Bulky N-heterocyclic Carbene Ligands Research Articles

Divergent and Selective Light Alkene Cross-Coupling.

Light olefins are abundantly manufactured in the petroleum industry and thus represent ideal starting materials for modern chemical synthesis. Selective and divergent transformations of feedstock light olefins to value-added chemicals are highly sought-after but remain challenging. Herein we report an exceptionally regioselective carbonickelation of light alkenes followed by in situ trapping with three types of nucleophiles, namely a reductant, base, or Grignard reagent. This protocol enables efficient 1,2-hydrofunctionalization, dicarbofunctionalization, and branched-selective Heck-type cross-coupling of light alkenes with aryl and alkenyl reagents to streamline access to diverse alkyl arenes and complex alkenes. Harnessing bulky N-heterocyclic carbene ligands with acenaphthyl backbones for nickel catalysts is crucial to attain high reactivity and selectivity. This strategy provides a rare, modular, and divergent platform for upgrading feedstock alkenes and is expected to find broad applications in medicinal chemistry and industrial processes.

Bulky NHC–Cobalt Complex-Catalyzed Highly Markovnikov-Selective Hydrosilylation of Alkynes

The hydrosilylation of alkynes is one of the most attractive and, at the same time, most challenging catalytic transformations, usually demanding the use of noble transition metals. We describe a catalytic system, based on cobalt(0) complex and bulky N-heterocyclic carbene (NHC) ligands, permitting the highly effective hydrosilylation of a broad scope of alkynes and silanes. The application of bulky NHC ligands allowed a decrease in the amount of cobalt necessary for an effective reaction run to 2.5 mol% and provided excellent selectivity towards challenging α-vinylsilanes. The developed method tolerates a number of substituted aryl, alkyl, and silyl acetylenes. Moreover, it is suitable for both tertiary and secondary silanes. Our findings confirm that steric hindrance around the metal center can effectively increase the activity of a catalyst and ensure better selectivity than those of analogous complexes bearing smaller ligands.

A pyridine promoted ‘weak base route’ to (NHC)2NiCl2 complexes with bulky N,N'-diaryl carbene ligands

A simple one-step synthesis of (NHC)2NiCl2 complexes containing bulky N-heterocyclic carbene (NHC) ligands relies on the C–H metallation of N,N'-diaryl-substituted azolium salts, NHC-precursors, with NiCl2Py2 in the presence of Cs2CO3 and pyridine. This procedure allows one to implement all synthetic manipulations in air.

Multidynamic Crystalline Molecular Rotors Comprising an N‐Heterocyclic Carbene Binuclear Au(I) Complex Bearing Multiple Rotators

AbstractWe report multidynamic molecular rotations in crystals using a concave‐shape N‐heterocyclic carbene (NHC) binuclear Au(I) complex rotor bearing pyrazine and tetrahydrofuran (THF) molecules as multicomponent rotators. Single‐crystal X‐ray diffraction (XRD) measurements revealed that two THF molecules are located near the central pyrazine encapsulated by two bulky NHC ligands. From 2H solid‐state NMR analysis, it was observed that the pyrazine rotated in a 2‐fold site exchange with a 180° rotational angle and a 31 kJ mol−1 energy barrier, while the THF molecules showed a 23°‐38° libration with a lower energy barrier (14 kJ mol−1). Interestingly, the pyrazine rotation was accelerated when the THF molecules rotated in fast site exchange with a large angle of libration, suggesting that the rotators exhibit multidynamics in a correlated manner.

Catalyst-Controlled Nickel-Catalyzed Intramolecular endo-Selective C–H Cyclization of Benzimidazoles with Alkenes

Compared with the widely explored exo-selective C-H cyclization, transition metal-catalyzed endo-selective C-H cyclization of benzimidazoles with alkenes has been a formidable challenge. Previous efforts mainly rely on substrate-controlled methods, rendering the product complexity restricted. Herein we report a catalyst-controlled method to facilitate endo-cyclization, in which a bulky N-heterocyclic carbene ligand and tBuOK base-enabled Ni-Al bimetallic catalyst prove critical to the endo selectivity.

Synthesis of Indolo[2,1-a]isoquinolines by Nickel-Catalyzed Mizoroki-Heck/Amination Cascade Reaction.

An efficient Mizoroki-Heck/amination cascade reaction of o-dihaloarenes with cyclic imines was realized by combining nickel and a sterically bulky N-heterocyclic carbene ligand. This protocol provides access to a variety of indole[2,1-a]isoquinolines from readily available starting materials. This cascade approach could be applied to produce straightforward synthesis of the natural product cryptowoline.

Ruthenium olefin metathesis catalysts bearing two bulky and unsymmetrical NHC ligands

The activity profile of the new indenylidene complexes containing two unsymmetrical N‐heterocyclic carbene (NHC) ligands was investigated. They turned out to be effective in both straightforward olefin metathesis reactions, such as ring‐closing metathesis (RCM) of diethyl diallylmalonate, as well as in more demanding processes, such as RCM of 2,2‐di‐(2‐methylallyl)tosylate leading to a product bearing a tetrasubstituted double bond, and in selective ethenolysis of methyl and ethyl oleate.

Solvent-free hydroboration of alkynes catalyzed by an NHC-cobalt complex.

A new cobalt complex bearing a bulky N-heterocyclic carbene (NHC) ligand is described as a pre-catalyst for alkyne hydroboration. The proposed catalytic system, synthesized using easily accessible reagents, allowed obtaining a series of mono- and dialkenylboranes in solvent-free conditions with excellent efficiency and selectivity. The results have been compared to those obtained in the presence of the same cobalt complex containing smaller NHC ligands and those achieved for the catalytic system based on a CoCl2 – NHC precursor.

Application of Bulky NHC-Rhodium Complexes in Efficient S-Si and S-S Bond Forming Reactions.

The efficient and straightforward syntheses of silylthioethers and disulfides are presented. The synthetic methodologies are based on new rhodium complexes containing bulky N-heterocyclic carbene (NHC) ligands that turned out to be efficient catalysts in thiol and thiol–silane coupling reactions. These green protocols, which use easily accessible reagents, allow obtaining compounds containing S–Si and S–S bonds in solvent-free conditions. Additionally, preliminary tests on coupling of mono- and octahydro-substituted spherosilicates with selected thiols have proved to be very promising and showed that these catalytic systems can be used for the synthesis of a novel class of functionalized silsesquioxane derivatives.

Direct Synthesis of Ketones from Methyl Esters by Nickel-Catalyzed Suzuki-Miyaura Coupling.

The direct conversion of alkyl esters to ketones has been hindered by the sluggish reactivity of the starting materials and the susceptibility of the product towards subsequent nucleophilic attack. We have now achieved a cross-coupling approach to this transformation using nickel, a bulky N-heterocyclic carbene ligand, and alkyl organoboron coupling partners. 65 alkyl ketones bearing diverse functional groups and heterocyclic scaffolds have been synthesized with this method. Catalyst-controlled chemoselectivity is observed for C(acyl)-O bond activation of multi-functional substrates bearing other bonds prone to cleavage by Ni, including aryl ether, aryl fluoride, and N-Ph amide functional groups. Density functional theory calculations provide mechanistic support for a Ni0 /NiII catalytic cycle and demonstrate how stabilizing non-covalent interactions between the bulky catalyst and substrate are critical for the reaction's success.

Physical and photophysical properties of a linear copper(I) complex of a bulky acenapthene-based NHC ligand

We report the first example of a charge-neutral linear two-coordinate copper(I) complex bearing a sterically demanding acenaphthoimidazolylidene-based N-heterocyclic carbene (NHC) ligand. The identity and geometry of the complex are confirmed by single-crystal X-ray diffraction (XRD) analysis. The complex is poorly emissive at room temperature, showing either ligand-centered (LC) emission around 340 nm when excited at 300 nm or ligand-to-ligand charge transfer (LLCT) emission at 540 nm when excited at 420 nm; in chloroform, dual emission is observed upon photoexcitation at 300 nm. Nanosecond emission lifetimes were recorded for these processes. This is the first example of emissive linear copper(I) complexes containing this bulky NHC ligand.

Efficient Z-Selective Olefin-Acrylamide Cross-Metathesis Enabled by Sterically Demanding Cyclometalated Ruthenium Catalysts.

The efficient Z-selective cross-metathesis between acrylamides and common terminal olefins has been developed by the use of novel cyclometalated ruthenium catalysts with bulky N-heterocyclic carbene (NHC) ligands. Superior reactivity and stereoselectivity are realized for the first time in this challenging transformation, allowing streamlined access to an important class of cis-Michael acceptors from readily available feedstocks. The kinetic preference for cross-metathesis is enabled by a pivalate anionic ligand, and the origin of this effect is elucidated by density functional theory calculations.

Highly Active Manganese-Based CO2 Reduction Catalysts with Bulky NHC Ligands: A Mechanistic Study.

Because of the strong σ-donor and weak π-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO2 to CO with high activity. However, some NHC-based manganese complexes showed low catalytic activity and required very negative potentials. We report herein that complex fac-[MnI(bis-MesNHC)(CO)3Br] [1; bis-MesNHC = 3,3-bis(2,4,6-trimethylphenyl)-(1,1'-diimidazolin-2,2'-diylidene)methane] could catalyze the electrochemical reduction of CO2 to CO with high activity (TOFmax = 3180 ± 6 s-1) at a less negative potential. Due to the introduction of the bulky Mes groups, a one-electron-reduced intermediate {[Mn0(bis-MesNHC)(CO)3]0 (2•)} was isolated as a packed "dimer" and crystallographically characterized. Stopped-flow Fourier-transform infrared spectroscopy was used to prove the direct reaction between doubly reduced intermediate fac-[Mn(bis-MesNHC)(CO)3]- and CO2; the tetracarbonyl Mn complex [Mn+(bis-MesNHC)(CO)4]+ ([2-CO]+) was captured, and its further reduction proposed as the rate-limiting step.

Enhanced π-back-donation resulting in the trans labilization of a pyridine ligand in an N-heterocyclic carbene (NHC) PdII precatalyst: a case study.

The molecular structure of the benzimidazol-2-ylidene-PdCl2-pyridine-type PEPPSI (pyridine-enhanced precatalyst, preparation, stabilization and initiation) complex {1,3-bis[2-(diisopropylamino)ethyl]benzimidazol-2-ylidene-κC2}dichlorido(pyridine-κN)palladium(II), [PdCl2(C5H5N)(C23H40N4)], has been characterized by elemental analysis, IR and NMR spectroscopy, and natural bond orbital (NBO) and charge decomposition analysis (CDA). Cambridge Structural Database (CSD) searches were used to understand the structural characteristics of the PEPPSI complexes in comparison with the usual N-heterocyclic carbene (NHC) complexes. The presence of weak C-H...Cl-type hydrogen-bond and π-π stacking interactions between benzene rings were verified using NCI plots and Hirshfeld surface analysis. The preferred method in the CDA of PEPPSI complexes is to separate their geometries into only two fragments, i.e. the bulky NHC ligand and the remaining fragment. In this study, the geometry of the PEPPSI complex is separated into five fragments, namely benzimidazol-2-ylidene (Bimy), two chlorides, pyridine (Py) and the PdII ion. Thus, the individual roles of the Pd atom and the Py ligand in the donation and back-donation mechanisms have been clearly revealed. The NHC ligand in the PEPPSI complex in this study acts as a strong σ-donor with a considerable amount of π-back-donation from Pd to Ccarbene. The electron-poor character of PdII is supported by π-back-donation from the Pd centre and the weakness of the Pd-N(Py) bond. According to CSD searches, Bimy ligands in PEPPSI complexes have a stronger σ-donating ability than imidazol-2-ylidene ligands in PEPPSI complexes.

Palladium-Catalysed Amination of Hindered Aryl Halides with 9H-Carbazole

Palladium-catalysed Buchwald-Hartwig amination of ortho-substituted hindered aryl bromides or chlorides with 9H-carbazole has been investigated. In the amination of 1-bromo- or chloronaphthalene with 9H-carbazole, the combined use of Pd2(dba)3 as a Pd precursor, Buchwald ligands with two tert-butyl groups and LiOtBu or lithium hexamethyldisilazide as a base led to satisfactory yields. N,N’-Bis[2,6-bis(diphenylmethyl)-4-methoxyphenyl]imidazol-2-ylidene (IPr*OMe), which is a bulky N-heterocyclic carbene ligand, showed similar activity as Buchwald ligands with two tert-butyl groups. In contrast, only IPr*OMe provided satisfactory yields in the amination of 2-bromo-1,1′-biphenyl with 9H-carbazole. The amination of 2-bromo- or chlorotoluene and 1-(2-bromo- or chlorophenyl)naphthalene with 9H-carbazole proceeded smoothly when the IPr*OMe ligand was used.

Pd-catalyzed enantioselective cyclopropanation of nitriles with mono substituted allyl carbonates enabled by the bulky N-heterocyclic carbene ligand.

A highly efficient catalyst for Pd-catalyzed cyclopropanation was developed using a bulkier N-heterocyclic carbene ligand, with which the nitriles reacted with mono substituted allyl reagents to afford cyclopropanes in high yields with high cyclopropanation/allylation and enantioselectivities. The reasons for cyclopropanation were investigated and the usefulness of the products was demonstrated.

Monothiolate ruthenium alkylidene complexes with tricyclic fluorinated N-heterocyclic carbene ligands

New monothiolate ruthenium alkylidene complexes bearing bulky tricyclic N-heterocyclic carbene ligands decorated with two geminal trifluoromethyl groups were synthesized. Their catalytic activity in representative olefin metathesis reactions, such as ring closing metathesis of diallyltosylamine and selfmetathesis of allylbenzene, has been evaluated.

Synthesis of ultra‐high‐molecular‐weight ethylene‐propylene copolymer via quasi‐living copolymerization with N‐heterocyclic carbene ligated vanadium complexes

ABSTRACTThe quasi‐living copolymerization of ethylene with propylene was achieved by using N‐heterocyclic carbene (NHC) ligated vanadium complex (V3, VOCl3[1,3‐(2,6‐iPr2C6H3)2(NCH)2C:]) due to the stabilization of active center by the introduction of bulky and electron rich NHC ligand with bulky isopropyl substituents at the ortho positions of the phenyl rings. The weight‐average molecular weight (Mw) of the resulting copolymer increases linearly with its weight in 20 min. The ultra‐high‐molecular‐weight (UHMW) ethylene‐propylene copolymer (Mw = 1612 kg mol−1) can be synthesized with V3/Et3Al2Cl3 catalytic system. The novel complex V4′ (VCl3[1,3‐(2,4,6‐Me3C6H2)2(NCH)2C:]·2THF) was constructed by the introduction of two coordinated tetrahydrofuran molecules and decrease in steric hindrance at the ortho positions of phenyl rings. The UHMW ethylene‐propylene copolymer (Mw = 1167 kg mol−1) can also be synthesized by using V4′/Et3Al2Cl3 catalytic system. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 553–561

Glycerol to lactic acid conversion by NHC-stabilized iridium nanoparticles

Hydrogen reduction of an Ir(I) complex featured by a bulky N-heterocyclic carbene (NHC) ligand in dichloromethane gave small-sized (1.8 nm) Ir nanoparticles (NPs) decorated with NHC ligands (IrNHC). 1,4-Dioxane solutions of the latter particles were successfully applied to convert glycerol into lactic acid in the presence of NaOH (i.e. 1 mol equivalent with respect to glycerol). IrNHC showed an atom-related TOF value of almost 104 h−1, an almost exclusive formation of liquid reaction products, a high selectivity for lactic acid (93.0%) and a complete recyclability in air atmosphere. Attempts to synthesize analogous NHC-stabilized Ir NPs on a high surface area carbon support (CK) by reducing the same Ir(I) precursor, supported onto CK, prior to the hydrogen reduction in water, gave almost naked CK-supported Ir NPs (1.4 nm). Their catalytic activity tested for the same reaction in water as reaction medium, exhibited much lower catalytic activity (4 × 103 h−1), a lower percentage of liquid reaction products (i.e. 27.0% of the converted glycerol) and a lower selectivity for lactic acid compared to IrNHC.

Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand–VOF3 Bonding with Evidence of a Minute π Back-Donation of Fluoride

Reaction of vanadium(V) oxide trifluoride (VOF3) and the new "naked" fluoride reagent [(LDipp)H][F] (LDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2 H-imidazol-2-ylidene) leads to the isolation of [(LDipp)H][VOF4] (1) where the long sought discrete [VOF4]- anion was finally obtained. The neutral [(LDipp)VOF3] (2) complex was synthesized by a similar reaction between VOF3 and bulky N-heterocyclic carbene (NHC) ligand LDipp. In this context, we analyzed, by means of DFT calculations, intermolecular interactions between [(LDipp)VOF3] (2) complexes in the crystal structure and realized that these interactions have a significant effect on the V-Ftrans bond length. We further scrutinized ligand bonding within [(LDipp)VOF3] (2) and related complexes, because, in this kind of complexes, a rather short distance between CNHC and cis-halogen atoms has spurred some discussion about the type of interactions between them. We provide evidence of a minute π back-bonding into NHC ligands, which is larger for chloride [(NHC)VOCl3] than fluoride [(NHC)VOF3] complexes, although the fluoride ions are, counterintuitively and to a larger degree, involved in back-bonding than chloride ions. The influence of π back-bonding on V-Ftrans and V-Fcis bond lengths was also rationalized. Finally, the hydrolysis of [(LDipp)VOF3] (2) product was studied and [(LDipp)H][VO2F2] (3) salt was obtained and characterized as the most stable product in this system.