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  • General Chemistry  (6,850)
  • Animals  (2,869)
  • 1995-1999  (9,719)
  • 101
    Electronic Resource
    Electronic Resource
    Synthesis of the 2-Deoxyisomaltose Analogue of Acarbose by an Improved Route to Chiral Valieneamines (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 453-462 
    Details
    ISSN: 0947-6539
    Keywords: cleavage reactions ; enzyme inhibitors ; glycosides ; rearrangements ; transition states ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 2-deoxyisomaltose analogue of acarbose was stereoselectively synthesised in 11 steps with a total yield of 7% starting from 2,6-dibromo-2, 6-dideoxy-D-mannono-1,4-lactone (6). The latter was reduced to the lactol, converted to the methyl glycoside (7) and hydrogenated to the methyl 6-bromo-2,6-dideoxyglycoside (8). Benzylation of the hydroxy groups, elimination of bromine to a 5-ene and Ferrier carbocyclisation gave (2S, 3R)-2,3-bisbenzyloxycyclohex-5-enone (12). 1, 2-addition of benzyloxymethyl lithium at -110°C gave a 6:1 mixture of tertiary alcohols 13; the (1S) isomer was the major one. Reaction with trichloroacetyl isocyanate gave a carbamate 19, which, when dehydrated to the cyanate, spontaneously underwent [1,3] sigmatropic rearrangement to an isocyanate, which on addition of methanol gave the methylcarbamate 20. Basic hydrolysis of this compound gave (2R, 3R,5R)-5-amino-1-benzyloxymethyl-2,3
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030318
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  • 102
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    Synthesis of 1,2,3,4-Bisiminofullerene and 1,2,3,4-Bis(triazolino)fullerene - on the Mechanism of the Addition Reactions of Organic Azides to [60]Fullerene (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 744-748 
    Details
    ISSN: 0947-6539
    Keywords: azide adducts ; fullerenes ; reaction mechanisms ; regioselectivity ; thermolysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of organic azides with [60]fullerene have paved the way for the synthesis of adducts with a variety of structures. Treatment of [60]fullerene with 2,2-dibenzyl-1,3-diazidopropane (10) in refluxing chlorobenzene afforded three products, namely, 8, 9, and 11 in 18, 25, and 11% yields, respectively. Thermolysis of 9a in refluxing chlorobenzene gave a 40:54:6 mixture of 8, 11, and C60 in quantitative yield. No interconversion between 8 and 11 was observed. Whereas 11 was stable towards thermolysis, 8 decomposed to C60 (35% yield) on refluxing in chlorobenzene for 24 h, but it did not produce any 11. This indicates that 9a is an intermediate in the formation of 8 and 11. A general mechanism for the addition of azides to C60 is proposed. A similar mechanistic pathway is suggested for the thermolysis of 9a.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030514
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  • 103
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    A Packing Model for Interpenetrated Diamondoid Structures - an Interpretation Based on the Constructive Interference of Supramolecular Networks (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 765-771 
    Details
    ISSN: 0947-6539
    Keywords: aromatic stacking ; crystal engineering ; diamondoid networks ; packing model ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ability to predict and subsequently control solid-state structure has been identified as a major challenge in the field of crystal engineering. Here we suggest the concept of constitutive models as a tool for understanding crystal packings and for designing new solid-state structures. Such models are intended to relate molecular interactions and their geometrical constraints with solid-state organization. These models will most likely be of greatest use for crystals consisting of supramolecular networks, that is, infinite assemblies of small molecules associating through strong, directional, and selective noncovalent interactions. The concept of the constitutive packing model is illustrated for interpenetrated diamondoid coordination networks based on crystalline adducts of 4,4′-biphenyldicarbonitrile with silver(I) salts. Observed structural deformations induced by counterions of varying size may be understood in terms of the interference of two supramolecular networks within this system: the diamondoid metal-ligand coordination network and face-to-face aromatic stacks of the organic ligand. The constitutive model developed here has been applied to other diamondoid coordination networks in the literature and is found to be general.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030517
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  • 104
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    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 499-503 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030403
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  • 105
    Electronic Resource
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    Design of Sequence-Specific DNA-Binding Ligands (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 505-508 
    Details
    ISSN: 0947-6539
    Keywords: DNA recognition ; triplexes ; molecular recognition ; peptide nucleic acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Double-stranded DNA can be viewed as a multifunctional, modular receptor that can be read sequence-selectively in a digital way (base pair per base pair) by a complementary, similarly modular ligand. This principle has been exploited in several approaches to design sequence-specific DNA-binding ligands, such as triplex-forming oligonucleotides, peptide nucleic acids and minor groove binding polyamides.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030404
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  • 106
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    Triprotonated Hydrogen Sulfide: Pentacoordinate Sulfonium Trication SH3+5 and the Search for its Parent Pentacoordinate Oxonium Trication OH3+5Chemistry in superacids, part 29. For part 28, see: G. A. Olah, G. Rasul, Inorg. Chem. submitted. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1039-1041 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; gold ; isolobal relationship ; sulfonium trication ; superelectrophiles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The pentahydridosulfonium trication SH3+5 was found by ab initio MP2/ 6-31G** and QCISD(T)/6-311G** levels of calculation to be a stable minimum with Cs symmetric structure 1. It is isostructural with the parent pentacoordinate carbonium ion CH+5. Structure 1 resembles a complex between SH3+3 and molecular hydrogen forming a 2e-3c bond. This structure involving a pentaco-ordinate sulfur atom is unprecedented. Rotation of the H2 unit around the pseudo-C3 axis of SH3 is facile. The structure of SF3+5 was also calculated and compared with that of 1. In addition, our studies at the ab initio MP2/6-31G** level showed that the parent pentacoordinate oxonium trication OH3+5 is not a minimum on its potential energy surface, and dissociation into OH2+4 and H+ occurs upon optimization.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030709
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  • 107
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    A Novel Synthetic Route to Tantalum-Zinc Neophylidyne Complexes Stabilized by ortho-Chelating Arylamine Ligands; the X-Ray structure of [TaCl2(μ-C6H4CH2NMe2-2)(μ-CCMe2Ph)ZnCl(THF)] (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 817-822 
    Details
    ISSN: 0947-6539
    Keywords: bridging ligands ; intramolecular coordination ; organometallic complexes ; tantalum ; zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A dimeric tantalum-zinc neophylidyne complex proposed to be [{{TaCl2(μ-CCMe2Ph)(μ-Cl)(THF)2}{Zn(μ-Cl)}}2](5a), which can be prepared easily on a scale of 50 mmol, is an ideal starting material for the synthesis of mono- and bis(ortho)-chelated arylamine alkylidyne species such as the red Ta-Zn neophylidyne complexes [TaCl2(μ-C6H4CH2NMe2-2)(μ-CCMe2Ph)ZnCl(THF)] (6) and [TaCl2-{μ-C6H3(CH2NMe2)2-2,6}(μ-CCMe2Ph)ZnCl] (7), which have been isolated in high yields. Reaction of 7 with tmeda affords the Tav alkylidene complex [TaCl(=CHCMe2Ph){C6H3(CH2N(Me)CH2)-2-CH2NMe2)-6}] (8), which shows catalytic activity in the ring-opening metathesis polymerization of norbornene.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030522
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  • 108
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    Dicyanopolyynes: A Homologuous Series of End-Capped Linear sp Carbon (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1105-1112 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; alkynes ; carbon allotropes ; electronic structure ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, isolation, spectroscopic characterization, and computational investigations of the rod-shaped dicyanopolyynes 1-6 (C2nN2, n = 4-9), which are model substances for the hypothetical one-dimensional carbon allotrope carbyne sp-C∞, are described. Based on the trends in the spectroscopic behavior of 1-6 with increasing chain length, the electronic as well as the NMR properties of carbyne are predicted. For the investigation of the synthetic potential of this compound class, a first selected series of regioselective derivatizations is presented with the synthesis of the [4+2] and [3+2] cycloadducts 7-12.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030718
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  • 109
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    Molecular and Supramolecular Synthesis with Dibenzofuran-Containing SystemsMolecular meccano, part 18; for Part 17, see D. B. Amabilino, P. R. Ashton, S. E. Boyd, M. Gómez-López, W. Hayes, J. F. J. Org. Chem. 1997, 62, 3062-3075. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1136-1150 
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    ISSN: 0947-6539
    Keywords: crystal engineering ; dibenzofuran ; molecular quadrilaterals ; supra-molecular chemistry ; template synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The template-directed syntheses of two new tetracationic cyclophanes, cyclobis(paraquat-2,8-dibenzofuran) and cyclobis(paraquat-3,7-dibenzofuran), incorporating dibenzofuran subunits has been accomplished. Initially, the cyclophanes were self-assembled around a macrocyclic polyether template, bis-p-phenylene[34]crown-10 (BPP34C10), to form catenanes: the mechanical bond order of the catenane formed determined the requisite “amacrocyclic” templates for synthesis of the free cyclophane. X-ray crystallography shows that both of the cyclophanes possess rectangular covalent frameworks. Furthermore, these cyclophanes form self-assembled tapes in the solid state, since the dibenzofuran moieties have a tendency to associate with each other through crossed ∞-∞ stacks. The dibenzofuran-containing catenanes also form two-dimensional supramolecular arrays in the solid state on account of extended ∞-∞ stacking interactions. In addition, the serendipitous discovery of a plerotopic tecton (consisting of a dibenzofuran nucleus covalently linked from the 2- and 8-positions by methylene groups to 4,4′-pyridylpyridinium (hydrogen bond acceptor) and protonated bipyridinium (hydrogen bond donor) units) has been made. The tecton dimerizes in the solid state to form a supramolecular macrocycle, since its complementary hydrogen bonding sites are oriented in a horseshoelike fashion by the 2,8-disubstituted dibenzofuran unit. However, this superstructure is not retained in the 1:1 complex of the tecton with BPP34C10: cocrystallization of the tecton with this crown ether opens the macrocyclic two-component supermolecule to afford a hydrogen-bonded pseudopolyrotaxane.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030720
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  • 110
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    The C60 Core: A Versatile Tecton for Dendrimer Chemistry (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 561-567 
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    ISSN: 0947-6539
    Keywords: conformation ; dendrimers ; fullerenes ; molecular dynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, complete characterization and molecular dynamics simulations of dendrimers 4-8 involving [60]fullerene as core tecton and first- to thirdgeneration benzyl-ether-based dendrons as branches is described. In dendrimer 7 a core branching multiplicity of 12, the highest known to date, is realized for the first time with a Th-symmetrical C60 core having an octahedral addition pattern. This unique structural type of core building block is only possible on the base of C60 and has no precedent in organic chemistry. NMR investigations as well as molecular modelling studies show that, owing to high core branching multiplicity, dendrimer 7 and to a minor extent the mixed adduct 8 already represent globular and densely packed macromolecules, although only first-generation dendrons are involved in their construction.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030411
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  • 111
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    Adsorption of Ionophores and of Their Cation Complexes at the Water/Chloroform Interface: A Molecular Dynamics Study of a [2.2.2]Cryptand and of Phosphoryl-Containing Podands (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 552-560 
    Details
    ISSN: 0947-6539
    Keywords: extraction ; interfaces ; ionophores ; molecular dynamics ; counterion effect ; solvation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report molecular dynamics simulations on ionophores of different topologies and on their complexes with alkali and alkaline-earth cations, with or without counterion, at the water/chloroform interface. As ionophores we consider two phosphoryl-containing podands (the “chainlike” monopodand MP and the “octopuslike” tripodand TP) and the bicyclic cryptand 222. We find that all the solutes behave as surfactants: they remain adsorbed at the interface, without migrating to bulk phases. Their precise location and solvation depend on the nature and conformation of the ionophore, of the cation and of the counterion. Schematically, two types of solutes can be distinguished, depending on their hydrophilic/hydrophobic character. The first type (cryptand 222 and its complexes, or the [MP⋅K+] complex), which have a hydrophobic exterior, stay on the chloroform side of the interface and are partially hydrated by “water fingers”. The second type (free MP and TP, [MP⋅K+]Pic- and [MP⋅Sr2+](Pic-)2 complexes), which are more hydrophilic, are partitioned to a greater extent between the two liquid phases. The status of the ion pairs at the interface depends on the interplay between cation … anion and anion … solvent interactions. When cation-anion interactions are strong enough (as in [MP⋅Sr2+](Pic-)2), the ion pairs remain intimate. Otherwise they dissociate, leading to solvent-separated ion pairs adsorbed at the interface (in the [222⋅K+] Pic- complex) or to the migration of the anion to the water phase (in the [222⋅K+] Cl- complex).
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030410
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  • 112
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    Functionalized Lipid Tubules as Tools for Helical Crystallization of Proteins (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 620-625 
    Details
    ISSN: 0947-6539
    Keywords: helical structures ; lipids ; liposomes ; self-assembly ; tubules ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The development of functional supramolecular devices built by self-assembly of elementary molecules and with bioactive properties arouses considerable interest in the field of nanotechnology and new materials. We report here the formation of a new class of lipid tubules exhibiting both properties of molecular recognition and crystal formation for the protein streptavidin. These lipid tubules, made of biotin-containing dioctadecylamine molecules, are straight hollow cylinders with a constant diameter of 27 nm and variable length up to several micrometers. They are unilamellar with an inner diameter of about 16 nm, as shown by cryoelectron microscopy. Streptavidin binds to the biotinylated tubules and assembles spontaneously into ordered helical arrays at the tube surface. These crystals exhibit regular order up to about 1.5 nm resolution. In addition, the helical streptavidin arrays act as functionalized supramolecular devices that bind a wide variety of biotinylated objects, as demonstrated here with proteins and liposomes.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030419
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  • 113
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    Masthead (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030502
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  • 114
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    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.
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    URL: http://dx.doi.org/10.1002/chem.19970030421
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  • 115
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    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 672-676 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030503
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  • 116
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    Absolute Asymmetric Synthesis from Achiral Molecules in the Chiral Crystalline Environment (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 684-689 
    Details
    ISSN: 0947-6539
    Keywords: absolute asymmetric synthesis ; asymmetric synthesis ; chirality ; photochemistry ; solid-state chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The current status of absolute asymmetric synthesis in a chiral crystalline environment has been reviewed. A number of topochemically controlled four-center type photocycloadditions are described for a series of unsymmetrically substituted diolefin crystals and CT crystals. This concept has been applied to intramolecular photoreactions, and several successful absolute asymmetric syntheses have been achieved, involving Norrish Type II reaction, di-π-methane rearrangement, electrocyclization, thietane formation, oxetane formation, hydrogen abstraction by thiocarbonyl and alkenyl groups, and radical-pair intermediates.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030506
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  • 117
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    Manipulation of Electric Charge on Vesicles by Means of Ionic Surfactants: Effects of Charge on Vesicle Mobility, Integrity, and Lipid Dynamics (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 690-695 
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    ISSN: 0947-6539
    Keywords: liposomes ; phospholipids ; surfactants ; vesicles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A combination of electrophoresis, dynamic light scattering, conductometry, and fluorescence spectroscopy was applied to investigate vesicles (both in the “solid” and “liquid” states) that had been imparted with electric charge through the incorporation of ionic amphiphiles. These amphiphilic compounds comprised cardiolipin (with two negative charges), sodium dodecyl sulfate (with one negative charge), and cetylpyridinium bromide (with one positive charge). By this means it was discovered that negative vesicles could be converted into neutral vesicles, and then into positive vesicles, by the addition of a cationic surfactant. The amount of cationic surfactant required for the conversion depended upon the mobility of the surfactant within the bilayer. Vesicles were found to be capable of absorbing large amounts of surfactant, both cationic and anionic, before ultimately disintegrating and releasing their contents. Mixtures of cationic and anionic vesicles were able to exchange surfactant, and thereby neutralize each other's charges, without any concurrent vesicle fusion. This phenomenon is reliable only if the vesicles are in the liquid state. Finally, a biphasic exchange process was observed in which a surfactant rapidly departs from one bilayer and then enters another, while a fluorescently labeled lipid travels the reverse path only slowly.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030507
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  • 118
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    Addition of Organozincate Reagents to Imines Derived from (S)-1-Phenylethylamine and Ethyl (S)-Valinate - Synthesis of (S)-1-(2-Pyridyl)Alkylamines (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 726-731 
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    ISSN: 0947-6539
    Keywords: amines ; asymmetric synthesis ; C-C coupling ; chiral auxiliaries ; imines ; zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Triorganozincates were added to aliphatic aldimines derived from (S)-1-phenylethylamine and (S-valine esters in the presence of boron trifluoride to give secondary amines with low diastereoselectivies. From mixed zincates, most alkyl groups (methyl, ethyl, 1-heptynyl, but not tert-butyl) could be transferred. No addition to benzaldimines was observed, but the imines prepared from 2-pyridinecarboxaldehyde did not require activation by BF3 and underwent selective group transfer from mixed zincates at  -  78°C. Excellent diastereoselectivities were observed in the reactions of the 2-pyridine imine derived from ethyl (S)-valinate with mixed zincates, in which the methyl group was used as nontransferable ligand, allowing the transfer of alkyl and vinyl groups with excellent to complete selectivity. However, dimethyl(aryl)- and dimethyl-(1-heptynyl)zincates did not react. (S)-1-(2-Pyridyl)alkylamines were prepared with high optical purity by subsequent removal of the chiral auxiliary.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030512
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  • 119
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    Trapping of Radicals in the Coordination Spheres of Metals (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1005-1008 
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    ISSN: 0947-6539
    Keywords: alkali metals ; calcium ; ketyl radicals ; radicals ; samarium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The use of hexamethylphosphoramide (HMPA) or other sterically demanding groups as stabilizing ligands allows the successful isolation of a series of structurally characterizable complexes of ketyl radicals with alkali, alkaline earth, and lanthanide metal. It has been demonstrated that the stability and reactivity of the ketyl radicals strongly depend on both the nature of the metals to which they are bound and the steric and electronic properties of the ancillary ligands, as well as the structure of their parent ketones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030704
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  • 120
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    Structure and Chemical Bonding in K14Cd9Tl21, a Compound Containing Both Isolated Tl7-11 Clusters and 2=[Cd9Tl7-10] Metallic Layers (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 799-806 
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    ISSN: 0947-6539
    Keywords: cadmium ; clusters ; layered compounds ; semiempirical calculations ; thallium ; Zintl phases ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The compound K14Cd9Tl21 has been prepared by fusion of the elements in a tantalum reactor followed by slow cooling at the rate of 6° per hour to enable crystal growth. The X-ray crystal structure (hexagonal, P2m; a = 9.884(3), c =17.173(5) Å; Z = 2) was refined to reliability factors of R1 = 5.09, wR2 = 11.64%. The anionic substructure contains both isolated clusters of Tl7-11 and a layered network based on Cd5Tl2 pentagonal bipyramids. The structure-electron count relationship for the Tl7-11 cluster was analyzed in detail. The Cd9Tl7-10 layers are found to confer metallic properties to the material, and the bonding within such layers is found to be highly delocalized.
    Additional Material: 6 Ill.
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    Masthead (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030602
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    Molecular Structures and Energetics of Corrole Isomers: A Comprehensive Local Density Functional Theoretical Study (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 823-833 
    Details
    ISSN: 0947-6539
    Keywords: corroles ; density functional calculations ; metallocorroles ; porphyrinoids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By using geometry optimizations with local density functional theory and double-π plus polarization basis sets, an extensive study has been carried out on the molecular structures and stabilities of free-base and metal-complexed corrole isomers. The optimized structures of normal metallocorroles have been found to agree well with crystallographic results. For both free-base and metal-complexed derivatives, the [1.1.1] ring system is found to be the most stable. The [2.0.1]- and [2.1.0]corrole isomers are unequivocally predicted to exist as stable materials. Of these, the [2.0.1] ring system, known as isocorrole, has been recently synthesized. Various derivatives of these two ring systems lie only about 10-20 kcal mol-1 above analogous derivatives of normal corrole. In general, the cis- and trans-[3.0.0]corrole derivatives are predicted to be significantly less stable than the other corrole isomers. However, the ScIII complexes of these two stereoisomeric ring systems are surprisingly stable. Direct C-α-Cα linkages between pyrrole rings are identified as a principal source of strain in the molecular structures of corrole isomers. The N4 cores of [1.1.1]- and [2.0.1]corrole isomers are significantly smaller than the porphyrin core. Thus, these corrole isomers are predicted to have a strong preference for smaller metal ions such as GaIII. The [2.1.0] core is somewhat larger, as evidenced by longer metal-nitrogen distances in [2.1.0]-metallocorroles. These differences in core geometry account for an interesting reversal of the relative stabilities of [2.0.1]-and [2.1.0]metallocorroles with increasing ionic radius of the complexed metal ion. Analogous to porphyrin isomer chemistry, the trans stereoisomer of [3.0.0]-corrole is found to be more stable than the cis stereoisomer for the free-base and for the ScIII and InIII derivatives. For the free bases of any particular isomer, the tautomers are quite similar in energy, differing by only 2-7 kcal mol-1. This, together with the presence of short, strong N-H=N hydrogen bonds, suggests that N-H tautomerization in at least some free-base corrole isomers should be considerably faster than that in porphyrins. Overall, it has been possible in most cases to establish a good correlation between the energetics trends and structural differences among molecules.
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    A Density Functional Study of [Fe(PR3)4H3]+ Isomers: Comparing Model Compounds (r = H) with Real Molecules (R = CH3) (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 881-886 
    Details
    ISSN: 0947-6539
    Keywords: density functional calculations ; iron ; ligand effects ; molecular modeling ; phosphorus ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structures, energetics, and dynamics of various isomers of [Fe-(PR3)4H3]+ (R = H, Me) were studied by density functional theory. The conformations considered were derived from a square planar (P), tetrahedral (T) and C2v-butterfly (C) arrangement of the phosphine ligands. For PH3, the stability ranking PC〉T was obtained, whereas in the PMe3 case the P isomer was least stable, T〉C〉P. Mechanisms for the hydrogen exchange in the C and T isomers are discussed. For the latter, this process might be described as a tetrahedral jump, but could also be viewed as a dodecahedral distortion of a cubic arrangement. The theoretical findings are compared with the results of experimental studies of [Fe(PR3)4H3]+ (R = Me, Et).
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030608
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    Intermetallic s̰ and ∞ Communication in Heterodinuclear μ-Cyclooctatetraene ComplexesCooperative effects in ∞ ligand bridged dinuclear complexes, Part 21; Part 20: J. Heck, J. Körnich, J. Organomet. Chem. in press; Part 19: U. Hagenau, J. Heck, E. Hendrickx, A. Persoons, T. Schuld, H. Wong, Inorg. Chem. 1996, 35, 7863-7866. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1151-1159 
    Details
    ISSN: 0947-6539
    Keywords: bridging ligands ; cyclooctatetraene ; EPR spectroscopy ; heterodinuclear complexes ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of [(C5R5)Cr(∞6-Cot)] (R = H, Me; Cot = cyclooctatetraene) with [(CO)3Fe(∞-cis-cyclooctene)2] affords the heterodinuclear complexes [(C5R5)Cr{μ-∞5(Cr):∞3(Fe)-Cot}Fe(CO)3] (R = H: 1, R = Me: 2) in quite good yields. One of the CO ligands in 1 and 2 can be easily substituted by a phosphane ligand, PR3 (R = Me, Ph, OEt, F) to obtain [(C5R5)Cr{μ-∞5(Cr):∞3(Fe)-Cot}Fe-(CO)2PR3] (R = H, R = Me: 3a; R = H, R = Ph: 3b; R = Me, R = Me: 4a; R = Me, R = Ph: 4b; R = Me, R = OEt: 4c; R = Me, R = F: 4d). The X-ray structure determinations of 2, 3a, and 4c showed exclusively synfacial coordination of the two metal ligand moieties, despite the bulky C5Me5 (Cp*) ligand in 2 and 4c. However, the steric demand of Cp* gives rise to structural distortions in 2 and 4c, compared to the Cp-containing products 1 and 3a, and an elongation of the Cr-Fe distance from 293 pm to 303 and 304 pm, respectively. The heterodinuclear complexes were investigated by cyclic voltammetry and ESR spectroscopy in order to elucidate the role of the permethylation of the cyclopentadienyl ligand and the influence of phosphane ligands with different π-accepting abilities. The ESR spectroscopic results reveal surprisingly large 31P hyperfine coupling constants (hfcc). These can be explained by a superposition of two different electron spin transfer mechanisms, which include a s̰- and a π-bonding mode between the Cr and Fe centers.
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    Toward Controllable Molecular Shuttles“Molecular Meccano”, Part 10: For part 9 see P. R. Ashton, R. Ballardini, V. Balzani, M. Belohradsky, M. T. Gandolfi, D. Philp, L. Prodi, F. M. Raymo, M. V. Reddington, N. Spencer, J. F. Stoddart, M. Venturi, D. J. Williams, J. Am. Chem. Soc. 1996, 118, 4931-4951. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1113-1135 
    Details
    ISSN: 0947-6539
    Keywords: molecular devices ; nanostructures ; rotaxanes ; self-assembly ; translational isomerism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of nanometer-scale molecular assemblies, based on rotaxane-type structures, have been synthesized by means of a template-directed strategy from simple building blocks that, on account of the molecular recognition arising from the noncovalent interactions between them, are able to self-assemble into potential molecular abacuses. In all the cases investigated, the π-electron-deficient tetracationic cyclophane cyclobis(paraquat-p-phenylene) is constrained mechanically around a dumbbell-shaped component consisting of a linear polyether chain intercepted by at least two, if not three, π-electron-rich units and terminated at each end by blocking groups or stoppers. The development of an approach toward constructing these molecular abacuses, in which the tetracationic cyclophane is able to shuttle back and forth with respect to the dumbbell-shaped component, begins with the self-assembly of a [2]rotaxane consisting of two hydroquinone rings symmetrically positioned within a polyether chain terminated by triisopropylsilyl ether blocking groups. In this first so-called molecular shuttle, the tetracationic cyclophane oscillates in a degenerate fashion between the two π-electron-rich hydroquinone rings. Replacement of one of the hydroquinone rings - or the insertion of another π-electron-rich ring system between the two hydroquinine rings - introduces the possibility of translational isomerism, a phenomenon that arises because of the different relative positions and populations of the tetracationic cyclophane with respect to the π-donor sites on the dumbbell-shaped component. In two subsequent [2]rotaxanes, one of the hydroquinone rings in the dumbbell-shaped component is replaced, first by a p-xylyl and then by an indole unit. Finally, a tetrathiafulvalene (TTF) unit is positioned between two hydroquinone rings in the dumbbell-shaped component. Spectroscopic and electrochemical investigations carried out on these first-generation molecular shuttles show that they could be developed as molecular switches.
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    Cover page (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. cpi 
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    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/chem.19970030801
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    Bidirectional and Convergent Routes to oligo(Tetrahydrofurans) (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1170-1180 
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    ISSN: 0947-6539
    Keywords: oligomers ; polyethers ; tetrahydrofurans ; Williamson reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligo(tetrahydrofurans) (oligo-THFs) 8-12 have been synthesised stereoselectively. Multiple Williamson reactions were used as key steps. While oligo-THFs with an even number of THF rings like the bi-THFs 8 and 9 as well as the tetra-THFs 10 and 11 were obtained by a bidirectional strategy, the penta-THF 12 with an odd number of THF rings was prepared by a convergent strategy with a sulfone-aldehyde coupling as connecting step. The oligo-THF products are important structural features of natural (Annonaceae acetogenins) and non-natural (artificial ion channels) products.
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    Masthead (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/chem.19970030802
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    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1187-1191 
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    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/chem.19970030803
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    Synthetic Carbohydrate-Containing DendrimersThis paper should be regarded as part 4 of a series on synthetic carbohydrate dendrimers: For part 3, ref. [16]. A list of abbreviations used in this article is given in ref. [39]. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1193-1199 
    Details
    ISSN: 0947-6539
    Keywords: carbohydrates ; cluster glycosides ; convergent synthesis ; dendrimers ; divergent synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dendrimers coated with carbohydrates on their exterior surfaces have been constructed by using both convergent and divergent synthetic routes. Alternatively, cluster glycosides in the form of highly branched oligosaccharides can serve as dendritic wedges in the subsequent elaboration of fully carbohydrate dendrimers. It is anticipated that these novel saccharide-containing polymers, which are highly branched and water-soluble, will find applications of a biological nature as well as in the context of new materials.
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    Reaction Paths for the Conversion of Methane to Methanol Catalyzed by FeO+ (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1160-1169 
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    ISSN: 0947-6539
    Keywords: ab initio calculations ; methane ; methanol ; reaction mechanisms ; transition metals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We propose possible theoretical reaction paths for the conversion of methane to methanol catalyzed by FeO+. The geometric and electronic structures for the reactant, product, intermediates, and transition states were calculated and analyzed in detail by means of a hybrid Hartree-Fock/density functional method. Sextet and quartet spin states were taken into consideration in the analysis of the reaction paths. The conversion of methane to methanol was shown to proceed through basic concerted hydrogen- and methyl-shift reactions. A fragment molecular orbital analysis for the formation of the reactant complex, OFe+-CH4, which plays an important role in the initial stage of methane activation, was carried out in order to understand the nature of the interesting Fe-C bond. The five-coordinate methane in the reactant complex was calculated to have a C3v-type geometry. Each reaction path presented in this paper includes an important insertion species, HO-Fe+-CH3 or H-Fe+-OCH3, and two transition states. Thus, there are several kinds of reaction paths, if the high-spin sextet and low-spin quartet states are taken into consideration. A reaction towards the hydroxy intermediate, HO-Fe+-CH3, was found to be more favorable in both the sextet and quartet spin states from the viewpoint of activation energy, and this intermediate is extremely stable. It was found from intrinsic reaction coordinate (IRC) analyses that two basic reactions coexist, namely, hydrogen or methyl migration between the reactant and the methoxy intermediate, H-Fe+-OCH3. This transition state is interesting, because the two transition states resulting from C-H bond cleavage and methyl migration are located in the same region of space on the potential energy surfaces. IRCs are partially shown for the complicated first halves of the total reaction paths.
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    Control of the Helical Chirality in Octahedral Complexes by a Chiral Macrobicyclic Cavity Possessing Six Convergent Hydroxyl Groups (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1202-1202 
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    ISSN: 0947-6539
    Keywords: chirality ; coordination modes ; macrocyclic ligands ; octahedral complexes ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030806
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    Enantioselectivity Control with Metal Colloids as Catalysts (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1200-1202 
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    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; catalysis ; colloids ; hydrogenations ; platinum ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metal colloids protected with chiral molecules can lead to a new type of enantioselective catalyst combining good selectivity control with an extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidine.
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    Molecular Receptors for Adenine and Guanine Employing Metal Coordination, Hydrogen-Bonding and π-Stacking Interactions (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1203-1213 
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    ISSN: 0947-6539
    Keywords: hydrogen bonds ; metalloreceptor ; molecular recognition ; nucleobases ; pi interactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thiacyclophane ligands 1 and 2, containing a meta-xylyldithiaether unit, an aromatic spacing unit and a polyether chain, were prepared in good yield in a three-step synthesis. The macrocyclic organopalladium complexes [Pd(L)-(MeCN)][BF4] (3: L = 1; 4: L = 2) were prepared through palladation of the respective thiacyclophane ligand by reaction with [Pd(MeCN)4][BF4]2. These complexes act as metalloreceptors to aromatic amines such as p-aminopyridine (pap), m-aminopyridine (map) and the DNA nucleobases adenine and guanine. Binding occurs through simultaneous first- and second-sphere coordination. This involves three separate interactions: first-sphere σ donation from an aromatic N atom to the Pd centre, second-sphere hydrogen bonds between the NH2 group and polyether O atoms, and π stacking between the electron-poor aromatic rings of the substrate and the electron-rich aromatic spacing units of the receptor. 1H NMR spectra exhibit chemical shift changes indicative of the H-bonding and π-stacking interactions in solution. X-ray structures for thiacyclophane 1, metalloreceptor [Pd(1)(MeCN)][BF4] (3), metalloreceptor/model substrate complexes [Pd(1)(pap)][BF4 (5) and [Pd(2)-(pap)][BF4] (7), and metalloreceptor/nucleobase complexes [Pd(1)(adenine)][BF4] (13), [Pd(2)(adenine)][BF4] (14) and [Pd(1)(guanine-BF3)][BF4] (15b) show details of these interactions in the solid state.
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    Kinetics and Mechanism of the Reactions of Amine Boranes with Carbenium IonsDedicated to Professor Wolfgang Beck on the occasion of his 65th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1214-1222 
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    ISSN: 0947-6539
    Keywords: amine boranes ; carbocations ; kinetics ; linear free enthalpy relationship ; reductions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetics of the reactions of trialkylamine boranes and pyridine boranes with benzhydryl cations have been determined photometrically. Second-order rate laws are obeyed, first-order with respect to amine borane concentration and first-order with respect to carbocation concentration. As for other reactions of carbocations with neutral nucleophiles, the rates of these reactions are only slightly affected by solvent polarity. The structure - reactivity relationships and kinetic isotope effects are in accord with a polar mechanism proceeding through a transition state where the migrating hydride is partly bound to the entering carbon and to the leaving boron atom. The rate constants correlate linearly with the electrophilicity parameters E of the carbenium ions. It is therefore possible to use the linear free enthalpy relationship logk2 = s(E+N) for determining nucleophilicity parameters N for the amine boranes and to compare their hydride-donating abilities with those of other non-charged hydride donors (silanes, germanes, stannanes, and dihydropyridines).
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    Twelve-Membered O-Bridged Cyclic Ethers of Red Seaweeds in the Genus Laurencia Exist in Solution as Slowly Interconverting Conformers (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1223-1231 
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    ISSN: 0947-6539
    Keywords: conformation analysis ; heterocycles ; hydrogen bonds ; NMR spectroscopy ; obtusallenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The twelve-membered O-bridged cyclic ether obtusallene IV (1), a new isolate from the red seaweed Laurencia obtusa from Kaş in the Turkish Mediterranean, revealed temperature-dependent NMR signals attributable to a major conformer in equilibrium with a minor conformer by 180° flipping of the trans olefinic bond. This prompted us to reexamine the known congeners obtusallene I (2), 10-bromoobtusallene I (3), obtusallene II (4), and obtusallene III (5), isolated both from the same and taxonomically related seaweeds, as well as their semisynthetic derivative peracetylobtusallene III (6). Two conformers could in fact be directly observed at room temperature for 2-3 and at low temperature for 4. Marked cross-saturation-transfer effects between the couples of conformers confirmed these observations. Activation energies for processes involving 10-membered subunit rings (2-3) are higher than for 11-membered (4-5) analogues, where faster conformational motion occurs too resulting in mediated vicinal J couplings. 1,3-Dihydroxy substituents in 5 form an intramolecular hydrogen bond in low-polarity, non-H-bonding solvents; this results in the existence of two further conformers, giving more complex NMR spectra. Descriptions in the literature of single conformers for obtusallenes 2-5 must have resulted from overlooking minor NMR signals or attributing them to impurities.
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    New Strategies for the Generation of Large Heteropolymetalate Clusters: The β-B-SbW9 Fragment as a Multifunctional Unit (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1232-1237 
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    ISSN: 0947-6539
    Keywords: antimony ; heteropolyanions ; polyoxometalates ; synthesis design ; tungsten ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Proceeding from the α-B-SbW9O33 fragment 1, various novel large heteropolyanions ([Na2Sb8W36O132(H2O)4]22- (2), [Sb2W22O74(OH)2]12- (3) and [Sb2W20M2O70(H2O)6](14-2n)- (4); Mn+ = Fe3+, Co2+, Mn2+, Ni2+) have been obtained in crystalline form by systematic synthesis. Their unexpected structures have been studied by X-ray diffraction: 2, 3 and 4 contain β-SbW9 units connected to each other by Sb3+ ions, WO2/WO2OH groups, and WO2/Mn+ (H2O)3 groups, respectively. Structural details and especially the systematic synthesis strategy for the formation of these interesting polytungstates are discussed in order to explain the unusual behaviour of 1 in aqueous solution under different reaction conditions.
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    Synthesis of a Pentasaccharide-Oligonucleotide Conjugate: A Novel Antithrombotic Agent (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1572-1577 
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    ISSN: 0947-6539
    Keywords: antithrombotics ; enzyme inhibitors ; oligonucleotides ; oligosaccharides ; protecting groups ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Derivatization of the octadecathymidylate derivative 18 (T18 ODN) containing a free amine function with sulfo-SIAB® gave the corresponding iodoacetyl ODN 21. Conjugation of the latter with the thiol-containing pentasaccharide 17c gave pentasaccharide-ODN conjugate III, which exhibited anti-Xa and antithrombin activities of 173 U mg-1 and 5 U mg-1, respectively.
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    Concave Hydrocarbons (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1585-1595 
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    ISSN: 0947-6539
    Keywords: concave hydrocarbons ; cyclophanes ; host/guest chemistry ; ion-selective electrodes ; prismands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Concave hydrocarbons, such as 1 (C36H36), 4 (C60H60), 5 (C54H48), and 6 (C60H52), represent three-dimensionally clamped analogues of π-prismands. They encapsulate small metal ions and accomplish metal-ion extraction from aqueous solution. Their remarkable selectivity allows applications such as incorporation in ion-selective electrodes. The synthetic route is based on well-established cyclophane methodology and, thus, offers a general approach to a whole family of concave hydrocarbons.
    Additional Material: 11 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021218
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    Covalent Bonding of Crown Ethers to γ-Zirconium Phosphate - New Layered Ion Exchangers Showing Selective Recognition (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1578-1584 
    Details
    ISSN: 0947-6539
    Keywords: crown ethers ; layered materials ; molecular recognition ; zirconium phosphate ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The topotactic exchange reactions of γ-zirconium phosphate and γ-zirconium phosphate/methylphosphonate with phosphonic acids derived from azacrown ethers of different sizes was examined. Prior intercalation of hexylamine was necessary in most cases to attain the maximum exchange level predicted by molecular modeling. Materials with imbricated or nonimbricated layers were obtained depending on crown size and exchange level. Exchange of methylphosphonates in γ-zirconium phosphate/methylphosphonate was only observed when hexylamine had previously been intercalated. The exchange with the bisphosphonic acid derived from 1,10-diaza[18]-crown-6 gave a material with an interlayer distance compatible with pillaring of the γ-phase. The materials obtained from the exchange reactions of γ-zirconium phosphate with phosphonic acids derived from [12]crown-4 and [18]crown-6 showed thermodynamic selectivities towards Na+ and K+, respectively, that were much higher than those observed for the simple crowns in solution. This observation suggests that the inorganic molecular framework enhances molecular recognition by minimization of solvation effects.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021217
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    Resolution of the Absorbance and CD Spectra and Formation Constants of the Complexes between Human Serum Albumin and Methyl OrangeA preliminary communication has been presented at the 12th Conference on Physical Organic Chemistry (IUPAC), Padua (Italy), 1994, Abstracts, p. 82. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 149-156 
    Details
    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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    URL: http://dx.doi.org/10.1002/chem.19960020206
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    Tricarbonyl(η6-1,2-dioxobenzocyclobutene)chromium(0): Preparation, Nucleophilic Addition Studies and Syntheses of cmplex Polycyclic Systems by Dianionic Oxy-Cope Rearrangement (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 182-190 
    Details
    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.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020210
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    Effect of the Position and Number of Chiral Carbons on Ferroelectric Liquid Crystals from Multichain Mononuclear ortho-Palladated Complexes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 214-220 
    Details
    ISSN: 0947-6539
    Keywords: ferroelectrics ; liquid crystals ; metallomesogens ; palladium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of twelve mononuclear ortho-palladated complexes incorporating a Schiff base and a β-diketone ligand have been synthesised. These compounds have four nonequivalent terminal chain positions and differ in the number and position(s) of chiral chains [(R)-2-methylheptyl] in the structure. A study of the ferroelectric properties of the compounds has been performed. It is found that the spontaneous polarisation is highly dependent not only on the number of chiral carbons present, but also on the position within the molecular core.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020214
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020301
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 251-254 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020303
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  • 146
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    The Pre-Reactive Complex of H2S and BrCl; Observation and Characterisation by Rotational Spectroscopy (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 265-270 
    Details
    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.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020306
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    Synthesis, Conformational Analysis and Comparative Protein Binding of a Galabioside and Its Thioglycoside Analogues (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 295-302 
    Details
    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.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020310
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    X-ray structural studies of highly enantioselective Mn(salen) epoxidation catalysts (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 974-980 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric eponidations ; Cstalysis ; manganese complexes ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(sa1en) complexes (1 - 10) was examined. The X-ray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6′= -tBu; 4,4 = -tBu]+ClO4-), and 10 (6,6′= -tBu; 4,4′=-Br) were determined. Catalysts 1 - 9 were derived from (R,R)-1, tdiaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethyIenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the orfho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five- and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1 - 10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6′ = -H; 4,4′ = -H),2(6,6′ = -CH3; 4,4′ = -CH3),3(6,6′=-tBu;4,4′=-H),4(6,6′=-tBu; 4,4′ =-CH3), 5 (6,6′=-tBu; 4,4′=-tBu), and 6 (6,6′=-tBu; 4,4′= -trityl), and electronically modifying the para substituents in 7 (6,6 = -tBu; 4,4 =-OMe) and 8 (6.6′ = -tBu; 4,4′=-OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.
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    Effects of nitrogen substitution in poly(pyrazolyl)borato ligands: From orbital energy levels to C-H ⃛ O hydrogen bondingDedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 992-1000 
    Details
    ISSN: 0947-6539
    Keywords: cyclovoltammetry ; electronic effects ; photoelectron spectroscopy ; pyrazole ligands ; Mössbauer spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic effect of substituting CH with N in poly(pyrazolyl)-borato ligands and their transition-metal complexes is shown to be a decrease in energy of the filled metal and ligand orbitals. This conclusion is based on the cyclovoltammograms and photoelectron spectra of bis(hydrotris(azolyl)borato)-iron(II) and -cobalt(II) complexes (azolyl = pyrazolyl and 1,2,4-triazolyl) and on MO calculations. 57Fe Mössbauer spectra of bis(hydrotris(1,2,4-triazolyl)-borato)iron(II) show that there is a fine-tuning of the HOMO-LUMO gap by a shift in transition temperature for the spin equilibrium. 15N NMR spectroscopy supports the assignment of a higher negative charge to the exodentate N-4 nitrogen than to the endodentate N-2 or N-3 positions in the poly(azolyl)borato anions, where azolyl is 1,2,4-triazolyl or tetrazolyl. The electron withdrawing effect of the additional nitrogen atoms and the in-corporation of water of crystallization by O-H ⃛ N bonding both assist in the formation of (azolyl)C-H ⃛ O bonds.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020813
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 909-912 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020819
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020820
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    A novel catalytic cycle for the synthesis of epoxides using sulfur ylides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1024-1030 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; diazo compounds ; epoxidations ; sulfur ylides ; synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel, neutral catalytic cycle for the synthesis of epoxides from carbonyl compounds and diazo compounds using catalytic quantities of transition metal salts and sulfides has been developed. In this catalytic cycle, the diazo compound is decomposed by the transition metal salt to give a metallocarbene, and this is picked up by the sulfide to give a sulfur ylide, which then reacts with the aldehyde to give an epoxide and returns the sulfide back into the catalytic cycle. To obtain good yields of epoxides it is necessary to maintain a low concentration of the diazo compound (by slow addition), otherwise dimerisation of the diazo compound is the dominant reaction. Factors affecting the outcome of the reaction were studied. The reactions are relatively insensitive to solvent, but are sensitive to the structure of the sulfide, the metal salt and the concentration. Unhindered sulfides give good yields of epoxides with any metal salt, but with hindered sulfides higher yields are obtained with Cu(acac)2 than with Rh2(OAc)4. The yields of epoxides are sensitive to sulfide concentration especially when using substoichiometric amounts of sulfides. Higher concentration leads to faster rates of formation and subsequent reaction of the sulfur ylides, and consequently to higher yields. This novel catalytic cycle has also been applied to base-sensitive aldehydes. We found that our new catalytic cycle for epoxidation gives much improved yields of epoxides compared to those obtained by traditional sulfur ylide chemistry and is tolerant to a wide variety of sensitive functional groups. Ketones also participate in the catalytic cycle, although they give reduced yields of epoxides compared to aldehydes and require a slightly elevated temperature.
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    URL: http://dx.doi.org/10.1002/chem.1460020817
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    TiIV-BINOLate-catalyzed highly enantioselective additions of β-substituted allylstannanes to aldehydesFirst presented at the „15th Regiosymposium of the Universities Basel, Freiburg, and Mulhouse“ in Falkau, September 27-29, 1995. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1077-1084 
    Details
    ISSN: 0947-6539
    Keywords: allylstannanes ; asymmetric allylations ; catalysis ; C-C bond formation ; homoallylic alcohols ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantiomerically pure homoallyl alcohols were prepared from aldehydes R1-CH=O (R1 = Ph, pentyl, Ph-CH=CH-, iPr) and β-substituted allyl-stannanes H2C=CR2-CH2-SnBu3 (R2 = pentyl, tBuPh2SiO-CH2-, tBuPh2SiO-CH2-CH2-, PhS-CH2-CH2-). These reactions were catalyzed by the same additives - Ti(OR)4 (10 mol%) and (R)-BINOL (20 mol%) - that Keck et al. used in analogous reactions with methallyl-and allyltributylstannane. To attain optimum ee values (96.4-99.2% in the reaction with hexanal) these additives had to be premixed for 2 h at room temperature. Ti(OEt)4 and Ti(OiPr)4 gave equally good results, while Ti(OMe)4, Ti(OCHEt2)4, and Ti(OtBu)4 were inferior. Our procedure works in the absence of molecular sieves [which were previously found to give rather unreliable results in reactions catalyzed by Ti(OiPr)4/(R)-BINOL] and can be extended to enantioselective addition reactions with allyl- and methallyltributylstannane, too (+ hexanal: 97.4 and 97.0% ee, respectively).
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    Catalytic properties of layered gold-palladium colloidsDedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1099-1103 
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    ISSN: 0947-6539
    Keywords: catalysis ; colloids ; gold-palladium colloids ; HRTEM ; hydrogenations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Layered bimetallic gold-palladium colloids in the size range of 20-56 nm have been synthesized by the seed=growth method: gold seeds were convered by palladium layers of various thickness and vice vaers. The outer metal was coordinated by trisulfonated triphenylphosphine and sodium sulfanilate ligands to stabilize the bimetallic particles to such an extent that they could be isolated in the solid state. Owing to the hydrophilic ligand shell, redispersion in water was posible in any concentration. High-resolution transmission electronn microscopy and energy-dispersive X-ray analysis was used to characterized the colloids. Stabilized and nonstabilized gold-palladium and palladium-gold systems on a TiO2 support were used as heterogeneous catalysts for the hydrogenation of hex-2-yne to cis-hex-2-ene. Both the palladium-plated gold seeds and the gold-plated palladium particles showed considerably increased activities compared with the pure metals. The ligand shell seems not to influence the catalytic behavior because protected and unprotected colloids behave very similarly.
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    A Convergent synthesis of carbohydrate-containing dendrimersThis paper should be regarded as Part 1 of a series on Synthetic Carbohydrate Dendrimers. A list of abbreviations used in this paper is given in ref. [40]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1115-1128 
    Details
    ISSN: 0947-6539
    Keywords: carbohydrates ; cluster glucosides ; convergent syntheses ; dendrimers ; neoglycoconjugates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of carbohydratecontaining dendrimers has been achieved by a convergent growth approach. The synthetic strategy involves: 1) the synthesis of the triglucosylated derivative of tris(hydroxymethyl)methylamine (TRIS), 2) the introduction of a glycine-derived spacer and 3,3′-iminodipropionic acid derived branching units on to the TRIS derivative by amide bond formation, 3) condensation of the above saccharidecontaining dendrons with a trifunctional 1,3,5-benzenetricarbonyl derivative, used as the core, by formation of amide bonds, and 4) deprotection of the saccharide units. A 9-mer and an 18-mer, carrying nine and eighteen saccharide units at the periphery, respectively, have been synthesized, in high yields at each step, by this synthetic strategy. By a variety of chromatographic and spectroscopic techniques, the dendrimers were shown to be structurally homogeneous, monodisperse, and error-free at all steps in their growth. These investigations were complemented by molecular modeling studies on the dendrimers. The presence of slightly distorted C3 symmetry was noted in both the 9-mer and the 18-mer.
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    The unimolecular chemistry of protonated glycinamide and the proton affinity of glycinamid - mass spectrometric experiments and theoretical model (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1143-1149 
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    ISSN: 0947-6539
    Keywords: ab initio calculations ; glycinamide ; mass spectrometry ; protonations ; quantum chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential energy hypersurface of protonated glycinamide (GAH+) has been investigated experimentally and theoretically. The calculated G2(MP2) value for the proton affinity of glycinamide, PAcalcd = 919 kJ mol-1, is in good agreement with the measured value of 908PAexp〈914kJ mol-1. The fact that the amide group is a better hydrogenbond acceptor explains why glycinamide has a higher PA than glycine. Proton transfer experiments with glycinamide performed in a Fourier transform mass spectrometer and analysis of metastable GAH+ ions in a four-sector mass spectrometer show that the lowest-energy unimolecular reactions are two distinct processes: 1) loss of CO, which has a substantial barrier for the reverse reaction, and 2) loss of CO plus NH3, which has no barrier for the reverse reaction. Ab initio quantum chemical calculations give a reaction model that is consistent with the observed fragmentation pattern.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020915
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    Stabilization of Octahedral Thorium Clusters by Interstitial Hydrogen (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 511-515 
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    ISSN: 0947-6539
    Keywords: chemical bonding ; hydrides ; interstitial atoms ; magnetic properties ; thorium clusters ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, structure, and characterization of the cluster compounds Th6HxBr15 (x = 5 and 7) are reported. The compounds can be prepared by a reversible hydrogenation/dehydrogenation process; their structures have been determined by X-ray and neutron diffraction methods. Both compounds form a body-centered cubic structure of octahedral cluster units with five or seven hydrogen atoms in eight disordered positions above the faces of the octahedron. The chemical bonding in these diamagnetic compounds has been analyzed: the simple counting rules for octahedral cluster compounds are fulfilled, and MO and band structure calculations confirm the hydridic nature of the bonding.
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    URL: http://dx.doi.org/10.1002/chem.19960020509
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    Conformers of Saturated Chains: Matrix Isolation, Structure, IR and UV Spectra of n-Si4Me10Dedicated to Professor H.-D. Scharf on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 529-538 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; conformation ; sigma conjugation ; matrix isolation ; oligosilanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Infrared and ultraviolet spectra of the gauche and anti conformers of matrix-isolated permethyl-n-tetrasilane have been obtained separately by taking advantage of thermally induced gauche-to-anti conversion and of wavelength-selective photochemical destruction of either conformer. The resolved UV spectrum of the gauche conformer provides the first piece of experimental evidence in favor of the recently proposed reinterpretation of conformational effects on tetrasilane electronic states. According to this, it is not the energy but the intensity of the lowest singlet excitation that changes dramatically as the SiSiSiSi dihedral angle is varied, as a result of an avoided crossing between s̰s̰* and s̰π* states. Implications for the general understanding of sigma conjugation in simple terms are discussed. Unconstrained MP2/6-31 G* optimization predicts the existence of a third backbone conformer (ortho), with a dihedral angle of about 90°. Its predicted (HF/3-21 G*) mid-IR spectrum is indistinguishable from that of the gauche conformer, and the matrix-isolation spectra thus provide no evidence for or against its presence.
    Additional Material: 10 Ill.
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    Coordination and Coupling of OH-Functionalized C2 Units at a Single Metal Center: The Synthesis of Alkynyl(Vinylidene), Alkynyl(Enyne), Bis(Alkynyl)Hydrido, Enynyl, and Hexapentaene Rhodium Complexes from Propargylic Alcohols as PrecursorsDedicated to Professor Martin A. Bennett on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 561-569 
    Details
    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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    Preparation, Characterization, and Reactivity of a Diamagnetic Vanadium Nitride (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1258-1263 
    Details
    ISSN: 0947-6539
    Keywords: dinitrogen activation ; nitrides ; structure elucidation ; vanadium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of (Ph2N)3V-(THF) (1) with [(CH3)3Si]N3 and [(CH3)3Si]CHN2 proceed at room temperature to afford the corresponding derivatives (Ph2N)3V[N-Si(CH3)3] (2a) and (Ph2N)3V[N2CHSi(CH3)3] (3). Reactions of both 2a and 3 with lithium isopropylamide provide the unprecedented anionic vanadium nitride species [(Ph2N)3V(μ-N)Li(THF)3]·(THF)0.5 (4), which displays moderate nucleophilicity. It reacts with (CH3)3SiCl and Mel to reform the complex (Ph2N)3V(N-R) [R = Si(CH3)3 (2a), Me (2b)]. Reactions with Cp2ZrCl2 and CpTiCl3 lead to the formation of the heterodinuclear bridged nitride complexes [(Ph2N)3V(μ-N)ZrClCp]2] [Cp = Cp]=Cp (6a), CH3C5H4 (6b)] and [{(Ph2N)3V(μ-N)}2TiClCp] (8).
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021012
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    Kinetic Analysis of the Stepwise Platination of Single- and Double-Stranded GG Oligonucleotides with Cisplatin and cis-[PtCl(H2O)(NH3)2]+ (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1283-1291 
    Details
    ISSN: 0947-6539
    Keywords: antitumour agents ; DNA ; kinetics ; nucleotides ; platinum complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report the first direct comparison of the kinetics of platination of defined single- and double-stranded DNA with the anticancer drug cisplatin. The courses of the reactions of the 14-mer duplex d(A-T-A-C-A-T-G-G-T-A-C-A-T-A)·d(T-A-T-G-T-A-C-C-A-T-G-T-A-T) with [15N]cisplatin and cis-[PtCl(H2O)-(15NH3)2]+ and of each of the single strands with [15N]cisplatin have been studied at 298 K, pH 6, by [1H, 15N] NMR spectroscopy. As expected the reactions of cisplatin proceed via cis-[PtCl(H2O)(NH3)2]+, and lead to two monofunctional adducts on the duplex and two on the GG single strand. In both the GG single strand and the duplex, one of the two G's is platinated faster than the other (by a factor of ca. 4). Remarkably, ring closure on the duplex to form the GG chelate occurs about an order of magnitude faster for one monofunctional adduct than for the other. The latter monofunctional adduct has distinctive 1H and 15N NMR chemical shifts for Pt-NH3, and is very long-lived (persists for 〉5 d). The Pt-Cl bond in this monofunctional adduct is protected from hydrolysis by the duplex. In contrast, the two monofunctional adducts on the GG single strand undergo ring closure at about the same rate. Equilibria between kinked and distorted forms of the GG platinated duplex, the platination of G's on the complementary strand, and the potential biological significance of long-lived monofunctional adducts of platinum drugs with DNA are discussed.
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    URL: http://dx.doi.org/10.1002/chem.19960021016
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    Me3TTF-PO3H2, a Redox Phosphonic Acid and Its Monoanilinium Salt [PhNH+3][Me3TTF-PO(OH)O-], the Electrocrystallized Neutral (Zwitterionic) π Radical [Me3TTF-PO(OH)O-]·+, and Their Associated Lamellar Constructions in the Solid State (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1275-1282 
    Details
    ISSN: 0947-6539
    Keywords: crystal engineering ; hydrogen bonds ; phosphonates ; radical cations ; tetrathia fulvalenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of Me3TTFLi (TTF = tetrathiafulvalene) with ClP(O)-(OEt)2 followed by hydrolysis with Me3SiBr affords the novel π-donor molecule trimethyltetrathiafulvalenylphosphonic acid (Me3TTF-PO3H2) in a partially oxidized form. Subsequent reduction and neutralization with aniline gives the corresponding phosphonate monoanilinium salt. A unique hydrogen-bonded hexagonal net is identified within the lamellar structure of [PhNH+3][Me3TTF-PO(OH)O-], which is described by analogy with the anti-CaSi2 structure type. Electrocrystallization of the former salt yields single crystals of a neutral (zwitterionic) π radical, formulated as [Me3TTF-PO(OH)O-]·+. Their structure reveals the presence of hydrogen-bonded molecular ribbons whose association creates a novel layered architecture similar to that obtained within radical cation salts of π-donor molecules of larger spatial extension. The analysis of the calculated HO-MO-HOMO intermolecular interaction energies demonstrates that these slabs contain strong π-π intermolecular interactions despite the nonexistence of any 2D network of short S ⃛S contacts. The spin susceptibility of [Me3TTF-PO-(OH)O-]·+, determined by single-crystal ESR measurements, is characteristic of triplet excitons, the origin of which may be understood from the electronic structure of the compound.
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    URL: http://dx.doi.org/10.1002/chem.19960021015
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    Novel Heterocumulenes: Bisiminopropadienes and Linear Ketenimines (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1318-1329 
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    ISSN: 0947-6539
    Keywords: cumulenes ; heterocumulenes ; ketenimines ; matrix isolation ; thermolysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Flash vacuum thermolysis (FVT) of suitably substituted isoxazol-5(4H)-ones 7-9 leads to three different types of ketenimines, namely, the isoxazolonoketenimines 2, the novel bisiminopropadienes RN=C=C=C=NR (5), and the C-cyanoketenimines 14, all characterized by a combination of FVT/matrix isolation/IR spectroscopy and FVT/MS. An unusual, linear C=C=N-C backbone in ketenimines 2g and 2h is revealed by their exceptional spectroscopic properties as well as an X-ray crystal structure of 2g, and confirmed by density functional calculations (B3LYP/6-31 G*); these compounds are best described as resonance hybrids of ketenimines and isonitrile ylides R2C-C≡N-R'. The identification of the highly reactive bisiminopropadienes 5 is supported by the observed shifts in the IR bands of the 15N and 13C isotopomers as well as theoretical calculations, tert-Butyl-substituted isoxazolones 7e and 7f, and 8i form the expected ketenimines 2, which then undergo elimination of isobutene and CO2 to generate C-cyanoketenimines 14 and 14i. N-Phenyl-dicyanoketenimine 32 is also described.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021020
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    Investigations into the biosynthesis of porphyrins and corrins - calculations on 1,3-allylic strain and [1,5]-sigmatropic rearrangements in pyrroles, furans, and thiophenesDedicated to Professor Hans Paulsen on the occasion of his 75th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 523-529 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; biosynthesis ; porphyrinogens ; rearrangements ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The [1,5]-sigmatropic rearrangements of hydrogen and other groups, such as methyl, tert-butyl, allyl, benzyl, and azafulvenium, in pyrroles and, to some extent, in furans and thiophenes, has been studied by using semiempirical and ab initio methods. These systems are used as models to explain the ring D inversion in the biosynthesis of uroporphyrinogen III and the stereoselective shift of a methyl group in the biosynthesis of vitamin B12. The difference in energy between the competing pathways of hydrogen and methyl shifts is significantly lower than for cyclopentadiene. The rearrangements are usually concerted, except for in strongly resonance-stabilized systems, such as azafulvenium cations. Furthermore, ab initio calculations of the 1,3-allylic strain for a range of substituted pyrroles has been performed, and the results compared with semiempirical data.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030407
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    Highly Stereoselective Synthesis of Bicyclo[n.3.0]alkanes by Titanium Tetrachloride Promoted [3 + 2] Cycloaddition of Allylsilanes and 1-AcetylcycloalkenesCycloadditions of Allylsilanes, part 9. Part 8: H.-J. Knölker, P. G. Jones, R. Graf, Synlett 1996, 1155. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 538-551 
    Details
    ISSN: 0947-6539
    Keywords: allylsilanes ; diastereoselective synthesis ; bicycloalkanes ; cycloadditions ; enones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The titanium tetrachloride promoted reaction of allylsilanes 1 with 1-acetylcyclohexene is shown to afford the silylbicyclo[4.3.0]nonanes 9 ([3 + 2] cycloaddition products) along with the 1-acetyl-2-allylcyclohexane 4 (Hosomi-Sakurai product). Here we report that systematic variation of the substituents at the silicon atom of 1 allows suppression of the classical Hosomi-Sakurai reaction in favor of the [3 + 2] cycloaddition. Cycloaddition of the allylsilanes 1 d, 1 i, and 1 k with 1-acetylcycloalkenes 10, containing a 5-, 6-, 7-, 8-, or 12-membered ring, gives rise to the corresponding silylbicyclo[n.3.0]alkanes 11 - 13. The cycloaddition of allyltriisopropylsilane (1 k) and 1-acetyl-2-methylcycloalkenes 15 provides silylbicyclo[n.3.0]alkanes 16 with two contiguous quaternary carbon centers. The stereochemistry of the silylbicyclo[n.3.0]alkanes 11 a - c and 14 is unambiguously determined by X-ray analysis and 13C NMR spectroscopy.
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    URL: http://dx.doi.org/10.1002/chem.19970030409
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    The Reaction of a Triorganochloroindate with Dioxygen (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 568-572 
    Details
    ISSN: 0947-6539
    Keywords: cesium ; indium ; layered compounds ; metalla(crown ethers) ; pi interactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of CsCl with In(CH2Ph)3 yields the triorganochloroindate Cs[(PhCH2)3InCl] (1). Treatment of In(CH2Ph)3 with dry O2 gives the alkoxide [(PhCH2)2InOCH2Ph]2 (2), whereas the reaction of 1 with O2 leads to Cs2[O{PhCH2In(OCH2Ph)2}4] (3). According to X-ray structural investigations, 1 contains Cs2Cl2 four-membered rings as basic structural motif. The dimers are connected by Cs-π-electron interactions to form a layer structure. Compound 3 can be described as a metalla(crown ether) fragment coordinating two Cs+ ions. Additional shielding of the Cs centers by weak Cs-π-electron contacts was observed.
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    URL: http://dx.doi.org/10.1002/chem.19970030412
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    On the Mechanism of Stereospecific Polymerization - Development of a Universal Model to Demonstrate the Relationship Between Metallocene Structure and Polymer Microstructure (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 585-591 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric catalysis ; metallocenes ; polymerizations ; molecular modeling ; reaction mechanisms ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With the discovery of stereorigid bridged metallocenes, soluble catalysts became available for the stereospecific polymerization of α-olefins. A relatively simple mechanism was used to explain the stereospecificity, primarily in terms of the catalyst symmetry. In this paper we demonstrate that the simple rule of thumb that C2-symmetric catalysts produce isotactic and Cs-symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [{iPr(CpFlu)}ZrCl2]/ MAO system (Flu = fluorenyl, MAO = methylalumoxan) reduces the Cs symmetry to C1, and the resulting catalyst produces hemiisotactic polypropylene. The analogous catalyst with a bulkier tert-butyl group at the Cp ring gives isotactic polypropylene. When the C2 symmetry of [{Me2Si(Ind)2}ZrCl2] (Ind = indenyl) is reduced to C1, a metallocene can be obtained that produces atactic polypropylene. We have broken away from the symmetry-based model and developed a universal model, which accurately describes the experimental microstructures of the polymers by considering the four lowestenergy conformers of the metallocene species coordinating to prochiral propene (Rre, Sre, Ssi, and Rsi) and the positional changes that the polymer chain undergoes during insertion. The relative energy levels of the four diastereomers can be determined by molecular modeling calculations; these energy gradations, in particular the size of the energy gaps, are decisive in determining the stereospecificity. Also, the model permits the stereoerrors to be classified and explained. Through this model the stereosequence of a polymer chain can be calculated and predicted.
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    Thermal Ring-Opening Polymerization of Hydrocarbon-Bridged [2]Ferrocenophanes: Synthesis and Properties of Poly(ferrocenylethylene)s and Their Charge-Transfer Polymer Salts with Tetracyanoethylene (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 573-584 
    Details
    ISSN: 0947-6539
    Keywords: ceramics ; ferrocenes ; iron ; magnetic properties ; polymers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The poly(ferrocenylethylene)s [Fe(η-C5H3RCH2)2]n 5a and 5b (a: R = H, b: R = Me) have been prepared by thermal ring-opening polymerization of the corresponding strained hydrocarbon-bridged [2]ferrocenophanes [Fe(η-C5-H3RCH2)2] (4a and 4b). An X-ray diffraction study of 4a indicated significant strain. Polymer 5a was crystalline and insoluble in common organic solvents and was characterized by solid-state 13C NMR. Polymer 5b, which was soluble in organic solvents, was characterized by 1H and 13C NMR, UV/visible spectroscopy and elemental analysis. Its molecular weight distribution was bimodal (gel permeation chromatography: Mw = 9.6 × 104, Mn = 8.6 × 104 for the high molecular weight fraction, Mw = 4.8 × 103, Mn = 3.5 × 103 for the oligomeric fraction), suggesting two polymerization mechanisms. The UV/visible spectrum implied a localized structure for the polymer backbone. Cyclic voltammetry revealed that 5b undergoes two reversible oxidations in CH2Cl2 solution at-0.25 and-0.16V. The redox coupling is indicative of only a small degree of interaction between the iron centres. Thermogravimetric analysis indicated that 5a and 5b are thermally stable to ca. 300-350°C under N2. At higher temperatures they yield ferromagnetic iron carbide ceramics 6a and 6b (ca. 50% and 32%, respectively, at 600°C) together with molecular depolymerization products. The reaction of 5b with tetracyanoethylene (TCNE) yielded insoluble and soluble oxidized products 11 and 12, which differed in the degree of oligomerization of the TCNEy-x counterions. These products were characterized by IR, elemental analysis, ESR spectroscopy, and magnetic susceptibility measurements. The last revealed the presence of significant antiferromagnetic interactions in 12.
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    Nature of the Interactions between Polar β-Substituents and Palladium in η3-Allylpalladium Complexes - A Combined Experimental and Theoretical Study (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 592-600 
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    ISSN: 0947-6539
    Keywords: allyl complexes ; chemoselectivity ; density functional calculations ; regioselectivity ; stereoelectronic control ; substituent effects ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Deuteromethanolysis of six β-methoxy-substituted η3-allylpalladium complexes (1-5) was studied under the same reaction conditions. The reaction rates depend on the ring size of the cyclic complexes, on the σ-donor/π-acceptor nature of the ancillary ligand, on the configuration of the allylic moiety, and on the position of the β-substituent with respect to the palladium atom. Replacement of the methoxy group proceeds about 1000 times faster in the trans-β-substituted cycloheptylallyl palladium complex 2 than in the cyclooctyl analogue 3; this indicates that the C-O bond strength is a function of the ring and substituent conformations. A theoretical analysis of the structure and stability of slightly simplified model compounds 6-10 was performed with density functional theory at the B 3 PW91 level in order to elucidate the relationship between the rate of deuteromethanolysis and the electronic interactions between the β-methoxy substituent and the palladium atom. It was concluded that the strength of these β-substituent effects critically depends on the relative position of the Pd-C3 and C4-O bonds. In cyclohexyl and cycloheptylallyl complexes 6 and 7, the trans-β-methoxy substituent adopts a conformation that is particularly favoured by the β-substituent effect. Since the η3-allylpalladium complexes studied and their derivatives are key intermediates of important palladium-catalysed transformations, the implications of the β-substituent effect for the regio- and chemoselectivity of nucleophilic attack are also discussed.
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    Chain Conformation and Metal…Metal Contacts in Dimers and Stacks of d8-ML4 Complexes: Electronic EffectsThe abbreviations used in this paper are given in the Appendix. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 655-664 
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    ISSN: 0947-6539
    Keywords: band theory ; electronic structure ; stacking interactions ; square-planar complexes ; one-dimensional complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A qualitative theoretical study is presented of the ligand effects on the strength of the M…M nonbonded interaction in dimers and stacks of 8-ML4 chains. It is found that π-acidic ligands enhance and π-basic ligands weaken the M…M bonding interaction. A large amount of structural data can be rationalized with the help of a simple orbital model. Other features of the crystal structures of such compounds are correctly accounted for by semiempirical molecular orbital and electronic band calculations (extended Hückel level). Electronic effects control the deviation from linearity of stacks of complexes with mixed ligands, whereas coupled electronic and steric effects determine the conformational preference. The predictive capability of theoretical band calculations is found to be good.
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    Nitridosilicates-A Significant Extension of Silicate Chemistry (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 679-683 
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    ISSN: 0947-6539
    Keywords: high-temperature synthesis ; materials science ; nitridosilicates ; nitrogen ; silicon ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Because of its low chemical reactivity Si3N4 has hardly been used as a starting material for the preparation of multinary silicon nitrides or nitridosilicates, but has gained increasing importance as a nonoxidic material for high-performance applications. A novel synthetic approach is presented, which starts from silicon diimide (Si(NH)2) and makes available a large variety of novel nitridosilicates. The structures of these nitridosilicates represent a significant extension from those found in the family of oxosilicates. Because of their outstanding chemical and physical stability, nitridosilicates might be of particular interest for the development of novel inorganic nitridic materials.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030505
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    An Analytical Study of the Redox Behavior of 1,10-Phenanthroline-5,6-dione, its Transition-Metal Complexes, and its N-Monomethylated Derivative with regard to their Efficiency as Mediators of NAD(P)+ Regeneration (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 79-88 
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    ISSN: 0947-6539
    Keywords: catalysis ; cobalt ; cofactors ; redox systems ; ruthenium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis as well as the chemical and electrochemical properties of homoleptic and heteroleptic (trispyridyl-methylamine as coligand) transition-metal complexes (Ru and Co) of 1,10-phenanthroline-5,6-dione (PD) and of its N-monomethylated derivative (PDMe+) are described. In particular, their ability to abstract hydride ions was studied. Electrochemical investigations with cyclic voltammetry, rotating disk electrode experiments, and spectroelectrochemical methods at different pH values gave an insight into the complex electrochemistry of the compounds described, which is strongly influenced by a hydration pre-equilibrium. The electrochemically active quinone form of the transition-metal complexes can be reduced to the hydroquinone state in acidic solution and to their transition-metal-stabilized semi-quinone states for neutral and basic solutions, whereas PDMe+ is reduced to the hydroquinone state in both acidic and neutral solutions. The compounds can also be reduced chemically, and are efficient catalysts for the indirect oxidation of the enzymatic cofactor NAD(P)H. For the indirect aerobic NAD(P)H oxidation, up to 900 turnovers per hour can be observed, an achievement yet to be reached by other catalyst systems.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030114
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    Organic Fluorine Hardly Ever Accepts Hydrogen Bonds (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 89-98 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; Cambridge structural database ; fluorine compounds ; hydrogen bonds ; protein data bank ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Statistical analysis of structural data and detailed inspection of individual crystal structures culled from the Cambridge Structural Database and the Brookhaven Protein Data Bank show that covalently bound fluorine (in contrast to anionic fluoride) hardly ever acts as a hydrogen-bond acceptor. The weakness of covalently bound fluorine as hydrogen-bond acceptor is backed by results of new molecular orbital calculations on model systems using ab initio intermolecular perturbation theory (IMPT), and is in accord with results of other physicochemical studies and with the physical properties of fluorinated organic compounds. Factors influencing the strength of hydrogen bonding in extended systems are discussed.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030115
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    Double Subroutine Self-Assembly; Spontaneous Generation of a Nanocyclic Dodecanuclear Cu1 Inorganic Architecture (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 99-104 
    Details
    ISSN: 0947-6539
    Keywords: bipyridines ; copper compounds ; selfassembly ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The newly synthesised ligand 2 combines binding components known to undergo specific and distinct self-assembly processes with Cu1 ions. It complexes Cu1 to form, in almost quantitative yield, a large inorganic architecture 1 made up from four ligand molecules and twelve metal ions. The structure of 1 was ascertained by X-ray radiocrystallography as well as by NMR spectroscopy and electrospray mass spectrometry. It consists of a macrocycle of nanometric dimension with an external diameter of 28 Å; the central cavity has a diameter of 11 Å, which contains four PF-6 anions as well as solvent molecules. The spontaneous formation of 1 results from a self-assembly process based on a “program” combining two assembly subroutines, each specific to one of the ligand subunits. Self-assembly through double or, more generally, multiple subroutines can be used to generate a wide variety of highly complex inorganic supramolecular architectures by combination of two or more assembly processes.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030116
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    Epoxidation and Oxygen Insertion Into Alkane Ch Bonds by Dioxirane Do Not Involve Detectable Radical Pathways (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 105-109 
    Details
    ISSN: 0947-6539
    Keywords: dioxiranes ; epoxidations ; insertions ; oxenoids ; radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dimethyldioxirane oxidation of α-methylstyrene, trans-cyclo-octene, and 1-vinyl-2,2-diphenylcyclopropane gave, under all reaction conditions employed, the corresponding epoxides in high yields. No radical products from allylic oxidation, from trans/cis isomerization, or from cyclopropylcarbinyl rearrangement (radical clock) were observed. Even for these alkenes, which are prone to radical reactions, the previously established electrophilic concerted mechanism applies, rather than the recently proposed radical mechanism. The selective hydroxylation of (-)-2-phenylbutane by dimethyldioxirane gave only (- )-2-phenylbutan-2-ol with complete retention of configuration and no loss of optical purity. Thus, a radical-chain oxidation is also discounted in the oxygen insertion into hydrocarbon C-H bonds for dioxiranes.
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    1′, 5′ -Anhydrohexitol Oligonucleotides: Synthesis, Base Pairing and Recognition by Regular Oligodeoxyribonucleotides and Oligoribonucleotides (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 110-120 
    Details
    ISSN: 0947-6539
    Keywords: antisense systems ; DNA recognition ; nucleic acids ; oligonucleotides ; RNA ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligonucleotides constructed of 1′, 5′-anhydrohexitol nucleoside building blocks (hexitol nucleic acids, HNA) are completely stable towards 3′-exonuclease and form very stable self-complementary duplexes as well as sequence-selective stable duplexes with the natural DNA and RNA. Triple-helix formation has also been observed. These hybridisation characteristics are highly dependent on the base sequence and the experimental conditions. When using a phosphate buffer containing 0.1M NaCl, a homopurine HNA dodecamer gives a δTm of +3.0 °C/ base pair with RNA as complement. These oligomers may therefore be of considerable interest as antisense constructs.
    Additional Material: 11 Ill.
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    The Unusual Properties of 5-Methyl-4,5,6,7-Tetrahydro-1H-Indazole in the Solid State (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 121-126 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; NMR spectroscopy ; proton transfer ; pyrazoles ; tautomerism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structure of the title compound was determined by X-ray analysis at 200 K. Three independent molecules form a trimer joined by strong and linear N-H … N hydrogen bonds. There is another centrosymmetrically related trimer in the unit cell. Both tautomers (1H and 2H) are present in each trimer. Disorder of the NH protons involved in the N-H … N hydrogen bonds has been observed. Solid-state 13C CPMAS NMR was used to establish the dynamic nature of the NH-proton disorder, the title compound being the first example of proton transfer in a tautomeric mixture of pyrazoles with an equilibrium constant other than 1.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030119
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    Enantioselective Construction of Highly Functionalized Indoloquinolizines - congeners to Polycyclic Indole Alkaloids (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 143-151 
    Details
    ISSN: 0947-6539
    Keywords: alkaloids ; asymmetric catalysis ; asymmetric synthesis ; indoles ; Mannich reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Indolo[2,3-a]quinolizines have been prepared in enantiomerically pure form by a very short and efficient synthetic sequence consisting of a) formation of imines of tryptophan esters, b) their enantioselective reaction with substituted silyloxydienes mediated by a chiral or an achiral boron Lewis acid, and c) subsequent ring closure initiated by conversion of the generated vinylogous amides into vinylogous imidoyl chlorides. With this strategy various substituents can be incorporated directly into the 1-position of the heterocyclic framework of complex indole alkaloids by the choice of an appropriate silyloxydiene, so that subsequent derivatization of the alkaloid precursor at this position is rendered unnecessary.
    Additional Material: 6 Ill.
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    Rhodium(I)-Assisted Stereoselective Coupling of an Alkyl, Aryl or Vinyl Group with a Vinylidene Ligand: A Novel Synthetic Route to π-Allyl and π-Butadienyl Rhodium ComplexesDedicated to Professor Wolfgang beck on the occasion of his 65th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 127-137 
    Details
    ISSN: 0947-6539
    Keywords: allyl complexes ; butadienes ; C-C coupling ; rhodium ; vinylidene complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the first part of this work, a general method for the preparation of aryl, methyl, vinyl and alkynyl(vinyl-idene)rhodium(I) complexes trans-[Rh(R')-(=C=CHR)(PiPr3)2] (8-14, 18-22) and trans-[Rh(R')(=C=CMe2)(PiPr3)2] (16, 17) from the corresponding chloro(vinyl-idene) derivatives and Grignard reagents is described. Whilst compounds 8 and 10-13 react with pyridine to give trans-[Rh(C≡CR)(py)(PiPr3)2] (23-25) by elimination of R'H, treatment of 8-11, 16, and 18 with carbon monoxide yields the square-planar η1-vinyl and η1-butadienylrhodiumcarbonyl complexes trans-[Rh{η1-(Z)-C(R')=CHR}(CO)(PiPr3)2] (27-32). The reaction of 8 or 18 with methyl or tert-butylisocyanide leads stereoselectively to the isocyaniderhodium(I) compounds trans- [Rh{η1-(Z)-C(R)=CHPh}(CNR') (PiPr3)2] (33-35). Acid-induced cleavage of the rhodium-carbon σ bond of 27, 30, or 31 with CH3CO2H gives trans-[Rh(η1-O2CCH3)-(CO)(PiPr3)2] (38) and the corresponding olefin or diene, respectively. In the absence of a Lewis base such as pyridine, CO, or CNR', compounds 18-20 rearrange in benzene at 40-50 °C to afford the isomeric π-allyl complexes [Rh(η3-1-RC3H4)(PiPr3)2] (40-42) almost quantitatively. The vinyl(vinylidene) compounds 11 and 12 also undergo an intramolecular rearrangement that leads to the η3-2,3,4-butadienyl- or to the alkynyl(ethene)rhodium(I) isomers, depending on the reaction conditions. In an analogous manner to the η1-vinyl-and η1-butadienyl(carbonyl) derivatives 27, 30, and 31, the π-allyl and π-butadienyl complexes also react with acetic acid to give [Rh(η1-O2CCH3)(PiPr3)2] (47) and the respective olefin.
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    The Importance of Magnetic Coupling Through Atoms with Large Spin Densities - Structure and Magnetic Properties of meso-Tetrakis-(4′-tert-Butylphenyl)Porphinatomanganese(III) Hexacyanobutadienide, [MnIIITtBuPP]+[C4(CN)6].- (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 138-142 
    Details
    ISSN: 0947-6539
    Keywords: electron transfer ; magnetic properties ; metalloporphryins ; polymers ; spin density ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: [MnIIITtBuPP]+[C4(CN)6].- · 5PhMe [MnIIITtBuPP = meso-tetrakis-(4′-tert-butylphenyl)porphinatomanganese(III)] has been prepared and structurally and magnetically characterized. The uniform, linear-chain (1-D) coordination polymer comprises alternating cations and anions. The bond lengths in planar ion [C4(CN)6].-]'- are 1.377(10) (CC-CC), 1.418(7) (C-CCC), 1.414 (C-CN), 1.457 (C-CNMn), 1.150 (C≡N), and 1.134 Å (C = NMn). The Mn-N-C angle is 172.3(4)°, and the intrachain Mn  -  Mn separation is 10.685 Å. Each [C4(CN)6].-]' unit is bonded to two MnIII atoms through the interior nitrogen atoms in a trans-μ2-N-σ manner with N-Mn bond lengths of 2.353 Å. The ṽCN absorptions are at 2217 (w, br) and 2190 (m) cm-1. Above 50 K the magnetic susceptibility of [MnIIITtBuPP]+[C4(CN)6].- can be fitted to the Curie-Weiss expression, χ∝1(T - θ), with an effective θ of -13 K. This is consistent with weak antiferromagnetic coupling, which is in contrast to the effective θ of +67 K for the uniform chain [MnIIIOEP]+[C4(CN)6].- [OEP = octaethylporphinato]. Here, the [C4(CN)6].-'- units are bonded to the MnIII centers through endo CN nitrogen atoms in a similar trans-μ2 manner. Density functional theory MO calculations reveal that the spin density of the CN nitrogen atom bound to [MnIIITtBuPP]+ (0.019 μBÅ-3) is significantly lower than that of the N atom bound to [MnIIIOEP]+ (0.102 μBÅ-3). This is consistent with the reduced spin coupling observed for [MnIIITtBuPP]+[C4(CN)6].-with respect to [MnIIITtBuPP]+[C4(CN)6].-, as evidenced by the lower θ value. The different orientations of the [C4(CN)6].- units - almost perpendicular (84.72°) for [MnIIITtBuPP]+[C4(CN)6].- and substantially tilted (32.1°) for [MnIIIOEP]+ [C4(CN)6].- may also contribute to the poorer overlap and weaker spin coupling. Hence, binding between sites with large spin densities is needed to stabilize strong ferromagnetic coupling.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030121
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    1-Azafagomine: A Hydroxyhexahydropyridazine That Potently Inhibits Enzymatic Glycoside Cleavage (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 940-947 
    Details
    ISSN: 0947-6539
    Keywords: chemoselectivity ; enzyme inhibitors ; glycosidases ; kinetics ; pyridazines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (3,4-trans-4,5-trans)-4,5-dihydroxy-3-hydroxymethylhexahydropyrida-zine (16) was synthesized in four steps from 2,4-pentadienol (22) and 4-phenyl-triazolin-3,5-dione (18) in an overall yield of 32%. In the first step a Diels-Alder reaction between 18 and 22 gave (π)-2-hydroxymethyl-8-phenyl-1,6,8-triazabicyclo[4.3.0]non-3-ene-7,9-dione (23c) in 88% yield. Epoxidation of 23c with trifluoromethyl(methyl)dioxirane, generated in situ, gave the trans epoxide 24c in 62% yield. Hydrolysis of the epoxide with perchloric acid gave stereoselectively (2,3-trans-3,4-trans)-3,4-dihydroxy-2-hydroxy-methyl-8-phenyl-1,6,8-triazabicyclo[4.3.0]-nonane-7,9-dione (26) in 73% yield. In the fourth and final step, hydrazinolysis of 26 gave 16 in 84% yield. Pyridazine 16 was found to be a potent inhibitor of α-and β-glucosidase, isomaltase and glyco-gen phosphorylase, while galactosidases and α-mannosidase were not inhibited. The inhibition of β-glucosidase is independent of pH, and was found to be due to unprotonated 16.
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    Molybdenum-Catalyzed Olefin Epoxidation: Ligand EffectsMetal-catalyzed oxidations, part 6. part 5: ref. [1]. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 696-705 
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    ISSN: 0947-6539
    Keywords: catalysis ; epoxidations ; molybdenum ; olefins ; peroxo complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We synthesized substituted pyrazolylpyridine ligands to examine their donor properties by spectroscopic (IR, NMR) and computational (AM 1) methods. The influence of the substitution patterns on spectroscopic and thermodynamic features of molybdenum oxobisperoxo complexes [(L-L)MoO(O2)2] (L-L=2-(1-alkyl-3-pyrazolyl)pyridine/pyrazine) correlates with the activities of the complexes in catalytic olefin epoxidation reactions. This further proof for the relation between the Lewis acidity and the catalytic activity of epoxidation catalysts supports a reaction mechanism in which the peroxo complex activates the oxidizing agent (H2O2, ROOH) instead of directly transferring an oxygen atom from a π2-peroxo ligand to the olefin.
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    A Theoretical Study of the Indigoid Dyes and Their Chromophore (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 717-725 
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    ISSN: 0947-6539
    Keywords: ab initio calculations ; chromophores ; dyes ; electronic structure ; indigo ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A theoretical study of the electronic spectra of indigo, bispyrroleindigo, and the H-chromophore, thought to be the basic structure leading to the intense absorption in the visible spectrum of the indigoid dyes, has been performed by means of the complete active space (CAS) SCF method and the multiconfigurational second-order perturbation theory (CASPT2). The calculations include excitation energies, oscillator strengths, and transition moment directions for the valence singlet states of the molecules. The assumption that the H-chromophore is responsible for the absorption in the visible spectrum is quantitatively revisited with the help of the modern ab initio methodologies and the main features of these molecules are theoretically analyzed, some of them for the first time.
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    URL: http://dx.doi.org/10.1002/chem.19970030511
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    Catalytic asymmetric synthesis of new halogenated chiral synthons (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 517-522 
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    ISSN: 0947-6539
    Keywords: asymmetric catalysis ; chiral synthons ; cyclic sulfates ; dihydroxylations ; organofluorine compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two-step and practical asymmetric syntheses of enantiomerically pure 4-trifluoromethyl-2,2-dioxo-1,3,2-dioxathiolane and 4-trichloromethyl-2,2-dioxo-1,3,2-dioxathiolane (〉98% ee) have been achieved. Catalytic asymmetric dihydroxylation (AD) of 3,3,3-trifluoropropene and 3,3,3-trichloropropene, respectively, is followed by direct cyclic sulfate formation by reaction with sulfuryl chloride. Opening of the cyclic sulfates with various nucleophiles provides easy access to important chiral synthons.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030406
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    New Aspects of the Photophysical Properties of 1′H-2′,3′-Dimethoxycarbonylspiro[fluoren-9,1′-pyrrolo[2,1-a]quinoline]Dedicated to Prof. M. Hanack on the occasion of his 65th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 509-516 
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    ISSN: 0947-6539
    Keywords: betaines ; fluorescence spectroscopy ; mechanistic approach ; photochromism ; spiro compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper we describe photophysical and mechanistic studies of spiro[fluorene-9,1′-pyrrolo[2,1-a]quinolines] (DHI) 1-4 and of their styryl homologues 5-11 (styryl DHI). DHIs 1-11 have low fluorescence quantum yields (approximately φf = 10-3). The position of the fluorescence emission band is temperature-dependent. The ring-opened betaines 1′-11′ do not fluoresce at all. DHIs 1-11 exhibit phosphorescence, which is hypsochromically shifted by NO2 substitution of the fluorene ring. The triplet can be populated only by sensitization (with benzophenone as sensitizer) and, therefore, the reaction that occurs under direct excitation of the molecule involves the singlet excited state only. On irradiation under time-resolved conditions, two transients were seen to be formed in the picosecond and microsecond domains. Slightly tilted educt or product-like geometries are attributed to these transients. An energy diagram is proposed for the photoreaction of the model compound spiro[pyrroloquinoline] 1 → 1′, which takes into account the singlet pathway and the different transients of ca. 100 ps lifetimes formed from the singlet excited state of 1-11.
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    Peptide Self-Replication Via Template-Directed Ligation (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1017-1024 
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    ISSN: 0947-6539
    Keywords: autocatalysis ; coiled coil ; kinetics ; peptides ; self-replication ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 32-residue α-helical peptide with a sequence similiar to that of the GCN4 leucine zipper region is shown to catalyze its own formation by accelerating the amide bond formation of a 17-residue peptide, preactivated as a thiobenzyl ester, and a 15-residue peptide with a N-terminal cysteine. The self-replication process displays parabolic growth characteristics as revealed by a detailed kinetic analysis. Control reactions with single-mutant peptides strongly support a mechanism in which a ternary and/or quaternary complex of the product with both peptide fragments act(s) as the catalytically active intermediate(s). Furthermore, these experiments reveal a remarkable sequence selectivity, as evidenced by the loss of autocatalytic activity as a result of a single replacement of leucine or valine residues with an alanine at the recognition interface.
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    Masthead (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997) 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/chem.19970031102
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    Proof of Inversion of Configuration on Stannylation of a Configurationally Stable, Tertiary Benzyllithium Compound from a Single-Crystal X-Ray Structure AnalysisChiral Carbanions, part 2. Presented at the Loschmidt Symposium, Vienna, June 1995; for part 1, see ref. [18]. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1728-1732 
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    ISSN: 0947-6539
    Keywords: benzoic acids ; carbanions ; lithiation ; stannylation ; stereochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Configurationally stable, dipole-stabilised benzyllithium compounds (R)-and (S)-1 b, prepared by deprotonation of the corresponding esters with sBuLi in toluene/di-ethyl ether (5:1), reacted with (-)-menthyldimethyltin bromide to afford the stannanes (-)-3 and ( + )-4, respectively. A single-crystal X-ray structure analysis of compound (-)-3 proved that stannylation occurred with inversion of the configuration at the benzylic centre, assuming retention for the deprotonation step. Lithio-destannylation and deuteration with MeOD follow a retentive course.
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    Reactive Pseudorotaxanes: Inclusion Complexation of Reduced Viologens by the Hosts β-Cyclodextrin and Heptakis(2,6-di-o-Methyl)-β-Cyclodextrin (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1052-1058 
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    ISSN: 0947-6539
    Keywords: cyclic voltammetry ; cyclodextrins ; host-guest chemistry ; pseudorotax-anes ; viologens ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complexation of three guests containing 4,4′-bipyridinium redoxactive residues by β-cyclodextrin (β-CD) and its heptakis-(2,6-O-dimethyl) analogue (DM-β-CD) was investigated by means of voltammetric techniques. The three 4,4′-bipyridinium (viologen) derivatives used as guests were designed to be water-soluble in all three accessible oxidation states. The N-substituents chosen to enhance aqueous solubility were: 2-(2-(2-ethoxy)ethoxy)ethanol (guest 12+), 6-hexanoate (guest 2), and 3-propanesulfonate (guest 3). Detailed analysis of the voltammetric results by digital simulation techniques revealed that the oxidized forms of the guests did not interact appreciably with either CD host; the two-electron reduced guests formed extremely stable inclusion complexes, with association constants in the range 103-104M-1, while the cation radical forms exhibited intermediate binding affinities (≍102M-1). In all cases, DM-β-CD was found to form more stable complexes than unmodified β-CD.
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    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1739-1743 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031103
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    The Quest For Extra-Large Pore, Crystalline Molecular Sieves (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1745-1750 
    Details
    ISSN: 0947-6539
    Keywords: large-pore materials ; materials science ; molecular sieves ; zeolites ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The definition and reasons for desiring extralarge pore, crystalline molecular sieves are ennumerated. A historical perspective on the development of these materials is presented with emphasis on critical features important for practical application. Based on the known physiochemical properties of extra-large pore materials, several avenues for their synthesis are suggested. Finally, a few issues of concern when dealing with this class of materials are presented.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031104
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  • 192
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    Carbohydrate-Modified Metal Carbenes: Synthesis And First Applications (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1751-1756 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; carbene complexes ; carbohydrates ; glycosylidenes ; spiro compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Carbohydrate-modified carbene complexes can be synthesised by combining a sugar electrophile with a metalate nucleophile or vice versa. Acyclic sugar skeletons adopt conformations that are controlled by the minimisation of 1,3-interactions and are not significantly affected by the incorporation of a metal fragment. Thus, the latter can be used exclusively for reactivity tuning. Sugar carbene complexes undergo regio- and stereoselective ligand- and metal-centred reactions such as C2-homologisation and benzannulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031105
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    Nondipolar Structures With Threefold Symmetry For Nonlinear Optics (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1765-1773 
    Details
    ISSN: 0947-6539
    Keywords: cyanines ; hyper-Rayleigh scattering ; nonlinear optics ; second-order polarizability ; triazines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The second-order polarizabilities β( -  2ω; ω, ω) of six nondipolar NLO chromophores - 1,3,5-tris(isopropyl-amino)-2,4,6-trinitrobenzene (1), 1,3,5-tris(isopropylamino)-2,4,6-tris(trifluoro-methylsulfonyl)benzene (2), tris(dimethyl-imoniomethyl)methide diperchlorate (3), 2,4,6-tris[4-(N,N-diethylamino)phenyl-ethynyl]-1,3,5-triazine (4), 2,4,6-tris[4-(N,N-diethylamino)phenyl]-1,3,5-triazine (5), and tris(2-dimethylaminoethenyl)-methinium perchlorate (6) - were studied by polarized hyper-Rayleigh scattering (HRS). The NLO activity of these two-dimensional NLO chromophores is attributed to three-level contributions to the β values, for which different optimization strategies are investigated. Exceptionally large second-order polarizabilities were observed for the conjugationally elongated structures: the β value of triazine 4 is 15 times larger than that of the one-dimensional two-level chromophore 4-nitroaniline. Results obtained with the polarized HRS experiment are discussed with respect to local field corrections, calibration, and systematic sources of error.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031107
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  • 194
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    The Stable (Phosphino)(Silyl)Carbene As A Useful Building Block: Synthesis And Reactivity of 2-Phosphorus-Substituted 2H-Azirines (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1757-1764 
    Details
    ISSN: 0947-6539
    Keywords: nitrogen heterocycles ; phosphinocarbenes ; ring expansions ; ylides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: [Bis(dicyclohexylamino)phosphino]trimethylsilylcarbene (1) reacts with benzonitrile leading to the corresponding 2-phosphino-2H-azirine 3 in 85% yield. Treatment of 3 with trifluoro-methanesulfonic acid, methyl trifluoro-methanesulfonate, or elemental sulfur leads to the P-hydrogeno-2-phosphonio-,P-methyl-2-phosphonio-, or 2-thioxo-phosphoranyl-2H-azirine (4, 5, and 7) in 77, 87, and 91% yields, respectively. Irradiation of 3 gives rise to the 1,2λ5-azaphosphete 8 (98% yield). Treatment of 3 with BF3·OEt2, BH3·SMe2, Lawesson's reagent, or methyl isothiocyanate gives heterocycles 9 (90% yield), 10 (76% yield), 12 (83% yield), or 13 (80% yield), while under the same experimental conditions, heterocycle 8 reacts with the same reagents to give 9 (82% yield), 11 (83% yield), 12 (86% yield), and 15 (56% yield), respectively. Thermolysis of the P-hydrogeno-2-phosphonio-2H-azirine 4 at 55°C leads to the cationic, four-membered heterocycle 17 (96% yield), while photolysis of the P-methylazirine analogue 5 in the presence of dimethyl acetylenedicarboxylate affords pyrrole 19 (64% yield). Irradiation of the thiox-ophosphoranyl azirine 7 gives the 1,3,5λ5-thiazaphosphole 20 in 79% yield. The influence of the coordination state of phosphorus on the reactivity of 2-phosphorus-substituted 2H-azirines is studied.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031106
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    Synthesis and Properties of O-Glycosyl Calix[4]Arenes (Calixsugars)This trivial name refers to calixarene derivatives in which one or more mono- or oligosaccharide moieties are bound to either the upper or lower rim by O-or C-glycosidic bond. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1774-1782 
    Details
    ISSN: 0947-6539
    Keywords: calixarenes ; carbohydrates ; glycosylations ; host-guest chemistry ; Mitsunobu reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Model O-glycosylation reactions at either rim of calix[4]arenes are described with the aim of providing access to a new family of carbohydrate-containing calixarene derivatives named calixsugars. One or two sugar moieties (D-mannofuranose and D-glucopyranose) were introduced at the lower rim of the parent calix[4]arene by glycosylation of the phenolic hydroxyl groups by means of a Mitsunobu reaction. Tetrapropoxy calix[4]arenes bearing two or four hydroxymethyl groups at the upper rim were coupled with perbenzoylated thioethyl D-galactoside and D-lactoside in the presence of the thiophilic promoter copper(III) triflate. In this way β-linked bis- and tetrakis-O-galactosyl calix[4]arenes were obtained in good yield, the latter showing some solubility in water. For the O-lactosyl derivatives only the bis-substituted compound could be obtained because of the competing formation of an intramolecular ether linkage between 1,3-hydroxymethyl groups. Preliminary binding studies showed some affinity of the galactose-containing calixsugars toward charged carbohydrates and dihydrogen phosphate anion.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031108
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    Synthesis, Structure and Hydrolysis Studies of Dimethyltris(Trimethylsilyl)Methylmetallanes of Aluminium and GalliumDedicated to Professor Manfred Weidenbruch on the occasion of his 60th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1783-1792 
    Details
    ISSN: 0947-6539
    Keywords: aluminium ; gallium ; hydrolyses ; Si ligands ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of [(Me3Si)3CLi2thf] with Me2MCl (M = Al, Ga) afford the mixed trialkylmetallanes [(Me3Si)3-CAlMe2·thf] (1) and [(Me3Si)3 CGaMe2·thf] (2) in high yields. The coordinated THF molecule of compound 2 can be removed by sublimation in vacuo to yield the solvent-free product [(Me3Si)3-CGaMe2] (3). Hydrolysis of compound 2 with one equivalent of water at 0°C gives the trimeric hydroxide [{(Me3Si)3CGaMe(μ-OH)}3] (4), while the reaction with two equivalents of water at room temperature yields the unusually stable gallium hydroxide-water complex [{(Me3Si)3CGaMe(OH)(μ-OH)MeGaC(SiMe3)3) H2O·2thf] (6). On heating, compound 6 is converted to the hydroxide [{(Me3Si)3C}4Ga4(μ-O)2(μ -OH)4] (5), which has a heteroadamantane-like core. The hydrolysis of compound 1 with one equivalent of water at - 25°C gives the dimeric hydroxide [{(Me3Si)3CAlMe(μ-OH)}2·2thf] (7), while the reaction with two equivalents of water results in the formation of the novel hydroxide [{(Me3Si)3C}4Al4(μ-O)2 (μ-OH)4] (8), which is isostructural to the gallium compound 6 with the adamantane-like structure. The molecular structures of compounds 1, 2, 4, 5·3THF, 6, 7 and 8·0.5 THF have been determined by X-ray structure analysis. Compound 7 is the first structurally characterised aluminium hydroxide containing methyl groups, and 8 is the smallest structurally characterised galloxane hydroxide described in literature.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031109
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    A Contribution to the Asymmetric Synthesis of 3-Amino β-Lactams: The Diastereoselective [2+2] Cycloaddition Reaction of Chiral Aminoketene Equivalents with Enolizable Aldehyde-Derived Imines (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1432-1441 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; cycloadditions ; imines ; lactams ; silicon ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: N-[Bis(trimethylsilyl)methyl]imines 9 show unique chemical properties when compared with conventional imines. Their reaction with optically pure aminoketenes derived from dehydrochlorination of 14 and 15 affords the corresponding 3-amino-4-alkyl-β-lactams 16 and 17 in good yields and high diastereoselectivities. The mild deprotection of bis(trimethylsilyl)methyl- and phenyloxazolidinone moieties with, respectively, cerium(IV) ammonium nitrate and lithium/ammonia or hydrogen/Pd(OH)2 allows the preparation of a variety of β-lactam antibiotic building blocks.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030909
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    The First Peralkylated Phosphino(stibino)methanes and Their Organometallic Rhodium ComplexesDedicated to the memory of Professor Geoffrey Wilkinson (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1442-1450 
    Details
    ISSN: 0947-6539
    Keywords: antimony ; arene complexes ; diene complexes ; rhodium ; ylides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first representatives of phosphino(stibino)methanes R2PCH2Sb-R′2 (3-5) with bulky alkyl or cycloalkyl groups R and R′ were prepared in two steps from Ph3SnCH2I via the isolated stannylated phosphanes Ph3SnCH2PR2 (1, 2) as intermediates. X-ray structural analysis of 5 (R = C6H11, R′ = tBu) reveals that the lone pairs and the substituents R and R′ at phosphorus and antimony and the hydrogen atoms of the CH2 bridge adopt staggered conformations. Treatment of [{C8H12RhCl}2] with 3-5 affords the neutral compounds [Rh-Cl(η4-C8H12)(κ-P-R2PCH2SbR′2)] (6-8), of which 7 and 8 react with CH3MgI to give the corresponding methylrhodium derivatives [RhCH3(η4-C8H12)(κ-P-R2-PCH2SbR′2)] (9, 10). Cationic complexes [Rh(η4-C8H12)(η2-P,Sb-R2PCH2SbR′2)]-X (X = PF6: 11a, 12a, 13; X = BPh4: 11b, 12b) containing the phosphino(stibino)methanes as chelating ligands were obtained either from [{C8H12RhCl}2], R2PCH2SbR′2 and MX, or (for 13) from 8 and AgPF6. Compound 12a (R = iPr, R′ = tBu) was characterized by X-ray crystallography. The PF6 salts 11a, 12a, and 13 react with CH2N2 by insertion of CH2 into the Rh-Sb bond to yield the complexes 14-16, the first examples of transition-metal compounds with Sb ylides as ligands. Treatment of BPh4 salts 11b and 12b with H2 gives the half-sand-wich-type complexes [(η6-C6H5BPh3)Rh(κ2-P,Sb-R2PCH2SbR′2)] (17, 18), in which the tetraphenylborate is coordinated like a substituted arene to the metal center.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030910
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    Tris(trimethylsilyl)silanides of the Heavier Alkali Metals - A Structural StudyDedicated to Professor Paul von Ragué Schleyer on the occasion of his 68th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1418-1431 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; alkali metals ; Si ligands ; silicon ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Transmetalation reactions between bis(hypersilyl)zinc Zn[Si(SiMe3)3]2 and alkali metals have already been established as a facile route to powders of the solvent-free potassium, rubidium, and cesium derivatives of tris(trimethylsilyl)-silane (hypersilane, (Me3Si)3SiH).[1,2] By the use of boiling n-heptane as the solvent, the hitherto unknown NaSi(SiMe3)3 (1) along with the previously synthesized KSi(SiMe3)3 (2) have now been obtained as colorless crystalline materials. Information from NMR and Raman spectra in conjunction with the acute Si-Si-Si angles found in their molecular structures indicate mainly ionic bonding. This was verified by population analyses of suitable model systems. Both hypersilanides[2] consist of cyclic dimers [MSi(SiMe3)3]2 (1a, M = Na; 2a, M = K) with almost planar M2Si2 rings (Na-Si = 299 pm (av); K-Si = 339 pm (av)), which are linked up into coordination polymers. In a similar manner to the related rubidium and cesium derivatives, a pentane suspension of the potassium compound afforded a yellow solution on addition of benzene, from which the crystalline, bright yellow tris(benzene) solvate 2·(benzene)3 (2b) was isolated. Complex 2b consists of monomers containing short K-Si bonds (332-334 pm) and three η6-bonded benzene molecules. No solvate of 1 was obtained under these conditions. However, on crystallization from pure benzene, crystals of (1)2·benzene (1b) were isolated (Na-Si = 302 pm (av)). Benzene was found to be intercalated between rods of coordination polymers of (1)2. In addition, the influence of π or s̰ donors on the molecular and crystal structures of hypersilylrubidium (3) and cesium (4) was investigated. The structures of the tetrahydrofuran solvate (4)2·THF (4c), the biphenylene/pentane complex (4)2·biphen·(pentane)0.5 (4b) along with the known toluene solvates (3)27dot;toluene (3a) and (4)2·(toluene)3 (4a)[1a] are compared. Finally, an example is presented of how the alkali metal hypersilanides could be utilized in preparing extremely bulky stannanide anions.
    Additional Material: 20 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030908
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    Biomimetic Crystallization of Calcium Carbonate Polymorphs by Means of Collagenous Matrices (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1807-1814 
    Details
    ISSN: 0947-6539
    Keywords: aragonite ; biomineralization ; calcite ; crystal growth ; peptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Crosslinked gelatin films with entrapped poly-L-aspartate were used to induce the crystallization of calcite and aragonite polymorphs. Calcite with high degrees of isomorphous substitution (up to 12mol% magnesium) has been obtained. The morphology and layered organization of magnesium calcite crystals grown inside the crosslinked gelatin films with entrapped poly-L-aspartate resemble some structural features of radial calcitic ooids. The concentration of entrapped poly-L-aspartate and the uniaxial deformation of the films control the growth of aragonite aggregates inside the films. Such aggregates grown inside uniaxially deformed films consist of parallel rods with an architectural assembly similar to that found in some marine organisms. The crystals' aggregation and the control of calcium carbonate polymorphism are related to the modelling of the nucleation sites by poly-L-aspartate structure and concentration, local supersaturation and microenvironment shape. The results indicate that the collagenous matrices with entrapped polyelectrolytes are versatile systems which can contribute to the elucidation of strategies for biomimetic materials chemistry.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031113
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