Hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands: Imidazole and oxazole ring Lewis basicity,Ni(II)/Pd(II) complex structures and spectra

Fourteen new organic molecules A1–A4, B1–B5, C1–C4 and D and a series of transition metal(II) complexes (Ni1–Ni9 and Pd1–Pd2b) were synthesized and studied in order to characterize the hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands (A1–A4 = 2-R²-6-(4,5-diphenyl-1R¹-imidazol-2-yl)pyridines, R¹ = H or CH₃, R² = H or CH₃; B1–B5 = 1-R²-2-(pyridin-2-yl)-1R¹-phenanthro[9,10-d]imidazoles/oxazoles, R¹ = H or CH₃, R² = H or CH₃; C1–C4 = 2-(6-R²-pyridin-2-yl)-1H-imidazo/oxazo[4,5-f][1,10]phenanthrolines, R² = H or CH₃; D = 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline). They were also used to study the substituent effects on the donor strengths as well as the coordination chemistries of the imidazole/oxazole fragments of the hemilabile ligands.All the observed protonation–deprotonation processes found within pH 1–14 media pertain to the imidazole or oxazole rings rather than the pyridyl Lewis bases. The donor characteristics of the imidazole/oxazole ring can be estimated by spectroscopic methods regardless of the presence of other strong N donor fragments. The oxazoles possessed notably lower donor strengths than the imidazoles. The electron-withdrawing influence and capacity to hinder the azole base donor strength of 4,5-azole substituents were found to be in the order phenanthrenyl (B series) > 4,5-diphenyl (A series) > phenanthrolinyl (C series). An X-ray structure of Ni5b gave evidence for solvent induced ligand reconstitution while the structure of Pd2b provided evidence for solvent induced metal–ligand bond disconnection.Interestingly, alkylation of 1H-imidazoles did not necessarily produce the anticipated push of electron density to the donor nitrogen. Furthermore, substituents on the 4,5-carbons of the azole ring were more important for tuning donor strength of the azole base. DFT calculations were employed to investigate the observed trends. It is believed that the information provided on substituent effects and trends in this family of ligands will be useful in the rational design and synthesis of desired azole-containing chelate ligands, tuning of donor properties and application of this family of ligands in inorganic architectural designs, template-directed coordination polymer preparations, mixed-ligand inorganic self-assemblies, etc.     

Synthesis of organosoluble polyamides with bulky triaryl imidazole pendent group

New unsymmetrical diamine monomer containing triaryl imidazole pendent group,4-[4-(4,5-diphenyl-1H-imidazol-2-yl)phenoxy] -1,3-benzenediamine,was synthesized via aromatic substitution reaction of 1-chloro-2,4-dinitrobenzene with 4-(4,5- diphenyl-1H-imidazol-2-yl)phenol,followed by palladium-catalyzed hydrazine reduction.This new monomer was further confirmed by FT-IR,~1H NMR and ~(13)C NMR.Novel polyamides having pendant triaryl imidazole group were prepared by the phosphorylation polycondensation of fou...     

Synthesis,spectral characterization and biological evaluation of novel 1-(2-(4,5-dimethyl-2-phenyl-1H-imidazol-1-yl)ethyl)piperazine derivatives

A simple, highly useful synthesis of 1-(2-(4,5-dimethyl-2-phenyl-1H-imidazol-1-yl)ethyl)piperazine derivatives is achieved by a four component cyclo condensation of diacetyl, aromatic aldehyde, 2-(piperazin-1-yl)ethanamine and ammonium acetate using SO₄²⁻/Y₂O₃ as a catalyst in ethanol. The synthesized compounds were characterized by IR, ¹H and ¹³C NMR and mass spectral studies. All the synthesized compounds were screened for their in vitro antimicrobial studies. Among the newly synthesized compounds 5d, 5e, 5h and 5m showed excellent antibacterial and antifungal activities when compared to the standard drugs.     

A novel synthesis of aryl tethered imidazo[4,5-b]pyrazin-2-ones through in situ ring construction and contraction

An innovative synthesis of aryl tethered 1,3-dimethylimidazo[4,5-b]pyrazin-2-ones 4 and 6 has been delineated through base catalyzed ring transformation of 6-aryl-4-(piperidin-1-yl)-2H-pyran-2-one-3-carbonitriles 1 and methyl 6-aryl-4-methylsulfanyl-2H-pyran-2-one-3-carboxylates 5 with 7-acetyl-1,3-dimethyllumazine 2 with subsequent ring contraction of the fused pyrimidine to an imidazole ring. An additional product, methyl [6-(1,3-dimethyl-2-oxo-2,3-dihydro-1H-imidazo[4,5-b]pyrazin-5-yl)-4-thiophen-2-ylpyran-2-ylidene]acetate 8b, was also isolated from the reaction of 5 and 2, as a minor constituent.     

Synthesis of light-emmting materials bis-[2′-2″-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) and bis-[2′-4″-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquinoline] zinc(II)

Two novel light-emitting materials bis-[2′-2″-(9H-fluoren-2-yl)-vinyl-8-hydroxyquinoline] zinc(II) (3) and bis-[2′-4″-(4,5-diphenyl-1H-imidazol-2-yl)styryl-8-hydroxyquinoline] zinc(II) (4) containting 8-hydroxyquinoline and fluorene or imidazole moieties have been synthesized. The optical properties of these complexes were influenced by the styryl substituents, and exhibited orange-emission. They have higher fluorescence quantum yields than Alq₃, and good stabilities with thermal decomposition temperatures 395 °C and 435 °C. The single-layer OLED fabricated by 3 emitted lemon-yellow, and exhibited good device performance with a maximum luminance of 489 cd m⁻², and luminance efficiency of up to 0.41 cd A⁻¹. The single-layer OLED fabricated by 4 emitted yellow-green, and exhibited good device performance with a maximum luminance of 323 cd m⁻², and luminance efficiency of up to 0.54 cd A⁻¹.     

1,10-(1-H-IMIDAZOL-5-YL)DECANEPHOSPHONIC ACID: A NEW COMPOUND WITH BASIC AND ACIDIC SITES TO FABRICATE PROTON-CONDUCTING SOLID ELECTROLYTES

2-(tert-butyldimethylsilyl)-5-(10-bromodecyl)-1-(N,N-dimethylsulfamoyl)-imidazole and NaPO₃ Et₂ react at room temperature to yield the corresponding imidazole diethyl phosphonate, which when refluxed with concentrated HBr yields 1,10-(1-H-imidazol-5-yl)decanephosphonic acid hydrobromide (VI). Compound VI titrated with the required mole equivalents of NaOH yields 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (VII) and 1,10-(1-H-imidazol-5-yl)decane (sodium)phosphonates (VIII). CP-MAS NMR (¹³C, ³¹P, and ¹⁵N) and IR spectroscopy indicate that only compound VII is likely to yield a polymeric structure where each phosphonic acid group is H bonded to the imidazole ring of the same molecule and to the imidazole ring of another molecule. For this reason, solid-state proton conductivity in VII is likely to be favored.     

Efficient synthesis of 1-R1-2-R-4,5-di(furan-2-yl)-1H-imidazoles and their luminescence properties

In this study, a series of 1-R₁-2-R-4,5-di(furan-2-yl)-1H-imidazole derivatives were synthesized in better yield 59.0%∼89.8% by the treatment of purified imidazole compounds with benzyl chloride or allyl chloride in the presence of sodium hydride, and were characterized by FT–IR, HRMS, ¹H NMR and ¹³C NMR spectroscopy. Furthermore, the luminescence properties of the synthesized products were investigated. It was found that N-substituted groups of imidazole have little influence on the absorption bands in a 0.1 N H₂SO₄ aqueous solution containing 0.5 mL of dissolved CH₃OH. However, the emission of some compounds in solution was sensitive to the polarity of the solvents.     

Synthesis of soluble and thermally stable polyimides from unsymmetrical diamine containing 2,4,5-triaryl imidazole pendent group

A novel unsymmetrical diamine monomer containing triaryl imidazole pendant group, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)phenoxy] benzene-1,3-diamine (DAI), was successfully synthesized via aromatic substitution reaction of 1-chloro-2,4-dinitrobenzene with 4-(4,5-diphenyl-1H-imidazol-2-yl)phenol, followed by palladium-catalyzed hydrazine reduction. The diamine monomer DAI polymerized with commercial available dianhydrides such as benzophenone tetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA) and bicyclo[2.2.2]-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) by using two step synthetic methods to obtain corresponding polyimides (PIs). PIs had inherent viscosity of 0.42-0.51 dL/g and exhibited excellent solubility in aprotic polar solvents such as N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), pyridine and methyl sulfoxide (DMSO). PIs showed high glass transition temperatures between 230 and 320 °C, and they were fairly stable up to a temperature above 300 and 450 °C depending on the dianhydride monomer used for the PIs preparation.     

Exploring facile synthesis and cholinesterase inhibiting potential of heteroaryl substituted imidazole derivatives for the treatment of Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder and cholinesterase (ChE) enzymes are considered as crucial targets for the treatment of AD. Herein, a series of heteroaryl substituted imidazole derivatives (5a-5x) was prepared using amino acid catalyzed, one-pot facile synthetic approach. In this context, the catalytic potentials of different amino acids were investigated and 15 mol% of glutamic acid was identified as the most suitable catalyst to obtain the target products in good yields up to 90 %. These structurally exciting heterocyclic hybrids were screened against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes. This series displayed moderate to excellent inhibitory potential against AChE with IC₅₀ values > 25 µM and the most active compound was 3-(4-(1-(3,5-dimethylphenyl)-4,5-diphenyl-1H-imidazol-2-yl)-1-phenyl-1H-pyrazol-3-yl)–2H-chromen-2-one (5x) with IC₅₀ value of 25.83 ± 0.25 µM.This inhibitory potential was attributed to hydrophobicity as the major contributory factor. The most potent compound against BChE was 1,3-diphenyl-4-(1,4,5-triphenyl-1H-imidazol-2-yl)-1H-pyrazole (5a) with IC₅₀ value of 0.35 ± 0.02 µM followed by other potent compounds 5p, 5 m, 5x, 5b, 5c, 5e and 5f with IC₅₀ values < 10 µM. SAR studies further revealed that coumarinyl moiety at R¹ position in the imidazolylpyrazole skeleton significantly improved the overall cholinesterase inhibitory potential. However, a simple phenyl ring attached at this R¹ site was highly effective and selective for BChE inhibition (5a) over AChE. Docking data also demonstrated the interaction of 5x and AChE with a docking score of 7564 and atomic contact energy (ACE) value of –291.90 kcal/mol whereas docking score for 5a against BChE was 7096 with ACE value of –332.95 kcal/mol. The results altogether suggest further investigations of the heteroaryl substituted imidazole core skeleton in search of potential leads towards designing of new anti-cholinesterase drugs for the treatment of AD.     

Transition-metal-free insertion of benzyl bromides into 2-(1H-benzo[d]imidazol-1-yl)benzaldehyde: One-pot switchable syntheses of benzo[4,5]imidazo[1,2-a]quinolin-5(7H)-ones and 3-arylquinolin-4-ones mediated by base

A transition-metal-free insertion of benzyl group between aldehyde and imidazole of 2-(1H-benzo[d]imidazol-1-yl)benzaldehyde was achieved for the first time. Two diverse sets of quinolin-4-one derivatives: benzo[4,5]imidazo[1,2-a]quinolin-5(7H)-ones (2) and 3-arylquinolin-4-ones (3) were synthesized based on identical starting materials 2-(1H-benzo[d]imidazol-1-yl)benzaldehydes (1) and benzyl bromides. In the preparations, two key intermediates I and II were involved and might be synthesized in situ through the reaction of an intra-Breslow intermediate with benzyl bromide via an enol attack in the presence of base or a NHC-based enamine attack in the absence of base, respectively, in which the intra-Breslow intermediate might function as a nucleophilic reagent by following two novel different pathways.     

Effect of ortho-substituents on the stereochemistry of 2-(o-substituted phenyl)-1H-imidazoline-palladium complexes

Palladium complexes composed of [Pd(Ln)₂Cl₂] (n = 1, 2, 3, 4, 6), [L5a]₂[PdCl₄] and [Pd(L5b)₂], where L1 = 4,5-dihydro-2-phenyl-1H-imidazole (=2-phenyl-1H-imidazoline), L2 = 2-(o-fluorophenyl)-1H-imidazoline, L3 = 2-(o-methylphenyl)-1H-imidazoline, L4 = 2-(o-tert-butylphenyl)-1H-imidazoline, L5a = 2-(o-hydroxyphenyl)-1H-imidazolinium, L5b = 2-(1H-imidazolin-2-yl)phenolate, and L6 = 2-(o-methylphenyl)-1H-imidazole, were synthesized. Molecular structures of the isolated palladium complexes were characterized by single crystal X-ray diffraction analysis. The effect of ortho-substituents on the phenyl ring on trans-chlorine geometry was noted for complexes [Pd(L1)₂Cl₂] 1a and 1b, [Pd(L2)₂Cl₂] 2 and [Pd(L6)₂Cl₂] 6, whereas cis-chlorine geometry was observed for [Pd(L3)₂Cl₂] 3 and [Pd(L4)₂Cl₂] 4. PdCl₂ reacts with 2-(o-hydroxyphenyl)-1H-imidazoline in DMF to give [L5a]⁺ and [L5b]^- so that [L5a]₂[PdCl₄] 5a and [Pd(L5b)₂] 5b were obtained. In complex 5b, as an N,O-bidentate ligand, two ligands L5b coordinated with the central Pd(II) ion in the trans-form. The coordination of PdCl₂ with 2-(o-hydroxyphenyl)-1H-imidazolines in solution was investigated by NMR spectroscopy.     

Synthesis of 2,6-di(pyrazol-1-yl)-4-bromomethylpyridine, and its conversion to other 2,6-di(pyrazol-1-yl)pyridines substituted at the pyridine ring

Two routes to 2,6-di(pyrazol-1-yl)-4-hydroxymethylpyridine (1) from 2,6-dihydroxy-isonicotinic acid, in four and six steps, are reported. Reaction of 1 with 48% HBr yields 2,6-di(pyrazol-1-yl)-4-bromomethylpyridine (2), which is a powerful precursor to a range of new tridentate ligands for transition metals functionalised at the pyridine ring. As a proof of principle, we describe the further elaboration of 2 to give two 2,6-di(pyrazol-1-yl)pyridines bearing nucleobase substituents, and the back-to-back ligand 1,2-bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]ethane. Crystal structures of two of these new derivatives are presented.     

Prolonging the Triplet State Lifetimes of Rhenium Complexes with Imidazole-Pyridine Framework for Efficient CO2 Photoreduction

The photocatalytic reduction of CO₂ into fuels offers the prospect for creating a new CO₂ economy. Harnessing visible light-driven CO₂-to-CO reduction mediated by the long-lived triplet excited state of rhenium(I) tricarbonyl complexes is a challenging approach. We here develop a series of new mononuclear rhenium(I) tricarbonyl complexes ( Re-1 − Re-4 ) based on the imidazole-pyridine skeleton for photo-driven CO₂ reduction. These catalysts are featured by combining pyridyl-imidazole with the aromatic ring and different pendant organic groups onto the N1 position of 1,3-imidazole unit, which display phosphorescence under Ar-saturated solution even at ambient conditions. By contrast, {Re[9-(pyren-1-yl)-10-(pyridin-2-yl)-9H-pyreno[4,5-d]imidazole)](CO)₃Cl} ( Re-4 ) by introducing pyrene ring at the N1 position of pyrene-fused imidazole unit exhibits superior catalytic performance with a higher turnover number for CO (TON_CO=124) and >99.9 % selectivity, primarily ascribed to the strong visible light-harvesting ability, long-lived triplet lifetimes (164.2 μs) and large reductive quenching constant. Moreover, the rhenium(I) tricarbonyl complexes derived from π-extended pyrene chromophore exhibit a long lifetime corresponding to its ligand-localized triplet state (³IL) evidenced from spectroscopic investigations and DFT calculations.     

Double nucleophilic reaction of amines to the imidazole nucleus and selective synthesis of 5-aminoimidazoles

Reaction of 2-(1-chloro-2,2-dimethylpropyl)-1-methyl-1H-imidazole with an excess of N,N-dimethylamine at room temperature gave an abnormal adduct, trans-4,5-bis(dimethylamino)-1-methyl-2,2-dimethylpropyl-2-imidazoline, which was derived from a serial, double nucleophilic addition into the imidazole nucleus in 74% yield together with a normal S_N product, 1-methyl-2-(1-dimethylamino-2,2-dimethylpropyl)-1H-imidazole in 15% yield. The former was easily converted to 1-methyl-5-(dimethylamino)-2-(2,2-dimethylpropyl)-1H-imidazole by only reflux in toluene in 90% yield. The scope, mechanism and limitation of these reactions are discussed.     

A new dimeric imidazole alkaloid plasmid conjugation inhibitor from Lepidium sativum

Phytochemical investigation of the methanolic extract of Lepidium sativum seeds led to the isolation of a new compound, named 2-(3-(3-((1H-imidazol-2-yl)methyl)-5-methoxyphenoxy)benzyl)-1H-imidazole and given the trivial name Lepidine AK (1), along with three known compounds; Lepidine E (2), Lepidine B (3) and 2-(3-(2-((1H-imidazol-2-yl)methyl)-6-methoxyphenoxy)benzyl)-1H-imidazole (4). The structures were elucidated based on NMR spectroscopy, UV, IR and high-resolution electrospray ionization mass spectrometry. The isolated compounds were tested for bacterial conjugation inhibition. Lepidine AK (1, 100?μg/mL) reduced the conjugal transfer of the IncI₂ plasmid TP114 to 44.7?±?3.5% but interestingly promoted the conjugation of the IncN plasmid pKM101 to greater than 120%.     

Highly effective, easily accessible screw-sense-determining end group in the asymmetric polymerization of 1,2-diisocyanobenzenes

Enantiopure helical poly(quinoxaline-2,3-diyl) was formed stereoselectively in the polymerization of 1,2-diisocyanobenzene using a new organopalladium initiator bearing the (4S,5S)-N-acyl-4,5-dihydro-4,5-diphenyl-1H-imidazol-2-yl group as the polymer-end screw-sense-determinant.     

Reaction of 5-(2-furyl)-1-methyl-1<Emphasis Type="Italic">H-</Emphasis> and 1-methyl-5-(2-thienyl)-1<Emphasis Type="Italic">H</Emphasis>-imidazoles with electrophilic reagents

5-(2-Furyl)-1-methyl-1H- and 1-methyl-5-(2-thienyl)-1H-imidazoles were synthesized. The electronwithdrawing effect of the 5- and 2-imidazole substituents on the furan ring was studied by ¹H NMR spectroscopy and quantum-chemical calculations. Some electrophilic substitution reactions were investigated (nitration, bromination, sulfonation, hydroxymethylation, formylation, and acylation). In some cases, depending on the reaction conditions, both the furan and thiophene ring and the imidazole fragment undergo electrophilic attack.     

Efficient enantiorecognition of ruthenium(II) complexes by silica-bound teicoplanin

A series of chiral tris-diimine ruthenium(II) complexes have been resolved by HPLC on a chiral stationary phase. The stationary phase (CSP1) was prepared by covalent attachment of the glycopeptide antibiotic teicoplanin to isocyanate activated silica gel. CSP1 selectively retains the enantiomers of [Ru(L)₃]²⁺ (L=2,2′-bypyridine (bpy), 1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline), with a preference for the Δ isomer. For the mixed-ligand complexes [Ru(bpy)₂pztr]⁺ and [Ru(bpy)₂pytr]⁺ (Hpztr=3-(pyrazin-2-yl)-1,2,4-triazole, Hpytr=3-(pyridin-2-yl)-1,2,4-triazole), where the triazole unit is bound to the metal centre either through the N² or the N⁴ nitrogen of the ring, CSP1 discriminates both the enantiomers and the regioisomers. Diastereo- and enantioselective association was also observed between CSP1 and the stereoisomers of the dinuclear complex ((Ru(bpy)₂)₂bpt]³⁺ (Hbpt=3,5-bis(pyridin-2-yl)-1,2,4-triazole), with differences in binding affinities of 1.4 kJ/mol between the homochiral enantiomers.     

Synthesis,studies and in vitro antibacterial activity of some 5-(thiophene-2-yl)-phenyl pyrazoline derivatives

In the present investigation, a novel series of pyrazolines 2a–2d were synthesized by the cyclization of various -1-[2-(alkoxy) phenyl]-3-(thiophen-2-yl) prop-2-en-1-one 1a–1d with N-substituted phenyl hydrazine and thiosemicarbazide in the presence of CH₃COOH and NaOH in ethanol which lead to the formation of new pyrazolines. The structures of these compounds were elucidated by, IR, ¹H-NMR, ¹³C-NMR, ESI-MS spectral data and their purities were confirmed by elemental analyes. The in vitro antibacterial activity of these compounds was evaluated against two Gram-positive and two Gram-negative bacteria Aeromonas hydrophila, Yersinia enterocolitica, Listeria monocytogenes, and Staphylococcus aureus by microdilution method and then the minimum inhibitory concentration (MIC) of these compounds was determined. The results showed that compounds 1-[2-(benzyloxy) phenyl]-5-(thiophen-2-yl)-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl (2b) and 1-[2-(naphthalen-2-ylmethoxy) phenyl]-5-(thiophene-2-yl)-1-phenyl-4,5-dihydro-1H-pyrazole-4-yl (2d) showed most promising antibacterial activity as compared to the antibiotics gentamicin and tetracycline in (Table 1, Table 2).     

Synthesis and in-vitro-antibacterial activity of [5-(furan-2-yl)-phenyl]-4,5-carbothioamide-pyrazolines

Cyclization of various different alkoxy [1-[2-(alkoxy)phenyl]-5-(furan-2-yl)-prop-2-en-1-one] chalcone with thiosemicarbazide in the presence of NaOH in ethanol afforded a series of novel 1-N-substituted cyclized pyrazoline analogues [5-(furan-2-yl)-3-[2-(alkoxy) phenyl]-4,5-dihydro-1H-pyrazole-1-carbothioamide 2a–2d. The structures of these compounds were elucidated by IR, ¹H NMR, ¹³C NMR, Fab mass spectrometry and their purities were confirmed by elemental analyses. In vitro antibacterial activity of these compounds were evaluated by the disk diffusion assay and then the minimum inhibitory concentration (MIC) strain of two Gram-positive and two Gram-negative bacteria like Aeromonas hydrophila, Yersinia enterocolitica, Listeria monocytogenes, and Staphylococcus aureus, among all the compounds, alkoxy [5-(furan-2-yl)-2-(benzyloxy)phenyl]-4,5-dihydro-1H-pyrazole-1-carbothioamide 2b and 5-(furan-2-yl)-1-[2-(naphthalen-2-ylmethoxy) phenyl]-4,5-dihydro-1H-pyrazole-1-carbothioamide 2d showed the most promising antibacterial agent when compared to gentamicin and tetracycline.