N-heterocyclic Carbene Pd Complexes Research Articles

Bis(imino)acenaphthene N‐Heterocyclic Carbene Palladium Complexes Bearing Tertiary Amines: Structural and Catalytic Studies on the Interaction between Soft Acid and Hard Base

AbstractThe HSAB principle in designing Bis(imino)acenaphthene N‐heterocyclic carbene (NHC) palladium complexes is presented. N‐donor ligands serve as hard bases to enhance the catalytic activities of NHC Pd complexes in Suzuki and Buchwald–Hartwig amination reactions. The Pd complex bearing tertiary amine ligands exhibited higher performance in coupling reactions than those exhibited by other soft base ligands reported in literature. 48 coupling products were generated in moderate‐to‐complete yields in the presence of aryl chlorides as substrates for Suzuki and amination reactions in air under mild conditions.

NHC Pdii complexes for the solvent-free telomerisation of isoprene with methanol

A comparative study of N-heterocyclic carbene Pd complexes in the head-to-head isoprene telomerization with methanol revealed significant impact of ligand structure as well as axial group structure on the catalyst activity. Some N,N'-diaryl substituted imidazol-2-ylidene, imidazolidin-2-ylidene and expanded-ring tetrahydropyrimidin-2-ylidene and tetrahydrodiazepin-2-ylidene based ligands were tested to explore the fundamental correlations between structure (ring carbene size along with the substituent sterical and electronic properties) and catalytic activity.

PEPPSI type Pd(II)NHC complexes bearing chloro-/fluorobenzyl group: Synthesis, characterization, crystal structures, α-glycosidase and acetylcholinesterase inhibitory properties

This work reported the synthesis of a new PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) type Pd(II)N-heterocyclic carbene (NHC) complexes bearing halo-benzyl (4-florobenzyl and 2-chloro-4-florobenzyl) group. These new complexes were synthesized from the florobenzyl / chlorofluorobenzyl substituted benzimidazolium salts, PdCl2 and pyridine. Characterizations of all the synthesized complexes were done using elemental analysis, 1H NMR, 13C NMR and FT-IR spectroscopy techniques. The molecular and crystal structures of the new PEPPSI type Pd(II)NHC complexes were determined by single-crystal X-ray diffraction method. X-ray studies show that molecular structures of three complexes comprise a palladium(II) atom with a slightly distorted square-planar coordination environment. These synthesized salts were found to be effective inhibitors for the α-glycosidase, and acetylcholinesterase (AChE) enzyme with Ki values in the range of 27.36 ± 5.06–124.88 ± 18.05 µM for α-glycosidase, and 0.78 ± 0.11–4.34 ± 1.02 µM for AChE, respectively. The significant group of drugs currently utilized for the therapy of Alzheimer's disease (AD) is acetylcholinesterase/cholinesterase inhibitor compounds. The first cholinesterase inhibitor licensed for symptomatic therapy of AD was tacrine. Inhibition acts of α-glycosidase enzyme by inhibitors tend to slow the breakdown and release of sugar molecules into the bloodstream and can be utilized as therapeutic factors in the therapy of obesity and diabetes.

Polymerization of 5-Ethylidene-2-norbornene in the Presence of Pd–N-Heterocyclic Carbene Complexes with Phosphine and Pyridine Ligands

The polymerization of commercial strained cycloalkene, 5-ethylidene-2-norbornene, in the presence of Pd–N-heterocyclic carbene complexes (two of which are synthesized for the first time) containing phosphine or pyridine ligands is studied. Upon activation with borate Na+[B(3,5-(CF3)2C6H3)4)]–, the studied Pd complexes catalyze the polymerization of 5-ethylidene-2-norbornene according to the addition scheme. Polymerization proceeds selectively and involves only the endocyclic (norbornene) double bond of the monomer, while the exocyclic (ethylidene) double bond remains unaffected. Pd–N-heterocyclic carbene complexes with phosphine ligands are inactive in polymerization, while similar complexes with 3-chloropyridine ligands show a higher activity. Two new Pd complexes are synthesized.

NHC Pd(II) and Ag(I) Complexes: Synthesis, Structure, and Catalytic Activity in Three Types of C-C Coupling Reactions.

Four bis-benzimidazolium salts, 1,4-bis[1′-(N-R-benzimidazoliumyl)methyl]-2,3,5,6-tetramethylbenzene 2X– (L1H2·(PF6)2: R = ethyl, X = PF6; L2H2·Br2: R = picolyl, X = Br; L3H2·Br2: R = benzyl, X = Br; and L4H2·Br2: R = allyl, X = Br), and their three N-heterocyclic carbene (NHC) Pd(II) and Ag(I) complexes, L1Pd2Cl4 (1), L2Ag2Br2 (2), and L4(AgBr)2 (3), as well as one anionic complex L3H2·(Ag4Br8)0.5 (4), have been synthesized and characterized. Complex 1 adopts a funnel-like type of structure, complex 2 adopts a cyclic structure, and complex 3 is an open structure. In the crystal packing of 1–4, one-dimensional polymeric chains and two-dimensional supramolecular layers are formed via intermolecular weak interactions, including hydrogen bonds, π–π interactions, and C–H···π contacts. The catalytic activities of NHC Pd(II) complex 1 in three types of C–C coupling reactions (Suzuki–Miyaura, Heck–Mizoroki, and Sonogashira reactions) were studied. The results show that this catalytic system is efficient for these C–C coupling reactions.

Addition Polymerization of 5-Ethylidene-2-Norbornene in the Presence of Pd N-Heterocyclic Carbene Complexes

New high-performance catalytic systems based on Pd N-heterocyclic carbene complexes for the selective addition polymerization of 5-ethylidene-2-norbornene (ENB) were proposed. With these catalysts, polymerization can be conducted at unprecedentedly high monomer/catalyst ratio (up to 5 × 105/1) and gives high-molecular-weight soluble polymers with good film-forming properties. Varying the polymerization conditions (reaction temperature, monomer and catalyst concentrations, monomer/Pd ratio) makes it possible to prepare soluble ENB-based addition polymers with specified molecular weights in reasonable yields.

Addition homo- and copolymerization of 3-triethoxysilyltricyclo[4.2.1.02,5]non-7-ene

New norbornene type monomer bearing reactive triethoxysilyl group was synthesized, and its addition homo- and copolymerization with 3-trimethylsilyltricyclonon-7-ene was studied. The target monomer was obtained using regio- and stereospecific [2σ+2σ+2π] cyclo-addition of quadricyclane with vinyltrichlorosilane followed by the reaction of the formed cycloadduct with ethanol in the presence of triethylamine. Addition polymerization was investigated over the three-component Pd-containing catalytic system (Pd complex, Na+[B(3,5-(CF3)2C6H3)4]–(cocatalyst) and tricyclohexylphosphine). The N-heterocyclic carbene Pd complex (SIPrPd(cinn)Cl) with high activity and tolerance to the Si—O—C moieties was used as a catalyst. The yields of the homo- and copolymers were 24—68% depending on the monomer (comonomer): Pd: B: PCy3 ratio. The obtained addition polymers are high-molecular-weight amorphous products, the glass transition temperature of which exceeds 300 °C. The presence of reactive Si(OC2H5)3 groups in the homo- and copolymers made it possible to carry out a hard-to-realize cross-linking involving side substituents and followed by the formation of insoluble polymers.

Designing Pd-N-Heterocyclic Carbene Complexes for High Reactivity and Selectivity for Cross-Coupling Applications.

Over the past decade, the use of Pd-NHC complexes in cross-coupling applications has blossomed, and reactions that were either not previously possible or possible only under very forcing conditions (e.g., > 100 °C, strong base) are now feasible under mild conditions (e.g., room temperature, weak base). Access to tools such as computational chemistry has facilitated a much greater mechanistic understanding of catalytic cycles, which has enabled the design of new NHC ligands and accelerated advances in cross-coupling. With these elements of rational design, highly reactive Pd-NHC complexes have been invented to catalyze the selective formation of single products in a variety of transformations that have the potential to afford multiple compounds (e.g., isomers). Pd-NHC catalysts may be prepared as stable Pd(II) precatalysts that are readily reduced to the active Pd(0) species in the presence of an organometallic cross-coupling partner or nucleophile possessing β-hydrogens. It has been found from computational and experimental results that Pd-NHC complexes bearing a single bulky NHC ligand are well-suited to tackle challenging cross-coupling reactions. N-Aryl-substituted imidazole-2-ylidenes with branched alkyl chains at the ortho positions of the aryl group are effective for the challenging couplings of hindered biaryls, secondary alkyl organozincs, electron-deficient anilines, α-amino esters, primary alkylamines, and ammonia. The bulk of the NHC has been tuned by increasing the size of the alkyl groups at the ortho positions and substituting the NHC core with chlorine substituents. All of the cross-coupling transformations studied benefit from the increased bulk when the ortho groups are changed from methyl to 2-propyl to 3-pentyl. However, there is a limit to the positive effect of steric bulk, as some reactions do not benefit from the increased size of the 4-heptyl group compared with 3-pentyl. Thus, there is an optimum size for the NHC ligand that depends upon whether reactivity (turnover frequency and turnover number), selectivity, or both are needed to obtain the desired reaction outcome. In the cases that we have studied, reactivity and selectivity increase together (i.e., the fastest catalyst is also the most selective), allowing cross-couplings to be carried out under mild conditions to obtain one product with high selectivity. This Account focuses on seminal literature reports that have disclosed new Pd-NHC complexes that have led to significant breakthroughs in efficacy for challenging couplings while demonstrating high selectivity for the desired target. These catalysts have been used widely in materials science, pharmaceutical, and agrochemical applications.

Palladium–N-heterocyclic carbene complexes for the Mizoroki–Heck reaction: An appraisal

This article presents a review of the most significant developments with N-heterocyclic carbene (NHC)–palladium catalytic systems used for the Heck reaction. For more than the past two decades, a large number of new NHC–Pd complexes have been synthesized and characterized. These studies focused on NHCs as a phosphine analogue, but the current review highlights the differences with particular ligands so as to attain a suitable balance between the electronic and bulky environments around the metal. NHCs have gained wide recognition as these ligands act as excellent σ-donors that form stable metal–NHCs with strong metal–carbon bonds. For this reason, metal–NHCs are commonly used as they are highly reactive and can selectively serve as catalysts for various chemical transformations. The objective of our article is to highlight significant recent progress in NHC–Pd(II) complexes and provide an overview of their extensive interaction in the Mizoroki–Heck reaction.

N-Heterocyclic Carbene Based Nickel and Palladium Complexes: A DFT Comparison of the Mizoroki–Heck Catalytic Cycles

A theoretical investigation has been performed on nickel– and palladium–N-heterocyclic carbene (NHC) complexes in the Mizoroki–Heck (MH) reaction. A density functional theory (DFT) comparison of reaction pathways in cationic and neutral catalytic cycles for both metals was made using the M06 functional. The oxidative addition barrier is much lower for NHC–Ni than that for NHC–Pd for both mono- and bis-ligand pathways; the bis-ligand pathway (i.e., where both NHCs remain coordinated) is energetically more favorable for both metals. The migratory insertion steps have lower free energy barriers for NHC–Ni than for NHC–Pd complexes for both neutral and cationic pathways. β-Hydride elimination is computed as thermodynamically difficult for NHC–Ni complexes, indicating a possible reluctance in the elimination of the Ni-based hydride intermediate and formation of the Heck adduct at room temperature. The base-assisted reductive elimination is easier for NHC–Pd than for NHC–Ni complexes in the neutral and cationic...

N-heterocyclic carbene-stabilized palladium complexes as organometallic catalysts for bioorthogonal cross-coupling reactions.

A small library of water-soluble N-heterocyclic carbene (NHC)-stabilized palladium complexes was prepared and applied for cross-couplings of biomolecules under mild conditions in water. Pd-NHC complexes bearing hydrophilic groups were demonstrated to be efficient catalysts for the Suzuki-Miyaura coupling of various unnatural amino acids and proteins bearing p-iodophenyl functional groups. We further utilized this catalytic system for the rapid bioorthogonal labeling of proteins on the surfaces of mammalian cells. These results demonstrated that NHC-stabilized metal complexes have potential utility in cellular systems.

N-Heterocyclic carbene palladium complex supported on ionic liquid-modified graphene oxide as an efficient and recyclable catalyst for Suzuki reaction

An N-heterocyclic carbene Pd complex immobilized on GO with an ionic liquid framework synthesized by the modification of GO through a silylation reaction displayed a high catalytic activity in the Suzuki reaction.

Amination with Pd–NHC Complexes: Rate and Computational Studies on the Effects of the Oxidative Addition Partner

Pd-PEPPSI-IPent, a recently-developed N-heterocyclic carbene (NHC) complex, has been evaluated in amination reactions with secondary amines and it has shown superb reactivity under the most mildly basic reaction conditions. Rate and computational studies were conducted to provide insight into the mechanism of the transformation. The IPent catalyst coordinates to the amine much more strongly than the IPr variant, thus favouring deprotonation with comparatively weak bases. Indeed the reaction is first order in base and only slightly more than zeroth order in amine.

Synthesis and characterization of amide-functionalized N-heterocyclic carbene–Pd complexes

Amide-functionalized N-heterocyclic carbene (NHC) precursors such as azolium compounds have been designed and synthesized. Reaction of PdCl 2(CH 3CN) 2 with the NHC–Ag complex derived from the azolium salt gave [(NHC)PdCl 2] 2 or (NHC) 2PdCl 2, whereas PdCl 2(PPh 3) 2 reacted with the Ag complex to afford a mixed carbene/phosphine complex such as (NHC)(PPh 3)PdCl 2 together with a cationic [(NHC)(PPh 3) 2PdCl] +Cl − whose structure was characterized by X-ray crystallographic studies. Thus, the library of NHC–Pd complexes with a tethered amide group has been successfully expanded.

Synthesis and characterization of bidentate NHC–Pd complexes and their role in amination reactions

The new well-defined and air-stable ortho-xylyl-linked N-heterocyclic carbene (NHC) Pd complexes ( 2a– d) have been synthesized and characterized by elemental analysis, 1H NMR, 13C NMR, IR spectroscopy, and single crystal X-ray diffraction studies. The palladium atom in the complex 2a lies on a crystallographic mirror plane and can be described as having a square-planar coordination environment with the carbene atoms of the benzimidazole rings of the ligand occupying two coordination sites in cis positions. Two further coordination sites are occupied by chloride ligands. The benzimidazole rings are connected to each other by an ortho-xylyl bridge. The catalytic activity of these palladium complexes has been tested in the coupling reactions of various N-containing substrates with bromobenzene. A preliminary catalytic study shows that the bis(NHC)–Pd complexes are highly active in the Buchwald–Hartwig amination reaction.

Polyphenylens, Cross‐Linked Through Pd‐Carbene Complexes Formation, for Catalysis in Suzuki‐Miyaura Reactions

AbstractPolyphenylenes supported N‐heterocyclic carbene Pd‐complexes were synthesized for catalysis of cross‐coupling Suzuki‐Miyaura reactions. Starting polyphenylenes were prepared by cyclocondensation reaction of diacetylaromatic with monoacetylaromatic compounds. N‐metylimidazole has been involved to the polymer through the mono‐functional acetyl monomer, in which in p‐position to acetyl group the group of haloalkyl was situated, and haloalkyl group interacted with N‐methylimidazole. N‐heterocyclic carbene complexes of Pd were synthesized usually by the reaction of imidazolium salts with the salts of transition metals in the presence of a base, obtaining the complex (N‐heterocyclic carbene)2PdX2. The catalysis reaction was carried out between arylhalides (iodo‐ or bromobenzene) and phenylboronic acid with the presence of 1 mol% of Pd. The yields of biphenyl are from 70 to 95%, which is comparable with homogeneous catalysis.

N-Heterocyclic carbenes: deducing σ- and π-contributions in Rh-catalyzed hydroboration and Pd-catalyzed coupling reactions

The effect of tuning the electronic properties of N-heterocyclic carbene (NHC) ligands was evaluated in multiple, mechanistically distinct, metal-mediated reactions. Hydroboration and Heck reactions, catalyzed by Rh–NHC and Pd–NHC complexes, respectively, were found to result in yields that were up to ten times lower when π-withdrawing substituents were incorporated into the NHC backbone relative to analogues bearing σ-withdrawing groups.

Highly effective silica gel-supported N-heterocyclic carbene–Pd catalyst for Suzuki–Miyaura coupling reaction

The immobilized N-heterocyclic carbene–Pd complex was readily prepared by reaction of silica gel-supported imidazolium chloride with Pd(OAc)2. The Pd complex exhibited excellent catalytic activity in the coupling reaction of aryl halides with arylboronic acid. The heterogeneous Pd catalyst was reusable as well as air-stable to allow easy use.

First synthesis of bidentate NHC–Pd complexes with anthracene and xanthene skeletons

We synthesized a series of new bidentate NHC (N-heterocyclic carbene) precursors with anthracene and xanthene skeletons. The corresponding NHC–Pd complexes were prepared from these precursors. The catalytic activity of these Pd complexes was examined.

N,N'-Di(alkyloxy)imidazolium Salts: New Patent-free Ionic Liquids and NHC Precatalysts

1-Hydroxyimidazole-3-oxides (2-H, 2-Me) were alkylated with (RO)2SO2 (R =Me, Et) to give the new 1,3-di(alkyloxy)imidazolium cations which were isolated as hexafluorophosphates. Ion metathesis yielded new hydrophobic ionic liquids (bis(trifluoromethanesulfonyl)imides, tris(pentafluoroethyl) trifluorophosphates). Bromination afforded 2-bromo derivatives which were converted to Ni and Pd N-heterocyclic carbene complexes by oxidative insertion. Fifteen crystal structures were determined by X-ray diffraction. The N-alkyloxy groups are twisted out of the imidazole ring plane and adopt either syn or anti conformations in the solid state.