Nickel N-heterocyclic Carbene Research Articles

Mass spectrometric support for a bifunctional catalysis mechanism for the base-free Michael addition by a nickel N-heterocyclic carbene complex: Detection of the catalytic intermediates

A bifunctional catalysis mechanism for an asymmetric base-free Michael addition reaction mediated by a chiral nickel N-heterocyclic carbene complex has been corroborated by mass spectrometry. In particular, for the first time, all of the intermediates for a proposed catalytic cycle for an asymmetric base-free Michael addition of α-methyl cyano ester and cyclic β-keto ester substrates with various activated olefins as catalyzed by a chiral nickel N-heterocyclic carbene complex namely, {[(1-R1-3-R2-acetamido)imidazol-2-ylidene][(1-R1-3-R2-acetamidato)imidazol-2-ylidene]}NiOH [R1=(1S,2S,5R)-menthyl, R2=2,6-i-Pr2C6H3 (3)] have been detected by Electrospray Ionization Mass Spectrometry (ESI-MS).

C-S cross-coupling of aryl halides with alkyl thiols catalyzed by in-situ generated nickel(II) N-heterocyclic carbene complexes

The C-S cross-coupling of aryl halides with alkyl thiols catalyzed by in-situ generated Ni (II) N-heterocyclic carbene (NHC) complexes is investigated. Good to excellent yields can be obtained for a variety of aryl halides when using 5mol% of the Ni (II)-NHC catalyst and 1.5eq. of KOtBu. Both the electronic and steric effects of the NHC ligands on the catalytic performance of Ni (II)-NHC, as well as the electronic effects of aryl halides on coupling reactivity are examined. The mechanism for Ni (II)-NHC catalyzed coupling reactions is also discussed.

Silver(I), nickel(II) N-heterocyclic carbene complexes based on bidentate bis-imidazolium salt with a quinoxaline linker: syntheses, structures, and characterization

Quinoxaline-bridged bidentate bis-imidazolium dicarbene ligand 1,1′-(quinoxaline-2,3-diyl)bis(3-methyl-1H-imidazol-3-ium) hexafluorophosphate salt H2L·2PF6 (3) was prepared by a two-step reaction based on 2,3-bis(imidazol-1-yl)quinoxaline (1). First, the 2,3-bis(imidazol-1-yl)quinoxaline reacted with CH3I resulting in the 1,1′-(quinoxaline-2,3-diyl)bis(3-methyl-1H-imidazol-3-ium) iodide salt H2L·2I (2), then through anion exchange reactions with NH4PF6 in water produced the desired bis-imidazolium bidentate ligand H2L·2PF6 (3). Reaction of the bidentate bis-imidazolium ligands H2L·2PF6 (3) with Ag2O in acetonitrile gave the macrocyclic binuclear silver(I) carbene complex [Ag2(L)2]·2PF6·CH3CN (4). Nickel carbene complex [Ni(L)PPh3Cl]·PF6·2DMSO (5) was obtained via transmetalation of 4 with Ni(PPh3)2Cl2 in DMSO. The bidentate carbene ligand is a chelating ligand in 5, while bridging in 4. The imidazolium ligand H2L·2PF6 (3) and transition metal carbene complexes 4 and 5 have been fully characterized by elemental analysis, NMR, ESI-MS spectroscopy, and X-ray diffraction analyses. Furthermore, the UV and luminescent properties of 3–5 were also studied.

Monomeric Three-Coordinate N-Heterocyclic Carbene Nickel(I) Complexes: Synthesis, Structures, and Catalytic Applications in Cross-Coupling Reactions

A series of three-coordinate monovalent nickel halide complexes bearing N-heterocyclic carbene (NHC) ligands, i.e., NiCl(IPr)(L) [L = pyridine, P(OPh)3, bis(diphenylphosphino)butane (dppb), IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], NiX(IMes)(PPh3) (X = Cl and Br, IMes = 1,3-bis(mesityl)imidazol-2-ylidene), were prepared. The complexes were identified using NMR spectroscopy, superconducting quantum interference device (SQUID), and X-ray crystallography. Additionally, ESR spectra of NiCl(IPr)(pyridine) were taken in toluene. These complexes had three-coordinate Y-shaped geometries in both the solid and solution states. The compounds containing IPr showed equilibrium between the monomeric and dimeric forms, with liberation of ligands. Addition of 1,2-bis(diphenylphosphino)ethane and 1,3-bis(diphenylphosphino)propane to the dinickel(I) IPr complex instead of dppb resulted in heterolytic cleavage to nickel(0) and nickel(II) species. Catalysis of Suzuki cross-coupling and Buchwald–Hartwig aminati...

Efficient N-heterocyclic carbene nickel pincer complexes catalyzed cross coupling of benzylic ammonium salts with boronic acids

Pyridine-bridged bis-benzimidazolylidene nickel complexes exhibited very high catalytic activity toward cross coupling of inactive (hetero)aryl benzylic ammonium salts with (hetero)aryl and alkenyl boronic acids under mild reaction conditions. Even at 2mol% catalyst loading, a wide range of substrates for both coupling partners with different steric and electronic properties were well tolerated.

ChemInform Abstract: Nickel N‐Heterocyclic Carbene‐Catalyzed C—Heteroatom Bond Formation, Reduction, and Oxidation: Reactions and Mechanistic Aspects

AbstractReview: 141 refs.

Synthesis of inexpensive chiral half-sandwich nickel N-heterocyclic carbene complexes: X-ray diffraction study of the D-menthyl-functionalized complex [Ni(iPr2Ph-NHC-CH2OMent)ClCp

The imidazolium salts, 1-(2,4,6-trimethylphenyl)-3-[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride (Mes-NHC-CH2OMent·HCl) (1a) and 1-(2,6-diisopropylphenyl)-3-[(1R,2S,5R)-(−)-menthoxymethyl]imidazolium chloride (iPr2Ph-NHC-CH2OMent·HCl) (1b), are readily accessible from inexpensive (1R,2S,5R)-(−)-menthol. They react with nickelocene to give two new chiral nickel-N-heterocyclic carbene (NHC) complexes, [Ni(Mes-NHC-CH2OMent)ClCp] (2a) and [Ni(iPr2Ph-NHC-CH2OMent)ClCp] (2b), in good yields. The new complexes were fully characterized by standard spectroscopic techniques and by a single crystal X-ray diffraction study on complex 2b. Complex 2b crystallizes in the chiral spacegroup P1, with two independent molecules in the unit cell, which are slightly different from each other. Preliminary studies show that these complexes are effective catalysts for the hydrosilylation of ketones. However no chiral induction was observed.

Nickel-Catalyzed Cross-Coupling of Organolithium Reagents with (Hetero)Aryl Electrophiles.

Nickel-catalyzed selective cross-coupling of aromatic electrophiles (bromides, chlorides, fluorides and methyl ethers) with organolithium reagents is presented. The use of a commercially available nickel N-heterocyclic carbene (NHC) complex allows the reaction with a variety of (hetero)aryllithium compounds, including those prepared via metal-halogen exchange or direct metallation, whereas a commercially available electron-rich nickel-bisphosphine complex smoothly converts alkyllithium species into the corresponding coupled product. These reactions proceed rapidly (1 h) under mild conditions (room temperature) while avoiding the undesired formation of reduced or homocoupled products.

Nickel N-Heterocyclic Carbene-Catalyzed C–Heteroatom Bond Formation, Reduction, and Oxidation: Reactions and Mechanistic Aspects

The chemistry of nickel N-heterocyclic carbene species has blossomed at the beginning of this century with a particularly rapid acceleration in the last 5 years, as indicated by the fact that more than 65% of the discussed research articles in this comprehensive review date from the period 2010–2015. The rapid evolution of this chemistry has led to an increasing number of applications in the field of catalytic C–heteroatom bond formation, reduction, and oxidation where the heteroatom is nitrogen, sulfur, oxygen, or boron. Thus, in addition to the development of aryl aminations, aryl thiolations, alkyne hydrothiolations, and transfer hydrogenations, which are the most ancient reactions of this type known for these systems, the last five years have seen the appearance of a number of novel interesting Ni(NHC)-catalyzed transformations such as the dehydrogenative cross-coupling of aldehydes and amines or alcohols, the hydroamination of alkenes, the hydroimination of alkynes, a one-step indoline synthesis, (cr...

ChemInform Abstract: Nickel N‐Heterocyclic Carbene Complexes and Their Utility in Homogeneous Catalysis

AbstractReview: 570 refs.

Nickel N-heterocyclic carbene catalyzed C-C bond formation: a new route to aryl ketones.

A novel nickel N-heterocyclic carbene catalyzed cross-coupling reaction of aryl aldehydes with boronic esters for the synthesis of aryl ketones was developed. This reaction provides a mild, practical method toward aryl ketones, which are versatile intermediates and building blocks in organic synthesis.

ChemInform Abstract: Nickel N‐Heterocyclic Carbene‐Catalyzed C—C Bond Formation: Reactions and Mechanistic Aspects

AbstractReview: 165 refs.

Nickel N-Heterocyclic Carbene-Catalyzed C–C Bond Formation: Reactions and Mechanistic Aspects

The chemistry of nickel N-heterocyclic carbene complexes is a research area that has blossomed over the last 10 years, and a large number of new complexes with a variety of architectural motifs are now known. The evolution of this chemistry has led to increasing applications of these complexes in catalytic bond formation. The rapid expansion of this field now calls for a review of the kinds of reactions that are catalyzed and a summary of the state of the art at this time. As the breadth of reactions catalyzed by such complexes is vast, this review specifically targets catalytic C–C bond formation, in particular C–C cross-couplings and C–C couplings via C–H bond activation, mediated by nickel–N-heterocyclic carbene complexes. A special emphasis is placed on mechanistic data, because this allows possible new insights into catalyst improvement.

An insight into a base-free Michael addition reaction as catalyzed by a bifunctional nickel N-heterocyclic carbene complex using density functional theory studies

A proposed catalytic pathway for a base-free Michael addition reaction mediated by a N/O-functionalized N-heterocyclic carbene (NHC) based bifunctional nickel precatalyst has been probed using density functional theory (DFT) studies. In particular, the base-free Michael addition of a β-dicarbonyl compound namely, 2-acetyl-cyclopentanone (a) with methyl vinyl ketone (b) as catalyzed by a representative bifunctional nickel precatalyst viz. [1-(Me)-3-N-(methylacetamido)imidazol-2-ylidene]2Ni (A) has been investigated. The modeling studies reveal that the nucleophilic attack of a metal bound enolate moiety of a 1,3-dicarbonyl adduct species (B) to the approaching activated olefinic substrate, methyl vinyl ketone (b), is the crucial rate-limiting step of the reaction yielding a Michael addition product adduct species (C). Interestingly, the subsequent intramolecular rearrangement of (C) to a different O-bound intermediate (D) exhibit nearly equal activation barrier.

Nickel-catalysed carboxylation of organoboronates.

A nickel/N-heterocyclic carbene (NHC) catalysed carboxylation of aryl-, heteroaryl- and alkenylboronates, affording the corresponding carboxylic acids, has been developed. This transformation proceeds under one atmosphere of CO2 with a broad range of substrates and exhibits good functional group compatibility.

Dehydrogenative coupling of aromatic thiols with Et3SiH catalysed by N-heterocyclic carbene nickel complexes

A series of new tetramethylcyclopentadienyl-functionalised N-heterocyclic carbene ligands with different wingtip substituents have been prepared and characterised. These ligands have been successfully coordinated to nickel affording complexes of the general type (Cp*-NHC(R))NiX (X = Cl, I). These well-defined nickel complexes selectively catalysed the coupling of aromatic thiols with Et3SiH to give the corresponding silylthioethers (RSSiEt3). The nickel complexes bearing ethyl, iso-butyl, and n-butyl wingtips displayed comparable catalytic efficiency, while the nickel complex bearing a methyl substituent on the wingtip was the worst performing catalyst.

Suzuki–Miyaura cross-coupling of bulky anthracenyl carboxylates by using pincer nickel N-heterocyclic carbene complexes: an efficient protocol to access fluorescent anthracene derivatives

A series of fluorescent (hetero)-aryl substituted anthracene derivatives were readily accessible from the corresponding bulky anthracen-9-yl carboxylates via Suzuki-Miyaura cross-coupling reactions by using pincer nickel N-heterocyclic carbene complex even at the catalyst loading as low as 0.1 mol% in the presence of catalytic amounts of PCy3.

Cyclopentadienyl N-heterocyclic carbene–nickel complexes as efficient pre-catalysts for the hydrosilylation of imines

The in situ generated nickel hydride complex, [Ni(Mes2NHC)HCp], and its cationic analogue, [Ni(Mes2NHC)(NCMe)Cp](PF6), are efficient and chemoselective pre-catalysts for the hydrosilylation of both aldimines and ketimines under mild conditions.

Chemical bonding and properties in [Ni(N-heterocylic carbene)(NO)(R)] (R = H, Me, HC=CH2, and C≡CH) complexes: Theoretical insights

A series of N-heterocyclic carbene nickel complexes of the type [Ni(N-heterocylic carbene)(NO)(R)] (R = H, Me, HC=CH2, and C≡CH) are examined to study the influence of a substituent on the molecular structure and bonding of these complexes. Geometrical and AIM analyses of the interaction between Ni and the carbene fragment reveal that for the metal-carbene bond donation is more important than back-donation. The NICS values suggest that aromaticity in the heterocyclic ring is less than in the free heterocycle.

Synthesis, Solid-state Structures, Solution Behaviour and Catalysis Studies of Nickel Complexes of Bis(benzimidazolin-2-ylidene)pyridine Pincer Ligands

N-Heterocyclic carbene–nickel complexes with five- and four-coordinate geometries [(CNC)NiBr2] and [(CNC)NiBr]X (X = PF6 or BPh4) have been prepared with the pincer ligands 2,6-bis(N-octylbenzimidazolin-2-ylidene)pyridine and 2,6-bis(N-butyl-5,6-dimethoxybenzimidazolin-2-ylidene)pyridine. The addition of the n-octyl substituent significantly extends the solubility of the complexes and has allowed UV-vis solution studies of the complexes in dichloromethane and methanol. The four- and five-coordinate species exist in equilibrium in solution and this equilibrium has been explored by UV-vis studies. The complexes have also been characterized by NMR studies, and single crystal X-ray diffraction studies have been performed on [(CNC)NiBr2] (where CNC = 2,6-bis(N-octylbenzimidazolin-2-ylidene)pyridine) and [(CNC)NiBr]BPh4 (where CNC = 2,6-bis(N-butyl-5,6-dimethoxybenzimidazolin-2-ylidene)pyridine).