leading to the formation of isotactic-rich stereoblock polylactide（Pi = 0.81） under optimized conditions with high activity（Conv. = 98% in 30 min） and narrow molecular weight dispersity（D = 1.05）.

Superatomic Aub clusters ligated by different N-heterocyclic carbenes and their ligand-dependent catalysis, photoluminescence, and proton sensitivity

We report herein a class of superatomic AU13 clusters stabilized by different N-heterocyclic carbenes(NHCs).The clusters show diverse metal surface structures,properties and functions as exemplified by:(1)the first anionic AU13 cluster[Au1g(NHC-1)sBr6],which has bulky NHC-1 ligands that lead to a rather open metal surface contributing to its high catalytic activity;(2)the tricationic cluster[Au13(NHC-2)sBr2]+which has bidentate,benzyl-rich NHC-2 ligands that make it ultra-stable and highly-luminescent,suitable for bio-imaging;and(3)by bearing two pyridyl groups on NHC-3,the dicationic cluster[Au:a(NHC-3)gClg]f+exhibits reversible and stable visible absorption and solubility responses to protonation/deprotonation cycles,making it a potential pH sensor(NHC-1=1,3-disopropylbenzimidazolin-ylidene;NHC-2=1,3-bis(1-benzyl-1H-benzimidazol-1-ium-yl)propane;NHC-3=1,3-bis(icolyI)benzimidazolin-ylidene).The study nicely demonstrates the importance of ligands in designing metal nanoclusters with desired functionalities.

N-Heterocyclic Carbenes with a Nido-C_(2)B_(9) Carborane Backbone

The steric and electronic properties of A/-heterocyclic carbenes(NHCs)can be modified by the exocyclic substituents at the nitrogen atoms,by 1-3 atoms'replacements of the five-membered imidazolium skeleton and by the changes of the backbones.Herein,we report the usage of nido-C_(2)B_(3) carbora ne anions as the backb ones of NHCs.Stirring the mixture of sec on dary amino o-carbora nes(lb-le),triethyl orthoformate and HBF4-Et2O results in the unexpected cage-opening of o-carboranes to afford the n/do-C_(2)B_(9) carbora ne anions supported A/-heterocyclic carbene precursors(4b-4e).Deprotonation of 4b with sodium hexamethyldisilazide at-78℃ affords the/V-heterocyclic carbene ligand,which was used to form an Au(I)NHC complex(5).DFT calculations revealed a high-lying lone pair orbitals of the carbene ligands,predicting their strong o-donating abilities.

Enantioselective benzoin condensation catalyzed by bifunctional N-heterocyclic carbenes

Benzoin condensation is an important carbon-carbon bond-forming reaction. The benzoin reaction with good yield, high enantioselectivity and broad substrate scope is highly desired in organic synthesis. The intermolecular benzoin condensation catalyzed by bifunctional N-heterocyclic carbenes has been investigated, and the corresponding α-hydroxyketones were obtained with yields up to 76% and enantioselectivities up to 99% ee.

N-heterocyclic carbene as a promising metal-free electrocatalyst with high efficiency for nitrogen reduction to ammonia

Electrocatalytic nitrogen reduction reaction(NRR)at ambient conditions holds great promise for sustainably synthesizing ammonia(NH3),while developing highly-efficient,long-term stable,and inexpensive catalysts to activate the inert N≡N bond is a key scientific issue.In this work,on the basis of the concept"N-heterocyclic carbenes(NHCs)",we propose a carbon decorated graphitic-carbon nitride(C/g-C3N4)as novel metal-free NRR electrocatalyst by means of density functional theory(DFT)computations.Our results reveal that the introduced C atom in g-C3N4 surface can be regarded as NHCs and catalytic sites for activating N≡N bond,and are stabilized by the g-C3N4 substrate due to sterically disfavored dimerization.Especially,this NHCs-based heterogeneous catalysis can efficiently reduce the activated N2 molecule to NH3 with a low overpotential of 0.05 V via an enzymatic mechanism.Our work is the first report of NHCs-based electrocatalyst for N2 fixation,thus opening an alternative avenue for advancing sustainable NH3 production.

Synthesis, Crystal Structure and Catalytic Behavior of 1-Ethyl-3-benyl-imidazolyl Tetranuclear N-Heterocyclic Carbene Silver Bromide

The title complex [Ag（carbene）2]2[Ag2Br4] has been synthesized by the reaction of Ag2O with 1-ethyl-3-benyl-imidazolium bromide in DMSO at room temperature, and characterized by elemental analysis, 1H NMR and single-crystal X-ray diffraction analysis. It crystallizes in triclinic, space group P with a = 10.1597（10）, b =11.0646（11）, c = 13.0245（14） , α = 102.230（2）, β = 90.606, γ = 113.9250（10）o, V = 1300.3（2） 3, Mr = 748.06, Z = 2, Dc = 1.911 g/cm3, μ（MoKα） = 4.60 mm-1 and F（000） = 728. The structure was refined to R = 0.0316 and wR = 0.0835 for 3744 observed reflections with I 〉 2σ（I）. The title compound crystallizes as a centrosymmetric tetranuclear compound. One half of the molecule comprises the asymmetric unit of the structure. The Ag（1） atom is nearly linear or T-shaped when the Ag（1）-Ag（2） interaction is taken into consideration, which is bi-coordinated by two carbene carbon atoms. The Ag（2） atom adopts tetrahedral geometry. The catalytic behavior of the title complex has been investigated, and the results indicate it has a highly catalytic activation for L-lactide polymerization.

Synthesis and Structural Study of a N-Heterocyclic Carbene Trinuclear Silver(Ⅰ)Complex

The bis-benzimidazolium salt bis[2-（N-ethylbenzimidazoliumyl）ethyl]amine hexafluorophosphate（LH2·（PF6）2） and its N-heterocyclic carbene silver（Ⅰ） complex[L2Ag3]（PF6）3（1）have been prepared and characterized.In complex 1,a trinuclear silver（Ⅰ） architecture is formed by two tridentate ligands and three silver（I） atoms,in which one 12- and two 7-membered rings are contained.In the crystal packing of 1,1D chain and 2D supramolecular layer are formed via intermolecular weak interactions,including π…π interactions and C-H…π contacts.Additionally,the fluorescence emission spectra of LH2-（PF6）2 and 1 are described.

Transformations of biomass-based levulinate ester into γ-valerolactone and pyrrolidones using carbon nanotubes-grafted N-heterocyclic carbene ruthenium complexes

As a renewable biomass-based compound with wide applications in food additives,fine chemical synthesis and fuels,γ-valerolactone(GVL)has attached much attention.While,pyrrolidones are widely used in pharmaceutical,agrochemical,material industrial and other chemical production.In this research,we demonstrated transformations of biomass-based ethyl levulinate(EL)into GVL and pyrrolidones by using heterogeneous catalysts(CNT-Ru-1)with N-heterocyclic carbene ruthenium(NHC-Ru)complex grafted on multi-walled carbon nanotube(CNT).The Ru catalyst showed high efficiency on EL hydrogenation to GVL with both EL conversion and GVL yield exceeding 99%.Moreover,the Ru catalyst readily promoted reductive amination of EL in the presence of various amines for pyrrolidone synthesis.Finally,the Ru catalyst was also applicable to hydrogenation of various carbonyl compounds for the synthesis of the corresponding alcohols with excellent catalytic performance.The research provides insight for heterogenizing the homogeneous noble metal-based catalysts with high catalytic active for biomass-based transformations.

Co(Ⅲ)/Zn(Ⅱ)-catalyzed dearomatization of indoles and coupling with carbenes from ene-yne ketones via intramolecular cyclopropanation

A straightforward and efficient protocol for dearomatizing indoles is described.The reaction,catalyzed by an inexpensive Co(III)/Zn(II)catalyst,starts from easily accessible N‐pyrimidinyl indoles and ene‐yne ketones.Mild reaction conditions,high diastereoselectivity,a broad substrate scope,effective functional group tolerance,and reasonable to remarkable yields were observed.

NEW HETEROCYCLIC SYSTEM SYNTHESIS I REACTION OF DIHYDRO-1,5-BENZO-DIAZEPINES AND THIAZEPINES WITH (ETHOXYCARBONYL)CARBENES

Dihydro-1-benzoyl-1,5-benzodiazepines react with (ethoxycarbonyl)carbenes to give 4H-azirino-[1,2-a][1,5]-benzodiazepines and unexpected new tin9 system 1H-pyrrolo-[1,2-a][1,5]-benzodiazepines. Dihydro-1,5-benzothiazepines react with (ethoxycarbonyl)carbenes in cyclohexane to gire unexpected rin9 cleavage products ethyl (2E, 4E)-3-aryl-2-arylthiohexadienoates.

SOMO-HOMO level conversion in triarylmethyl-cored N-heterocyclic carbene-Au(I)complexes triggered by selecting coordination halogens

Conventionally,organic radicals adhere to the Aufbau principle,the energy level of the singly occupied molecular orbital(SOMO)is not below the highest occupied molecular orbital(HOMO),but somewhat abnormal phenomena have appeared recently.In this study,we introduce a novel strategy by incorporating unique NHC-Au-X units into a tris(2,4,6-trichlorophenyl)methyl(TTM)system to create metal-involved open-shell complexes,denoted as TTM-NHC-Au-X(X=I,Br,or Cl).Density-functional theory calculations were used to predict an inversion in the energy of the SOMO and highest doubly occupied molecular orbital(HOMO)of TTM-NHC-Au-I,which is supported by experimental results.Organometallic radicals TTM-NHC-Au-X demonstrated distinct properties with different coordinated halides.The radical behaviors have been investigated by EPR,UV-vis spectroscopy and cyclic voltammetry,additional structural information provided by structurally comparing related the precursor complexes given by X-ray crystallography.TTM-NHC-Au-I with SOMO-HOMO conversion(SHC)features a highly thermal decomposition temperature up to 305℃.Furthermore,the photostability of TTM-NHC-Au-I was found to be 75and 23 times greater than that of TTM-NHC-Au-Br and TTM-NHC-Au-Cl,respectively.These findings provide valuable insights into the structural and electronic design principles governing the occurrence of SOMO-HOMO conversion in open-shell systems.

N-Heterocyclic carbene-catalyzed enantioselective(dynamic) kinetic resolutions and desymmetrizations

N-heterocyclic carbene-catalyzed enantioselective kinetic resolutions,dynamic kinetic resolutions,and desymmetrization reactions are systematically reviewed.The content is organized according to the activation modes involved in these transformations.Future advances within this highly active research field are discussed from our perspectives on the topic.

Ab initio Study of Mechanism of Forming Germanic Bis-Heterocyclic Compound Between Dichloro-Germylene Carbene （Cl2Ge=C：） and Formaldehyde

The mechanism of the cycloaddition reaction of forming germanic bis-heterocyclic compound between singlet dichloro-germylene carbene and formaldehyde has been investigated with CCSD（T）//MP2/6-31G^＊ method, from the potential energy profile, we predict that the reaction has two competitive dominant reaction pathways. The presented rule of this reaction： the 2p unoccupied orbital of the C atom in dichloro-germylene carbene insert the π orbital of formaldehyde from oxygen side, resulting in the formation of intermediate. In the intermediate and between two reactants, because of the two bonding π orbital in dichloro-germylene carbene and formaldehyde have occurred [2＋2] cycloaddition reaction, forming two four- membered ring compounds in which Ge and O are in the opposite orientation and in the syn-position, respectively. Because of the unsaturated property of C atom from carbene in the two four-membered ring compounds, they further reacts with formaldehyde, resulting in the generation of two germanic bis-heterocyclic compounds.

DFT Study of Mechanism of Extraction Reaction Between Germylene Carbene （H2Ge=C：） and Its Derivatives and Ethylene Oxide

The mechanism of the oxide extraction reaction between singlet germylene carbene and its derivatives X2Ge=C： （X=H, F, Cl, CH3） and ethylene oxide has been investigated with B3LYP/6-311G（d,p） method. The results show that this kind of reaction has similar mechanism, the shift of 2p lone electron pair of O in ethylene oxide to the 2p unoccupied orbital of C in X2Ge=C： gives a p→p donor-acceptor bond, thereby leading to the formation of intermediate. As the p→p donor-acceptor bond continues to strengthen, that is the C-O bond continues to shorten, the intermediate generates product （P＋C2H4） via transition state. It is the substituent electronegativity that mainly affect the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater.

Density Functional Theory Study of Mechanism of Cycloaddition Reaction Between Dimethyl-Silylene Carbene and Acetone

The mechanism of the cycloaddition reaction between singlet dimethyl-silylene carbene and acetone has been investigated with density functional theory, From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. The presented rule of this reaction： the [2＋2] cycloaddition effect between the πorbital of dimethyl-silylene carbene and the π orbital of π-bonded compounds leads to the formation of a twisty four-membered ring intermediate and a planar four-membered ring product; The unsaturated property of C atom from carbene in the planar four-membered ring product,resulting in the generation of CH3-transfer product and silicic bis-heterocyclic compound.

Enantioselective Alkylation of Aldehydes with Organoborons Enabled by Nickel/N-Heterocyclic Carbene Catalysis

Transition-metal-catalyzed asymmetric alkylation of aldehydes represents a straightforward strategy for the synthesis of chiral secondary alcohols.However,efficient methods using organoborons as coupling reagents are rare.Herein,we report a highly enantioselective nickel-catalyzed alkylation reaction of aldehydes,using readily available alkylborons as nucleophiles.A wide variety of chiral secondary alcohols were prepared from commercially available aldehydes with high yields.The key to the excellent enantioselectivity and chemoselectivity was the employment of a bulky C2-symmetric chiral NHC ligand.This protocol features excellent enantiocontrol,mild conditions,and good functional group compatibility.

Recent advances in the synthesis and application of N-heterocyclic carbene-based molecular cages

N-Heterocyclic carbene(NHC)-based cages have emerged as a prominent and dynamic research area within the research field of chemistry. Leveraging the distinctive electronic and steric properties of NHC ligands, the design, synthesis, and application of these corresponding cages have garnered substantial scholarly interest. In recent years, we have witnessed the successful fabrication of diverse NHC-based cages through a range of synthetic methodologies, which hold significant potential for applications in molecular recognition and catalysis. In this review, we delve into the foundational synthetic strategies that underlie the creation of NHC-based cages, employing approaches encompassing metal–NHC chelation, coordination assembly,and covalent bonding. Additionally, we compile the diverse applications of these cages within catalytic processes and molecular recognition. Lastly, we shed light on the current limitations of synthesis and outline future trends in the development of NHCbased cages.

Chemoselective Transfer Hydrogenation of Nitroarenes with Ammonia Borane Catalyzed by Copper N-Heterocyclic Carbene Complexes

Herein,we present a method for the homogeneous hydrogenation of nitroarenes to produce anilines using low catalyst loading(1 mo%)of copper N-heterocyclic carbene complexes as the catalyst and ammonia borane as the source of hydrogen.A wide range of nitroarenes,featuring diverse functional groups,were selectively transformed into their corresponding primary aromatic amines with high yields.This process can be readily scaled up and exhibits compatibility with various sensitive functional groups,including halogen,trifluoromethyl,aminomethyl,alkenyl,cyano,ester,amide,and hydroxyl.Notably,this catalytic methodology finds application in the synthesis of essential drug compounds.Mechanistic investigations suggest that the in-situ-generated Cu-H species may serve as active intermediates,with reduction pathways involving species such as azobenzene,1,2-diphenylhydrazine,nitrosobenzene,and N-phenylhydroxylamine.