Carbene Palladium Complexes Research Articles

Solvent‐Free Buchwald–Hartwig (Hetero)arylation of Anilines, Diarylamines, and Dialkylamines Mediated by Expanded‐Ring N‐Heterocyclic Carbene Palladium Complexes

AbstractA highly efficient solvent‐free protocol for the Buchwald–Hartwig (hetero)arylation of anilines, diarylamines, and dialkylamines mediated by the expanded‐ring N‐heterocyclic carbene palladium complex (THP‐Dipp)Pd(cinn)Cl [THP‐Dipp = 1,3‐bis(2,6‐diisopropylphenyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene; cinn = cinnamyl, 3‐phenylallyl] was developed. The catalytic protocol was efficient for the coupling of amines and (hetero)aryl chlorides and bromides bearing donor, acceptor, and bulky substituents.

A new N–heterocyclic carbene palladium complex immobilized on nano silica: An efficient and recyclable catalyst for Suzuki–Miyaura C[sbnd]C coupling reaction

A new catalytic system based on Pd–NHC complex utilizing (4–pyridyl)bis(imidazolyl)methane as a bidentate NHC ligand supported on nano‒silica is introduced. The catalyst was characterized by FT–IR spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy, energy dispersive X–ray analysis, transmission electron microscopy and elemental analysis. This heterogeneous catalytic system exhibited excellent activity in the Suzuki–Miyaura coupling reaction of various aryl halides with phenylboronic acid, and was reusable several times without significant loss of its catalytic activity.

Synthesis of conjugated Mn porphyrin polymers with p-phenylenediamine building blocks and efficient aerobic catalytic oxidation of alcohols

A series of conjugated metalloporphyrin polymers (MnP-AMPs) were synthesized based on Buchwald-Hartwig aromatic amination with p-phenylenediamine and manganese tetraphenylporphyrin as building blocks, and N-heterocyclic carbene-palladium complex as catalyst. Brunauer–Emmett–Teller surface area analysis, scanning electron microscopy and transmission electron microscopy showed that MnP-AMPs had large surface areas and uniform pore sizes. The stable porous and superreticular conjugated structure of MnP-AMP further activated manganese ions in the porphin ring compared with those in the original monomanganeseporphyrin. Moreover, introducing CN bonds in MnP-AMP increased its polarity and affinity to alcohols, so MnP-AMP exhibited high catalytic activity when applied to aerobic oxidation of alcohols. For the catalysis of selective oxidation of benzyl alcohol to benzaldehyde with air, the conversion and aldehyde selectivity both reached up to 100% under mild condition in acetonitrile at 40°C. MnP-AMP also remained stable in oxidation reaction system and still had high catalytic activity after several recycles.

E-H (E = B, Si, Ge) bond activation of pinacolborane, silanes, and germanes by nucleophilic palladium carbene complexes.

The reactivity of two nucleophilic palladium carbenes, [PC(sp(2))P]Pd(PMe3) and [PC(sp(2))P]Pd(PPh3), where [PC(sp(2))P] = bis[2-(di-iso-propylphosphino)phenyl]methylene, toward the E-H bond activation of Ph4-nEHn (E = Si, Ge; n = 1-3) and pinacolborane (HBpin) is discussed. Unlike previous reports, both types of isomer species, hydride [PC(EHn-1Ph4-n)P]PdH or [PC(Bpin)P]PdH and silyl/germyl [PC(H)P]Pd(EHn-1Ph4-n), were observed depending on the substrate and the phosphine ligand, showing that the polarity of the Pd-C bond can be tuned by the phosphine substituents.

Amino acid-derived N-heterocyclic carbene palladium complexes for aqueous phase Suzuki–Miyaura couplings

Five palladium complexes featuring N-heterocyclic carbene ligands derived from sustainable precursors have been synthesized and applied in aqueous Suzuki–Miyaura couplings.

Porphyrins fused to N-heterocyclic carbene palladium complexes as tunable precatalysts in Mizoroki–Heck reactions: How the porphyrin can modulate the apparent catalytic activity?

Porphyrins fused to N-heterocyclic carbenes (NHCs) 1M, with inner metal M = Ni, Cu, Zn and H2, were used to synthesize palladium(II) complexes. These complexes behaved as precatalysts for high-temperature Mizoroki–Heck reactions between iodobenzene and styrene. Despite similar electronic properties between 1M ligands, catalytic activity was found to significantly depend on the presence, or not, of a central metal incorporated in the porphyrin core: a conversion of iodobenzene between 80–90% was observed with metalloporphyrins (M = Ni, Cu and Zn), whereas only 30% was observed with free-base porphyrins (M = H2) after 10 h. Palladium leaching and subsequent trapping of free palladium in the inner core of the free-base porphyrin modified the concentration of Pd(0) active species and may explain the lower catalytic activity observed. Des porphyrines conjuguées à des carbènes N-hétérocycliques 1M avec M = Ni, Cu, Zn et H2 ont été utilisées pour synthétiser des complexes de palladium(II). Ces complexes sont des pré-catalyseurs de la réaction de couplage de Heck entre l’iodobenzène et le styrène à haute température. Malgré des propriétés électroniques similaires entre les ligands 1M, les activités catalytiques dépendent significativement de la présence, ou non, d’un métal central dans la porphyrine : une conversion de 80–90% de l’iodobenzène est observée avec les métalloporphyrines (M = Ni, Cu et Zn), tandis qu’une conversion de seulement 30% est observée avec des porphyrines bases libres (M = H2) après 10 heures de réaction. La libération de palladium et son piégeage dans la porphyrine base libre modifie la concentration d’espèces actives de Pd(0) et peut expliquer la plus faible activité catalytique observée.

Homoleptic chelating N-heterocyclic carbene complexes of palladium immobilized within the pores of SBA-15/IL (NHC–Pd@SBA-15/IL) as heterogeneous catalyst for Hiyama reaction

In this work, SBA-15 was selected as a support for the homoleptic chelating N-heterocyclic carbene complexes of palladium. Here, Pd2+ ions were coordinated to N-heterocyclic carbene (NHC–Pd) which also contain ionic liquid. Then, this NHC–Pd@SBA-15/IL catalyst (NHC–Pd active part of the catalyst) was successfully applied in the catalysis of Hiyama coupling reaction with lower mol% Pd rather to previously reported literatures. The role of TBAF and Cs2CO3 in the decrease of Pd mol% was crucial. Recyclability test showed a successful reusability for 5 runs. Simultaneous application of TBAF and NaOH did destroy the silica structure of SBA-15 in the first run.

Palladium Complexes with Tetrahydropyrimidin-2-ylidene Ligands: Catalytic Activity for the Direct Arylation of Furan, Thiophene, and Thiazole Derivatives

The synthesis and characterization of novel 1,3-benzyl-3,4,5,6-tetrahydropyrimidin-2-ylidene-based N-heterocyclic carbene palladium(II) complexes (1a–d) were described. The crystal structure of trans-dichlorobis[1,3-bis(4-methylbenzyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]palladium(II) was presented. Pd(II) complexes 1a–d were tested as catalysts in the direct C5 or C2 arylation of furans, thiophenes, and thiazoles, with various aryl bromides at 150 °C for 1 h. These complexes exhibited moderate to high catalytic activities under the given conditions.

Imino-N-Heterocyclic Carbene Palladium(II) Complex-Catalyzed Direct Arylation of Electron-Deficient Fluoroarenes with “On and Off” Chelating Effect Assistance

An imino-N-heterocyclic carbene palladium(II) complex with a bulky substituted group on the imino nitrogen was used to catalyze the direct arylation of electron-deficient fluoroarenes with aryl halides. A series of electron-poor substrates and aryl bromides could be coupled in good to excellent yields with satisfactory position selectivity (20 examples, up to 93%). These arylations could proceed at a relatively low temperature (80 °C, 20 examples, up to 95%) with mono-N-protected amino acid assistance. Some of them even gained higher yields than those at high temperature (110 °C). Otherwise, some aryl iodides can forge cross-coupling products in yields of nearly 30% under the optimized conditions. The rate profiles for arylation of electron-poor arenes were measured in the presence of the imino-N-heterocyclic carbene palladium(II) complex or Pd(OAc)2 as the catalyst, which showed that the former could keep catalytic activity for a longer time. Computational studies indicated that the imino nitrogen in the imino-N-heterocyclic carbene ligand can detach from and attach to the central metal in the catalytic cycle. Thus, the coordination site could be protected, and this effect may be responsible for decreasing the rate of palladium black formation.

Palladium carbene complexes as persistent radicals.

A series of palladium(ii) radical carbene complexes, [PC˙(sp2)P]PdI, [PC˙(sp2)P]PdBr, and [PC˙(sp2)P]PdCl (PC(sp3)H2P = bis[2-(di-iso-propylphosphino)-phenyl]methane), is described. Compound [PC˙(sp2)P]PdI dimerizes to {[PC(sp2)P]PdI}2 in the solid state, akin to the formation of Gomberg's dimer. While the bromo and the iodo derivatives could be obtained from the oxidation of [PC(sp2)P]Pd(PMe3) by the respective dihalogens, a halogen transfer reaction from CH2Cl2 was used for the formation of [PC˙(sp2)P]PdCl. The halogen transfer from CH2X2 (X = Cl, Br, I) could be used to obtain all three radical carbene palladium complexes and also allowed the isolation of [PC(CH2)P]Pd(PMe3), which is the result of methylene group transfer from CH2X2. Compound [PC(CH2)P]Pd(PMe3) was independently synthesized from [PC(CH3)HP]PdCl2, which contains a supporting ligand analogous to that of the radical carbene complexes but has one of the hydrogen atoms replaced by a methyl group. All three carbene radical species abstract a hydrogen from 9,10-dihydroanthracene or n Bu3SnH.

New Progress in C—C Coupling Reactions Catalyzed by Supported N-Heterocyclic Carbene Palladium Complex

The C—C coupling reactions catalyzed by supported N-heterocyclic carbene palladium complex have attracted considerable attention because catalytic activity is excellent and supported catalyst can be separated easily and reused several times. The performance of the carriers has an important effect on the catalytic activity and reusability of the supported catalyst. In this paper, the advances in the C—C coupling reactions catalyzed by supported N-eterocyclic carbene palladium complex have been reviewed. Polymer, silica, carbon and magnetic nanoparticles were used to support the N-heterocyclic carbene pal- ladium catalysts. Finally, the direction of the supported N-heterocyclic carbene palladium complex was also put forward.

Palladium-catalysed Suzuki--Miyaura cross-coupling with imidazolylidene ligands substituted by crowded resorcinarenyl and calixarenyl units

Two $N$-heterocyclic carbene (NHC) palladium complexes of formula [PdBr$_{2}$(NHC)(pyridine)] in which the carbenic ring is flanked by sterically crowded cavitand substituents were prepared from appropriate imidazolium salts bearing either two resorcinarene or a combination of resorcinarene and calixarene fragments. Both complexes displayed high stability and good activities in the cross-coupling of aryl bromides with phenyl boronic acid. One of the imidazolium salts was characterised by an X-ray diffraction study.

Redox-induced umpolung of transition metal carbenes.

Metal carbene complexes have been at the forefront of organic and organometallic synthesis and are instrumental in guiding future sustainable chemistry efforts. While classical Fischer and Schrock type carbenes have been intensely studied, compounds that do not fall within one of these categories have attracted attention only recently. In addition, applications of carbene complexes rarely take advantage of redox processes, which could open up a new dimension for their use in practical processes. Herein, we report an umpolung of a nucleophilic palladium carbene complex, [{PC(sp2)P} tBuPd(PMe3)] ({PC(sp2)P} tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene), realized by successive one-electron oxidations that generated a cationic carbene complex, [{PC(sp2)P} tBuPdI]+, via a carbene radical, [{PC˙(sp2)P} tBuPdI]. An EPR spectroscopic study of [{PC˙(sp2)P} tBuPdI] indicated the presence of a ligand-centered radical, also supported by the results of reactions with 9,10-dihydroanthracene and PhSSPh. The cationic carbene complex shows electrophilic behavior toward nucleophiles such as NaH, p TolNHLi, PhONa, and PMe3, resulting from an inversion of the electronic character of the Pd-Ccarbene bond in [{PC(sp2)P} tBuPd(PMe3)]. The redox induced umpolung is reversible and unprecedented.

Frustrated Lewis pair-like reactions of nucleophilic palladium carbenes with B(C6F5)3.

The reactions of two nucleophilic palladium carbene complexes with the strong Lewis acid B(C6F5)3 afforded two zwitterionic products. One of them features a remote nucleophilic attack at the para-carbon of the supporting ligand, while the other indicates C-F activation of B(C6F5)3. Both behaviours are reminiscent of the reactivity of frustrated Lewis pairs due to the steric inaccessibility of the nucleophilic carbon center, but are unprecedented for transition metal carbene complexes. Furthermore, when those reactions are carried out in the presence of H2, products resulting from H2 splitting are observed.

ChemInform Abstract: Dinuclear N‐Heterocyclic Carbene Palladium(II) Complexes as Efficient Catalysts for the Buchwald—Hartwig Amination.

Abstract A series of dinuclear N-heterocyclic carbene (NHC) palladium complexes [PdCl2(NHC)]2(μ-L) (1–6, L = diphosphine ligands) has been synthesized and fully characterized by 1H NMR, 13C NMR and 31P NMR spectroscopies, elemental analysis, IR spectroscopy, and X-ray crystal structural study. The solid-state structures of 1–6 show dinuclear frameworks with two palladium(II) centres were held together by bridging diphosphine ligands. Each palladium centre was coordinated by an N-heterocyclic carbene ligand, a phosphorus atom and two chloro ions in a trans-arrangement. Further explorations of the catalytic potential of the dinuclear carbene palladium complexes as catalysts for Pd-mediated transformations have been examined under microwave irradiation conditions and the title complexes (1–6) displayed good catalytic activities in Buchwald–Hartwig aminations of aryl chlorides.

Reversible chelating in acyclic diaminocarbene palladium complex containing hydrazide fragment

Study of molar conductivity of several palladium(II) complexes has revealed that cationic acyclic diaminocarbene complexes of palladium(II) containing carbohydrazide fragment in the carbene ligand are capable of reversible cyclization to form a six-membered C,O-chelate. This process is favored by factors enhancing dissociation of the anionic ligand in the starting neutral compound. DFT simulation of cationic carbene complexes of palladium has confirmed possibility of such chelating.

Synthesis of Imidazole-Based Functionalized Mesoionic Carbene Complexes of Palladium: Comparison of Donor Properties and Catalytic Activity toward Suzuki–Miyaura Coupling

Three different backbone-monofunctionalized imidazolium salts have been synthesized using the metal–halogen exchange procedure, and their corresponding mesoionic carbene complexes with palladium were prepared via oxidative addition without protection of the C2 position. The donor properties were evaluated with 31P NMR spectroscopy of the respective palladium complexes. The catalytic activity of these complexes toward Suzuki–Miyaura coupling of aryl bromides was also explored. Also, in one case, a comparison of donor properties was made with those of a “normal” carbene with similar steric bulk.

Synthesis and Reactivity of a Nucleophilic Palladium(II) Carbene

Two formal palladium carbene complexes, [PC(sp2)P]Pd(PR3) (3: R = Me; 4: R = Ph) were isolated and characterized from [PC(sp3)H2P] ([PC(sp3)H2P] = bis[2-(di-isopropylphosphino)phenyl]methane, iPr2P-C6H4-CH2-C6H4-PiPr2). Structural studies and DFT calculations indicate that the interaction between palladium and carbon is best described as a single bond, associated with nucleophilic character at that carbon atom. The characteristics of 3 were probed by reactions with electrophiles (MeI), acids (MeOH and HCl), and para-toluidine.

ChemInform Abstract: Synthesis, Characterization and X‐Ray Structures of N‐Heterocyclic Carbene Palladium Complexes Based on Calix[4]arenes: Highly Efficient Catalysts Towards Suzuki—Miyaura Cross‐Coupling Reactions.

AbstractA novel calix[4]arene tagged palladium NHC complex is found to be the most efficient catalyst for Suzuki—Miyaura cross‐coupling reactions.

A new library of arsine, stibine-stabilized N-heterocyclic carbene palladium complexes: synthesis, structures and activities in C-C and C-N coupling reactions.

A series of new N-heterocyclic carbene palladium complexes bearing AsPh3 and SbPh3 functions PdCl2(IMes)(AsPh3) (1a), PdCl2(IMes)(SbPh3) (1b), PdCl2(IPr)(AsPh3) (1c), PdCl2(IPr)(SbPh3) (1d), PdCl2(SIMes)(AsPh3) (2a), PdCl2(SIMes)(SbPh3) (2b), PdCl2(SIPr)(AsPh3) (2c), and PdCl2(SIPr)(SbPh3) (2d) (IMes = N,N'-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr = N,N'-bis-(2,6-di(iso-propyl)phenyl)imidazol-2-ylidene; SIMes = N,N'-bis-(2,4,6-trimethylphenyl)imidazolidin-2-ylidene; SIPr = N,N'-bis-(2,6-di(iso-propyl)phenyl)imidazolidin-2-ylidene) was synthesized and fully characterized by (1)H NMR, (13)C NMR, and FT-IR spectroscopy, elemental analysis and single-crystal X-ray crystallography. The catalytic potential of these arsine and stibine-stabilized carbene palladium complexes for Pd-mediated transformations was investigated with various substrates in the Hiyama cross-coupling reactions and Buchwald-Hartwig aminations. The results demonstrate that these complexes show high catalytic activity and good tolerance to various chemical functions.