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  • 1995-1999  (828)
  • 1915-1919  (17)
  • 1905-1909  (48)
  • General Chemistry  (665)
  • Theoretical, Physical and Computational Chemistry  (228)
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  • 1
    Electronic Resource
    Electronic Resource
    Insertion Reactions into Palladium-Carbon Bonds of Complexes Containing Terdentate Nitrogen Ligands; Experimental and Ab initio MO Studies (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 1129-1143 
    Details
    ISSN: 1434-1948
    Keywords: Insertion ; N ligands ; Terdentate ligands ; Rigid ligands ; Palladium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel methyl complexes [Pd(Me)(N-N-N)]X (N-N-N = flexible or rigid terdentate nitrogen ligand, X = Cl, SO3CF3, BAr′4) have been synthesized and fully characterized. All complexes readily underwent insertion of carbon monoxide resulting in the quantitative formation of complexes [Pd{C(O)Me}(N-N-N)]X [X = Cl (1d-6d), BAr′4 (1e-6e)]. Subsequently, complexes 2e-6e underwent quantitative insertion of norbornadiene, resulting in complexes [Pd{C7H8C(O)Me}(N-N-N)]BAr′4 (2f-6f). Unexpectedly, these complexes, including even those containing rigid terdentate nitrogen ligands, possess a structure in which the nitrogen ligand is coordinated in a bidentate fashion. A kinetic study of the reaction of norbornadiene with complexes 1e-6e revealed that the reactivity of complexes 1e-6e toward norbornadiene increases with increasing rigidity of the terdentate ligand, i.e. with increasing strain in the PdN3 moiety, which indicates that insertion very likely occurs via a mechanism involving nitrogen dissociation. This is fully supported by ab initio MO calculations on CO and ethylene insertion into carbon-palladium bonds of cationic model systems containing a rigid terdentate nitrogen ligand, which showed that the lowest-energy pathway for both insertion reactions consists of substitution of one of the distal nitrogen atoms of the rigid terdentate nitrogen ligand by the substrate, followed by a rate-determining migratory insertion of the substrate into the carbon-palladium bond.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Methyl Aryl Ketones in the Synthesis of Hexaazabicycloicosane Cage ComplexesThis paper is dedicated to Gottfried Huttner, a stimulating teacher, researcher and companion (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1283-1291 
    Details
    ISSN: 0947-6539
    Keywords: cage compounds ; cobalt ; cyclic voltammetry ; fluorescence spectroscopy ; template synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The template syntheses of various CoIII-hexaazaicosane-type cage complexes with aromatic substituents are described. The parent template, [Co(sen)]3+ ion {sen = 4,4′,4″-ethylidynetris(3-azabutan-1-amine)}, paraformaldehyde, a methyl aryl ketone and a base were reacted in acetonitrile to give aroyl substituents attached to the fully saturated sarcophagine cage (1-aroyl-8-methyl-3,6,10,13,16,19-hexaazabicyclo[6.6.6]-icosane cobalt(III) ion) and aryl substituents attached to an analogous imine-type cage (2-aryl-8-methyl-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosa-2-ene cobalt(III) ion). Phenyl, napthyl, phenanthryl, anthracenyl and anthraquinonyl substituents have been bound to the cage simply by this route to give molecules that can act as intercalators in DNA or as photosensitizers attached to reversible redox-active metal centres and can couple an organic two-electron reagent with an inorganic one-electron reagent. An X-ray crystallographic analysis of a phenanthroyl cage complex has also been carried out. These substances and their progeny should be useful as biological probes and fluorescent indicators and for energy capture and conversion.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030816
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  • 3
    Electronic Resource
    Electronic Resource
    Influence of sodium dodecyl sulphate micelles on the kinetics of complex formation between Pd(H2O)42+ and S-carboxymethyl-L-cysteine (1996)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 9 (1996), S. 398-402 
    Details
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of complex formation between Pd(H2O)42+ and S-carboxymethyl-L-cysteine (SCMCH2) was investigated in the presence of sodium dodecyl sulphate in the acidity range between 2 M HClO4 and pH 5. Although the mechanism was not affected by the presence of anionic micelles, retardation (2·2 〈 pH 〈 5) and acceleration (pH 〈 2·2) of the complex formation were observed compared with its rate in aqueous solution. These effects were interpreted in terms of the long-range electrostatic interactions between the negatively charged micellar surface and ligand species which can be in different ionic forms depending on the pH.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Calix[4]arene-Based (Hemi)carcerands and Carceplexes: Synthesis, Functionalization, and Molecular Modeling Study (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 639-654 
    Details
    ISSN: 0947-6539
    Keywords: calixarenes ; carcerands ; inclusion compounds ; molecular devices ; resorcinarenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of 11 calix[4]arene-based carceplexes obtained by solvent or doped inclusion is reported. Carceplexes with amides, for example, DMF, NMP, and 1,5-dimethyl-2-pyrrolidinone, and sulfoxides, for example, DMSO and thiolane-1-oxide, were obtained by solvent inclusion. In these cases the yield of the carceplex decreases with increasing guest size. Potential guests that do not form carceplexes by solvent inclusion, such as 2-butanone and 3-sulfolene, could be incarcerated by doped inclusion with 1,5-dimethyl-2-pyrrolidinone as a solvent “doped” with 5-15 vol% of potential guest. The amide bridges of the carceplexes were converted into thioamide bridges in essentially quantitative yield by means of Lawesson's reagent in refluxing xylene. The dynamic properties of the incarcerated guests were examined by 2D NMR spectroscopy. Whereas for most guests a preference for one orientation inside the calix[4]arene-based (thia)carcerands was observed, for DMA, NMP, and ethyl methyl sulfoxide inside calix[4]arene-based (thia)carcerands two different orientations were present. The energy barriers for interconversion between the various orientations of DMA, NMP, and ethyl methyl sulfoxide inside calix[4]arene-based (thia)-carcerands were determined with 2D EXSY NMR. The energy barriers are higher for the thiacarcerands than for the corresponding carcerands with amide bridges. This may be due to the stronger hydrogen-bond-donating character of the thioamide group. Furthermore, molecular modeling simulations indicate that in case of the thiacarcerand the cavity is smaller as a result of a smaller diametrical distance between the NH atoms. Our results demonstrate that molecular modeling can be used to estimate the energy barriers for interconversion; the calculated activation energies showed good quantitative agreement with the experimental values.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030421
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  • 5
    Electronic Resource
    Electronic Resource
    Polystyrene-Poly(propylene imine) Dendrimers: Synthesis, Characterization, and Association Behavior of a New Class of Amphiphiles (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1616-1626 
    Details
    ISSN: 0947-6539
    Keywords: amphiphiles ; block copolymers ; dendrimers ; micelles ; surfactants ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new class of amphiphilic macromolecules has successfully been synthesized by creating well-defined diblock copolymers of polystyrene (PS, Mn=3.2 × 103, Mw/Mn=1.04) with poly-(propylene imine) dendrimers. A poly-styrene core molecule with a primary amine end-group was prepared by a quantitative three-step modification procedure of acid-functionalized polystyrene. On this core molecule, five different generations from PS-dendr-NH2 up to PS-dendr-(NH2)32 were constructed in high yields. The molecular structure of the block co-polymers was analyzed in detail with NMR and IR spectroscopy and electrospray mass spectrometry. With conductivity measurements and monolayer pressure-area isotherm determinations, we observed generation-dependent amphiphilic behavior. Dynamic light scattering and transmission electron microscopy showed that amphiphile geometry had a distinct effect on aggregation behavior; this is in qualitative agreement with Israelachvili's theory. Critical association concentrations determined with the pyrene probe luminescence technique were as low as 5 × 10-7 M. The amphiphiles presented here, therefore, show a resemblance to traditional block copolymers with regard to size and stability, and are similar to surfactants with respect to tuning of the shape.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021221
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  • 6
    Electronic Resource
    Electronic Resource
    Synthesis and Biological Evaluation of a Backbone-Modified Phytoalexin Elicitor (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 920-929 
    Details
    ISSN: 0947-6539
    Keywords: bioorganic chemistry ; carbohydrates ; glycosides ; peptide bonds ; phytochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two suitably protected building blocks (11 and 33) for the preparation of amide-linked heptaglucoside mimetic 2, an analogue of the naturally occurring phytoalexin elicitor 1a, were readily accessible by glycal chemistry. Sequential elongation of terminal glucuronide 21 with laminaribiosyl hemiaminal 33 and anomeric amine 11 by EDC/HOBt-catalyzed condensation and two-step conversion of the C6-OTr moiety into the corresponding carboxylate function afforded homogeneous carbopeptoid 2 in high overall yield. It was found that replacement of the acetal linkages by the more rigid amide bonds destroys the phytoalexin-elicitor activity.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030614
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  • 7
    Electronic Resource
    Electronic Resource
    Concerted base-promoted elimination in the decomposition of N-halo amino acids (1996)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 9 (1996), S. 552-560 
    Details
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: N-Chloroamino acids are unstable in aqueous solution and decompose through different pathways depending on the reaction conditions, yielding precursors of carcinogenic and/or mutagenic compounds. One of these pathways is a 1,2-elimination process, which has scarcely received any attention and for which no systematic analysis is available. The process is first order relative to the N-chloroamino acid and to that of hydroxide ion. The use of 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol buffer solutions established that the process is general-base catalysed. The reaction rate is affected by the presence of a methyl group on the nitrogen atom and the nature of the leaving group. The results show an important steric effect due to the alkyl substituents on the α-carbon. With bulky alkyl substituents on the α-carbon, and in particular in the case of N-alkylamino acids, the catalytic effect increases as the base strength decreases. To characterize the transition state, Brønsted's β and βlg were used. A More O'Ferrall-Jencks diagram shows the transition state structure changing from carbanion-like to nitrenium-like, a large perpendicular effect being evident. The reaction proceeds through a concerted mechanism AxhDHDN instead of the stepwise AxhDH
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Homogeneous and Supported Copper Complexes of Cyclic and Open-Chain Polynitrogenated Ligands as Catalysts of Cyclopropanation Reactions (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2347-2354 
    Details
    ISSN: 1434-1948
    Keywords: Cyclopropanation ; Copper ; N ligands ; Macrocycles ; Supported catalysts ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: CuI and CuII complexes of cyclic and open-chain polyaza compounds have been tested as catalysts in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate. In general, only small amounts of copper are needed to promote the reaction. The catalytic activity depends on the structure of the ligand, e.g. amine-amides are more efficient than polyamines, and on the oxidation state of copper, CuII being more active than CuI. Given that CuI is the active species, these changes of behavior must be related to the stabilities of the complexes. The nature of the counterion also has a noticeable influence on the catalytic activity, the role of which is discussed. XAS measurements suggest the formation of oligomeric species. Some of the chiral ligands lead to small enantiomeric excesses. Open-chain ligands can easily be supported on organic polymers and their complexes can be used as catalysts. Furthermore, cyclic and acyclic complexes can be supported on clays by cation exchange and the solids obtained tend to promote the reaction with a decrease in the trans/cis ratio.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Exploration of High-Pressure Cycloadducts of Furans and Citraconic Anhydride as Precursors for CD-Ring Fragments of Paclitaxel and Its Analogues (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 2483-2492 
    Details
    ISSN: 1434-193X
    Keywords: Paclitaxel (Taxol®) ; Paclitaxel CD-ring ; High-pressure Diels-Alder reactions ; Furan cycloadditions ; Ether cleavage in 7-oxabicyclo[2.2.1]heptanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The high-pressure promoted Diels-Alder reactions between several furans and citraconic anhydride have been studied and the cycloadducts obtained have been explored in new straightforward routes to the CD-ring fragment of paclitaxel. The reaction between furan and citraconic anhydride afforded the exo cycloadduct diastereoselectively, whereas a variety of 2-substituted furans afforded approximate 1:1 mixtures of exo regioisomers. Separation of both regioisomers was accomplished after either diastereoselective esterification or regioselective reduction of the anhydride function. Ether cleavage of the bicyclic compounds by either high-pressure promoted ether cleavage or Boord elimination afforded several potential CD-ring precursors which can be used in the total synthesis of paclitaxel analogues.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Diastereoselective Conjugate Addition Reactions of 2′-Hydroxypropiophenone to 2′-Hydroxychalcones - Synthesis and Structural Characterization of the Diastereomers of (±)-3-Aryl-1,5-bis(2-hydroxyphenyl)-2,4-dimethyl-1,5-pentanediones (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1739-1744 
    Details
    ISSN: 1434-193X
    Keywords: Addition reactions ; Diastereoselectivity ; 2′-Hydroxychalcones ; 3-Aryl-1,5-bis(2-hydroxyphenyl)-2,4-dimethyl-1,5-pentanediones ; NMR spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The diastereoselective synthesis of 3-aryl-1,5-bis(2-hydroxyphenyl)-2,4-dimethyl-1,5-pentanediones has been carried out by conjugate addition reactions of 2′-hydroxypropiophenone to 2′-hydroxy-α-methylchalcone derivatives, or in single one-pot reactions of 2′-hydroxypropiophenone with appropriate benzaldehyde derivatives. The structures and stereochemistry of the obtained diastereomers have been determined using various NMR techniques, and the factors determining their formation are investigated and discussed.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
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