Chiral Imidazolium Salts Research Articles

Chiral imidazolium and triazolium salts as NHC and aNHC ligand precursors: A promising framework for asymmetric epoxidation catalysis

Two chiral imidazolium salts and one chiral triazolium salt were synthesized containing a chiral cyclohexane bridge based on the salen ligand framework of the Jacobsen-Katsuki epoxidation catalysts. They are promising NHC and aNHC ligand precursors for asymmetric epoxidation or C–H oxidation catalyzed by organometallic compounds. The ligand framework offers plenty of opportunities for modification.

2-Unsubstituted Imidazole N-Oxides as Novel Precursors of Chiral 3-Alkoxyimidazol-2-ylidenes Derived from trans-1,2-Diaminocyclohexane and Other Chiral Amino Compounds.

‘Desymmetrization’ of trans-1,2-diaminocyclohexane by treatment with α,ω-dihalogenated alkylation reagents leads to mono-NH2 derivatives (‘primary-tertiary diamines’). Upon reaction with formaldehyde, these products formed monomeric formaldimines. Subsequently, reactions of the formaldimines with α-hydroxyiminoketones led to the corresponding 2-unsubstituted imidazole N-oxide derivatives, which were used here as new substrates for the in situ generation of chiral imidazol-2-ylidenes. Upon O-selective benzylation, new chiral imidazolium salts were obtained, which were deprotonated by treatment with triethylamine in the presence of elemental sulfur. Under these conditions, the intermediate imidazol-2-ylidenes were trapped by elemental sulfur, yielding the corresponding chiral non-enolizable imidazole-2-thiones in good yields. Analogous reaction sequences, starting with imidazole N-oxides derived from enantiopure primary amines, amino alcohols, and amino acids, leading to the corresponding 3-alkoxyimidazole-2-thiones were also studied.

Chiral Imidazolium Salts Derived from Amino Acids: Synthesis Characterization and Structure

Chiral Imidazolium Salts Derived from Amino Acids: Synthesis Characterization and Structure

ChemInform Abstract: Chiral NHC Ligands Bearing a Pyridine Moiety in Copper‐Catalyzed 1,2‐Addition of Dialkylzinc Reagents to β‐Aryl‐α,β‐unsaturated N‐Tosylaldimines.

Asymmetric 1,2-addition of dialkylzinc reagents to α,β-unsaturated N-tosylaldimines was catalyzed by copper salt in the presence of chiral imidazolium salts having a pyridine ring, which were derived from amino acid, to afford the corresponding chiral allylic amines with up to 91% ee in reasonably high yields. The chiral N-heterocyclic carbene (NHC) ligand played an important role in controlling chemoselectivity.

Chiral NHC Ligands Bearing a Pyridine Moiety in Copper-Catalyzed 1,2-Addition of Dialkylzinc Reagents to β-Aryl-α,β-unsaturated N-Tosylaldimines.

Asymmetric 1,2-addition of dialkylzinc reagents to α,β-unsaturated N-tosylaldimines was catalyzed by copper salt in the presence of chiral imidazolium salts having a pyridine ring, which were derived from amino acid, to afford the corresponding chiral allylic amines with up to 91% ee in reasonably high yields. The chiral N-heterocyclic carbene (NHC) ligand played an important role in controlling chemoselectivity.

ChemInform Abstract: Chiral N‐Heterocyclic Carbene Ligands Bearing a Pyridine Moiety for the Copper‐Catalyzed Alkylation of N‐Sulfonylimines with Dialkylzinc Reagents.

AbstractAmino acid‐derived chiral imidazolium salts, each bearing a pyridine ring, are developed as N—heterocyclic carbene ligands.

Chiral bicyclic NHC/Ir complexes for catalytic asymmetric transfer hydrogenation of ketones.

A diverse series of chiral bicyclic NHC/Ir complexes were prepared via a previously developed divergent synthesis of chiral imidazolium salts. Among the complexes, 8dz was found to be an excellent catalyst precursor for the Ir-catalyzed asymmetric transfer hydrogenation of ketones. The reaction of ketones with 8dz proceeded smoothly to give corresponding alcohols with high enantioselectivities (up to 98%) and productivities (TON up to 4500).

Chiral N-heterocyclic carbene ligands bearing a pyridine moiety for the copper-catalyzed alkylation of N-sulfonylimines with dialkylzinc reagents.

Amino acid-derived chiral imidazolium salts, each bearing a pyridine ring, were developed as N-heterocyclic carbene ligands. The copper-catalyzed asymmetric alkylation of various N-sulfonylimines with dialkylzinc reagents in the presence of these chiral imidazolium salts afforded the corresponding alkylated products with high enantioselectivity (up to 99 % ee). The addition of HMPA to the reaction mixture as a co-solvent is critical in terms of chemical yield and enantioselectivity. A wide range of N-sulfonylimines and dialkylzinc reagents were found to be applicable to this reaction.

Synthesis of Chiral Imidazolium Salts from a Carbohydrate and Their Application in Pd-Catalyzed Suzuki–Miyaura Reaction

A series of chiral 1-(acetylated glucopyranosyl)-3-substituted-imidazolium salts [3-substitute = n-butyl (1a), 3-bromopropyl (1b), 2-chloromethyl benzyl (1c), and 4-chloromethyl benzyl (1d)] have been synthesized. Preliminary catalytic studies show that these imidazolinium salts are remarkably efficient in Pd-catalyzed Suzuki–Miyaura reaction. Functionalized aryl boronic acids reaction with aryl halides (including aryl iodides, aryl bromides and activated aryl chlorides) using environmentally friendly conditions (ethanol aqueous and ambient). The excellent isolate yields reveal that the bulky carbohydrate unit is promising for the construction of highly active transition-metal catalyst. Four C-1 bonded sugar-containing chiral imidazolium salts 1a–d were synthesized and they all exhibited excellent catalytic activity in Pd-catalyzed Suzuki reactions. Pd/1a is most effective for the coupling of functionalized arylboronic acids with arylhalides including activated arylchlorides in ethanol aqueous under air.

ChemInform Abstract: Enantioselective Conjugate Addition of Alkylboranes Catalyzed by a Copper—N‐Heterocyclic Carbene Complex.

AbstractFor the first time the title reaction is developed in a catalytic manner by using a new chiral imidazolium salt.

Chiral N-Heterocyclic Carbene Gold Complexes: Synthesis, Properties, and Application in Asymmetric Catalysis

Nine new N-heterocyclic carbene gold chloride complexes (10–18) were synthesized starting from bulky chiral imidazolium salts (1–9) developed in this laboratory. Full characterization of all complexes, including the X-ray structures of gold(I)(1,3-bis((S)-1-(2-methoxyphenyl)-2,2-dimethylpropyl)-1H-imidazol-2(3H)-ylidene) chloride (13) and gold(I)(1,3-bis((S)-2,2-dimethyl-1-(naphthalen-1-yl)propyl)-1H-imidazol-2(3H)-ylidene) chloride (16), is reported. The complexes 10–18 were applied in the methoxycyclization of 1,6-enynes using AgNTf2 as an additive. Synthesis of the N-heterocyclic carbene gold triflimidate (19) was achieved by treating complex 12 with AgNTf2. The complex gold(I)(1,3-bis((R)-1-(2-methoxyphenyl)-2,2-dimethylpropyl)imidazolidin-2-ylidene)(1,1,1-trifluoro-N-(trifluoromethylsulfonyl)methylsulfonamido) (19) was isolated and spectroscopically and structurally (X-ray) characterized.

Quaternary Chiral β2,2-Amino Acids with Pyridinium and Imidazolium Substituents

The reactions of cyclic sulfamidates as electrophiles with a variety of nitrogen-containing aromatic heterocycle nucleophiles, such as pyridines, N-alkylimidazoles and N-methylbenzimidazol, was explored. In all cases, although the nucleophilic substitution reactions occurred on quaternary centres, elimination products were not detected. The inversion of configuration at this quaternary centre was determined by X-ray diffraction analysis and the enantiomeric excess of the reactions was checked by chiral HPLC. This synthetic approach allowed us to obtain a new family of chiral charged β(2,2)-amino acids, including a new bisamino acid that incorporates an imidazolium salt as a cross-linker. In this context, the treatment of these chiral imidazolium salts with Ag(2)O opens the way to new chiral N-heterocyclic carbenes, which are important substrates in the fields of organometallic and organocatalytic chemistry. Additionally, we have done a thorough conformational analysis of these β-amino acid derivatives, both in the solid state and in solution. The most important conformational feature of these acyclic systems is the rigidity of the N-CH(2)-C-N(+) dihedral angle, which is essentially due to the gauche effect.

Enantioselective Conjugate Addition of Alkylboranes Catalyzed by a Copper–N-Heterocyclic Carbene Complex

The first catalytic enantioselective conjugate addition of alkylboron compounds has been achieved. Reactions between alkylboranes and imidazol-2-yl α,β-unsaturated ketones proceeded with high enantioselectivity under the influence of a Cu(I) catalyst system, prepared in situ from CuCl, a new chiral imidazolium salt as a precursor for the N-heterocyclic carbene ligand, and PhOK. Alkylboranes are widely obtained via alkene hydroboration. A variety of functional groups are tolerated in alkylboranes and α,β-unsaturated ketones.

Asymmetric Aldol Reaction Catalyzed by the Anion of an Ionic Liquid

Herein we report the synthesis of a chiral imidazolium salt derived from trans-L-hydroxyproline and its applications as a catalyst for the asymmetric aldol reaction. By performing the aldol reaction in [Bmim]NTf(2) as a solvent, we report excellent isolated yields of the aldol product (up to 99%), as well as modest to excellent selectivities (dr superior to 99:1. ee up to 89%). Mechanistic insights and the origins of the selectivity of the aldol reaction are discussed on the basis of the results obtained with two catalytic imidazolium salts having different H-bonding potential.

Antimicrobial toxicity studies of ionic liquids leading to a ‘hit’ MRSA selective antibacterial imidazolium salt

Imidazolium salts can be classed as surfactants, detergents, ionic liquids, reagents, catalysts or solvents. A study of the toxicity and ecotoxicity of these salts yields valuable information for their use as pharmaceuticals as well as impact on the environment. Our approach to screen a series of chiral imidazolium salts for toxicity to bacteria and fungi, including clinical pathogen strains, has led to the identification of a ‘hit’ MRSA selective antimicrobial compound. Preliminary structure–activity-relationship (SAR) information (required position of L-phenylalanine and L-valine group) is also elucidated within this first generation of compounds. Conversely, most of the imidazolium salts were nontoxic (IC95 > 2 mM) to the 12 fungi strains and 8 bacteria strains screened, and we propose that they are suitable candidates for ‘green chemistry’ applications. Ecotoxicity studies (Biodegradation ISO 14593 ‘CO2 Headspace Test’) of two bromide ionic liquids containing L-phenylalanine residues indicate that these ionic liquids passed the test (>60% in 28 days) and classed as readily biodegradable.

Comparison of Superparamagnetic Fe3O4‐Supported N‐Heterocyclic Carbene‐Based Catalysts for Enantioselective Allylation

Once, twice, three times a catalyst: Iron oxide nanoparticles (NPs) were functionalized through a silane linker by a chiral imidazolium salt on the surface, a latent N-heterocyclic carbene precursor. This system was utilized in three ways in the enantioselective allylation of 4-nitrobenzaldehyde: as an organocatalyst (1); as a molecular metal complex catalyst (2), when together with [Pd(OAc)2]; and as a matrix for the formation of Pd NPs and, subsequently, as an active chiral NP catalyst (3). Enantiomeric excesses of up to 74 % were obtained.

Grignard allylic substitution catalyzed by imidazol-2-ylidene- and imidazol-4-ylidene-magnesium complexes

In the presence of a catalytic amount of 1,2-disubstituted or 1,2,3-trisubstituted imidazolium salts, γ-substituted allyl chlorides reacted with alkyl Grignard reagents to undergo substitution reactions in an S N2′-selective fashion, where the magnesium ate complexes [( N-heterocyclic carbene-MgR 3) −(MgX) +] of imidazol-2-ylidenes or imidazol-4-ylidenes, generated in situ, were postulated as the active species. It was observed that the reactions with imidazol-4-ylidene catalysts were faster than those with imidazol-2-ylidenes. Enantioselective catalysis using a chiral imidazolium salt was preliminarily investigated.

Planar chiral imidazolium salts based on [2.2]paracyclophane in the asymmetric rhodium-catalyzed 1,2-addition of arylboronic acids to aldehydes

Abstract A series of planar chiral imidazolium salts derived from [2.2]paracyclophane have been synthesized and characterized. By using these imidazolium salts as carbene precursors, the Rh-catalyzed 1,2-addition of arylboronic acids to aldehydes proceeded readily with low catalyst loadings (0.03–0.3 mol %) and gave a variety of chiral diarylmethanols in excellent yields and moderate enantioselectivities.

Synthesis of novel room temperature chiral ionic liquids: application as reaction media for the heck arylation of aza-endocyclic acrylates

New achiral and chiral RTILs were prepared using novel and/or optimized synthetic routes. These new series of imidazolinium, imidazolium, pyridinium and nicotine-derived ionic liquids were fully characterized including differential scanning calorimetry (DSC) analysis. The performance of these achiral and chiral room temperature ionic liquids (RTILs) was demonstrated by means of the Heck arylation of endocyclic acrylates employing arenediazonium salts and aryl iodides. The Heck arylations performed in the presence of these ionic entities, either as a solvent or as an additive, were effective leading to complete conversion of the substrate and good to excellent yield of the Heck adduct. In spite of the good performances, no asymmetric induction was observed in any of the cases studied. Two new diastereoisomeric NHC-palladium complexes were prepared in good yields from a chiral imidazolium salt and their structure characterized by X-ray diffraction. Overall, the Heck arylations employing arenediazonium tetrafluoroborates in RTILs were more effective than the traditional protocols employing aryl iodides in terms of reactivity and yields.

Synthesis of new chiral imidazolium salts derived from amino acids: their evaluation in chiral molecular recognition

A family of new imidazolium salts derived from natural amino acids has been synthesized and tested for NMR enantiodiscrimination, as chiral shift reagents, of carboxylic acids. These imidazolium receptors contain different structural modifications and the splitting of the signals of the acids, after addition of the corresponding CSRs, depends on these structural variables. Compound 8b exhibited the strongest chiral solvating properties for racemic Mosher acid and was recognized as a suitable CSR for the determination of its enantiomeric composition.