Palladium(II)-N-heterocyclic carbene complexes derived from proline: synthesis and characterization

Pd(II)-N-heterocyclic carbene complexes derived from proline have been successfully synthesized in good yields and their structures have been characterized by X-ray single crystal diffraction. It was found that the substituents on the N-atom of the pyrrolidine skeleton dramatically affect on the coordination pattern of the palladium complexes. In a word, when an electron-rich group as benzyl group was attached on the N-atom, both of the N-atom and NHC were coordinated to the Pd(II) center; while when an electron-poor group as Ts group was attached, a dimeric mono-coordinated Pd(II)-NHC was obtained exclusively.     

Facile one-step synthesis of bis(NHC) ruthenium benzylidene catalyst for ring-closing metathesis

Bis(NHC) ruthenium benzylidene complex (H₂IMe)₂(Cl)₂RuCHPh (9) [H₂IMe = 1,3-bis(2,6- dimethylphenyl)-4,5-dihydroimidazol-2-ylidene] was synthesized facilely by one-step reaction of (PPh₃)₂(Cl)₂RuCHPh (7) with N-heterocyclic carbene (NHC) H₂IMe (6). Complex 9 proved to exhibit remarkable catalytic activity for ring-closing metathesis (RCM) reaction at increased temperature.     

Synthesis of unsymmetrical benzils using N-heterocyclic carbene catalysis

In this paper, we propose a novel and efficient method for the preparation of various unsymmetrical benzils. We first demonstrate the nucleophilic aroylation of N-phenylbenzimidoyl chlorides with aromatic aldehydes using N-heterocyclic carbene as the catalyst to afford 1-aryl-2-phenyl-2-(phenylimino)ethanones. These iminoethanones were then converted to 1,2-diaryl-1,2-diketones by acid-promoted hydrolysis.     

γ-Selective allylic substitution reaction with Grignard reagents catalyzed by copper N-heterocyclic carbene complexes and its application to enantioselective synthesis

The reaction of allylic compounds with alkyl Grignard reagents in the presence of a catalytic amount of copper N-heterocyclic carbene (NHC) complexes proceeded predominantly in an S_N2′ reaction pathway to give γ-substituted product in excellent yield. The method was applied to asymmetric reaction by using optically active NHC ligands.     

Facile route to benzils from aldehydes via NHC-catalyzed benzoin condensation under metal-free conditions

A simple and efficient one-pot procedure for the synthesis of α-diketones from aldehydes via benzoin condensation under the influence of a catalytic amount of azolium salt combined with DBU has been developed. Thus, aldehyde was allowed to react with the azolium salt/DBU catalytic system at room temperature, and then the reaction mixture was heated to 70 °C under air atmosphere to afford the corresponding 1,2-diketone in good yield. This would be an efficient alternative method of synthesizing α-diketones from aldehydes under metal-free conditions.     

N-Heterocyclic carbene-Pd(II) complex derived from proline for the Mizoroki-Heck reaction in water

N-Heterocyclic carbene-Pd(II) complex 1 derived from proline was found to be an efficient catalyst in the Mizoroki-Heck reaction of aryl bromides and iodides performed in water. The reactions can tolerate various functional groups in the substrates and all gave the corresponding coupling products in good to high yields.     

Silver(I)-promoted intramolecular addition of N-heterocyclic carbenes towards unsaturated esters in water

Silver(I)-promoted intramolecular addition reaction between N-heterocyclic carbene and unsaturated esters in water is described. The reaction leads to 1H-imidazo[1,2-a]indole derivatives in moderate to excellent yields.     

Highly efficient trialkylsilylcyanation of aldehydes, ketones and imines catalyzed by a nucleophilic N-heterocyclic carbene

The synthetic utility of N-heterocyclic carbenes was demonstrated by the trialkylsilylcyanation of aldehydes, ketones and imines. In the presence of a catalytic amount of 3a, the reactions with Me₃SiCN proceeded smoothly to give the corresponding cyanohydrin trimethylsilyl ethers or amino nitrile derivatives in good to excellent yields.     

Palladium(II)-N-heterocyclic carbene complex derived from proline towards Suzuki-Miyaura coupling reaction in water at room temperature

Palladium(II)-N-heterocyclic carbene complex 1 derived from proline has been proved to be a highly effective catalyst in the Suzuki-Miyaura coupling reaction of aryl iodides and bromides with arylboronic acids in water at room temperature. The reactions are tolerant towards various functional groups in the substrates. Moreover, the complex 1-catalyzed medium-scale (10.0 mmol) Suzuki-Miyaura reactions were also carried out and it was found that the complex was also effective enough in these cases.     

Solvent-controlled selective synthesis of a trans-configured benzimidazoline-2-ylidene palladium(II) complex and investigations of its Heck-type catalytic activity

Reaction of N,N′-dimethylbenzimidazolyl iodide (A) with Pd(OAc)₂ in DMSO gives selectively trans-bis(N,N′-dimethylbenzimidazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isomer and thus the cis-trans rearrangement is driven by the insolubility of trans-2 in DMSO. X-ray single-crystal diffraction analysis and ¹³C NMR spectroscopy confirm the trans-geometry of the square planar Pd(II) complex. Catalytic studies show that cis-1 and trans-2 are highly efficient in the Mizoroki-Heck coupling reaction of aryl bromides and activated aryl chlorides both in DMF and [N(n-C₄H₉)₄]Br as ionic liquid. The catalytic activities of Pd(II) complexes with N-heterocyclic carbene ligands derived from benzimidazole are comparable to their imidazole-derived analogues.     

N-Functionalised heterocyclic dicarbene complexes of silver: Synthesis and structural studies

A range of new diimidazolium salts, held together by an alkyl unit and bearing alcohol pendant arms on their nitrogen, was synthesized. A short modular reaction pathway leads to the N-heterocyclic carbene (NHC) precursors, differing by the flexibility of the bridge, the steric bulk of substituents in α-position of the OH groups and the anions. Treatment of diimidazolium salts with Ag₂O yields Ag^I(carbene)₂ complexes. The related trimethylene-bridged bis-NHC silver complexes 6 and 7 were crystallised with di-tosylate and di-hexafluorophosphate anions, respectively. Their X-ray structures revealed dimeric species, involving two ligands with different arrangements around the Ag cations, leading to crossed and parallel conformations.     

Anellated N-heterocyclic carbenes: 1,3-Dineopentyl-benzimidazol-2-ylidene,structural aspects of C-protonated precursor salts and an AgCl complex

Access to 1,3-dineopentyl-benzimidazol-2-ylidene (1) by deprotonation of various benzimidazolium salts was studied. [Ag(1)Cl] was prepared from the corresponding benzimidazolium chloride. X-ray crystal structure analyses of benzo-, naphtho- and quinoxalino-anellated imidazolium salts displayed neither significant changes nor systematic trends of bond lengths and angles within the five-membered ring. Consistently downfield shifts of the ¹³C¹H(2) nuclei reflect the influence of electron-withdrawing anellation.     

Efficient catalysis of Ullmann-type arylation reactions by a novel trinuclear copper(I) complex with a chelating tricarbene ligand

A novel trinuclear copper(I) complex with a chelating tricarbene ligand is shown to be an efficient catalyst for the arylation of different classes of compounds containing N-H or O-H functions. Different kinds of azole rings (pyrazole, imidazole, 1,2,4-triazole) can be arylated with comparable efficiencies at relatively mild temperatures (100 °C). The catalyst activates aryl iodides, bromides and even chlorides for the reaction. An unusually strong influence of the nature of the aryl substituent on the reaction yield is observed. The synthetic protocol can be extended to other substrate classes, such as phenols and amides, although the catalytic efficiency with amides is significantly reduced.     

An improved synthesis of Biginelli-type compounds via phase-transfer catalysis

3,4-Dihydropyrimidin-2(1H)-one, 3,4-dihydropyrimidin-2(1H)-thione, and 3,4-dihydropyrimidin-2(1H)-imine derivatives were synthesized by modified Biginelli cyclocondensation reaction in a time-efficient manner with near quantitative yields starting from appropriately substituted aromatic aldehyde, β-ketoester and either urea, thiourea or guanidine as constituent synthons using tetra-butyl ammonium bromide (TBAB) as catalyst for the first time.     

Well-defined NHC-Pd(II)-Im (NHC=N-heterocyclic carbene; Im=1-methylimidazole) complexes catalyzed amination of aryl chlorides

A new class of well-defined NHC-Pd(II)-Im complexes was synthesized and was found to be an efficient catalyst for the amination reactions of aryl chlorides. Under the optimal reaction conditions, a range of amines can be coupled to give the amination products in good to high yields.     

Sequential one-pot and three-component reactions of an N-heterocyclic carbene to form 4-(1,2,4-triazol-5-ylidene)pyrrolidine-2,5-diones: a tandem umpolung/annulation sequence via deoxy-Breslow intermediates

The tandem sequence of the umpolung of α,β-unsaturated esters by an N-heterocyclic carbene (NHC) followed by annulation is demonstrated. The deoxy-Breslow intermediates are selectively generated from the reaction of NHC with methyl methacrylate, dimethyl itaconate, or methyl crotonate. Subsequently, they undergo annulation with isocyanates to give 4-(1,2,4-triazol-5-ylidene)pyrrolidine-2,5-diones in good isolated yields. In addition, the stoichiometric three-component reaction of NHC, methyl methacrylate, and phenyl isocyanate proceeded, accompanied by the formation of the 1:2 adduct of NHC with phenyl isocyanate.     

Use of tetrahydropyrimidinium salts for highly efficient palladium-catalyzed cross-coupling reactions of aryl bromides and chlorides

New, sterically demanding 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (2) as NHC precursors have been synthesized and characterized. These salts, in combination with palladium acetate, provided active catalysts for the cross-coupling of aryl chlorides and bromides under mild conditions. The catalytic system was applied to the Heck, Suzuki and benzaldehyde (Kumada) coupling reactions. Catalyst activity was found to be influenced by the presence of a methoxy group on the ring of the p-position of benzyl substituent of the ligand precursor.     

Cobalt-catalyzed sequential cyclization/cross-coupling reactions of 6-halo-1-hexene derivatives with Grignard reagents and their application to the synthesis of 1,3-diols

Cobalt/N-heterocyclic carbene system or cobalt/diamine combination effectively catalyzes sequential cyclization/cross-coupling reactions of 6-halo-1-hexene derivatives with trialkylsilylmethyl, 1-alkynyl, and aryl Grignard reagents. The sequential cyclization/cross-coupling reactions are applied to the synthesis of 1,3-diols starting from siloxy-tethered 6-halo-1-hexene derivatives.     

Pd-NHC-catalyzed synthesis of diaryl ketones

With N-hetereocyclic carbene and palladium catalysis, diaryl ketones with a variety of functional groups that span from electron withdrawing to electron donating substitutions can be conveniently synthesized using the corresponding aryl boronic acid and N-acyloxyphthalimide.     

The synthesis and X-ray structure of trans-NiCl2(1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)2; attempts to polymerize olefins utilizing a nickel(II) complex of a sterically demanding N-heterocyclic carbene

The compound trans-NiCl₂(IPr)₂ (IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) is synthesized and characterized crystallographically. In combination with MAO, it is converted to an intermediate which catalyses the dimerization of ethylene but which not the polymerization of ethylene, propylene or 1-hexene.