Presence Of N-heterocyclic Carbene Research Articles

Promoted Thermal Reduction of Copper Oxide Surfaces by N-Heterocyclic Carbenes

The influence of metallic and oxide phases coexisting on surfaces is of fundamental importance in heterogeneous catalysis. Many reactions lead to the reduction of the oxidized areas, but the elucidation of the mechanisms driving these processes is often challenging. In addition, intermediate species or designed organic ligands increase the complexity of the surface. In the present study, we address the thermal reduction of a copper oxide overlayer grown on Cu(111) in the presence of N-heterocyclic carbene (NHC) ligands by means of scanning tunneling microscopy (STM) and density functional theory (DFT). We show that the NHC ligands actively participate in the copper oxide reduction, promoting its removal at temperatures as low as 470 K. The reduction of the oxide was tracked by employing scanning tunneling spectroscopy (STS), providing a chemical identification of metallic and oxide areas at the nanometric scale.

Access to Sulfoxides under NHC/Photocatalysis via a Radical Pathway.

A photocatalyzed transformation from sulfinic acids to sulfoxides under visible-light irradiation in the presence of N-heterocyclic carbene is established. Various alkyl groups from four-substituted Hantzsch esters or Meyer nitriles are smoothly converted to the corresponding sulfoxides through a radical coupling pathway in the presence of 1,1-carbonyldiimidazole. This method allows sulfoxide synthesis to refrain from relying on the oxidation of sulfides and provides an alternative route for the preparation of sulfoxides.

N-heterocyclic Carbene Containing Homogeneous Ru Catalyst for Aqueous Atom Transfer Radical Polymerization of Water-soluble Vinyl Monomers

This report presents aqueous atom transfer radical polymerization (ATRP) of commonly used acrylate monomers in the presence of N-Heterocyclic carbene (NHC) containing homogeneous Ru catalyst in neat water. The newly synthesized Ru catalyst includes NHC, which possesses poly(ethylene glycol) (PEG) as a water-soluble component and benzylidene. The water-soluble Ru catalyst enables the polymerization of acrylamide, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), and PEG methyl ether methacrylate. Polymerization with this new catalyst displayed typical ATRP behavior such as a low PDI, a direct linear relationship between molecular weight and conversion, and first-order kinetics. The polySBMA (Mn 335.5 kDa and PDI 1.66) was prepared with 91% conversion at 6 h of reaction time with 0.05 mol% catalyst loading in neat water ([catalyst]:[monomer] = 1:2000 at 80 °C). Without the Ru catalyst, the thermally initiated polymerization displayed uncontrolled polymerization (e.g., non-linear kinetics, high PDI, and too fast or slow polymerizations). The Ru catalyst also demonstrated ATRP of the PEG methyl ether methacrylate in an organic solvent, acetonitrile. More detailed studies of initiator amount, solvent concentration, and catalyst amount effects were performed. In turn, the new ATRP will add prominent value to the polymer science field as an initial example of Ru-benzylidene catalyzed ATRP in neat water. Additionally, this will promote various applications of environmentally benign polymerization as well as applications in the biomedical field with high-cost efficiency.

A computational study on NHC-Catalyzed [3+4] annulation between isatin-derived enal and aurone-derived azadiene: Insights into mechanism and stereoselectivity

Using density functional theory (DFT), a systematic computation has been conducted on the reaction between an enal and an azadiene in the presence of N-heterocyclic carbene (NHC) to explore the catalytic mechanisms and origins of stereoselectivity. Several possible pathways leading to benzofuroazepinone have been investigated. We found that the preferred pathway proceeds via six steps, i.e. (1) the nucleohilic addition wherein the NHC adds to the enal, (2) 1,2- proton transfer allows to form the Breslow intermediate, (3) the carbon−carbon bond formation, (4) 1,3- proton transfer, (5) intramolecular cyclization and (6) catalyst regeneration that eventually leads to the RR-configurational cycloadduct benzofuroazepinone. The DFT results agree well with the experimentally observed stereoselectivity. We also found that HOAc plays a vital role as proton shuttle in proton transfer steps. The 1,3-proton transfer was the stereoselectivity-determining step. Furthermore, we conducted global reactivity index (GRI), distortion/interaction and noncovalent interaction (NCI) analyses to identify the role of the NHC in the reaction as well as the origin of the experimentally observed stereochemistry.

NHC-copper-catalyzed asymmetric conjugate silylation of access chiral α-aminosilanes

Chiral α-aminosilane and its derivatives have found potential applications in pharmaceuticals. Yet there are rare examples have been reported for the synthesis of these molecules. Herein we report a catalytic asymmetric conjugate silylation reactions of β-amido-acrylonitriles and β-amido-acrylates for the first time in the presence of catalytic amount of chiral N-heterocyclic carbene (NHC)/Copper(I) complex. A variety of functionalized chiral α-aminosilanes were obtained at room temperature in good yields with high enantioselectivities in the presence of NHC (10 mol%), CuCl (10 mol%), NaOtBu or NaOMe (20 mol%), and MeOH (2 equivalents).

Stabilization of Nanoparticles Produced by Hydrogenation of Palladium-N-Heterocyclic Carbene Complexes on the Surface of Graphene and Implications in Catalysis.

Palladium nanoparticles (NPs) have been obtained by decomposition of well-defined palladium complexes noncovalently anchored onto the surface of reduced graphene oxide. Morphological analysis by microscopy showed the presence of small palladium NPs homogeneously distributed on the support. Characterization by X-ray photoelectron spectroscopy confirmed that palladium NPs contain Pd(2+) and Pd(0) oxidation states and the presence of N-heterocyclic carbene and bromo ligands. The catalytic properties of the NPs with and without the support have been evaluated in the hydrogenation of alkynes. Supported palladium NPs showed increased activity versus the nonsupported ones and could be recycled up to 10 times without the loss of catalytic activity. The composition of the palladium NPs is different for each catalytic cycle indicating a dynamic process and the formation of different catalytic active species. On the contrary, the unsupported palladium NPs showed limited activity caused by decomposition and could not be recycled. The role of the support has been investigated. The results indicate that the support influences the stability of palladium NPs.

Mechanistic Insights into the Nickel-Catalyzed Cross-Coupling Reaction of Benzaldehyde with Benzyl Alcohol via C–H Activation: A Theoretical Investigation

With the aid of density functional theory (DFT) calculations, mechanistic investigations have been carried out for the nickel-catalyzed dehydrogenative cross-coupling reaction of benzaldehyde with benzyl alcohol in the presence of N-heterocyclic carbene (NHC) ligand. The overall Ni(0)/Ni(II) catalytic cycle consists of four basic steps: ligand exchange, oxidative addition, hydrogen transfer, and reductive elimination. Considerable interests are paid on detecting the transition state of the rate-determining step, with particular emphasis on the structural and electronic properties, together with clarifying the important roles of external oxidant and hydrogen acceptor. The hydrogen transfer process in the oxidative addition step is rate-determining in the whole catalytic cycle, which is accomplished by C-Ha (active Ha) activation without generating the high energy nickel hydride intermediate. Such process could be understood as the direct hydrogen transfer, instead of general concerted oxidative addition to low valent transition metal. The analysis of the bond distances, electrondistributions, and orbital interactions highlights the direct hydrogen transfer mechanism. Furthermore, by exploring the influences from the electronic effect of different substrates on the reaction energy barriers, the a,a,a-trifluoroacetophenone could accelerate the direct hydrogen transfer with low activate energy.

N-Heterocyclic Carbene Promoted Decarboxylation of Lignin-Derived Aromatic Acids

Decarboxylation is an important reaction in organic synthesis and drug discovery, which is typically catalyzed by strong bases or metal-based catalysts bearing low yield and selectivity. For the first time, we demonstrated a new strategy of decarboxylation of hydroxyl cinnamic acids such as p-coumaric acid, ferulic acid, sinapinic acid, and caffeic acid in the presence of N-heterocyclic carbene (NHC) precursors (i.e., 1-ethyl-3-methyl imidazolium acetate [C2C1Im][OAc]), achieving high yields and selectivities up to 100% under relatively mild conditions. [C2C1Im][OAc] showed excellent recyclability as organocatalysis during three times of recycling using biphasic reaction system. A mechanistic study revealed that the decarboxylation was catalyzed by NHCs that were in-situ generated by self-deprotonation of [C2C1Im][OAc]. Our demonstrated route is especially appealing for the production of lignin-derived renewable aromatics.

Synthesis of 4-Difluoromethylquinolines by NHC-Catalyzed Umpolung of Imines.

The N-heterocyclic carbene (NHC)-catalyzed umpolung of aldimines for the synthesis of 4-difluoromethylquinoline derivatives is reported. In the presence of NHCs, the intramolecular cyclization of aldimines bearing a moderately electron-poor double bond due to the presence of the -CF3 group likely proceeds via the intermediacy of the aza-Breslow intermediate. The key to the success of this aza-Stetter type transformation is the NHC generated from the bicyclic triazolium salt using DBU as the base.

Computational Study on γ-C-H Functionalization of α,β-Unsaturated Ester Catalyzed by N-Heterocyclic Carbene: Mechanisms, Origin of Stereoselectivity, and Role of Catalyst.

The N-heterocyclic carbene (NHC)-catalyzed γ-C-H deprotonation/functionalization of α,β-unsaturated esters with hydrazones leading to the δ-lactams has been theoretically investigated by using density functional theory. Three possible reaction mechanisms including Mechanism A, for which the NHC catalyst serves as a nucleophilic catalyst to attack on the carbonyl carbon atom to initiate the reaction, Mechanism B, in which NHC triggers the reaction through the hydrogen bond, and Mechanism C, which is the direct deprotonation/functionalization process without the presence of NHC, have been suggested and studied in detail. The most favorable Mechanism A was identified to proceed through the following processes: nucleophilic attack on the carbonyl carbon of the ester by NHC, γ-deprotonation, formal [4 + 2] cycloaddition of dienolate with hydrazone, and regeneration of NHC. Multiple possible deprotonation pathways were explored, and the additive base such as K2CO3 would significantly lower the energy barrier. The formal [4 + 2] cycloaddition step is the stereoselectivity-determining step, and R-configured rather than S-configured product was preferentially generated. In addition, the C-H···O, C-H···N, LP···π, and C-H···π interactions have been identified in the most energetically favorable transition state involved in the stereoselectivity-determining step. The additional analysis indicates that NHC strengthens the acidity and electrophilicity to promote the deprotonation, indicating this is not a simple NHC-catalyzed umpolung carbonyl reaction. The mechanistic insights and the significant role of NHC obtained in this study should provide valuable insights for understanding the organocatalytic γ-C(sp3)-H bond functionalization reaction.

Sequential Addition of Phosphine to Alkynes for the Selective Synthesis of 1,2-Diphosphinoethanes under Catalysis. Well-Defined NHC-Copper Phosphides vs in Situ CuCl2/NHC Catalyst

The well-defined NHC-copper phosphides [(NHC)CuPPh2]3 (1, NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr); 2, NHC = N,N-di-tert-butylimidazol-2-ylidene (ItBu)) have been prepared by the reaction of simple copper halides with HPPh2 in the presence of N-heterocyclic carbenes (NHCs). Complexes 1 and 2 enabled catalytic double hydrophosphination of alkyl and aryl terminal alkynes to yield 1,2-diphosphinoethanes selectively in good yields. On the basis of these results, the most efficient and pratical in situ CuCl2/NHC catalyst has been developed. It catalyzes the selective double hydrophosphination of the alkynes with high efficiency and a wide substrate scope and exhibits even better performance than the well-defined NHC-Cu phosphides. The mechanistic studies disclosed that the formation of a copper acetylide in the catalytic cycle played an important role in the acceleration of the catalytic process.

Studies of the Ligand Effect on the Synthesis of Dialuminoxanes by Various β-Diketiminato Ligands

Reactions of LH (L = HC[C(Me)N(2,6-Me(2)C(6)H(3))](2)) with Me(n)AlCl(3-n) in diethyl ether afforded the adducts LH·AlMe(n)(Cl)(3-n) (n = 2, 3; 1, 4; 0, 5) in good yields. Treatment of 3 at elevated temperatures in toluene resulted in LAlMeCl (2) by intramolecular elimination of methane. The controlled hydrolysis of LAlMeCl (2) with equimolar amounts of water in the presence of N-heterocyclic carbene (NHC) gave a mixture of [LAl(Me)](2)(μ-O) (7) and dimeric [LAlMe(μ-OH)](2) (8). A convenient route for the preparation of [LAlMe(μ-OH)](2) (8) was the NHC-assisted controlled hydrolysis of LAlMeI (9). Stepwise hydrolysis of LAlH(2) (11) gave dialuminoxane hydride [LAl(H)](2)(μ-O) (12) and dialuminoxane hydroxide [LAl(OH)](2)(μ-O) (13), respectively. Anhydrous treatment of LAlCl(2) (1) or LAlMeCl (2) with Ag(2)O afforded chlorinated dialuminoxane [LAl(Cl)](2)(μ-O) (14) and [LAl(Me)](2)(μ-O) (7), respectively.

N-Heterocyclic Carbene-Catalyzed Redox Reactions of α-Functionalized Aldehydes

This review summarizes the recent advances on the redox reactions involving α-functionalized aldehydes catalyzed by N-heterocyclic carbenes. In the presence of N-heterocyclic carbene, α-functionalized aldehydes such as α-haloaldehydes, α-ring-fused aldehydes, α-aryloxyacetaldehydes and α-aroyloxyaldehydes undergo the redox reactions affording novel reaction types.

Facile Synthesis of (Imido)vanadium(V)−Alkyl, Alkylidene Complexes Containing an N-Heterocyclic Carbene Ligand from Their Trialkyl Analogues

(Imido)vanadium(V)−alkyl, alkylidene complexes containing an N-heterocyclic carbene (NHC) ligand, V(CHSiMe3)(NR)(CH2SiMe3)(NHC) [R = 1-adamantyl (Ad) or 2,6-Me2C6H3 (Ar), NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], have been prepared from the trialkyl analogues, V(NR)(CH2SiMe3)3, by α-hydrogen elimination in C6D6 in the presence of NHC. The V−C(NHC) bond distance [2.172(2) A] in V(CHSiMe3)(NAd)(CH2SiMe3)(NHC) is much longer than the V−C(alkylidene) [1.829 A] and the V−C(alkyl) [2.069 A] distances, indicating that NHC plays an important role in the stabilization of the high-oxidation-state vanadium(V)−alkylidene as a neutral carbene ligand.

Ti(NMe 2) 4-catalyzed Markovnikov hydroamination of alkynes in the presence of N-heterocyclic carbenes and LiN(SiMe 3) 2

Intermolecular hydroamination of alkynes catalyzed by Ti(NMe 2) 4 was much improved with N-heterocyclic carbenes and LiN(SiMe 3) 2, by which high Markovnikov selectivity was attained for the coupling of nearly all alkynes and amines.

Construction of Tetrasubstituted Carbon by an Organocatalyst: Cyanation Reaction of Ketones and Ketimines Catalyzed by a Nucleophilic N-Heterocyclic Carbene

A method for cyanation reaction of ketones and ketimines having lower reactivity than aldehydes and aldimines with TMSCN in the presence of N-heterocyclic carbene prepared from 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and potassium tert-butoxide, as a nucleophilic organocatalyst, is described. These cyanations of ketones and ketimines afford the corresponding products in good yields under mild reaction conditions.

Strecker Reaction of Aldimines Catalyzed by a Nucleophilic N-Heterocyclic Carbene

The first method for Strecker reaction (cyanation of imines) of aldimines with TMSCN in the presence of N-heterocyclic carbene prepared from 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and t-BuOK, as a nucleophilic catalyst, is described.

Synthesis, Characterization, and X-ray Crystal Structure of a Gallium Monohydroxide and a Hetero-bimetallic Gallium Zirconium Oxide

A monomeric hydroxide of gallium, LGa(Me)OH, containing terminal hydroxide and methyl groups was prepared by the hydrolysis of LGa(Me)Cl in the presence of N-heterocyclic carbene and water [L = HC{(CMe)(2,6-i-Pr2C6H3N)}2] in high yield and in a pure form. LGa(Me)OH was used as a synthon to assemble the first hetero-bimetallic compound with a Ga-O-Zr core, [(LGaMe)(Cp2ZrMe)](mu-O).

Alkyl-Alkyl Negishi Cross-Coupling Utilizing a Pd-NHC Catalyst

This work is the first example of Pd-catalyzed sp3-sp3 cross-coupling reaction in the presence of N-heterocyclic carbenes as ligands. The process is performed at room temperature and highly tolerant to functional groups like ester, nitrile and phthalimide.

N-Heterocyclic Carbene Palladium Complexes Bearing Carboxylate Ligands and Their Catalytic Activity in the Hydroarylation of Alkynes

The synthesis and reactivity of palladium acetate/trifluoroacetate complexes stabilized by the presence of N-heterocyclic carbene (NHC) ligands are described. The structures and coordination characteristics of both (IPr)Pd(OAc)2 (1) and (IPr)Pd(OOCCF3)2(H2O) (2) were elucidated by spectroscopic and X-ray diffraction studies. The structure of 1 shows a novel coordination of the anions in a monomeric complex, with one acetate anion acting as a monodentate ligand while the second one coordinates through both oxygens. The NHC ligands in 1 and 2 are stable under acidic conditions. The complexes were used as precatalysts in the hydroarylation of alkynes. Using this simple protocol, a number of arenes react with various alkynes to produce stilbenes.