N-heterocyclic Carbene Palladium Research Articles

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.

Enantioselective Desymmetrization of Bicyclic Hydrazines using a C2-Symmetric N-Heterocyclic Carbene (NHC) Palladium Complex as Catalyst

The first example of palladium-catalyzed enantioselective desymmetrization of 2,3-bicyclic hydrazines with arylboronic acids through a ring-opening process is described by using a chiral C2-symmetr...

Synthesis and catalytic activity of pyrazolyl-functionalized N-heterocyclic carbene palladium complexes

Three pyrazolyl-functionalized N-heterocyclic carbene (NHC) palladium complexes based on 1-[2-(pyrazol-1-yl)phenyl]imidazole have been synthesized and characterized by physico-chemical and spectroscopic methods, and the structures of two of the complexes have been confirmed by single-crystal X-ray diffraction. The pyrazolyl-functionalized NHCs act as chelating N,C-bidentate ligands in these three complexes. Catalytic tests have proved that these complexes exhibit highly effective catalytic activity for the Suzuki–Miyaura and Mizoroki–Heck coupling reactions in water or aqueous/organic media under air. The substituents on the pyrazolyl ring exert different influences on the catalytic activity of the complexes in these coupling reactions.

(N-Heterocyclic carbene)Pd(triethylamine)Cl2as precatalyst for the synthesis of Poly(triarylamine)s

The use of a highly active, air-, and moisture-stable N-heterocyclic carbene-palladium(II) complex for the polymerization of anilines and aryl dibromides or dichlorides by Buchwald-Hartwig coupling is presented. In comparison to previous catalytic systems, higher molecular weight polymers with improved yields were obtained in shorter reaction times. The first examples of fabrication of OFETs with polytriarylamines synthesized with aryl dichlorides and anilines are also presented. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4904–4911

Carbon–Sulfur Bond Formation Catalyzed by [Pd(IPr*OMe)(cin)Cl] (cin = cinnamyl)

The newly prepared complex [Pd(IPr*(OMe))(cin)(Cl)] provides high catalytic activity for carbon-sulfur cross-coupling reactions. Nonactivated and deactivated aryl halides were successfully coupled with a large variety of aryl- and alkylthiols using this well-defined palladium N-heterocyclic carbene (NHC) complex.

Suzuki-MiyauraCross-Coupling Reaction of Naphthyl Triflate with Indole Boronic Acids Catalyzed by a Recyclable Polymer-Supported N-Heterocyclic Carbene-Palladium Complex Catalyst: Synthesis of Naphthalene-Linked Bis-Heterocycles

In this study, the Suzuki–Miyaura cross-coupling reaction of naphthyl triflate with indole boronic acids catalyzed by a recyclable polymer-supported Pd–NHC complex catalyst is presented. The polymer-supported catalyst can be reused several times retaining high activity for the transformation. The structures of all the synthesized compounds were established by elemental analysis and from their mass, 1H-NMR, and 13C-NMR spectra.

Synthesis and Structure of Imine–N-Heterocyclic Carbene Palladium Complexes and Their Catalytic Behavior in Norbornene Polymerization

On the basis of the steric effects of ligand, a series of imine–N-heterocyclic carbene (NHC) ligands and their corresponding five-membered palladium complexes with bulky substituents on both the imine and the NHC moieties were synthesized and characterized. Transpalladation of silver carbene complexes with (COD)PdCl2 and (COD)PdMeCl afforded the palladium dichloride and methylpalladium complexes, respectively. Bulky cationic palladium complexes were further obtained by treatment of the methylpalladium complexes with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF) in CH3CN. Well-defined cationic palladium complexes were confirmed by X-ray crystal diffraction to have trans forms. Palladium dichloride complexes and methylpalladium complexes after activation with MMAO show high activity for norbornene polymerization, whereas cationic palladium complexes can polymerize norbornene alone without any cocatalysts and exhibit a high thermostability. Norbornene polymerization with the cationic palladium catalyst was proven to proceed through a coordination–insertion mechanism by NMR studies. Analysis of oligomers obtained by polymerizing the monomer in the presence of H2 reveals the existence of a C7 linkage in the polynorbornene (PNB) by σ-bond metathesis, which may be the reason for the insolublity of polynorbornenes obtained by palladium catalysts.

(π‐Allyl)Pd Complexes Containing N‐Heterocyclic Carbene and Pseudohalogen Ligands – Synthesis, Reactivity toward Organic Isothiocyanates and Isocyanides, and Their Catalytic Activity in Suzuki–Miyaura Cross‐Couplings

AbstractDinuclear (π‐allyl)palladium chlorides, [(π‐allyl)Pd(μ‐Cl)]2, were cleaved by N‐heterocyclic carbenes (NHCs) to give mononuclear (π‐allyl)palladium–NHC chlorides, [(π‐allyl)Pd(Cl)(NHC)] (1–6) [NHC = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPR), 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidine (SIPR), 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes)]. Complexes 1–6 were subsequently treated with aqueous NaN3, KSCN, KOCN, and CF3COOAg to produce the corresponding mononuclear (π‐allyl)palladium–NHC pseudohalogen complexes, [(π‐allyl)Pd(X)(NHC)] (X = N3, NCS, SCN, NCO, CF3COO) (7–30). These products could also be obtained by treating dinuclear pseudohalogen‐bridged Pd complexes, [(π‐allyl)Pd(μ‐X)]2, which were prepared by replacing the μ‐Cl ligand in [(π‐allyl)Pd(μ‐Cl)]2, with aqueous NaN3, KSCN, KOCN, or CF3COOAg, followed by cleavage with the NHCs. Reactions of [(π‐allyl)Pd(N3)(NHC)] with organic isothiocyanates (R–NCS) or CH3O(CO)C≡CO(CO)CH3 resulted in selective 1,3‐dipolar cycloaddition into the Pd–azido bond to give heterocyclic compounds. By contrast, analogous reactions of [(η3‐allyl)Pd(N3)(IPr)] with an organic isocyanide (R–NC: R = tert‐butyl, benzyl) gave the adduct [(η3‐allyl)Pd(N3)(IPr)]·(R–NC) as the only product or a mixture of the adduct and a dipolar cycloaddition product, [(η3‐allyl)Pd{CN4(R)}(IPr)], depending on the isocyanides used. Finally, a series of (π‐allyl)Pd–NHC pseudohalogen complexes, [(π‐allyl)Pd(X)(NHC)], exhibited high catalytic activity in Suzuki–Miyaura cross‐coupling reactions of aryl chlorides with arylboronic acids.

Mild Negishi Cross-Coupling Reactions Catalyzed by Acenaphthoimidazolylidene Palladium Complexes at Low Catalyst Loadings

Considering that the strong σ-donor property of ylidenes derived from π-extended imidazolium salts is conducive to increasing the catalytic activity of the resulting palladium N-heterocyclic carbene complexes, robust acenaphthoimidazol-ylidene palladium complexes 3a-c with varying bulky substituted groups were prepared from the corresponding acenaphthoimidazolium chlorides by heating with PdCl2 and K2CO3 in neat 3-chloropyridine in satisfactory yields. Even at a catalyst loading as low as 0.25 mol %, complex 3a exhibited extremely high catalytic activity toward Negishi cross-coupling of alkylzinc reagents with a wide range of (hetero)aryl halides under mild reaction conditions within 30 min. Besides a great number of bromoarenes, various less expensive and inactive (hetero)aryl chlorides were coupled successfully with the alkyl- and arylzinc reagents, in which active functional groups (such as -NH2) were well tolerated even in one-pot dicoupling transformations without protection. In addition, in the case of coupling with secondary alkylzinc reagents, undesired β-hydride elimination leading to isomerized linear products was efficaciously suppressed. The catalyst system also displayed superiority in the construction of heterobiaryls through the coupling of heteroarylzinc reagents and heterocylic chloroarenes which were hardly accessible from the corresponding organoboron reagents by Suzuki-coupling reactions. Therefore, the protocol described in this paper represents a mild, general, and scalable approach to access various structurally intriguing and functionalized (hetero)aryls.

Catalyst-Controlled Divergent C–H Functionalization of Unsymmetrical 2-Aryl Cyclic 1,3-Dicarbonyl Compounds with Alkynes and Alkenes

Achieving site-selective, switchable C-H functionalizations of substrates that contain several different types of reactive C-H bonds is an attractive objective to enable the generation of different products from the same starting materials. Herein, we demonstrate the divergent C-H functionalization of unsymmetrical 2-aryl cyclic 1,3-dicarbonyl compounds that contain two distinct, nonadjacent sites for initial C-H functionalization, where product selectivity is achieved through catalyst control. By use of a palladium-N-heterocyclic carbene complex as the precatalyst, these substrates undergo oxidative annulation with alkynes to provide spiroindenes exclusively. In contrast, a ruthenium-based catalyst system gives benzopyrans as the major products. Examples of divergent, oxidative C-H alkenylations of the same substrates are also provided.

ChemInform Abstract: N‐Heterocyclic Carbene—Palladium(II)‐1‐methylimidazole Complex Catalyzed Suzuki—Miyaura Coupling of Benzylic Chlorides with Arylboronic Acids or Potassium Phenyltrifluoroborate in Neat Water.

An easily available N-heterocyclic carbene–palladium(II)–1-methylimidazole complex 1 showed efficient catalytic activity in the Suzuki–Miyaura coupling of benzylic chlorides with arylboronic acids or potassium phenyltrifluoroborate in neat water under mild conditions, providing an alternative method for the synthesis of diarylmethane derivatives, which widely exist in molecules with pharmaceutical activities and are also frequently found as subunits in supramolecules. Under the optimal conditions, all reactions performed well to give the desired products in moderate to almost quantitative yields in an environmentally benign medium within 12 h, extending their applications toward potentially industrial processes.

N-Heterocyclic carbene–palladium(ii)–1-methylimidazole complex catalyzed Suzuki–Miyaura coupling of benzylic chlorides with arylboronic acids or potassium phenyltrifluoroborate in neat water

An easily available N-heterocyclic carbene-palladium(II)-1-methylimidazole complex showed efficient catalytic activity in the Suzuki-Miyaura coupling of benzylic chlorides with arylboronic acids or potassium phenyltrifluoroborate in neat water under mild conditions, providing an alternative method for the synthesis of diarylmethane derivatives, which widely exist in molecules with pharmaceutical activities and are also frequently found as subunits in supramolecules. Under the optimal conditions, all reactions performed well to give the desired products in moderate to almost quantitative yields in an environmentally benign medium within 12 h, extending their applications toward potentially industrial processes.

Synthesis and applications of polymeric N-heterocyclic carbene palladium complex-grafted silica as a novel recyclable nano-catalyst for Heck and Sonogashira coupling reactions

A new catalytic system based on palladium nanoparticles supported on polymeric N-heterocyclic carbene-grafted silica (Si–PNHC–Pd) is introduced. Aminopropylsilica was reacted with benzoylchloride to form acrylamidopropylsilica. Onto this functionalized silica, vinylimidazole monomer was copolymerized by free radical polymerization. Poly(vinylimidazole)-grafted silica was treated with methyl iodide to form a quaternary salt of poly(vinylimidazole). Si–PNHC–Pd was obtained by subsequent treatment of imidazolium salt with PdCl2. Determination of Pd content was performed using an inductively coupled plasma (ICP) analyzer and the crystalline character was determined by an XRD technique. The topography and particle size of the catalyst was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM images showed that palladium was dispersed through the polymer surface in nano particle size. This catalytic system exhibited excellent activity in Heck and Sonogashira coupling reactions with various aryl halides. The catalyst was successfully re-used up to 12 runs without appreciable change in its activity. High efficiency of the catalyst along with high yields of products, easy purification, large scale synthesis and high TON and TOF are among the advantages of this heterogeneous catalyst.

Graphene oxide supported N-heterocyclic carbene-palladium as a novel catalyst for the Suzuki–Miyaura reaction

In this paper, an N-heterocyclic carbene–palladium complex (NHC-Pd2+) was immobilized onto the surface of graphene oxide via a chemical bonding method for the first time. The resultant graphene oxide supported NHC–Pd2+ composite (GO supported NHC-Pd2+) was characterized by transmission electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy and infrared spectroscopy. It was demonstrated that the GO supported NHC-Pd2+ can act as an efficient catalyst for Suzuki–Miyaura coupling reactions. The yields of the products were in the range of 83% to 96%. The catalyst can be readily recovered and reused for at least 6 consecutive cycles without any significant loss in catalytic activity.

The regioselective Larock indole synthesis catalyzed by NHC–palladium complexes

The first practical and regioselective process for the synthesis of 2,3-disubstituted indoles from the reaction of o-iodoanilines or o-bromoanilines and their derivatives with symmetrical and unsymmetrical internal alkynes catalyzed by a ferrocene-functionalized N-heterocyclic carbene (NHC)–palladium complex has been developed, and the indoles were isolated in good yields with high regioselectivity.

Synthesis and characterization of a pyrazolium bearing N-heterocyclic carbene–palladium(II) complex

3,5-Dichloro-1,2-diphenylpyrazolium tetrafluoroborate 1 reacts at 0 °C with 2,6-dimethylphenol in the presence of triethylamine to give the 5-chloro-3-aryloxy-1,2-diphenylpyrazolium tetrafluoroborate 2 (58% yield). Heterocycle 2 reacts with 1-methylimidazole affording 5-(1-methyl-1H-imidazolium-3-yl)-3-aryloxy-1,2-diphenylpyrazolium chloride tetrafluoroborate 3 in 83% yield. When half an equivalent of allylchloropalladium dimer [Pd(allyl)Cl]2 is added to 3 in the presence of triethylamine, monocationic compound 4 featuring an N-heterocyclic carbene coordinated to allylchloropalladium(II) is isolated in 72% yield. All compounds have been fully characterized by multinuclear NMR spectroscopy. We also describe the X-ray crystal structure of 4, which crystallizes in the monoclinic C2/c space group.

N-Heterocyclic carbene–palladium(II)-1-methylimidazole complex catalyzed Mizoroki–Heck reaction of aryl chlorides with styrenes

A well-defined N-heterocyclic carbene–palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid.

Benzimidazole-based palladium–N-heterocyclic carbene: a useful catalyst for C–C cross-coupling reaction at ambient condition

Abstract A convenient way for the synthesis of benzimidazole-based Pd–N-heterocyclic carbene complex and its structural characterization are described. The complex efficiently catalyzes Suzuki cross-coupling reaction in a wide variety of substrates including heteroaromatic system at ambient condition. The catalyst is also effective for multi Suzuki cross-coupling reaction. In addition, the catalyst is equally active toward C–C cross-coupling reaction between acid chloride and arylboronic acid, giving the desired ketones in high yield.

N-Heterocyclic Carbene–Palladium(II)–1-Methylimidazole Complex-Catalyzed Suzuki–Miyaura Coupling of Aryl Sulfonates with Arylboronic Acids

A well-defined NHC-Pd(II)-Im complex 1 was found to be an effective catalyst for the Suzuki-Miyaura coupling of aryl sulfonates including tosylates and phenylsulfonates with arylboronic acids, giving the desired coupling products in good to high yields. Acceptable yields can also be achieved even by using the less reactive mesylates as the substrates. It is worthy of noting here that this is the first example of NHC-Pd(II) complex-catalyzed Suzuki-Miyaura coupling of aryl sulfonates with arylboronic acids, enriching an inexpensive, convenient, and alternative method for the synthesis of biaryl compounds.

ChemInform Abstract: The Development of Bulky Palladium NHC Complexes for the Most‐Challenging Cross‐Coupling Reactions.

AbstractReview: 148 refs.