Effect Of Ligand Structure Research Articles

Highly enantioselective Friedel–Crafts reaction of indoles with N-sulfonyl aldimines catalyzed by heteroarylidene malonate-type bis(oxazoline) copper(II) complexes

The enantioselective Friedel–Crafts addition of indoles to N-sulfonyl aldimines was studied using a heteroarylidene malonate-type bis(oxazoline) as a chiral ligand. High to excellent enantioselectivities (up to >99% ee) were achieved using the complex of copper(II)– L1b with a benzyl group. The effects of ligand structure, solvent, temperature and substrate on the reaction were also investigated.

Advances of Chromium(III)-Based Catalysts with Multidentate Heteroatomic Ligands for 1-Hexene Production

According to the various multidentate heteroatomic ligands(based oxygen,nitrogen, phosphorus,mixed N and P,mixed N and S donors, composition, activity and overall selectivity to 1-hexene of chromium-based trimerization catalysts were reviewed. Commercial application foreground of these catalyst systems was also expected.It was found that effect of ligand structure on activity and selectivity for ethylene trimerization was the most predominant.The PNP （(Ph2P)2N(cyclopentyl)）ligands were prepared using inexpensive reagents and gave the corresponding complexes in high yields.Due to the inexpensive ligands coupled with low MAO （methyl aluminum oxide）requirements and high activity and selectivity of these systems, this has positive implication for a commercial process to 1-hexene.

New Chromium(III) Complexes with Imine−Cyclopentadienyl Ligands: Synthesis, Characterization, and Catalytic Properties for Ethylene Polymerization

A series of new half-sandwich chromium(III) complexes chelated with (2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl ligands, 2-(ArN═CH)C6H4Me4CpCrCl2 (Ar = 2,6-Me2C6H3 (1), 2,6-Et2C6H3 (2), 2,6-iPr2C6H3 (3), 4-MeC6H4 (4)), have been synthesized from the reaction of CrCl3 with the lithium salt of the corresponding ligand 2-(ArN═CH)C6H4Me4CpLi (Ar = 2,6-Me2C6H3 (LiL1), 2,6-Et2C6H3 (LiL2), 2,6-iPr2C6H3 (LiL3), 4-MeC6H4 (LiL4)). Free ligands HL1−HL4 were prepared by the condensation reaction of 2-(tetramethylcyclopentadienyl)benzaldehyde with 2,6-dialkylaniline. The free ligands were characterized by 1H NMR spectroscopy, while the chromium(III) complexes were characterized by elemental analyses and single-crystal X-ray crystallography. The X-ray crystallographic analysis indicates that the imine N atom in these complexes coordinates to the central chromium atom. Upon activation with AlR3 and Ph3CB(C6F5)4, these complexes exhibit high catalytic activity for ethylene polymerization and produce polyethylene with moderate to high molecular weights. These chromium(III) complexes can also be activated with AlR3 alone. In the latter case, they show slightly lower catalytic activity for ethylene polymerization in comparison to the AlR3/Ph3CB(C6F5)4 activated catalyst systems. The effects of ligand structure, polymerization temperature, AlR3, and Al/Cr molar ratio on the catalytic behavior of these complexes were examined.

Effect of ligand structure on the catalytic activity of Au nanocrystals

The surface coating-dependent catalytic activity of icosahedral Au nanoparticles (IAuNPs) for the reduction of 4-nitrophenol by NaBH 4 in aqueous solution was investigated. Cetyltrimethylammonium bromide (CTAB)-stabilized IAuNPs with average size of 50 nm were prepared by three-step seeding protocol. The CTAB was then exchanged with several amine derivatives such as pyrrole, pyridine, 2,2′-bipyridine, and 4,4′-bipyridine. The catalytic reaction rates of nanoparticles are in the order of IAuNPs@4,4′-bpy > IAuNPs@2,2′-bpy > IAuNPs@pyridine > IAuNPs@pyrrole > IAuNPs@CTAB, revealing that the catalytic activity of the nanoparticles highly depends on the type and structure of the ligand molecules.

Three novel supramolecular barium complexes with heterocyclic sulfonate ligands: Effect of ligand structure and auxiliary anions

Treatments of heterocyclic sulfonate ligands, viz. 4-(pyridin-2-yl) pyrimidin-2-sulfonate (L 1) and 4-(pyridin-3-yl) pyrimidin-2-sulfonate (L 2), with barium salts have resulted in three compounds ([BaL 1(NO 3)(H 2O)] n ( 1), {[BaL 1 2(H 2O)]·2H 2O} n ( 2) and [BaL 2(ClO 4)(H 2O)] n ( 3)). All the compounds have been structurally determined with interesting architectures, showing that the polarizability and the charge density of the Ba(II) ion, the positional modification of the pyridyl group about ligands as well as auxiliary anion effects play important roles in the formation of three supramolecular architectures.

Decreased STAT-1 phosphorylation by a thio analogue of beta-d-glucosylceramide is associated with altered NKT lymphocyte polarization

Glycolipid presentation to natural killer T (NKT) lymphocytes by CD1 proteins is important for antigen recognition. It was recently suggested that beta-glycolipids exert an immune-modulatory effect on NKT lymphocytes, alleviating immune-mediated disorders. The current study aimed to determine the effect of ligand structure on intrahepatic NKT lymphocyte function in concanavalin A (ConA) hepatitis, an NKT-mediated disorder. C57BL/6 mice were injected with a non-degradable β-d-glucosylceramide containing a thioglycosidic bond (GCT), β-d-glucosylceramide (GC), or vehicle. The phosphorylation of STAT-1, intrahepatic NKT lymphocyte number, and serum IFN-γ levels were determined. Liver damage was assessed by serum transaminases and the degree of apoptosis. The administration of GCT led to a significant inhibitory effect on intrahepatic NKT lymphocytes. Both GCT and GC led to a decrease in STAT-1 phosphorylation and a decrease in IFN-γ serum levels. These effects were associated with the alleviation of ConA immune-mediated hepatitis, as determined by a similar substantial decrease in serum transaminases and a profound decrease in apoptosis as noted by TUNEL assay, and were NKT-type I dependent. Conclusion: Alteration of the chemical structure of the GC by replacing the O-glycosidic bond with an S-glycosidic bond results in a non-degradable molecule GCT and significantly suppresses intrahepatic NKT lymphocytes. These results suggest that cellular events that alter the enzymatic pathways of glycosylceramides serve as an initial stimulus for NKT cell polarization in vivo.

Enhanced Hydrolysis of p-Nitrophenyl Picolinate by Schiff Base Mn(III) Complexes in Gemini 16-6-16 Micelles

Hydrolysis of p-nitrophenyl picolinate (PNPP) catalyzed by three Schiff base Mn(III) complexes (MnL2Cl) were investigated in Gemini 16-6-16 micelle media. , and systems give 835-, 969-, and 1931-fold rate enhancements in the hydrolysis of PNPP under the comparable conditions, respectively. Moreover, Effects of varied micelles on the reactivity of PNPP hydrolysis were especially studied. The results obtained indicate that rates of hydrolysis of PNPP are approximately 1.5- and 4-fold larger in Gemini 16-6-16 micelles than in CTAB micelles and Brij35 micelles, respectively. Effects of ligand structure on the activities of these three complexes are discussed in detail.

Probing the effects of ligand structure on activity and selectivity of Cr(iii) complexes for ethylene oligomerisation and polymerisation

A series of distorted octahedral Cr(III) complexes containing tridentate S-, S/O- or N-donor ligands comprised of three distinct architectures: facultative {S(CH(2)CH(2)SC(10)H(21))(2) (L(1)) and O(CH(2)CH(2)SC(10)H(21))(2) (L(2))}, tripodal {MeC(CH(2)S(n)C(4)H(9))(3) (L(3)), MeC(CH(2)SC(10)H(21))(3) (L(4))} and macrocyclic {(C(10)H(21))[9]aneN(3) (L(5)), (C(10)H(21))(3)[9]aneN(3) (L(6)), with [9]aneN(3)=1,4,7-triazacyclononane} are reported and characterised spectroscopically. Activation of [CrCl(3)(L)] with MMAO produces very active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence of the product distribution upon the ligand type present. The properties of the parent [CrCl(3)(L)] complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements, and the effects upon activation with Me(3)Al investigated similarly. Treatment with excess Me(3)Al leads to substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a change in the oxidation state of the Cr to either Cr(II) or Cr(IV)) and substantial dissociation of the neutral S and S/O-donor ligands.

Hybrid Quantum Mechanical/Molecular Mechanical Molecular Dynamics Simulations of HIV-1 Integrase/Inhibitor Complexes

Human immunodeficiency virus (HIV)-1 integrase (IN) is an attractive target for development of acquired immunodeficiency syndrome chemotherapy. In this study, conventional and coupled quantum mechanical and molecular mechanical (QM/MM) molecular dynamics (MD) simulations of HIV-1 IN complexed with 5CITEP (IN-5CITEP) were carried out. In addition to differences in the bound position of 5CITEP, significant differences at the two levels of theory were observed in the metal coordination geometry and the areas involving residues 116–119 and 140–166. In the conventional MD simulation, the coordination of Mg 2+ was found to be a near-perfect octahedral geometry whereas a distorted octahedral complex was observed in QM/MM. All of the above reasons lead to a different pattern of protein-ligand salt link formation that was not observed in the classical MD simulation. Furthermore to provide a theoretical understanding of inhibition mechanisms of 5CITEP and its derivative (DKA), hybrid QM/MM MD simulations of the two complexes (IN-5CITEP and IN-DKA) have been performed. The results reveal that areas involving residues 60–68, 116–119, and 140–149 were substantially different among the two systems. The two systems show similar pattern of metal coordination geometry, i.e., a distorted octahedron. In IN-DKA, both OD1 and OD2 of Asp-64 coordinate the Mg 2+ in a monodentate fashion whereas only OD1 is chelated to the metal as observed in IN-5CITEP. The high potency of DKA as compared to 5CITEP is supported by a strong salt link formed between its carboxylate moiety and the ammonium group of Lys-159. Detailed comparisons between HIV-1 IN complexed with DKA and with 5CITEP provide information about ligand structure effects on protein-ligand interactions in particular with the Lys-159. This is useful for the design of new selective HIV-1 IN inhibitors.

Ethylene Trimerization with Cr−PNP and Cr−SNS Complexes: Effect of Ligand Structure, Metal Oxidation State, and Role of Activator on Catalysis

Selected PNP and SNS ethylene trimerization ligands have been coordinated to CrII and CrIII, and further reactions of these complexes have been studied. The ligands are easily deprotonated to afford monoanionic tridentate ligands. All prepared complexes gave ethylene trimerization catalysis with varying degrees of activity upon activation with both MAO and AlR3/B(C6F5)3. The results of this study show that the role of MAO during activation is one of deprotonation, Cr reduction, and cation generation. A CrII → CrIV cationic mechanism is suggested.

Effect of Ligand Structure on Activation Rate Constants in ATRP

Activation rate constants (kact) for Cu complexes with various nitrogen-based ligands for Cu-mediated ATRP were determined at 35 °C in acetonitrile. The ratio of kact for complexes with different ligands exceeds 1 million times. The general order of Cu complex activity for ligands is tetradentate (cyclic-bridged) > tetradentate (branched) > tetradentate (cyclic) > tetradentate (linear) > tridentate > bidentate ligands. The nature of nitrogen atoms in ligands also plays a significant role in the activity of the Cu complexes and follows the order pyridine ≥ aliphatic amine > imine.

Synthesis of stilbenoids via the Suzuki–Miyaura reaction catalysed by palladium N-heterocyclic carbene complexes

The Suzuki–Miyaura reaction of aryl halides with trans-2-phenylvinylboronic acid using a series of related in situ generated N-heterocyclic carbene palladium(II) complexes was studied in order to evaluate the effect of ligand structure and electronics on the catalytic activity and to investigate the nature of the catalyst species. The nature of the substituents of the carbene ligand was found to be critical. Specifically, the presence of alkyl groups on the ortho positions of the phenyl substituents was a requisite for obtaining the most efficient catalyst systems.

Ligand Recognition by Vitamin D Receptor: Total Alanine Scanning Mutational Analysis of the Residues Lining the Ligand Binding Pocket of Vitamin D Receptor

We performed total alanine scanning mutational analysis (ASMA) of the residues lining the ligand binding pocket (LBP) of the human vitamin D receptor (hVDR) to investigate allosteric effects of ligands in the function of nuclear receptors (NRs). This was accomplished for the first time in the NR superfamily. The effects of ligand structure were also examined in this system (termed 2D-ASMA) using 8 representative VDR ligands. The results clearly revealed the role and importance of all amino acid residues lining the LBP and the relationships between ligand binding and transcriptional potency. 2D-ASMA indicated ligand-specific ligand-protein interactions, which are essential in determining the transactivation potency of the ligand. Taking the results as a whole, we suggest a ligand-mediated allosteric network, which allows transmission of information from ligands to the interfaces of the VDR in association with protein cofactors and was shown to be linked to a part of the network identified by statistical coupling analysis (SCA).

Asymmetric Friedel−Crafts Alkylations of Indoles with Nitroalkenes Catalyzed by Zn(II)−Bisoxazoline Complexes

[reaction: see text] A novel asymmetric Friedel-Crafts alkylation of indoles with nitroalkenes catalyzed by Zn(II)-bisoxazoline complexes has been developed. The nitroalkylated indoles are synthesized in excellent yields and high enantioselectivities (up to 90% ee). The effects of ligand structure, metal salt, and solvent on the reaction are discussed. The substrates of the reaction can be aromatic, heteroaromatic, and even aliphatic nitroalkenes. The high reactivity and selectivity of the reaction are presumptively attributed to the activation and asymmetric induction of chiral Lewis acids coordinated by nitroalkene substrates through a 1,3-metal bonding model.

Catalysts for Suzuki—Miyaura Coupling Processes: Scope and Studies of the Effect of Ligand Structure.

Catalysts for Suzuki—Miyaura Coupling Processes: Scope and Studies of the Effect of Ligand Structure.

Catalysts for Suzuki—Miyaura Coupling Processes: Scope and Studies of the Effect of Ligand Structure.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of Ligand Structure on the Pathways for Iron Release from Human Serum Transferrin

Rate constants for the removal of iron from N-terminal monoferric transferrin have been measured for a series of phosphate and phosphonocarboxylic acids in pH 7.4 0.1 M hepes buffer at 25 degrees C. The bidentate ligands pyrophosphate and phosphonoacetic acid (PAA) show a combination of saturation and first-order kinetics with respect to the ligand concentration. Similar results are observed following a single substitution at the 2-position of PAA to give 2-benzyl-PAA and phosphonosuccinic acid. In contrast, disubstitution at the 2-position to form 2,2-dibenzyl-PAA leads to a marked reduction in iron removal via the first-order pathway. Rate constants were also measured for tripolyphosphate and phosphonodiacetic acid, which are elongated versions of PP(i) and PAA. In both cases, this elongation completely eliminates the first-order component for iron release while having relatively little impact on the saturation pathway. The sensitivity of the first-order component to the structure of the ligand strongly indicates that this pathway involves the binding of the ligand to a specific site on the protein and cannot be attributed to changes in the overall ionic strength of the solution as the ligand concentration increases. It is proposed that this structural sensitivity reflects steric restrictions on the ability of the incoming ligand to substitute for the synergistic carbonate anion to form a relatively unstable Fe-ligand-Tf ternary intermediate, which then dissociates to FeL and apoTf.

In Vivo Evaluation of Gadolinium Hydroxypyridonate Chelates: Initial Experience as Contrast Media in Magnetic Resonance Imaging

The effect of ligand structure on the magnetic resonance (MR) imaging and biodistribution of six gadolinium (Gd) chelates based on a hydroxypyridonate-terephthalimide (HOPO-TAM) ligand design was investigated. Modifications to the molecular structure of the Gd-HOPO-TAM chelates (hydrophilicity and aromatic group substitution) significantly influence the efficacy of imaging and biodistribution. MR imaging was performed on female mice after intravenous (iv) injection of 100 micromol of Gd/kg of body weight of the different complexes. The biodistribution results indicate that the liver uptake of the complexes is enhanced by a short poly(ethyleneoxy) (PEO) chain, while blood pool localization is facilitated by a very long PEO chain. There is a direct correlation between the blood pool localization of the complexes and the signal intensity of blood vessels in the MRI. The imaging results are consistent with in vitro NMR measurements that indicate long PEO chains increase image enhancement capabilities in the presence of serum albumin.

Catalysts for Suzuki−Miyaura Coupling Processes: Scope and Studies of the Effect of Ligand Structure

Suzuki-Miyaura coupling reactions of aryl and heteroaryl halides with aryl-, heteroaryl- and vinylboronic acids proceed in very good to excellent yield with the use of 2-(2',6'-dimethoxybiphenyl)dicyclohexylphosphine, SPhos (1). This ligand confers unprecedented activity for these processes, allowing reactions to be performed at low catalyst levels, to prepare extremely hindered biaryls and to be carried out, in general, for reactions of aryl chlorides at room temperature. Additionally, structural studies of various 1.Pd complexes are presented along with computational data that help elucidate the efficacy that 1 imparts on Suzuki-Miyaura coupling processes. Moreover, a comparison of the reactions with 1 and with 2-(2',4',6'-triisopropylbiphenyl)diphenylphosphine (2) is presented that is informative in determining the relative importance of ligand bulk and electron-donating ability in the high activity of catalysts derived from ligands of this type. Further, when the aryl bromide becomes too hindered, an interesting C-H bond functionalization-cross-coupling sequence intervenes to provide product in high yield.

Enantioselective Cyanosilylation of Ketones Catalyzed by Double‐Activation Catalysts with N‐Oxides.

AbstractFor Abstract see ChemInform Abstract in Full Text.