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  • General Chemistry  (77,716)
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  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. 3-3 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 2
    ISSN: 0947-6539
    Keywords: chlorine complexes ; ethene complexes ; intermediates ; rotational spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A complex of ethene and Cl2 has been characterised in the gas phase. Rotational spectra of the isotopomers C2H4…35Cl2, C2H4…35Cl37Cl and C2H4…37Cl35Cl were recorded by using a fast-mixing nozzle in an FT microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl nuclear quadrupole coupling constants. χgg (Cl) are reported in each case. The complex is of the π-donor-acceptor type and has a C2v geometry in which Cl2 lies along the C2 axis perpendicular to the plane of the C2H4 nuclei. The binding is weak, and only small changes in the χgg (Cl) attend complex formation. A simple model attributes these changes to a transfer of around 0.02e from the inner to the outer Cl nucleus, thus confirming that the complex is of the Mulliken outer type. Similarities in the properties of C2H4… Cl2 and C2H4… HCl indicate that the angular geometry is in both cases determined mainly by the electrostatic part of the interaction. The distance from the π-bond midpoint to Cl decreases from C2H4… HCl to C2H4… Cl2; this suggests that Cl2 is “snub-nosed”.
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. 7-11 
    ISSN: 0947-6539
    Keywords: density-functional calculations ; ion-molecule reactions ; relativistic effects ; thermochemistry ; uranium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The nitridooxouranium cation NUO+ has been prepared as a new isoelectronic homologue to the uranyl dication UO22+ by a sequence of ionmolecule reactions starting from the atomic uranium cation U+. Collision-induced dissociation experiments and ligand-exchange reactions serve to unambiguously identify the ion as a species in which the uranium cation is formally inserted in the NO molecule. New thermochemical data for NUO+ and UN+ are given, and the stability of NUO+ with respect to UO22+ is investigated by means of quasi-relativistic density-functional calculations.
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  • 4
    ISSN: 0947-6539
    Keywords: amethyrin ; orangarin ; porphyrinoids ; terpyrroles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new, general synthesis of the first β-substituted tetra- and hexaalkyl terpyrroles is described. Also described are two new classes of expanded porphyrins derived from the hexaalkyl terpyrrole. The key step in the terpyrrole formation is the copper(II)-mediated oxidative coupling of the LDA-derived enolates of α-keto pyrroles. The first new expanded porphyrin reported here, the so-called “orangarin”, contains five pyrrolic subunits and two bridging carbon atoms, and is formally a 20π-electron nonaromatic macrocycle. The second new class of expanded porphyrins, the “amethyrins”, are 24π-electron nonaromatic macrocycles containing six pyrrole units. Both of these new macrocycles, as well as one of the new terpyrrolic precursors have been structurally characterized by single crystal X-ray diffraction analysis.
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  • 5
    ISSN: 0947-6539
    Keywords: porphyrinoids ; Rothemund synthesis ; sapphyrin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Rothemund-type condensation of pyrrole and benzaldehyde yields, apart from 5,10,15,20-tetraphenylporphyrin (TPPH2) and inverted tetraphenylporphyrin 2-aza-21-carba-5,10,15,20-tetraphenylporphyrin (CTPPH2), a unique pentapyrrolic macrocyclic molecule with the aromatic nucleus of sapphyrin, namely, 5,10,15,20-tetraphenylsapphyrin (TPSH3). Its unorthodox structural skeleton with an inverted pyrrole ring lying opposite to the bipyrrole unit accounts for the spectroscopic properties of the novel sapphyrin. The diprotonation of TPSH3 acts as a trigger for a structural transformation involving a flip of the pyrrole units, which relocates the 27-NH pyrrolic nitrogen from the periphery into the center of the macrocycle. The formation of 5,10,15,20-tetraphenylsapphyrin proves that the pentapyrrolic product is accessible by the mechanism of the Rothemund synthesis.
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  • 6
    ISSN: 0947-6539
    Keywords: catenanes ; cyclodextrins ; macrocycles ; orientational isomerism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel synthetic approach is described for the construction of catenanes in aqueous solution from a partially methylated cyclodextrin (CD)-namely, heptakis(2,6-di-O-methyl-β-cyclodextrin) (DM-β-CD)-and a range of substrate molecules that contain a hydrophobic central core in the form of a 4,4′-disubstituted biphenyl unit (usually bitolyl) carrying two hydrophilic polyether side chains terminated by primary amine functions. In water, the amphiphilic catenane precursors form 1:1 complexes with β-CD and DM-β-CD and 2:1 (guest: host) complexes with the larger γ-CD. Macrocyclizations of the biphenyl-containing substrates with aromatic diacid chlorides in aqueous solution and in the presence of DM-β-CD under Schotten-Baumann conditions afforded-in low yields-a range of [2]- and [3]catenanes. When a consitutionally asymmetrical diamine was employed as the substrate, orientational isomers of a [2]catenane were obtained. A [3]catenene incorporating a macrocyclic tetralactam was found to exist as a mixture of head-to-head and head-to-tail isomers, which could be separated by high pressure liquid chromatography and identified unambiguously by nuclear magnetic resonance spectroscopy. One of the [2]catenanes afforded good single crystals from which the solid state structure was determined by X-ray crystallography. Other techniques which aided the characterization of these novel compounds included ultraviolet/visible and luminescence spectroscopy, dynamic nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Generally speaking, the catenated cyclodextrins are soluble in halogenated and aromatic hydrocarbons as well as in hydroxylic solvents. The existence of these new compounds gives us a unique insight into the nature of the noncovalent bonding interactions that cyclodextrins employ in binding substrate molecules.
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  • 7
    ISSN: 0947-6539
    Keywords: chelate ligands ; hydrogen bonds ; ligand design ; mass spectrometry ; stability constants ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of compounds containing the mononuclear complexes [M(tdci)2]3+ (tdci = 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol, M = Al, Fe, Ga, In) and [M(tdci)2]4+ (M = Ti, Ge, Sn) was prepared and characterized by elemental analysis, NMR spectroscopy, and FAB mass spectrometry. Characteristic fragmentation reactions in the mass spectra were elucidated. X-ray analysis of the AlIII, FeIII, GaIII, and InIII complexes revealed that two neutral, zwitterionic tdci ligands coordinate to the metal cation exclusively through deprotonated alkoxo groups. The six coordinated oxygen donors and the six N—H protons form a hydrophilic pocket, whereas the two cyclohexane rings and the twelve methyl groups form two hydrophobic shells. The hydrophilic pocket is filled with twelve water molecules, which are arranged as a second and a third coordination sphere around the metal cation. The reactivity in aqueous solution was investigated by potentiometric measurements. The bis complexes proved to be stable at pH 7. The evaluated formation constants show an increase of stability in the order AlIII〈InIII〈GaIII〈FeIII. The measurements established that tdci is one of the most effective tridentate ligands for small (r ≤ 0.8Å) and highly charged cations. The different chelating properties of tdci and of the unmethylated 1,3,5-triamino- 1,3,5-trideoxy-cis-inositol are discussed in terms of different steric requirements and different types of solvation of the corresponding complexes in aqueous solution.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. cpi 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 9
    ISSN: 0947-6539
    Keywords: charge transfer ; EPR spectroscopy ; organometallic compounds ; platinum compounds ; spectroelectrochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In spite of their very similar cyclic voltammograms, absorption spectra, and solvatochromic behavior, the two 1,4-diazabutadiene title complexes exhibit markedly different photoreactivities and underlying electronic structures, as evident from absorption and EPR spectra of the persistent anion radical forms. The lowest excited state of the nonphotoreactive PtII system [(CyN=CH—CH=NCy)-PtMe2] has MLCT (metal-to-ligand charge-transfer, 5d → π*) character, and the EPR spectrum of the corresponding anion radical at 〈g〉 = 2.016 exhibits sizable metal/ligand orbital mixing. On the other hand, the structurally characterized PtIV complex [(CyN=CH—CH=NCy)-PtMe4] (C2/c; a = 2021.6(2), b = 805.3(1), c = 1254.2(1) pm; β = 111.05(1)°; V = 1905.7(4) × 106 pm3; Z = 4) has a lowlying photoreactive LLCT (ligand-to-ligand charge-transfer, σPt—C → π*) excited state in which the axial Pt—C bonds are activated, as already suggested by the longer Pt—C(ax) bonds (214.0(8) pm) relative to Pt—C(eq) in the ground state (204.5(5) pm). The anion radical of the PtIV complex has lost the long-wavelength absorption band in the visible; it shows a well-resolved EPR spectrum at 〈g〉 = 1.9945 with π-ligand and 195Pt hyperfine structure and a small g anisotropy. A qualitative MO scheme is presented to account for the similar frontier-orbital energy differences despite dissimilar underlying electronic structures.
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 11
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. cpi 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 12
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. cpi 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 13
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 14
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 15
    ISSN: 0947-6539
    Keywords: ab initio calculations ; copper compounds ; organometallic compounds ; tetrahydroborato ligand ; theoretical chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: All-electron ab initio calculations (MP4/MP2 level) have been performed on [Cu(BH4)(PH3)n] (n = 1, 2, 3) complexes. Full-geometry optimizations were carried out in each case, and the stationary points were characterized by the diagonalization of the analytically calculated Hessian matrix. The η2 coordination mode, with a tetrahedral arrangement around the copper atom, is the most stable structure for n = 2, while for n = 3 a strongly nonlinear η1 coordination mode is preferred. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental data, the η3 structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (η1 → η2 for n = 3 and η3 → η2 for n = 1) are small. Finally, a mechanism for the exchange between terminal and bridging hydrogen atoms is proposed for each complex under study.
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  • 16
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. cpi 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 17
    ISSN: 0947-6539
    Keywords: asymmetric ; syntheses ; enzyme inhibitors ; squalestatins ; total syntheses ; zaragozic acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel synthetic approach to the construction of the zaragozic acids, which was used for the asymmetric synthesis of zaragozic acid A/squalestatin S1 (1), is described. Fragment 5, representing the tricarboxylic acid core portion, is assembled in three key steps: 1) Stille coupling to establish the carbon framework; 2) enantioselective dihydroxylation to introduce the absolute stereochemistry; and 3) diastereoselective dihydroxylation to complete the required carbon-oxygen connectivity. The convergency of this synthesis is demonstrated by the dithiane addition of a variety of C 1 side chains (e.g., 78) to advanced intermediate 5. A multi-event acid-catalyzed rearrangement yielded the zaragozic acid core 86, which was converted to an intermediate obtained from degradation of zaragozic acid A. A second-generation synthesis of the core of the zaragozic acids is also described. When aldehyde 90 was used instead of 5, both the yield and diastereoselectivity of the dithiane addition reaction were improved, although the degree of convergency was slightly lower.
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  • 18
    ISSN: 0947-6539
    Keywords: exchange coupling ; ferromagnetic properties ; ligand design ; magnetic properties ; multimetallic complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An approach is suggested for using ligands to control exchange coupling in multinuclear ions. The idea arose from structural, EPR, and magnetic studies of [PPh4]3 (Scheme 1). Ferromagnetic coupling has been found between the CoII and each CoIII in 3 with J = -22 ± 5 cm-1 (JS1 · S2). It is suggested that dominant antiferromagnetic superexchange is absent because of the strong σ-donor capacity of the tetradentate ligand [k4-PAC*]4- (Fig. 1). The ligand interacts at CoIII primarily with a single d orbital; it is thus best able to participate in superexchange. The interaction makes the unique d orbital strongly σ-antibonding and empty for each d6, S = 1, CoIII ion in 3, that is, unavailable for antiferromagnetic coupling, but available for ferromagnetic pathways by a Goodenough-Kanamori mechanism. By corollary, when any [k4-PAC*]4--type ligand with any magnetic ion Ma in the tetradentate site binds any magnetic ion Mb in the bidentate site, ferromagnetic coupling should be favored provided Ma is not a d9 ion.
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  • 19
    ISSN: 0947-6539
    Keywords: electronic structure ; frontier orbitals ; metalloporphyrins ; radical cations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of magnesium and zinc tetraarylporphyrins and their porphyrinoxidized derivatives were studied by UV/Vis, ESR, and resonance Raman spectroscopy at various temperatures. The series included tetra(meta-dichlorophenyl)porphyrinatozinc (5), tetra(ortho-dichlorophenyl)porphyrinatozinc (6), tetra(ortho-difluorophenyl)porphyrinatozinc and -magnesium (9 and 10), and tetra(pentafluorophenyl)porphyrinatozinc and -magnesium (7 and 8). The radical cations (3a-10a) were isolated by chemical one-electron oxidation of their neutral precursors (3-10). Despite the structural similarity of all these radicals, their electronic ground state varied within the series. The position of the chloro groups was found to play a key role. While the radical cation of the meta-dichloro-substituted derivative 5a exhibited A2u spectroscopic features, the ortho-dichlorophenyl derivative (6a) showed A1u spectral features. Radicals of the fluoro-substituted porphyrins, especially that of 10, were found to have state-admixed (A1u/A2u) electronic structures, and the relative contributions of the two states was found to vary with temperature and to depend on the axial ligand. The results indicate that the fluoro-substituted porphyrins are primarily A2u at low temperature, even though their room temperature spectroscopic features resemble those of A1u cations. The elucidation of factors that affect the electronic structures of the radicals in the present series is helpful in providing a greater understanding of the spin-spin interactions in the intermediates of heme-dependant enzymatic reactions and their synthetic analogues.
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  • 20
    ISSN: 0947-6539
    Keywords: allylic substrates ; carbon-carbon coupling ; catalysis ; copper compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regioselectivity in the arenethiolatocopper(I)-catalyzed crosscoupling reaction of allylic substrates was studied. It was found that allylic acetates gave highly γ-selective reactions in Et2O at 0°C with slow addition of the Grignard reagent, whereas α-selective reactions were obtained in THF at -30°C with fast addition of the Grignard reagent. It is proposed that the formation of an intermediate in Et2O, in which the allylic acetate coordinates in a bidentate fashion the arenethiolatocopper(I) catalyst, dramatically increases the reactivity of the leaving group and results in excellent γ-selectivity. The remarkable observation that an allylic acetate can be made more reactive than an allylic chloride by using the arenethiolatocopper(I) catalyst 1a supports the theory of a bidentate coordination of the substrate to the catalyst through its double bond and acetate oxygen.
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  • 21
    ISSN: 0947-6539
    Keywords: asymmetric syntheses ; alkenylations ; SAMP/RAMP hydrazones ; selenyl aldehydes ; sphingosine ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: L-threo-sphingosine and its D-erythro isomer (1) are subunits of many glycosphingolipids, gangliosides and ceramides. This paper describes the highly diastereo- and enantioselective synthesis of both isomers (de, ee 〉 98%). The key steps in the synthesis are the aldol reaction of the SAMP hydrazone (S)-2 with racemic α-phenylselenylpentadecanal 3, the diastereoselective triacetoxyborohydride reduction of ketone 5 and exclusive (E) C—C double bond formation in the elimination of hydroxyl and selenyl moieties promoted by methanesulfonyl chloride. Mesylate 8 was then readily converted via the 1,3-O-acetonide-protected azidosphingosine 9 to L-threo-sphingosine. Conversion to the known 1-O,2-N-diacetyl-protected sphingosine 13 with subsequent Mitsunobu inversion of the C3—OH centre afforded the D-erythro-sphingosine epimer.
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  • 22
    ISSN: 0947-6539
    Keywords: ab initio calculations ; azaenolates ; NMR chemical shifts ; peptides ; Ramachandran maps ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structures and conformations of the azaenolate lithium salts of amides (formamide, acetamide, and N-methylacetamide) and of the dipeptide model N-formylalaninamide were investigated by means of ab initio MO theory. Four possible structures of the lithiated C-enolates of acetamide were also included in the study. All structures were calculated at the HF/6-31+G(d) and MP2(fc)/6-31 + G(d)/HF/6-31 + G(d) levels; the lithiated azaenolates of formamide were also investigated at higher theoretical levels (up to MP4(fc)/6-311 + G(d,p)/MP2(fc)/6-311 + G(d,p)). For the lithiated azaenolates of all amides investigated, the most stable structure contains a four-membered ring in which the lithium ion is complexed by the oxygen and nitrogen atoms; the substituents attached to the carbon and nitrogen atoms of the azaenolate are in a cis arrangement. The lithiated azaenolates of acetamide are predicted to be more stable than the corresponding C-enolates. To simulate solvation, calculations on complexes of the lithiated azaenolates of formamide with up to three molecules dimethyl ether were also performed, and all azaenolates of amides were also reoptimized by ab initio reaction-field calculations. Both solvation models reduce the preference for lithium-chelated cis structures. The Ramachandran maps of the dilithiated bis(azaenolate) of N-formylalaninamide (having cis or trans arrangements of the azaenolate substituents) were scanned by MNDO calculations for conformational accessible regions. Thirteen stable structures were subsequently optimized at the HF/6-31 + G(d) ab initio level. The global minimum resembles a peptide in C7 conformation, but other conformations, not known for peptides, are close in energy. The structures of dimers of the lithiated azaenolates of N-methylacetamide and of glycinaldehyde were also calculated. The NMR chemical shielding of carbon, nitrogen, and oxygen atoms in all structures were predicted ab initio by using the gauge-including atomic orbital (GIAO) method.
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  • 23
    ISSN: 0947-6539
    Keywords: alkenes ; catalysis ; dihydroxylations ; electrophilicity ; ruthenium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidative ruthenium catalysis (0.07 molequiv RuCl3·(H2o)3, 1.5 molequiv NaIO4, EtOAc/CH3CH3CN/H2O 3:3:1), beyond the usual C—C bond cleavage to give dicarbonyls, has been shown to syn-dihydroxylate a wide range of alkenes (except for strained bicyclic alkenes, sterically hindered trisubstituted alkenes, and most tetrasubstituted alkenes) to give vicinal diols rapidly (within minutes) and efficiently. The minor products are the usual oxidative fission products, namely, ketones and aldehydes or carboxylic acids, and sometimes ketols. Longer reaction times lower the yields of most diols, probably owing to oxidative glycol cleavage. Reactions with substrates containing one or more electron-with-drawing groups in conjugation with or adjacent to the alkene moiety are generally slower but give better yields. The diastereoselectivity of the present “flash” dihydroxylation, anti to the existing α-stereogenic center, with cycloalkenes is excellent whereas that with acyclic alkenes is moderate to poor. Sodium metaperiodate is still the best co-oxidant for the catalytic reaction. Aqueous acetonitrile (approximately 86%) as an alternative solvent system was found to give better yields of 1,2-diols than the original solvent system in some cases.
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  • 24
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 83-87 
    ISSN: 0947-6539
    Keywords: enzyme models ; iron complexes ; molybdenum complexes ; nitro genase ; semi-empirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A model for the active site of nitrogenase is suggested and examined by means of the intermediate neglect of differential overlap (INDO) quantum mechanical method. The initial steps of the nitrogen fixation process are discussed within the framework of the present model, and it is shown that of several binding sites, initial location of the nitrogen molecule inside the MoFe cofactor is favored. Possible pathways for electron and proton delivery to the active site are also suggested on the basis of electrostatic potential calculations.
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  • 25
    ISSN: 0947-6539
    Keywords: azadienes ; azepines ; cycloadditions ; Fischer carbenes complexes ; reaction mechanisms ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 4-Amino-1-azadienes 1 react with α,β-unsaturated Fischer carbene complexes at -40°C to give stereoselectively a variety of substituted 3H-4,5-dihydroazepines 3; similarly, 1-hydroxy-1-azadienes (α,β-unsaturated oximes) 6 afforded the corresponding azepine derivatives 7. Chiral, nonracemic carbene complexes 11 gave azepines 12-13 (d.e. = 40-44%) upon reaction with oxime 6a; the major isomers were obtained in a diastereomerically and enantiomerically pure form (45-50% overall yield) after crystallization. An X-ray structure of 12a allowed assignment of the absolute stereochemistry. The acid hydrolysis of azepines synthesized provided racemic and enantiomerically pure 1,6-dicarbonyl compounds (±)-5, (±)-9, and (-)-14, as well as diol (-)-15. The mechanism of the reaction of 1 and 2 was investigated by multinuclear (1H, 13C, 15N, and 183W) NMR characterization of four intermediates (A, B, C, and D) at low temperature. The experimental sequence of events involves: i) 1,2-nucleophilic addition of the unsubstituted imine nitrogen of 1 to the metal carbene function (zwitterion A, -60°C), ii) cyclization to the seven-membered ring with 1,2-migration of the pentacarbonyl metal (zwitterion B, -40°C), iii) reductive elimination and coordination of the metal to the amine nitrogen (intermediate C, -40°C), and iv) thermal decomplexation and tautomerization (intermediate D and compound 3, above -20°C).
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  • 26
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 27
    ISSN: 0947-6539
    Keywords: bimetallic complexes ; NLO materials ; sandwich complexes ; sesquifulvalenes ; solvatochromism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to construct bimetallic organometallic donor-acceptor complexes with nonlinear optical (NLO) properties, the bimetallic sesquifulvalene complexes [{CpFe(η5-C5H4)}Z{η7-C7H6)Cr-(CO)3}]BF4 (2BF4) have been synthesized with different bridging functions Z (2a: Z = - 2b: Z = C2; 2c: Z = (E)-C2H2). X-ray structure determinations show that 2a BF4 and 2c BF4 crystallize in acentric space groups (P41, P212121), whereas crystalline 2b BF4 has the centric space group P21/c. The organometallic fragments in 2a adopt a transoid conformation, those in 2b and 2c are cisoid. The cyclo-C5 and cyclo-C7 rings of the sesquifulvalene ligands are almost coplanar; this allows an optimum electronic interaction between the ferrocenyl donor and the [η7-C7H6)Cr(CO)3]+ acceptor moiety in the crystalline state. Cyclovoltammetric studies show an electrochemically reversible one-electron oxidation ( 〉 0 mV vs. FeH/FcH+, FcH = ferrocene) and an irreversible one-electron reduction (≤-900 mV), which are assigned to the ferrocenyl and [(η7-C7H6)Cr(CO)3]+ units, respectively. UV/vis spectra reveal low-energy absorptions for 2a-c at about λ=600 nm, showing strong negative solvatochromism. These absorption maxima are related to a charge-transfer excitation between the ferrocenyl and the [(η7-C7H6)Cr(CO)3]+ groups, indicating considerable nonlinear optical properties. Determination of the first hyperpolarizability β by means of hyper Rayleigh scattering yields exceptionally large β values: β(2b) = 570 × 10-30 esu and β(2c) = 320 × 10-30 esu. These unexpectedly large β values are explained by resonance enhancement.
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  • 28
    ISSN: 0947-6539
    Keywords: azo dyes ; circular dichroism ; data fitting ; formation constants ; protein complexation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Difference absorbance and circular dichroism techniques show that two complexes are formed between human serum albumin (HSA) and Methyl Orange (MO). The stoichiometries of the two HSA-MO complexes (1:1 (C1) and 1:2 (C2)), their association constants (K1, 1 = 2.32 (0.18) × 105 M-1 and K1, 2 = 1.12 (0.15) × 1011 M-2), and both absorbance and dichroic spectra have been determined by a computational approach. Nearly 900 experimental points, consisting of absorbance and CD measurements registered in the 340-550 nm interval and over a wide range of concentrations of protein and ligand, have been included in a unique fitting procedure. The Scatchard plot indicates the existence of a unique binding site which can accommodate up to two molecules of MO in a positive co-operative process. Calculation of the CD spectrum for the C2 complex according to the DeVoe method reproduces the fitted dichroic spectrum for the same complex. The shapes of the fitted absorbance and dichroic spectra, as well as the influence of concentrated NaCl or ethylene glycol on the absorbances of both free MO and HSA-MO mixtures are consistent with the presence of dominant electrostatic interactions in C1. The C2 complex can be envisaged as a unique chromophore, consisting of two MO units associated in a stacking process into the same binding site of HSA, leading to a well-defined chirality. The general validity of this multitechnique, multiwavelength approach in the investigation of protein-ligand complexes is discussed.
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  • 29
    ISSN: 0947-6539
    Keywords: catalysis ; crystal structure ; oxidations ; rhenium peroxo complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rhenium oxides Re2O7 and ReO3 react with hydrogen peroxide solutions yielding peroxo complexes that efficiently catalyze the oxidation of olefins, aromatics, and certain organometallic compounds. In contrast, related oxides of molybdenum (MoO3) and vanadium (V2O5) do not activate H2O2 under comparable conditions. The active rhenium peroxo complex was isolated from the system Re2O7/H2O2: the crystalline red-orange, explosive compound of formula H4Re2O13 is the most oxygen rich rhenium compound isolated to date. Its structure resembles a “peroxo perrhenic acid”. The binuclear compound could be isolated in the form of a diglyme adduct, structurally defined as two corner-sharing pentagonal bipyramids with apical oxo and aquo ligands; the equatorial positions are occupied by the bridging oxygen and by n2-peroxo groups (two [O2]2- ligands per rhenium). In contrast to the known complex [CH3ReO(O2)2].H2O, the new peroxo species [O{ReO(O2)2.H2O}2] decomposes hydrolytically during the catalytic cycle and can thus not compete in terms of catalytic activity in oxidation reactions involving H2O2. Hydrolysis yields “perrhenic acid” Re2O7.2H2O, the diglyme adduct of which compound was also characterized by means of an X-ray diffraction analysis.
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  • 30
    ISSN: 0947-6539
    Keywords: asymmetric syntheses ; chromium complexes ; diones ; oxy-Cope rearrangements ; polycycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tricarbonylchromium complex 3 of 1,2-dioxobenzocyclobutene (1) is easily prepared by hydrolysis of diacetal 4. Complex 3 stereoselectively undergoes both single and double nucleophilic addition reactions at the keto groups. As a consequence, addition of excess alkenyllithium to 3 gives rise to a dianionic oxy-Cope rearrangement yielding benzocyclooctenedione complexes with good diastereoselectivity. Trapping of the bis(enolate) intermediate with chlorotrimethylsilane gives bis(enolether) 23. Subsequent addition of two different alkenyllithium derivatives yields the asymmetrically substituted rearrangement product 15. To some extent, dependent on the method of hydrolysis, an intramolecular aldol addition follows the dianionic oxy-Cope rearrangement. This results in the formation of benzoanellated bicyclo[3.3.0]octane derivatives and, in the case of 1-cyclopentenyllithium as the alkenylmetal, in the tetraquinane system 19. The trans-anellation of one of the cyclopentane rings in trans-20 has been verified by an X-ray structural analysis. The highly substituted rearrangement product 21/22 is formed only in small yield; in solution the bis(enol) tautomeric form 22 is favored over diketone 21.
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  • 31
    ISSN: 0947-6539
    Keywords: eliminations ; orbital interactions ; solvent effects ; substitutions ; theoretical chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The model reaction system F- +C2H5F + nHF (n = 0-4) has been investigated by use of a density-functional method, in order to achieve a qualitative understanding of the effect of solvation on the E2 and SN2 reactions. Two characteristic effects already occur upon monosolvation: a) the activation energies of the E2 and SN2 pathways increase significantly and even become positive, because reactants are more strongly solvated than transition states; b) the SN2 transition state is stabilized much more and becomes lower in energy than the anti-E2 transition state. This agrees with general experience from gas- and condensed-phase experiments. The solvation is analyzed from two complementary viewpoints: a) as an interaction between solvent molecules and the F-/C2H5F reaction system; b) as an interaction between the [F-, nHF] solvated base and the C2H5F substrate. The extent to which condensed-phase characteristics can be modeled by this microsolvation approach is discussed.
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  • 32
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 33
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 34
    ISSN: 0947-6539
    Keywords: cyclic compounds ; medium-sized ; rings ; tetraketones ; transannular interactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By using a fast-mixing nozzle in a Fourier transform microwave spectrometer, any chemical reaction accompanying mixing of H2S and BrCl was prevented, thus allowing the observation of the pre-reactive complex H2S ··· BrCl. The rotational spectra of eight isotopomers of the complex were recorded. The analysis of the determined spectroscopic constants shows that the atoms S ··· Br—Cl are collinear or nearly so and that the H2S plane is approximately perpendicular to the S ··· Br—Cl axis with r(S ··· Br) = 3.094 (7) Å. This geometry is in agreement with previously stated rules for B ··· XY complexes, where B is a Lewis base and XY is an (inter)halogen molecule. The intermolecular interaction is shown to be relatively weak, both in terms of the intermolecular stretching force constant kσ and the intramolecular electric change redistribution δ within the BrCl subunit that accompanies formation of H2S ··· BrCl.
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  • 35
    ISSN: 0947-6539
    Keywords: carbonyl oxides ; dioxiranes ; mechanistic studies ; oxidations ; thianthrene 5-oxide ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thianthrene 5-oxide (SSO) constitutes a useful mechanistic tool for the assessment of the electronic character of oxygen transfer agents by means of their Xso values, which reflect the extent of nucleophilic oxidation at the SO site in SSO. Treatment of dioxiranes 1 a-d with the SSO probe confirms that these are electrophilic oxidants (Xso 〈 0.15). Dioxirane sulfoxidation is sensitive to protic solvents and acids, which implies a polar mechanism (SN2) with nucleophilic attack of the sulfide electron pair on the dioxirane peroxide bond. In contrast, the carbonyl oxides 2 a and b, diethyl persulfoxide (3), and adamantylideneadamantane perepoxide (4) are nucleophilic oxidants (Xso 〉 0.85). However, the cyclopentadienone carbonyl oxides 2 c and d show low Xso values typical for electrophilic oxygen transfer agents. For these carbonyl oxides, photoisomerization to the respective dioxiranes is proposed under the photooxidation conditions of the diazoalkanes. Additionally, the trans/cis ratio of bissulfoxides (SOSO) formed provides valuable information on the stereochemical course of the oxygen-transfer process due to the steric requirements of the oxidant. Thus, the electrophilic dioxiranes preferentially attack the axial lone pair of SSO because of repulsion by the peri hydrogen atoms to afford trans-SO-SO.
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  • 36
    ISSN: 0947-6539
    Keywords: conformational analysis ; galabioside ; hydrogen bonds ; protein recognition ; thioglycosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The two thio analogues (2 and 3) of TMSEt galabioside [2-(trimethylsilyl)ethyl 4-O-(α-D-galactopyranosyl)-β-D-galactopyranoside, 1], having anomeric sulfur instead of anomeric oxygen atoms, were synthesized and their conformations investigated by NMR and computational (MM 3) methods. A spacer galabioside was covalently coupled to aminated microtiter plates, and binding of a bacterial pilus adhesin (PapG) to the plates was inhibited by the soluble ligands 1, 2 and 3. The ligand 2, which has an intersaccharidic sulfur linkage, was a much less efficient inhibitor than 1, which has the natural oxygen linkage. The inhibitory power of ligand 3 was only slightly less than that of 1. An NMR experiment with 1 and 2, in which hydroxyl-group hydrogens had been partially (50%) substituted by deuterium, demonstrated the presence (in 1) and absence (in 2) of an intramolecular (HO 2′ - HO 6) hydrogen bond. This result indicates that the conformations of 1 and 2 are different and that the difference is sufficient to cause the observed (≈ 30 times) reduction of the saccharide-protein binding strength.
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  • 37
    ISSN: 0947-6539
    Keywords: enzyme inhibitors ; niobium complexes ; peptide aldehydes ; pinacol coupling ; vanadium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Peptide aldehydes 15 a-c are prepared without epimerization from enantiomerically pure (S)-α-amino acids (Scheme 3). Reductive pinacol homocoupling of 15 a-c, induced by vanadium complex 11 or niobium complex 16 in refluxing THF, yields C2-symmetrical (S,R,R,S)-configurated 6a, 6b and 2, respectively, with moderate to high stereoselectivity (Scheme 4). In a novel protocol for the preparation and utilization of THF solutions of 11, the isolation of air-sensitive intermediates can be avoided and the potent HIV protease inhibitor 2 prepared in enantio- and diastereomerically pure form on a kilogram scale without chromatographic purification. The (S,R,R,S) selectivity of the pinacol homocouplings is confirmed by means of an independent, stereochemically unequivocal synthesis of 6 a and 2 from D-mannitol 4 (Scheme 1).
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  • 38
    ISSN: 0947-6539
    Keywords: carbonyl ligands ; organometallic compounds ; phosphorus ligands ; tantalum compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reduction of [TaCl5] by six equivalents of alkali metal naphthalenide in 1,2-dimethoxyethane at -60°C followed by treatment with gaseous PF3 provides the first homoleptic phosphane complex containing tantalum in the -1 oxidation state, [Ta(PF3)6]-. This can be protonated by concentrated sulfuric acid to yield the previously unknown highly acidic and volatile hydride [HTa(PF3)6]. An improved normal-pressure synthesis of [Ta(CO)6]- is described. Reduction of the latter species by sodium in liquid ammonia gives the carbonyl trianion [Ta(CO)5]3- which undergoes monoprotonation and stannylation to form [HTa(CO)5]2- and [Ph3SnTa(CO)5]2-, respectively. The hydride is a useful precursor to [(Ph3PAu)3Ta(CO)5], the only known gold cluster of tantalum.
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  • 39
    ISSN: 0947-6539
    Keywords: alkenyl ; Fischer-Tropsch synthesis ; labelling studies ; rhodium compounds ; vinyl ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results consistent with the participation of vinyls in the initiation and of alkenyl species in the propagation steps of the Fischer-Tropsch reaction are reported. Substantial incorporation of 13C2 into the alkene and alkane (C3-C7) hydrocarbon products occurred when doubly labelled vinyls (13C2H3Br. (13C2H3)4Si, or 13C2H4) were added as molecular probes to the hydrogenation of carbon monoxide over rhodium/ceria/silica catalysts (1 atm, 220°C). There was, by contrast, no significant incorporation of 13C1 into any of the organic products; thus cleavage of the C2 probe did not occur. The degree of 13C2 incorporation decreased with increasing molecular mass of the hydrocarbon; this indicates that the probe molecule initiated but did not propagate. A mathematical model based on polymerisation of surface methylenes initiated by a vinyl, propagated by alkenyls and terminated by reaction with a surface hydrogen or by coupling has been developed to explain the 13C2 incorporation data. Under the conditions of the experiments, the relative ability of the probes to initiate is: vinyl bromide (60%)〉tetravinylsilane (30%)〉ethene (15%). Substantial formation of 13C4 products also occurred when vinyl bromide or tetravinylsilane were used as probes; this arises from a dimerisation of the vinyl on the surface, a process which has been modelled in homogeneous systems and also by other workers in studies on single crystal surfaces. There was no significant 13C incorporation into the oxygenates (methanol, ethanol, acetaldehyde); these products are formed by a different path.
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  • 40
    ISSN: 0947-6539
    Keywords: aniline ; aromatization ; hydrogen transfer ; mechanistic studies ; radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nitro-, nitroso- and azobenzene are reduced almost quantitatively to aniline when heated to 230-300°C with 9,10-dihydroanthracene (DHA), xanthene or tetralin. From the effect of polar substituents and polar solvents on the reactivity and from the isotope effect kH/kD ≍ 2.4 (280°C), a hydrogen-atom transfer from the H donor to the acceptor (retrodisproportionation) is proposed as the rate-determining step. The lower reactivity of xanthene compared with 9,10-dihydroanthracene eliminates the possibility of a rate-determining hydride transfer. The observation of an intense ESR signal of 9-xanthyl radicals during the reaction in diphenyl ether and the typical products support the proposed homolytic mechanism.
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  • 41
    ISSN: 0947-6539
    Keywords: biphenyls ; bipyridinium salts ; conformation ; semiempirical calculations ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of substituted N-aminopyridinium compounds yields bipyridinium salts. After anion exchange Br- → CIO4-, it was possible to grow single crystals from the 4,4′-di(tert-butyl) derivative. The structure determined at 100 K shows a dihedral angle of 84° between the molecular halves connected by a shortened N—N bond of 143 pm in length. This result is contrary to that expected from the isoelectronic correspondence 〉C—C〈↔〉N—N〈, which would suggest planarity, as has been observed experimentally both in solid biphenyl, and in tetraalkylhydrazine dications and radical cations. Lattice packing analysis, however, reveals the presence of hydrogen bonds C(H)…O between the phenyl rings and the perchlorate anions. AM 1 enthalpy hypersurface calculations for the isoelectronic series H5C5X-YC5H5 (X-Y = C—C, +N—C, +N—N+, -B—N+, B—C, and B—B-) predict single-minimum potentials for both the dication 〉N—N〈 as well as the dianion 〉B—B〈 with the molecular halves twisted perpendicular to each other. In detailed model calculations, counteracting effects of π electron density delocalization vs. H/H repulsion of the ortho ring hydrogens adjacent to the central bond seem to dictate the delicate balance of the biphenyl twisting. For further experimental confirmation, the structure of the isosteric molecule 4,4′-di(tert-butyl)biphenyl has been determined: in contrast to the unsubstituted π hydrocarbon with a latticeenforced dihedral angle of 0°, close to the calculated value, the two molecular halves are twisted by 40° - as in biphenyl in the gas phase.
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  • 42
    ISSN: 0947-6539
    Keywords: dinitrogen complexes ; EPR spectroscopy ; iron compounds ; reductions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of [Fe(CO)2-L2X2] (L = P(OMe)3 X = Br (1a), I (1b); L = P(OiPr)3, X = Br (2a), I (2b); L = PEt3, X = Br (3a), I (3b)) with Zn in dioxane (1a,b-2a,b) or PhLi in ether (3a,b) led to formation of the corresponding dicarbonyl(halo)bis(phosphorus donor)iron(I) complexes (L = P(OMe)3, X = Br (4a), I (4b); L = P(OiPr)3, X = Br (5a), I (5b); L = PEt3, X = Br (6a), I (6b)). Slightly contaminated 5a,b and pure 6a,b were isolated as stable crystalline blue or blue-green complexes. Complexes 4a,b were obtained, with only minor impurities, by comproportionation of 1a,b and the dicarbonyl(halo)bis-(trimethylphosphite)ferrate anions 7a,b and characterized in THF solution. The comproportionation products 5a,b-6a,b were obtained in high yields by reaction of 2a,b-3a,b with the dinitrogen complexes 13-15. Further reduction of 4a,b-6a,b or exhaustive reduction of 1a,b-3a,b with sodium amalgam or iBuLi in THF afforded the nonisolable dicarbonyl-(halo)bis(phosphorus donor)ferrate(o) anions (7a,b-9a,b). The latter were characterized by acidification with trifluoroacetic acid or acetic acid yielding stable dicarbonyl (halo) hydridobis (phosphorus donor)iron(II) complexes (L = P(OMe)3, X = Br (10a), I (10b); L = P(OiPr)3, X = Br (11a), I (11b);L = PEt3, X = Br (12a), I (12b)). In the presence of N2, 8a was transformed into dinitrogen complex [Fe(CO)2{P(OiPr)3}2N2] (13). With 9a,b the reaction led to formation of [Fe-(CO)2(PEt3)2)2N2] (14) and [{Fe(CO)2-(PEt3)2}2(μ-N2)] (15). In solution at low temperature (-90 to 0°C), the dinitrogen complexes 14 and 15 are in equilibrium with each other. Complexes 13, 14, and 15 were characterized by IR, 1H, 13C, 31P, and 15N NMR spectroscopy. The structures of 5a, 6b, 14, and 15 were determined by X-ray diffraction studies.
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  • 43
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. cpi 
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 44
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 45
    ISSN: 0947-6539
    Keywords: allylic substrates ; amidinomycin ; asymmetric syntheses ; carbanucleosides ; palladium catalysts ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The amino acid derivative in the title represents an important building block for the synthesis of a number of biologically important targets such as the antiviral carbanucleosides and amidinomycin. By using asymmetric palladium-catalyzed desymmetrization of meso-2-ene-1,4-diols, cis-1,4-dibenzoyloxy-2-cyclopentene can be converted to the enantiomerically pure title compound in only four steps. Chemoselective ester reduction allows entry into the domain of carbanucleosides, whereas double-bond reduction provides the precursor for amidinomycin. In an ancillary study, a facile diastereoselective cis-hydroxylation provides aminocyclopentitols, compounds that have proven to be potent glycosidase inhibitors.
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  • 46
    ISSN: 0947-6539
    Keywords: electron transfer ; iron-sulfur proteins ; NMR spectroscopy ; proteins ; solution structures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the 1H NMR spectrum of the oxidized form of the high-potential iron-sulfur protein (HiPIPI) from Ectothiorhodospira halophila, 91% of the total proton resonances and 100% of the residues have been assigned. The standard COSY, NOESY, and TOCSY sequences have been optimized for the paramagnetism of the molecule. Extensive assignment of the 15N NMR spectrum has been obtained through HMQC spectra. With 1437 dipolar connectivities, of which about 10% involved fast-relaxing protons, a family of 18 structures was generated with an RMSD of 0.65 Å by using the programs developed by Wüthrich. The family of structures was further refined by various calculation steps; the final RMSD was 0.48 Å. The structures appear to be very similar but not equal to the structures of the reduced protein. Despite the similarity in structure, significant variations in the chemical shifts are observed. A similar behavior was observed for the homologous protein from Chromatium vinosum. It is concluded that NMR is a sensitive tool to monitor differences between oxidized and reduced proteins; however, the detailed structural variations should be evaluated with caution at the present level of resolution, which roughly corresponds to a resolution of 2.5 Å in an X-ray structure.
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  • 47
    ISSN: 0947-6539
    Keywords: copper compounds ; exchange coupling ; heterometallic compounds ; lanthanide compounds ; magnetic properties ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and structural characterisation of three copper-lanthanoid complexes are reported. The compounds, of general formula [Cu3M(chp)8(NO3)(S)] [M = Gd, S = H2O in 2; M = Dy, S = H2O in 3; M = Er, S = (H2O)0.5(MeOH)0.5 in 4; chp = anion of 6-chloro-2-pyridone], are made by reaction of [Cu2(chp)4] (1) with the hydrated lanthanoid nitrate salt in methanol. Structural studies reveal the three copper atoms lie in an approximate hemisphere about a central lanthanoid atom. Magnetic studies on 2 and two further Cu-Gd complexes show ferromagnetic coupling between the 3d and 4f metals. Consideration of these results along with magnetic data previously reported for Cu-Gd compounds leads to a correlation between the magnitude of this exchange coupling and the exponential of the Cu…Gd distance. This is the first magneto-structural correlation reported for mixed d-block/f-block metal complexes.
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  • 48
    ISSN: 0947-6539
    Keywords: chemical bonding ; electron localization ; scanning tunnelling microscopy ; surface structures ; tight-binding calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The advantage of computer graphics in the visualization of tight-binding calculations is highlighted in a model study of the reconstruction of the Si(100) surface. Three different surface models - the unreconstructed surface Si(100)-(1 × 1), and symmetric and asymmetric pairing of surface atoms Si(100)-(2 × 1) - are investigated on the basis of density of states (DOS), local (projected) density of states (LDOS) and crystal orbital-overlap population (COOP) analysis. For the visualization of the real-space properties of tight-binding calculations, two- and three-dimensional images of the total (TED) and partial electron densities (PED) are shown. The PED calculated near the Fermi level are compared to densities of HOMOs and LUMOs in molecular systems and used to analyse constant current mode STM images, obtained by applying bias voltages of different sign. They show excellent agreement with STM experiments. The electron-localization function (ELF) has been shown to describe chemical bonds in molecules and solids surprisingly well. Here, the ELF is calculated for surfaces. In order to visualize the shape of the “dangling” surface bonds and bonds connecting surface atoms, two- and three-dimensional representations of the ELF are discussed. Using the reconstruction of the Si(100) surface as an example, we show that combining methods for extracting information from quantum mechanical calculations, such as PED, TED and ELF, leads to a more comprehensive description of the electronic surface structure. With the help of computer graphics, chemical concepts routinely used for describing local properties of molecules can be transferred very effectively to extended systems.
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  • 49
    ISSN: 0947-6539
    Keywords: chelate ligands ; coordination ; hydrogen bonds ; ligand fields ; polymers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1- and 2-D coordination polymers [Mn{HB(C2H2N3)3}2(H2O)2].4H2O (11) and [Ni{H2B(CHN4)2}2-(NH3)2] (13), respectively, and the chelate complex [Ni{H2B(C2H2N3)2}2(H2O)2] · 2H2O (12) were synthesized and structurally characterized. The compounds contain ambidentate poly(azolyl)borato ligands (azolyl = triazolyl or tetrazolyl), which can chelate or bridge metal centers. The metal-ligand structures in 11-13 differ from the known coordination modes of the poly(azolyl)borates towards other metal centers. We describe how a change in the metal and/or the conditions of crystallization affects the ligand-field stabilization energy and favors one type of nitrogen donor atom over the other for the poly(triazolyl)borato ligands. The crystal structures of 11 and 12 contain additional water of crystallization; this leads to hydrogen-bonded solvent substructures. In the case of the bis(triazolyl)borato ligands. The crystal structures of 11 and 12 contain additional water of crystallization; this leads to hydrogen-bonded solvent substructures. In the case of the bis(tetrazolyl)borato ligand the water substructure is shown to function as a “reinforcing bar” that symmetrizes the metal-ligand grid sheet.
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  • 50
    ISSN: 0947-6539
    Keywords: alkynes ; asymmetric syntheses ; cyclopropanes ; spiro compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of chiral, nonracemic 2-alkoxy-1-alkynylcyclopropanes 7 were synthesized in good to very good yields from enantiomerically pure glycidol derivatives (glycidol tosylate, epichlorohydrin) by boron trifluoride promoted addition of lithium trimethylsilylacetylide followed by protection of the secondary hydroxyl group and finally a diastereoselective γ-elimination. The 2-ethoxy derivative (S,R)-7 b was deprotonated with n-butyllithium, and the resulting 1-lithio-2-ethoxy derivative (S,R)-20 functionalized by treatment with oxygen followed by tosyl chloride. Protodesilylation and catalytic hydrogenation smoothly furnished 1-ethenylcyclopropyl sulfonates, which underwent a clean Pd0-catalyzed SN2′-type substitution with dimethyl propargylsodiummalonate to give the (E)-configurated enyne (R,E)-26 with a methylenecyclopropane end group. A diastereoselective Pauson-Khand reaction completed the sequence to give the enantiomerically pure spirocyclopropaneannelated bicyclo[3.3.0]octane derivative 31.
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  • 51
    ISSN: 0947-6539
    Keywords: clusters ; gas-phase chemistry ; ionmolecule reactions ; iron complexes ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the gas phase reactions of small Fe+n clusters (n = 2-10) with O2, N2O and CO2 in an FT-ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2-4) appeared to activate CO2. For all X-O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well-known X-O bond energies were used to calculate Fen-O+ bond dissociation energies (BDEs) for sizes n = 2-6; these amount to roughly 550 kJ mol-1 and thus are considerably higher than the BDE of the Fe-O+ ion. All oxidation reactions of the Fe+n clusters (n = 2-6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1-4). These “microoxides of iron” are not able to activate any further X-O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
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  • 52
    ISSN: 0947-6539
    Keywords: alkyne complexes ; carbon-carbon coupling ; pentaenes ; rhodium complexes ; vinylidene complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of [Rh(η3-C3H5)-(PiPr3)2] (1) with HC≡C—CH(Ph)OH yields the alkynyl(vinylidene) complex trans-[Rh{C≡C—CH(Ph)OH}{=C=CH-CH(Ph)OH}(PiPr3)2] (2), while from 1 and HC≡C—CPh2OH the alkynyl-(enyne)metal derivative trans-[Rh(C≡C—CPh2OH){n2-(E)-Ph2(OH)C—C≡C—CH=CH-CPh2OH} (PiPr3)2] (3) is obtained. On treatment with 1-alkyn-3-ols HC≡C—CR2OH (R = Me, Ph, iPr), the highly reactive π-benzyl compound [Rh(n3-CH2Ph)(PiPr3)2] (4) yields the five-coordinate complexes [RhH(C≡C—CR2OH)2(PiPr3)2] (5-7) of which those with R = Me and Ph can be converted to the alkynyl(vinylidene)metal isomers trans-[Rh(C≡C—CR2OH)(=C=CH-CR2OH)-(PiPr3)2] (8, 9). Compounds 8 and 9 react with L′ = CO and isocyanides by migration of the alkynyl ligand to the vinylidene unit to give the enynylrhodium(I) complexes trans-[Rh{n1-(Z)-C(C≡C—CR2—OH)=CH-CR2OH}(L′)(PiPr3)2] (10, 11: L′ = CO; 12-15: L′ = CNR′). Cleavage of the Rh-C s̰-bond of 10 with CF3-CO2H affords trans-[Rh(n1-O2CCF3)-(CO)(PiPr3)2] (16) and the enyne (E)-Me2(OH)C—C≡C—CH=CH-CMe2OH (17). Compounds 5-7 react with L′ = CO and isocyanides to give the octahedral 1:1 adducts [RhH(C≡C—CR2OH)2(L′)(P-iPr3)2] (18-20 and 24-27), of which the CO derivatives 18-20 readily eliminate HC≡C—CR2OH to yield trans-[Rh-(C≡C—CR2OH)(CO)(PiPr3)2] (21-23). On treatment of 6 or 9 (R = Ph) with Al2O3 in the presence of chloride ions, besides trans-[RhCl(=C=C=CPh2)(P-iPr3)2] (28) the hexapentaenerhodium(I) complex trans-[RhCl(n2-Ph2C=C=C=C=C=CPh2)(PiPr3)2] (29) is formed. The kinetically preferred isomer trans-[RhCl(n2-Ph2C=C=C=C=C=CPh2)(P-iPr3)2] (33) has been prepared from [RhCl(PiPr3)2]2 and Ph2C=C=C=C=C=CPh2; it rearranges smoothly at room temperature to the thermodynamically more stable isomer 29. The molecular structures of 7 and 29 have been determined.
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  • 53
    ISSN: 0947-6539
    Keywords: charge transfer complexes ; diimine complexes ; emission spectroscopy ; manganese complexes ; zeolites ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Manganese complexes of 2,2′-bipyridine (bpy) and 1,10-phenantroline (phen) have been synthesised in the supercages of cubic NaX and NaY and in the hypercages of the hexagonal NaEMT faujasites. The coordination and redox chemistry were studied with ESCA, CA, FT-IR, FT-Raman, diffuse reflectance and emission techniques. FT-IR/FT-Raman shows cis coordination for all complexes and a high Mn-N stretching frequency in the phen complexes as a result of steric constraints imposed by the ligand. [Mn(bpy)2]2+ in the different zeolites shows metal-to-ligand charge transfer (MLCT; at 495 nm); for [Mn(phen)2]2+-NaY the MLCt is broadened owing to complex distortion. On MLCT excitation [Mn(bpy)2]2+ complexes show an ipsochromic shift in the emission and an increase in quantum yield with increasing steric restrictions imposed by the zeolite. The ipsochromic shift of the emission band of [Mn(phen)2]2+ in NaY results from the combined effect of the ligand field (this suggests emission from a CT state) and of coordinative distortion. The key factor influencing the emission properties is found to be the overall matrix-induced complex distortion. Cation stabilisation of the ligand anion affects emission indirectly. The decay times for [Mn-(bpy)2]2+-NaY are in the millisecond range (7.5-11.5 ms). A proposed model for excitation and emission properties of zeolite-occluded MnII complexes involves excitation of a quartet CT state, intersystem crossing and subsequent emission. The enhanced stability of the coordination sphere in the zeolite allows complexes to luminesce from a CT state, which is not detected in solution. The zeolite behaves as a supramolecular cryptating agent, protecting complexes from photodissociation.
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  • 54
    ISSN: 0947-6539
    Keywords: barbiturates ; macrocycles ; molecular recognition ; receptors ; zinc complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new bis-zinc(II) receptor (Zn2L), which has two macrocyclic 12-membered tetraamine (cyclen) ZnII complexes connected through a p-xylene bridge, has been synthesized as a novel host molecule to recognize barbiturates (such as barbital (bar)) in aqueous solution. Each of the zinc(II) ions in the biszinc(II) receptor was originally intended to match the dianionic barbital anion (bar2-) with supplementary hydrogen bonds between the cyclen NH's and the three carbonyl oxygens in complementary positions to yield a 1:1 complex, Zn2L-bar2-. From an aqueous solution of equimolar Zn2L and barbital at pH 8, however, a cyclic 2:2 complex, (Zn2L-bar2-)2, was isolated and characterized by X-ray crystal analysis. The NMR study in 10% (v/v) D2O/H2O has revealed dissociation of (Zn2L-bar2-)2 solely into the original target 1:1 complex Zn2L-bar2- and established the dimerization constant for 2Zn2L-bar2- → (Zn2L-bar2-)2, Kd ( = [(Zn2L-bar2-)2]/[Zn2L-bar2-]2) to be 103.4 M-1. The thermodynamic parameters were evaluated from the NMR measurements at 25, 35, 45, and 55°C: ΔG = -1.9 × 104 J mol-1, ΔH = - 3.3 × 104 J mol-1, ΔS = - 49 J mol-1 K-1 at 25°C. Potentiometric pH titration of Zn2L (1 mM) and barbital (1 mM) disclosed extremely facile deprotonation of the two imido groups of barbital at pH less than 7 to form the dianionic barbital-bound ZnII complexes Zn2L-bar2- and (Zn2L-bar2-)2, where-by the barbital binding affinity for Zn2L was estimated to be Kbar ( = [Zn2L-bar2-]/[uncomplexed Zn2L][uncomplexed barbital]) = 105.8 M-1 at pH 8 and 25°C with I = 0.10 (NaNO3). The significance of the bis-zinc(II) receptor in stabilizing the dianionic barbital is evident by comparison with the interaction of ZnII-cyclen complex (ZnL) with barbital, which yields only a 1:1 monoanionic barbital complex, ZnL-bar- (Kbar = [ZnL-bar-]/[uncomplexed ZnL][uncomplexed barbital] = 104.2M-1 at pH 8 and 25°C with I = 0.10 (NaNO3)).
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  • 55
    ISSN: 0947-6539
    Keywords: binding assays ; immunosensors ; electrochemical polymerisations ; peptide derivatives ; peptide immobilisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the first time, antigenic peptides have been immobilised by electrochemical polymerisation after having been modified with a polymerisable functional group. 3-Hydroxyphenylacetic acid was chosen as the novel polymerisable group. The synthetic peptides represent epitopes of the bovine foot and mouth disease virus and of the sodium channel of the cardiac muscle. The polymerisation was performed by applying a constant anodic potential or by cyclic voltammetry. A combination of these two methods was also employed, that is, cyclic voltammetry with a delay at the anodic vertex potential. No additional free phenolic monomer was required for the polymerisation. The layers formed by the polymerisation were recognised by specific antibodies. The specific binding of the antibodies to the polymer film could be demonstrated by ELISA, an enzyme-linked amperometric immunoassay, and electrochemical impedance measurements, as well as by fluorescence-labelled antibodies. A peptide derived from laminine was also immobilised by electrochemical polymerisation. It could be shown that neuroblastoma cells adhere to this layer.
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  • 56
    ISSN: 0947-6539
    Keywords: amino alcohols ; asymmetric syntheses ; dihydroxylations ; diol cleavage Horner-Wittig reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spiroepoxide 1 was prepared from quinidine and converted into β-amino alcohol 3 (86% over two steps). Dihydroxylation of enantiopure oxazatricylic olefin (E)-4 provided diastereomeric diols 5a and 5b. Stereospecific conversion of 1,2-secondary, tertiary diol 5b into tetracyclic spiroepoxide 6 was accomplished in high yield by a one-pot tosylation-cyclization procedure. 1,2-Diol cleavage with NaIO4 in 80% acetic acid afforded the new tricyclic dehydrorubanone 7, containing the 4-oxa-7-azatricyclo[4.3.1.03,7]-decan-2-one core structure. Similarly, acetylated rubanone 9 was prepared on a 20 g scale. Reduction with NaBH4 in the presence of CeCl3 provided rubanols 10a and 10b (1:1.1). Horner-Wittig reaction of 9 with diethyl cyanomethylphosphonate was (Z)-selective, furnishing unsaturated nitrile (Z)-13. Conversion into the α,β-unsaturated aldehyde (Z)-14 and reduction afforded enartiopure allylic alcohol (Z)-12, which is a new isomer of the key quinidine metabolite 15.
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  • 57
    ISSN: 0947-6539
    Keywords: crystal structure ; density functional calculations ; polyarsenido complexes ; polyphosphido complexes ; Zintl anions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Zintl anion P3-7 reacts with complexes of transition metal halides to form multinuclear metal phosphorus clusters. Reaction of Li3P7·3 DME with [FeCp(CO)2Br] or [NiCl2(PBu3)2] leads to the formation of [P7{FeCp(CO)2}3] (1) and [{Ni(PBu3)2}4P14] (2), respectively. X-ray structure determinations show that in 1 the P7 cage of Li3P7 remains intact but in 2 a P14 framework is formed by linkage of two norbornadiene-like P7 units. The P14 skeleton coordinates to four Ni(PBu3)2 groups. LiCp* and CoCl2 react with P7(SiMe3)3 to give [(Cp*Co)3-(P2)3] (3) with three Cp*Co groups bridged by P2 units. Reaction of FeCl2 and LiCp* with P7(SiMe3)3 yields [(Cp*Fe)3P6][FeCl3(thf)] (4) or [(Cp*Fe)3{n3-P3)Fe}P6] (5), depending on the reaction conditions. In 4 the structure of the Fe3P6 core of the [(Cp*Fe)3P6]+ cation is comparable to closo-B9H92-, while in 5 the Fe4P6 core does not obey the Wade rules. The unusual Co-As cluster [Co6As12(PEt2Ph)6] (6), prepared from K, As, and [CoCl2(PEt2-Ph)2], can be described as a Co6As6 heteroicosahedron linked to two Co3As3 octahedra by common Co3 faces. A theoretical treatment within the density functional approximation reproduces the experimental structures of 2 and 6 and allows an interpretation of molecular electronic structures. In 2 one finds P-P double bonds that are delocalized to some extent into vacant Ni AOs. For the cage compound 6 the Co 3 d AOs participate in cage bonding and 38 electrons can be assigned to cage bonds; this is in accord with the (2N + 2) rule for 18-atom cages but not with the usual electron counting rules.
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  • 58
    ISSN: 0947-6539
    Keywords: biomineralization ; medium-sized rings ; calcite ; crystal morphology ; glycoproteins ; polysaccharides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of 4,4,9,9-tetramethyl-1-oxa-cycloundecane-5,6,7,8-tetrone (9) and 5,5,10,10-tetramethyl-1-oxa-cyclotridecane-6,7,8,9-tetrone (10) has been achieved in a multistep procedure. The key steps in this synthesis were the ring closure of 23 and 24 to 25 and 26, respectively, and the oxidation of the triple bond with RuO2/NaIO4 to the dihydroxydiketones 31 and 32. Compound 9 is the first cyclic tetraketone for which an intramolecular donor-acceptor stabilization has been found. A strong transannular interaction between the ether oxygen and the C4O4 unit in 9 was detected by X-ray studies on single crystals of 9. The transannular distance is 2.7-2.8 Å. Further evidence for a strong transannular interaction was obtained from the comparison of the reduction potential and the first band in the UV/Vis spectrum with the corresponding values from other openchain tetraketones. These findings were substantiated by PE investigations on 9. The crystal structures of the dihydroxy-diketones 31 a and 32 a showed that, in the case of the eleven-membered ring (31 a), there are also short transannular distances between the ether oxygen and the C2O2 moiety (2.5 Å and 2.9 Å). In the case of the thirteen-membered ring (32 a), no transannular interactions were found in the solid state.
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  • 59
    ISSN: 0947-6539
    Keywords: computer simulations ; high-pressure chemistry ; lanthanide complexes ; ligand exchange ; mechanistic studies ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(III) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell (τres (1st shell) = 1577, 170 and 410 ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12-18 ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H2O)9]3+ aqua ion follow a dissociatively activated Id mechanism via an eightfold-coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and 10 ps. The assignment of an Id mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (1800) and by the fact that the calculated average activation volume ΔV≠ = + 4.5 cm3 mol-1 is clearly smaller than the estimated activation volume ΔV≠lim ≈ΔV0 = + 7.2 cm3 mol-1 for a limiting D process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H2O)8]3+ and a [Sm(H2O)9]3+ aqua ion. The resulting trajectory pattern of incoming and leaving water molecules is an alternation of elimination and addition reactions and cannot be classified into the scheme of D, I or A mechanisms for substitution processes. The reaction volume ΔV0 for the coordination equilibrium [Sm(H2O)8]3+ + H2O → [Sm(H2O)9]3+ can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H2O)8]3+ exhibit the characteristics of an Ia mechanism. The water exchange takes place via a transition-state geometry close to that of a tricapped trigonal prism and involves a slightly negative activation volume.
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  • 60
    ISSN: 0947-6539
    Keywords: ab initio calculations ; dihydrogen complexes ; isotope effects ; NMR spectroscopy ; osmium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of complex [OsHCl-(CO)(PiPr3)2] (1) with hydrogen gives the trans-hydridodihydrogen complex [OsH-Cl(n2-H2)(CO)(PiPr3)2] (2). The H-H distance in the dihydrogen ligand, determined by variable-temperature 1H T1 measurements, is 0.8 Å. The fast-spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the n2-D2 ligand of [OsDCl(n2-D2)(CO)(PiPr3)2] ([D3]2). Measurements of the equilibrium constants for formation of 2 (K) give ΔH0 = - 14.1 ± 0.5 kcal mol-1 and ΔS0 = - 30 ±1 e.u. An equilibrium isotope effect Kd/K of 2.8 is found for this reaction. The activation parameters for the H2 loss from 2 are ΔH
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  • 61