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  • General Chemistry  (2,015)
  • Cell & Developmental Biology  (441)
  • 1995-1999  (2,456)
  • 1945-1949
  • 1930-1934
  • 1997  (2,456)
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  • 1995-1999  (2,456)
  • 1945-1949
  • 1930-1934
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  • 101
    Electronic Resource
    Electronic Resource
    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 6-10 
    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.19970030103
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  • 102
    Electronic Resource
    Electronic Resource
    Localization Patterns in Interstitial Space: A Special Property of the Electron Localization Function (ELF) (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 985-991 
    Details
    ISSN: 0947-6539
    Keywords: electron localization ; hydrides ; structure elucidation ; Zintl anions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron localization function (ELF) is a new descriptor of chemical bonding in real space which has turned out to be enormously powerful. We show here that the ELF also has predictive power. By analysis of the interstitial space, undetected atoms can be recognized by localization patterns in such regions. The importance of this approach is that the correct number of valence electrons is unknown before the calculations. The method was tested for the compounds Ca4Sb2O, Ba3Ge4X, Ba10Ge7O3, and for the so-called second SrSi modification, which, according to our analysis, very probably has the composition Sr10Si10X. In these cases the positions of the heteroatoms O and X were located and, in the case of X, proposals for a preferred atom type or group are made. Even in cases where the missing atoms are the majority component the approach was quite successful. As an example, the structure of CaH2 was generated from the known Ca positions by a two-step iterative procedure.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030621
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  • 103
    Electronic Resource
    Electronic Resource
    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 999-1003 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 20 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030703
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  • 104
    Electronic Resource
    Electronic Resource
    Synthesis, Structure, and Reactions of the First Rotational Isomers of Stable Thiobenzaldehydes, 2,4,6-Tris[bis(trimethylsilyl)methyl]thiobenzaldehydes (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 62-69 
    Details
    ISSN: 0947-6539
    Keywords: competition experiments ; conformation determination ; isomerizations ; rotamers ; thioaldehydes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first rotational isomers of thiobenzaldehydes, TbtCH—S (2a and 2b; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), were synthesized and isolated as stable crystalline compounds by the desulfurization of the corresponding overcrowded cyclic polysulfides TbtCHSn (n = 5 or 8) with phosphine reagents. The molecular structures of 2a and 2b in the solid state, determined by X-ray crystallographic analysis, differed in their conformations, which were essentially identical with those in solution as revealed by 1H{1H} nuclear Overhauser effect (NOE) experiments. The isomeric thiobenzaldehydes 2a and 2b were found to undergo thermal interconversion. A kinetic study of this process gave reasonable kinetic and thermodynamic parameters for conformational isomerizations of this type. Interesting differences in reactivity among the two isomers 2a,b and 2,4,6-tri-tert-butylthiobenzaldehyde (1) were shown in the reactions with hydrazine and m-chloroperoxybenzoic acid.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030111
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  • 105
    Electronic Resource
    Electronic Resource
    Cover page (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), 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.19970030201
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  • 106
    Electronic Resource
    Electronic Resource
    The Oxide Anion Accelerated Retro-Diels-Alder Reaction (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 187-192 
    Details
    ISSN: 0947-6539
    Keywords: Diels-Alder reactions ; pericyclic reactions ; retro reactions ; oxide anion ; synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The widespread application of the retro-Diels-Alder reaction in synthesis has been hampered by the high temperatures usually required to effect cycloreversion. The discovery of the anionic oxy-Cope reaction was followed by predictions that the accelerating effect of the oxide anion should also be observed with other pericyclic reactions. Recently, such predictions have been confirmed for the retro-Diels-Alder reaction, which often proceeds rapidly at room temperature by oxide anion rate acceleration. Such mild retro-Diels-Alder reactions have now been employed in the synthesis of a range of molecular targets, including temperature-sensitive enediynes.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030204
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  • 107
    Electronic Resource
    Electronic Resource
    Theoretical Investigation of the Mechanism of the Baeyer-Villiger Reaction in Nonpolar Solvents (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 210-218 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; Baeyer-Villiger reactions ; migrations ; peroxyacids ; substituent effects ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Baeyer-Villiger reaction of p-anisaldehyde with peroxyacetic acid in nonpolar solvents to give p-anisylformate was examined on the basis of ab initio molecular orbital calculations. To explain the experimental observations, the free-energy change was evaluated for each case in the absence and in the presence of an acid catalyst. It was found that, without catalysts, the rate-determining step corresponds to the carbonyl addition of peroxyacetic acid to p-anisaldehyde and the reaction hardly occurs. Acetic acid was found to catalyze the carbonyl addition and change the rate-determining step from the carbonyl addition to the migration of the carbonyl-adduct intermediate. Trifluoroacetic acid was observed to catalyze both the carbonyl addition and migration, and the carbonyl addition was demonstrated to be a rate-determining step. The results provided a convincing explanation of the complex kinetics seen experimentally. Further calculations were performed for the reaction of benzaldehyde with peroxyacetic acid to give phenylformate. Migratory aptitude was found to depend on the catalyst. Isotope effects were also investigated, and the exceptional isotope effect observed experimentally was shown to be due to the rate-determining carbonyl addition caused by autocatalysis. It is concluded that the mechanism of the reaction varies with catalysis or substituent effects.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030208
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  • 108
    Electronic Resource
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    The Vinylketene-Acylallene Rearrangement: Theory and Experiment (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 237-248 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; allenes ; ketenes ; matrix isolation ; rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alkoxyvinylketenes4are generated by flash vacuum thermolysis (FVT) or photolysis of 3-alkoxycyclobutenones3. The thermal interconversion of4and allene carboxylic acid esters5under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxy-vinylketene4bundergoes thermal elimination of ethene with formation ofs-cis-ands-trans-acetylketene(8). An analogous aminovinylketene-to-allenecarbox-amide conversion is observed on FVT of 3-dimethylaminocyclobutenone3c. A facile 1, 3-chlorine migration in 2, 3-buta-dienoyl chloride(5d)is also reported. Consistent with the experimental observations, 1, 3-methoxy, 1, 3-chloro, and 1, 3-dimethylamino migrations in vinylketene are calculated (G2(MP2, SVP) level) to have moderate barriers of 169, 157, and 129 kJ mol-1, respectively, significantly less than the corresponding 1, 3-H shift barrier (273 kJ mol-1). The stabilization of the four-center transition structures is rationalized in terms of the donor-acceptor interaction between the lone pair electrons of the migrating donor substituent and the vacant central carbon p orbital of the ketene LUMO. The predicted migratory aptitude in the series of substituted vinylketenes, R-C(=CH2)-CH=C=O, is in the order N(CH3)2〉SCH3〉SH〉Cl〉NH2〉OCH3〉OH〉F〉H〉CH3, and correlates well with the electron-donating ability of the R group.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030212
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  • 109
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    Structural Effects in the Reductive Activation of (Indenyl)RhL2 Complexes: The Reduction of [Rh(η5-C9H7)(η4-cod)] (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 279-285 
    Details
    ISSN: 0947-6539
    Keywords: electron transfer ; indene ; kinetics ; rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of the indenyl complex [Rh(η5-C9H7)(η4-cod)] has been investigated in the context of structural effects induced by the transfer of one electron. The reduction of this complex occurs in two steps, leading first to the radical anion and then to the highly frangible dianion. Both species eliminate the indenyl anion. In the presence of free cyclooctadiene, the related cleavage leading to the indenyl anion and bis-cyclooctadiene rhodium fragments now follows a Michaelis-Menten-type mechanism involving precoordination of one extra COD ligand to the initial radical anion. These results suggest the modification of the hapticity of the indenyl ligand in connection with 17- and 19-electron metalcentered intermediates.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030217
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  • 110
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    Effect of Melamine-Amphiphile Structure on the Extent of Two-Dimensional Hydrogen-Bonded Networks Incorporating Barbituric Acid (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1077-1082 
    Details
    ISSN: 0947-6539
    Keywords: amphiphiles ; barbituric acid ; hydrogen bonds ; melamines ; molecular recognition ; monolayers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four alkyl melamine amphiphiles each containing identical triads of hydrogen-bonding sites (hydrogen donor, acceptor, and donor) but different numbers of alkyl chains were examined in order to determine their monolayer properties and binding behavior towards barbituric acid (BA). Their structural organization in supramolecular assemblies at the air-water interface was affected by the bulkiness of the hydrophobic part of the amphiphile. Aqueous BA and amphiphiles with two or three alkyl chains formed a 1:1 alternate network structure. In contrast, a melamine amphiphile with four alkyl chains formed a 2:1 (BA:amphiphile) complex rather than a 1:1 alternate network structure. The 2:1 complex appears to behave like an independent molecular entity without further networking. The results point to the importance of size matching between the hydrophobic part of the monolayer and the underlying hydrogen-bonded network in order to maintain the overall supramolecular structure.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030715
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  • 111
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    Development of Tools for the Design of Selectin AntagonistsDedicated to Professor Hans Paulsen on the occasion of his 75th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1571-1578 
    Details
    ISSN: 0947-6539
    Keywords: carbohydrates ; molecular modeling ; preorganization ; sialyl Lewisx ; selectins ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A molecular modeling tool for the rational design of E-selectin antagonists based on the lead structure sialyl Lewisx has been developed. The binding affinity to the receptor is considerably influenced by the entropy and consequently by the antagonist's ability to place its pharmacophores in an optimal spatial arrangement, i.e., by its preorganization for binding. The computational model assesses the preorganization of a potential selectin antagonist with the aid of Monte Carlo (jumping between wells)/stochastic dynamics [MC(JBW)/SD] simulations. The model has been validated by correlating preorganization and bioactivity of several selectin antagonists. The results suggest that only preorganized compounds are likely to bind to E-selectin.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031006
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  • 112
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    A Pyrrole-Imidazole Polyamide Motif for Recognition of Eleven Base Pair Sequences in the Minor Groove of DNADedicated to Dieter Seebach on the occasion of his 60th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1600-1607 
    Details
    ISSN: 0947-6539
    Keywords: DNA recognition ; hydrogen bonds ; ligand design ; molecular recognition ; sequence-specificity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new upper limit of binding site size is defined for the 2:1 overlapped polyamide: DNA motif. Eight-ring polyamides composed of four-ring subunits containing pyrrole (Py) and imidazole (Im) amino acids linked by a central β-alanine (β) spacer („4-β-4 ligands“) were designed for recognition of eleven base pair sequences as antiparallel dimer (4-β-4)2.DNA complexes in the minor groove. The DNA binding properties of three polyamides, ImPyPyPy-β-PyPyPyPy-β-Dp, ImImPyPy-β-PyPyPyPy-β-Dp, and ImImImPy-β-PyPyPyPy-β-Dp, were analyzed by footprinting experiments on DNA fragments containing the respective match sites 5'-AGTAATTTACT-3', 5'-AGGTATTACCT-3', and 5'-AGGGATTCCCT-3' (Dp = dimethylaminopropylamide). Quantitative footprint titrations reveal that each polyamide binds its respective target site with subnanomolar affinity and 7-fold to over 30-fold specificity over double-base-pair mismatch sites. A 20-fold decrease in binding affinity is observed for placement of a side-by-side β-β pairing opposite G.C/C.G relative to placement opposite a A.T/T.A base pair. The use of side-by-side antiparallel β-alanine residues as an A.T/T.A-specific DNA binding element provides a new pairing rule for polyamide design. Expanding the DNA binding site size targeted by pyrrole-imidazole polyamides represents an important step in the development of cell-permeable synthetic ligands for the control of gene-specific regulation.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031009
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  • 113
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    In Search for Mononuclear Helical Lanthanide Building Blocks with Predetermined Properties: Triple-stranded Helical Complexes with N,N,N',N'-tetraethylpyridine-2,6-dicarboxamide (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1646-1659 
    Details
    ISSN: 0947-6539
    Keywords: carboxamides ; helical structures ; lanthanides ; luminescence ; tridentate ligands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ligand N,N,N',N'-tetra-ethylpyridine-2,6-dicarboxamide (L9) reacts with trivalent lanthanide ions (LnIII) to give stable mononuclear triple-stranded helical complexes [Ln(L9)3]3+ (Ln = La to Lu). The crystal and molecular structures of [La(L9)3](ClO4)3.2.5C2H5CN (8) and [Eu(L9)3](TfO)3.2THF (9) show that the three ligand strands are each meridionally tricoordinated to produce a pseudo-tricapped trigonal prismatic arrangement of the nine donor atoms (six amide O and three pyridine N) around LnIII. The distortions in the LaIII coordination sphere of 8 are more significant than for EuIII in 9, and the photophysical studies on Eu-doped (2%) La, Gd, and Lu complexes confirm a better structural match of L9 for the heavy LnIII ions. The separation of contact and pseudo-contact contributions to the induced lanthanide paramagnetic NMR shifts in [Ln(L9)3]3+ shows that the triple-helical structure is maintained in acetonitrile, but a minor structural change relative to that observed in the solid state occurs between TbIII and ErIII leading to two distinct isostructural series for Ln = La-Tb and Ln = Er-Lu. The origin of this effect together with its consequences for the dynamic helical (P→M) interconversion and stability of [Ln(L9)3]3+ in solution are discussed. A detailed investigation of the emission properties of [Ln(L9)3]3+ (Ln = Eu, Tb) shows that mixed pyridine-carboxamide ligands can be used to simultaneously optimize the structural and photophysical properties in mononuclear triple-helical lanthanide building blocks.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031014
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  • 114
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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 
    Details
    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.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030807
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  • 115
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    Self-Assembly of Bisurea Compounds in Organic Solvents and on Solid Substrates (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1238-1243 
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    ISSN: 0947-6539
    Keywords: gels ; electron microscopy ; scanning tunneling microscopy ; self-assembly ; ureas ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New low molecular weight gelators based on the structure R—NHCONH—X—NHCONH—R have been synthesized and tested for their ability to cause gelation of organic solvents. Compounds 2 (R = n-dodecyl, X = -(CH2)9-), 3 (R = n-dodecyl, X = -(CH2)12-), 4 (R = n-dodecyl, X = 4, 4′-biphenyl), and 5 (R = benzyl, X = -(CH2)9-) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mgmL-1. Depending on the nature of the R and X groups, the solvents that undergo gelation include hexadecane, p-xylene, 1-octanol, n-butyl actetate, cyclohexanone, and tetralin. The gels are stable up to temperatures well above 100°C, but are easily disrupted by mechanical agitation. Light microscopic investigations revealed that compounds 2-5 spontaneously aggregate to form thin flat fibers, which can be several hundreds of micrometers long and only 2-10 μm wide. Depending on the solvent, multiple twists in the fibers are observed. In the gels, these fibers form an extended three-dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electron microscopy and X-ray powder diffraction revealed that the fibers consist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for 2 and 3, respectively. Scanning tunneling microscopic investigations of 2 absorbed on graphite showed that 2 forms long ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrangement, with the long molecular axis perpendicular to the long ribbon axis. The two urea groups within a given molecule are each part of mutually parallel extended chains of hydrogen bonds. Based on these observations a model is proposed for the arrangement of the molecules in the fibers. In this model the bisurea molecules aggregate through hydrogen-bond formation into long ribbons, which assemble into sheets. In these sheets the ribbons are tilted. Finally, the sheets stack to form long thin fibers. This model is supported by molecular dynamics simulations.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030811
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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 
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    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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  • 117
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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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  • 118
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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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    Aggregation, Reaggregation and Degradation of a Trifunctional Thallium(I) Amide Induced by Weak TlI ⃛ TlI AttractionDedicated to Professor Mary McPartlin on the occasion of her 65th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1801-1806 
    Details
    ISSN: 0947-6539
    Keywords: aggregations ; amides ; metal-metal interactions ; subvalent compounds ; thallium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metal exchange of the solvated tripodal lithium amide [H3CC{CH2N(Li-solv)SiMe3}3] (1, solv = ether solvent) with thallium(I) chloride leads to the previously reported pentametallated dimeric thallium amide [{H3CC(CH2NSiMe3)3}2(H)Tl5] (2) in high yield. That the redox-induced partial demetallation of the amide presumably occurs at an intermediate stage of the metal exchange is inferred from the isolation and structural characterization of the mixed Tl-Li amide [H3CC{CH3N(Tl) SiMe3}3][H3CC{CH2NSiMe3}3(H)(Tl)(Li-thf)]·(toluene) (3), which has a crystal structure closely related to that of 2. In both cases the central structural motif, defined by weakly attractive Tl⃛ Tl contacts, is a tetrahedral metal array in which a triangular thallium triamide unit is capped by an exposed Tl atom of a second building block. Compound 3 may be converted to 2 by metal exchange with TlCl. The two supramolecular components of 2 reaggregate upon recrystallization in a nonpolar solvent such as pentane to yield the dimeric aggregate of the fully metallated thallium amide [H3CC{CH2N(Tl)-SiMe3}3]2 (4). An X-ray crystallographic study of 4 established the existence of dimeric aggregation through Tl ⃛ Tl interactions leading to a more open dimeric form than 2 and 3, which is interpreted as being due to a structural mismatch of the building blocks. Compound 4 may be thermally degraded by prolonged stirring in toluene at ambient temperature to yield the previously reported mixed-valence TlITlII amide [H3CC(CH2NSiMe3)3Tl2] (5).
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031112
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    A Quintupled Superstructure of the KHg2 Type Realized for the New Stannide EuAuSn (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1852-1859 
    Details
    ISSN: 0947-6539
    Keywords: conductivity ; europium ; gold ; magnetic properties ; superstructures ; tin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound was prepared from the elements by reaction in a sealed tantalum tube at 1320 K followed by slow cooling to 970 K. EuAuSn crystallizes with a pronounced subcell of space group Imma (KHg2 type). Additional very weak reflections required a quintupled b axis. The superstructure was refined from single-crystal four-circle diffractometer data (Imm 2, a = 479.1(1) pm, b = 3833.6(5) pm, c = 820.1(1) pm, Z = 20, wR2 = 0.0834, 3675 F2 values and 94 variables). Six crystallographically different europium sites occur in the superstructure. Each europium site has an ordered near-neighbour environment of six gold and six tin atoms in the form of two tilted hexagons. Magnetic susceptibility measurements show Curie-Weiss behaviour above 50 K with an experimental magnetic moment of 7.6(1) μB/Eu, indicating divalent europium. EuAuSn orders antiferromagnetically at about 12 K and undergoes a metamagnetic transition at a critical field of 2.0(2) T. Electrical conductivity measurements show metallic behaviour with a room temperature value of 260 μΩcm. 151Eu and 119Sn Mössbauer spectroscopic experiments are compatible with divalent europium and show complex magnetic hyperfine field splitting at low temperature.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031118
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    Poly(triacetylene) Oligomers: Synthesis, Characterization, and Estimation of the Effective Conjugation Length by Electrochemical, UV/Vis, and Nonlinear Optical MethodsDedicated to Professor Dieter Seebach on the occasion of his 60th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1505-1512 
    Details
    ISSN: 0947-6539
    Keywords: conjugation length ; electrochemistry ; nonlinear optics ; oligomers ; poly-(triacetylene)s ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Poly(triacetylene)s [PTAs, —(C≡C—CR=CR—C≡C)n-] are a new class of linearly conjugated polymers with a nonaromatic all-carbon backbone. To explore structure-property relationships in PTAs, we prepared a series of monodisperse oligomers ranging from monomer to hexamer by oxidative Glaser-Hay coupling of a bifunctional („chain-forming“) (E)-hex-3-ene-1,5-diyne in the presence of an endcapping unit. All six oligomers are amazingly stable towards exposure to light, air, and temperatures beyond their melting points. They have been fully characterized and are readily soluble in a wide range of solvents. The conjugated rods are reversibly reduced in one-electron transfer steps and cannot be oxidized below +1.23 V vs. Fc/Fc+. The effective conjugation length in PTAs was estimated from the electronic absorption (UV/Vis) data, and various evaluation methods yielded convergence of the optical properties in the range of 7 to 10 monomer units. The nonresonant second-order molecular hyperpolarizability γ was measured in CHCl3 by means of the third harmonic generation (THG) at Δ = 1.907 μm. A plot of γ/n vs. n revealed a power law γ ≈ na for γ with a fitted exponent a = 2.5 ∓ 0.1. From the THG measurements, an effective conjugation length of about 10 monomer units was found, in surprisingly good agreement with the value obtained from UV/Vis spectroscopy data.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030919
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    Strong Binding of Alkylguanidinium Ions by Molecular Tweezers: An Artificial Selective Arginine Receptor Molecule with a Biomimetic Recognition Pattern (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1537-1541 
    Details
    ISSN: 0947-6539
    Keywords: arginine ; guanidines ; molecular recognition ; receptors ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bisphosphonates 2 and 3 represent the first artificial receptor molecules for alkylguanidinium ions. They bind to the guanidinium moiety by forming a 1:1 chelate complex, stabilized by a planar network of electrostatic interactions and hydrogen bonds. This hydrogen bonding configuration is identical to the „arginine fork“ postulated by Frankel as a key element in RNA-protein recognition of the AIDS virus. Our guanidinium-bisphosphonate complexes thus constitute the first synthetic model for this important biological interaction and demonstrate that the high binding energy can be a driving force for a conformational change in the receptor (induced fit, e.g., in the RNA). Although binding of monosubstituted alkylguanidines is generally strong (Ka ≈ 10 000 in DMSO), molecular tweezer 3 recognizes N- and C-amide-protected arginine derivatives especially well (Ka ≈ 300 000 in DMSO), because an additional hydrogen bond is formed between the amide and the phosphonate. Since 3 does not bind amines effectively, it is highly selective for arginine, even in the presence of lysine or other amino acids. For di-, tri-, and tetrasubstituted guanidines the association constant remains low (Ka≤1000 in DMSO) reflecting the increase in the steric bulk of the guest.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19970030923
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    BIPNOR: A New, Efficient Bisphosphine Having Two Chiral, Nonracemizable, Bridgehead Phosphorus Centers for Use in Asymmetric Catalysis (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1365-1369 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric catalysis ; hydrogenations ; palladium ; phosphorus ligands ; rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically active phosphorus ligands are widely used in homogeneous asymmetric catalysis. However, among the numerous available structures of this type, the subclass of optically active bisphosphines with at least one chiral phosphorus atom is rather underdeveloped. A bisphosphine with two chiral, nonracemizable bridgehead phosphorus centers, (meso,d/l)-2,2′,3,3′-tetraphenyl-4,4′,5,5′-tetramethyl-6,6′-bis-1-phosphanorborna-2,5-dienyl (BIPNOR), can be obtained by thermolysis of 1,1′-bisphospholyl with diphenylacetylene. Here, we report the resolution of the d/l isomer by means of a chiral palladium complex to give the two optically active forms of BIPNOR. We then investigate the catalytic properties of BIPNOR, incorporated in RhI and RuII catalysts for the hydrogenation of olefins and ketones and in a PdII catalyst for asymmetric alkylation reactions. BIPNOR is shown to give good results in these catalytic reactions.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030824
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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.19970030902
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    The Phosphodiester Groups of d(TpT)- and d(TpG)- Coordinate to Platinum(II) in N,N-Dimethylformamide (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1405-1409 
    Details
    ISSN: 0947-6539
    Keywords: antitumor agents ; DNA ; NMR spectroscopy ; platinum ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The propensity of the phospho-diester group to bind platinum(II) was investigated by examining the reactions of the dinucleotide salts NH4{d(TpT)} and NH4{d(TpG)} with the complexes [Pt-(NO3)(NH3)3]NO3 (1) and cis-[Pt(NO3)2-(NH3)2] (2) in N,N-dimethylformamide. d(TpT)- coordinates to the platinum atom of both 1 and 2 through either of the two nonbridging phosphoryl oxygen atoms, as revealed by 31P NMR spectroscopy. Conversely, d(TpG)- binds to 1 through the guanine N7 atom. With 2, d(TpG)- forms monodentate adducts bound to N7(G), as well as two distinct macrochelates, in which N7(G) and one of the nonbridging phosphoryl oxygens are crosslinked by platinum. We suggest that guanine-phosphoryl macrochelate complexes play a role as intermediates in rearrangements of platinum monoadducts formed with DNA.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030906
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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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    Theory of the d10-d10 Closed-Shell Attraction: 1. Dimers Near EquilibriumTheory of the d10-d10 closed-shell attraction, part 1. Part 2: following paper. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1451-1457 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; bond theory ; closed-shell attraction ; gold ; relativistic effects ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We study the dependence of the aurophilic attraction (AuI-AuI) in perpendicular model systems of the type [(ClAuPH3)2] on the ab initio method, basis set and different pseudopotentials used, and on relativity. The effects of varying the „halogen“ (X = F, H, Cl, Me, Br, -C≡CH, I), the „phosphine“ (L = PH3, PMe3, -N≡CH) and the metal (M = Cu, Ag, Au) on the M-M′ interaction of the [(XML)2] dimer are also studied. The depth of the interaction potential increases with the softness of the group X. It decreases by 27% for M = Au, X = Cl and L = PH3 if relativistic effects are omitted at fixed geometry.
    Additional Material: 11 Ill.
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    A Square As4 and a Prismatic As6 Structure as Complex Ligands (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1494-1498 
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    ISSN: 0947-6539
    Keywords: arsenic ; cobalt ; density functional calculations ; structural elucidation ; Zintl anions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: [As7(SiMe3)3] reacts with [(Cp*CoCl)2] to give black crystals of the ionic compound [(Cp*Co)3As6][CoCl4] (1). The cation [(Cp*Co)3As6]2+ consists of an As6 prism, with squares capped by [Cp*Co] fragments. When [(CptBuCoCl)2] is used instead of [(Cp*CoCl)2] in the reaction, dark red crystals of [(CptBuCo)3As6][CoCl3(thf)]2 (2) and green-black crystals of [(CptBuCo)2As4][Co3Cl8(thf)2] (3) can be obtained. The cation in 2 shows a structure very similar to that of 1. The [(CptBuCo)2As4]2+ cations in 3 can be described as a triple-decker sandwich complex with two [CptBuCo]+ fragments bridged by an As4 ligand. Density functional calculations reproduce the experimental data of 1 and 3 and allow an interpretation of molecular electronic structure and bonding in these and related compounds with As replaced by P and Co by Fe.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030917
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    Reaction of „Jordan′s Cation“ [Cp2ZrCH3(thf)]+ with Amino Acid-Derived Isocyanates - A First Synthetic Route to Oligopeptide-Modified Group 4 Bent Metallocene Cation SystemsDedicated to Professor Waldemar Adam on the occasion of his 65th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1521-1525 
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    ISSN: 0947-6539
    Keywords: isocyanates ; „Jordan′s cation“ ; metallocenes ; peptides ; zirconium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The organometallic cation [Cp2ZrCH3(thf)]+, employed as the tetraphenylborate salt (1), reacts cleanly in 1:1 stoichiometry with the isocyanates 2 derived from valine methyl ester or valylvaline methyl ester, respectively. In each case addition of the Zr-CH3 group to the isocyanate sp-carbon center is observed with formation of a functionalized zirconocene cation derivative containing a chelating N-metallated N-acetylvaline methyl ester (3a) or N-acetylvalylvaline methyl ester (3b) moiety, respectively, coordinated in the bent metallocene s̰-ligand plane. The spectroscopic data of 3, supported by an X-ray crystal structure analysis of the zirconated dipeptide derivative 3b, have revealed the presence of chelating (η1-O:η1-N)-coordination of the terminal N-acetyl groups in addition to a Zr-O=C interaction with the adjacent valyl amido group.
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    URL: http://dx.doi.org/10.1002/chem.19970030921
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    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1552-1556 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031003
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    The Nature of the Metal - Silicon Bond in [M(SiR3)H3(PPh3)3] (M = Ru, Os) And the Crystal Structure of [Os{Si(N-pyrrolyl)3}H3(PPh3)3] (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1608-1616 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; hydrides ; osmium ; ruthenium ; silicon ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The compounds [M(SiR3)H3(PPh3)3] (1: M = Ru, R = 1 - NC4H4 = pyr; 2a-c: M = Os, R = pyr, Et, Ph) are prepared through reaction of either [RuH2(PPh3)4] or [OsH4(PPh3)3] with the appropriate silane HSiR3 (3a-c: R = pyr, Et, Ph). The X-ray structure analysis of compound 2 a and ab initio calculations on the model compounds [Os(SiR3)H3(PH3)3] (4a-c: R = H, NH2, pyr) reveal a trigonal distortion along the Os-Si axis from an idealised tetrahedral geometry for the central OsSiP3 heavy-atom skeleton. The structure can be described as two face-shared octahedra, one based on osmium (OsH3P3) and the other based on silicon (SiH3N3). Studies of the bonding situation in 2 a reveal that the N-pyrrolyl substituents have a marked shortening effect on the osmium-silicon distance (229.3(3) pm) and that each of the three hydride ligands participates in partial three-centre bonding involving osmium, silicon and hydrogen. 1H, 13C, 29Si and 31P NMR spectra were used to determine the solution structures of complexes 1 and 2 a.
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    Investigation of the Formation Reaction and Structural Characterization of the „Platinum Grignard Reagent“ [Pt(MgCl)2(THF)x] by Extended X-ray absorption fine structure (EXAFS) and Other methods (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1710-1718 
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    ISSN: 0947-6539
    Keywords: clusters ; EXAFS spectroscopy ; Grignard reactions ; magnesium ; platinum ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The „platinum Grignard reagent“ [Pt(MgCl)2(THF)x] (2), obtained by the reaction of PtCl2 and Et2Mg in a 1:2 molar ratio, as well as finely divided platinum (Pt*) - a possible intermediate formed during the preparation of 2 - have been investigated by EXAFS spectroscopy at the PtLII edge. Parallel investigations were carried out on Pt* obtained from PtCl2 and (9,10-dihydro-9,10-an-thracenediyl)tris(tetrahydrofuran)magnesium (MgA), and on 2 obtained from Pt*, MgA, and MgCl2. The EXAFS results suggest that Pt* consists of extremely small particles (≈ 5-11 Å) with strongly reduced Pt-Pt distances compared to bulk Pt (≈ 0.09 Å). The EXAFS spectra of 2 indicate the presence of Mg shells in addition to Pt shells in the Pt environment; Mg atoms are at a bonding distance from Pt atoms (2.78-2.80 Å). These results suggest that 2 consists of very small Pt-Mg clusters and confirm their formation from organomagnesium reagents and PtCl2 or Pt*.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031020
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    The Comparative Solvatochromism of Arylazo and Heteroarylazo Compounds Based On N,N-Diethyl-m-acetylaminoaniline and N,N-Diethyl-m-toluidine (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1719-1727 
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    ISSN: 0947-6539
    Keywords: azo compounds ; dyes ; heterocycles ; correlation analysis ; solvato-chromism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Azobenzene dyes derived from various anilines and aminothiaheterocycles azo-coupled with commercially important N,N-diethyl-m-toluidine (T series) and N,N-diethyl-m-acetylaminoani-line (A series) are positively solva-tochromic. The visible spectra of 16 pairs of derivatives have been measured in up to 22 solvents, and the transition energies related to Kamlet-Taft solvent polarity parameters. In general, A-series dyes are more bathochromic than their T-series counterparts in nonpolar solvents, consistent with colour chemistry tradition. However, in more dipolar solvents the more bathochromic T-series representatives unexpectedly become more bathochromic than their A-series partners. The relative solvatochromic shifts of the A and T series are related to their respective dipole moments. These in turn are distinguished by the effect of the anilide carbonyl group dipole moment, which is antiparallel to, and thus reduces, the dipole moment of the chromogen.
    Additional Material: 8 Ill.
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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 
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    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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    Nondipolar Structures With Threefold Symmetry For Nonlinear Optics (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1765-1773 
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    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.
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    URL: http://dx.doi.org/10.1002/chem.19970031107
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    Two New Closely Related MoV Hydroxymonophosphates Built Up of Cd[Mo6P4O25(OH)6]2 And Cd[Mo6P4O26(OH)5]2 ClustersDedicated to Professor H. G. von Schnering on the occasion of his retirement (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1797-1800 
    Details
    ISSN: 0947-6539
    Keywords: cadmium ; crystal structure ; hydrothermal synthesis ; hydroxyphosphate ; molybdenum ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new molybdenum(v) hydroxyphosphates have been synthesized hydrothermally, Na2Cd3(Mo2O4OH)6-(PO4)2(PO3OH)6[N(CH3)4]4·10H20 (1) and Cd9(Mo2O4OH)12(PO4)6(PO3OH)10-[N(CH3)4]8·15H2O (2). Their structures have been determined from single-crystal X-ray diffraction. The water molecules and hydroxyl groups have been deduced from valence calculations. Both compounds crystallize in the triclinic space group P1, with the cell parameters for 1 a = 12.340(2), b = 12.596(1), c = 14.717(2) Å, α = 107.24(1)°, β = 89.83(1)°, γ = 114.31(1)°, V = 1972.3(4) Å3, and for 2 a = 11.942(1), b = 13.339(2), c = 26.765(3) Å, α = 85.33(1)°, β = 86.87(1)°, γ = 64.08(1)°, V = 3821.3(9) Å3. The two frameworks can be described on the basis of similar [Mo6P4X31]n- (X = O, OH) anionic clusters, but 1 is a tridimensional structure, whereas 2 exhibits a monodimensional structure.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031111
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    Noncovalent Assembly of Functional Groups on Calix[4]Arene Molecular Boxes (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1823-1832 
    Details
    ISSN: 0947-6539
    Keywords: calixarenes ; hydrogen bonds ; molecular boxes ; noncovalent assembly ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3-bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen-bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen-bonded assembly, but this effect very much depends on the exact positions of the groups.
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    URL: http://dx.doi.org/10.1002/chem.19970031115
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    Electronic and Geometrical Structures of Dialuminoxane, Diboroxane, and Their Sulfur Analogues: Ab Initio Study of H2X-Y-XH2 (X = Al, B; Y = O, S) Compounds (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1860-1865 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; aluminum ; boron ; pi interactions ; sulfur ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structures of H2X—Y—XH2 compounds (X = B, Al; Y = O, S) have been studied by means of ab initio calculations at the MP4/6-311 G**/MP2/6-311G** level. The potential energy surface (PES) of the aluminoxane species H2Al—O—AlH2 is rather flat: the energy differences between the various located extrema are less than 1.5 kcal mol-1, and this shows that the observed geometrical preference is not due to electronic factors. For the sulfur analogues H2X—S—XH2 (X = B, Al), three minima are located on the PES: a planar C2v structure, a Cs structure in which one hydrogen atom bridges the two X atoms, and a C2v structure with two bridging hydrogen atoms. For H2Al—S—AlH2, the Cs structure is the most stable, and the planar C2v and the dibridged structures lie 3.2 and 6.7 kcal mol-1 higher, respectively. Qualitatively similar results are obtained for H2B—S—BH2: the Cs and planar C2v structures are nearly isoenergetic, and the dibridged minimum lies 19.4 kcal mol-1 above the planar C2v minimum. These results are rationalized by analyzing the ability of these systems to bend, depending on the nature (oxygen or sulfur) of the central Y atom. The conjugation in these species is also discussed, and calculations on model systems H2X—YH (X = B, Al; Y = O, S) are presented.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031119
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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 
    Details
    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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    Hexabromotricyclobutabenzene and Hexabromohexaradialene: Their Nickel-Mediated One-Pot Syntheses and Crystal Structure (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 208-210 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; cyclobutenes ; nickel ; radialenes ; radical reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of hexakis(dibromomethyl)benzene with [(Bu3P)2-Ni(COD)] (COD = 1,5-cyclooctadiene) in DMF at 65-70°C yielded a mixture of the title compounds. The mixture was separated by column chromatography to yield hexabromotricyclobutabenzene (3 a) and hexabromohexaradialene (4) in 24 and 16% yields, respectively. 1H and 13C NMR spectroscopy suggest that 3 is obtained as the syn-all-trans isomer 3 a, and the symmetric anti-all-trans isomer 3 b is not obtained at all. The X-ray structures of 3 a and 4 are reported. The hexaradialene 4 has a chair conformation, and deviates from planarity by 43.6°. Heat or radical impurities cause the clean transformation of 3 a to 4.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030207
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    The Thiazole Ylide: A Frequently Invoked Intermediate Is a Stable Species in the Gas Phase (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 232-236 
    Details
    ISSN: 0947-6539
    Keywords: carbenes ; density functional calculations ; mass spectrometry ; thiamin ; ylides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1, 2-hydrogen shift isomers of neutral (singlet and triplet) thiazole (1) and its radical cation have been investigated by a combination of mass spectro-metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C3H3NS and C3H3NS.+ isomers and transition state structures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcalmol-1, it is expected to be capable of independent existence, since the 1, 2-hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol-1. The other 1, 2-hydrogen shift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2-ylidene structure (3), which is apparently unstable, but rather to the ring-opened species HSCH=CHNC (4), which is 34.5 kcalmol-1 higher in energy than 1. The barrier in this case is lower but still large (54.9 kcalmol-1). The triplet ground states of 1, 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol-1) relative to their singlet states. Interestingly, in addition to 2.+ and 4.+, the cyclic radical cation 3.+ is predicted to be stable although it is substantially higher in energy than ionised thiazole 1.+ (by 53.9 kcalmol-1), whereas 2.+ and 4.+ are much closer in energy (only 10.2 and 27.0 kcalmol-1 higher, respectively). Dissuading 2.+ and 3.+ from isomerising to 1.+ are energy barriers of 52.6 and 15.3 kcalmol-1, respectively. Experimentally, dissociative ionisation of 2-acetylthiazole enabled the generation of 2.+, which could be differentiated from 1.+ by collisional activation mass spectrometry. Reduction of the ylide ion 2.+ in neutralisation-reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.
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    URL: http://dx.doi.org/10.1002/chem.19970030211
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    On the Role of Structural Zinc in Bis(Cysteinyl) Protein SequencesZinc complexes of amino acids and peptides, part 10. part 9, ref. [1]. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 261-267 
    Details
    ISSN: 0947-6539
    Keywords: NMR spectroscopy ; peptides ; protein structures ; structure elucidation ; zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Besides its functional role in many hydrolytic metalloenzymes, zinc acts as a structural component by being attached to bis(cysteinyl) protein sequences in some of the same enzymes, and in other metalloproteins and zinc fingers, and by being an essential constituent in metallothioneins. It is not always obvious whether the zinc-binding proteins are pre-organized for the incorporation of the metal or whether the zinc ion provides the structurizing power and stability for the observed peptide conformations. We have addressed the coordination chemistry aspects of this question by synthesizing zinc complexes of small model peptides and by determining their structures in solution by 2D NMR spectroscopy. The peptides chosen were of the terminally protected bis(cysteinyl) type: Cys-Cys, Cys-Gly-Cys, Cys-Phe-Cys, and Cys-Gly-Ile-Cys. The zinc ions fold these peptides into structures that can be superimposed on those of the natural proteins.
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    Quantum-Chemical Investigations of Stabilizing Interactions in μ-Diborylcarbene Dicobalt Complexes with a Planar Tetracoordinate Carbon AtomDedicated to Professor Gerhard Herberich on the occasion of his 60th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 294-299 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; cobalt ; planar carbon ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum-chemical methods have been employed to study the nature of stabilization in dinuclear cobalt complexes of the general formula [{(C5H5)Co}2(μ-CR12BCBR2R3)] (6) as well as the “antivan't Hoff-Le Bel” configuration of the planar tetracoordinate carbon (ptC) atom of the bridging diborylcarbene ligand 9. Extended Hückel and ab initio Hartree-Fock calculations have been carried out for the model compounds 6b (R1 = R2 = R3 = H) and 6c (R1 = R2 = H; R3 = C6H5). Ab initio electron deformation density maps and natural population analysis calculations show that complexes 6 are stabilized through push-pull effects by which the ptC experiences π electron density delocalization and σ electron density accumulation. The calculated electronic configuration of the ptC in the free ligand 9b is σ2.978π1.501, and in 6b σ3.944π1.356. Electron density donation from one cobalt atom to an aryl group on the bridging ligand further contributes to the stabilization of the complexes 6.
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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.19970030302
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    Regioselective Deuteration and Resonance Raman Spectroscopic Characterization of Biliverdin and PhycocyanobilinAbbreviations: A-B, B-C, C-D, dipyrrole units of tetrapyrrole compounds; e.g., A-B, rings A and B. FT IR, Fourier-transform infrared. HOOP, hydrogen-out-of-plane. PED, potential energy distribution. RR, resonance Raman. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 363-367 
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    ISSN: 0947-6539
    Keywords: isotopic labeling ; phytochrome ; Raman spectroscopy ; tetrapyrroles ; vibrational analysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5-Deuteriophycocyanobilin (4), 5,15,182,182-tetradeuteriobiliverdin IXα dimethyl ester (6), and 5,15-dideuteriomesobiliverdin IXα dimethyl ester (8) have been prepared and analyzed by resonance Raman spectroscopy in the C-H out-of-plane region, based on a previous normal-mode analysis of biliverdin XI α dimethyl ester (5). In this way most of the modes containing out-of-plane vibrations of the methine bridges could be assigned unambiguously. These results are of special relevance with regard to the interpretation of the resonance Raman spectra of the chromoprotein phytochrome.
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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.
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    The Reaction of a Triorganochloroindate with Dioxygen (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 568-572 
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    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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    Homoleptic Cuprates(II) with Multiply Deprotonated α-Cyclodextrin LigandsPolyol metal complexes, part 22. Part 21: J. Burger, P. Klüfers, Angew. Chem. Int. Ed. Engl. 1997, 36, Issue 7. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 601-608 
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    ISSN: 0947-6539
    Keywords: copper ; cuprates ; cyclodextrins ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the presence of cupric ions, α-cyclodextrin (α-CD) is multiply deprotonated in alkaline aqueous solution. With lithium or sodium as the counterions, trinuclear sandwich-type cuprate ions of the formula [Cu3(α-CDH_6)2]6- are formed (1 and 2), whereas dinuclear [Cu2(α-CDH_4)2]4- ions are isolated in the form of the potassium (3a) and the rubidium salts (3b). The structures of the cyclodextrinatocuprate ions resemble cylinders, since the usual conical shape of a cyclodextrin is altered due to the formation of shorter hydrogen bonds on the O2/O3 rim of the α-cyclodextrin torus. Despite anion formation, the cavity of the cyclodextrin is still hydrophobic, giving rise to the intercalation of acetone guests in 2 and 3.
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    Screening of Various Catalysts for the Asymmetric Cyclopropanation of Structurally Typical Silyl Enol Ethers - Scope and Limitations of Evans's Bisoxazoline Copper CatalystSynthesis of optically active 2-siloxycyclopropanecarboxylates by asymmetric catalysis, part 3 (based on dissertations by r. schumacher[1] and F. Dammast[2]). For Part 2 see ref. [7]. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 614-619 
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    ISSN: 0947-6539
    Keywords: asymmetric catalysis ; cycloadditions ; cyclopropanecarboxylates ; diazoacetates ; silyl enol ethers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The scope and limitations of asymmetric syntheses of 2-siloxycyclopropanecarboxylates 3 were examined by combination of structurally typical silyl enol ethers and methyl diazoacetate with various chiral catalysts. It was found that the Schiff base complex 5ṁCu(OAc)2 and the bisoxazoline complex 6ṁCuOTf gave the highest stereoselectivities (e.g., 72% ee for cis-3d with 5ṁCu(OAc)2 as catalyst or 76% and 73% ee for cis- and trans-3a, respectively, with 6-CuOTf as catalyst). High enantioselectivities or diastereoselectivities were obtained. Other catalysts based on copper, rhodium or ruthenium complexes afforded significantly lower values. In further investigations, Evans's bisoxazoline complex 6-CuOTf proved to be limited to 1,1-disubstituted silyl enol ethers (e.g. 1a and 1e) for high enantiomeric excesses (up to 〉 95%, poor diastereoselectivities) and to 1,2-substituted enol ethers (e.g. 1e) for high diastereoselectivities (cis:trans 〈 3:97, eetrans = 49%).
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    Dimethyl Peroxide Radical Cation: A New Theoretical and Experimental Approach to the C2H6O•+2 Potential Energy Surface (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 626-638 
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    ISSN: 0947-6539
    Keywords: ab initio calculations ; dimethyl peroxide ; mass spectrometry ; peroxides ; radical ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure and the unimolecular fragmentations of the metastable dimethyl peroxide radical cation have been investigated by mass spectrometric and isotopic labeling methods as well as high-level ab initio calculations. In line with the theoretical results, neutralization-reionization and charge reversal experiments suggest that ionized dimethyl peroxide bears a CH3OOCH•3 connectivity. In the cation the O-O bond dissociation energy is larger than that of the neutral counterpart; in contrast, the C-O bond strength is slightly and that of the C-H bond significantly reduced upon ionization. These energetic changes upon one-electron oxidation of CH3OOCH3 are also reflected in the NR and CR mass spectra of CH3OOCH•+3. Further, for metastable CH3OOCH•3 two major fragmentation pathways are observed: 1) Loss of a hydrogen atom by cleavage of a C-H bond is associated with a skeletal reorganization, which gives rise to a proton-bound formaldehyde dimer. 2) The expulsion of a CH3O• radical leads to protonated formaldehyde in a surprisingly specific double hydrogen transfer involving a [CH3OH/CH2O]• ion/dipole complex as central intermediate; this complex also accounts for other minor fragmentation channels. The structures of intermediates and transition states are calculated with the BECKE 3LYP density-functional method employing a 6-311++G** basis.
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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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    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 
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    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).
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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.
    Additional Material: 17 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030414
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    Biosynthesis of Nitric Oxide - Quantum Chemical Modelling of Nω-Hydroxy-l-arginine Formation (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 609-613 
    Details
    ISSN: 0947-6539
    Keywords: amino acids ; biosynthesis ; hydroxylations ; nitric oxides ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure (charge distribution, bond indices, character of the frontier orbitals) and geometry (bond distances and angles) of L-arginine and N-methyl-L-arginine were determined by means of the INDO procedure. The method was also adopted to model the conversion of L-arginine into N-hydroxy-L-arginine in biological systems. This revealed that the approach of diatomic O species does not result in reaction, whereas the approach of either an O atom or an O2- ion leads to insertion of oxygen and formation of hydroxy-L-arginine. The insertion of oxygen between the nitrogen and hydrogen atoms leads to more stable products than insertion into the C-H bond. The same results were obtained for N-methyl-L-arginine, and are consistent with the hypothesis that the inhibitive effect of N-substitution in L-arginine is of no importance for the first step in the biosynthesis of NO (hydroxylation process). The mechanistic considerations based on the charge distribution and frontier orbital characteristics led to the conclusion that the most probable mechanism of L-arginine hydroxylation consists in electrophilic attack of [FeO]3+ at the Nω-H bond, initiated by the reduction of L-arginine+, followed by insertion of oxygen and product oxidation.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030417
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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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    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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    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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    Dendritic Bipyridine Ligands and Their Tris(Bipyridine)Ruthenium(II) Chelates - Syntheses, Absorption Spectra, and Photophysical Properties (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 706-712 
    Details
    ISSN: 0947-6539
    Keywords: bipyridine ligands ; dendrimers ; dioxygen quenching ; luminescence ; ruthenium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several synthetic strategies have been explored to prepare dendrimers having the [Ru(bpy)3]2+ complex as their core (bpy = 2,2′-bipyridine). Dendritic ligands have been synthesized by attaching branches in the 4,4′-positions of bpy. The largest dendritic bipyridine ligand contains 54 peripherical methylester units. Four RuII dendritic complexes have been prepared. Their absorption and emission spectra are very similar to those of the unsubstituted parent RuII-bipyridine complexes. The large dendritic complexes, however, exhibit a more intense emission and a longer excited-state lifetime than [Ru(bpy)3]2+ in aerated solutions. This is due to the shielding effect of the dendrimer branches on the Ru-bipyridine core, which limits the quenching effect of molecular oxygen. For the largest dendritic complex, which contains 54 peripherical methylester units, the excited-state lifetime in aerated acetonitrile solution is longer than 1 μs, and the rate constant for dioxygen quenching is twelve times smaller than for [Ru(bpy)3]2+.
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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 
    Details
    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.
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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 
    Details
    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.
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    Design and Synthesis of Sugar-Responsive Semiartificial Myoglobin Triggered by Modulation of Apoprotein-Cofactor Interactions (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1025-1031 
    Details
    ISSN: 0947-6539
    Keywords: boronic acids ; heme proteins ; molecular recognition ; myoglobin ; saccharides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Phenylboronic acid groups as sugar recognition sites were successfully introduced into native myoglobin by a cofactor-reconstitution method. Spectrophotometric pH titration demonstrated the sugar-induced pKa shift of the H2O coordinated to the heme center of the semisynthetic myoglobin bearing phenyl-boronic acids (met-Mb(PhBOH)2). By means of circular dichroism (CD) and paramagnetic 1H NMR spectroscopies, it was proven that sugars that were bound to phenylboronic acid sites induced the rearrangement of the heme crevice to reinforce the heme cofactor-apoprotein interactions. The structural changes that were induced by the binding of sugars subsequently enhanced the dioxygen storage activity of Mb(PhBOH)2. Such sugar-induced structural and functional changes did not occur for other modified Mbs that had no sugar-recognition units. Interestingly, a randomly modified Mb with phenylboronic acid units did not show any sugar response. In Mb(PhBOH)2, the information from the sugar-binding event was efficiently transmitted to the active center, so that the activity was efficiently altered upon sugar binding. In conclusion, the active site specific incorporation of molecular recognition units as nonnatural functional molecules can provide a novel strategy for the design of stimuli-responsive semisynthetic proteins.
    Additional Material: 4 Ill.
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    Synthesis, Separation and Characterization of the Structural Isomers of Octa-tert-Butylphthalocyanines and Dienophilic Phthalocyanine Derivatives (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1042-1051 
    Details
    ISSN: 0947-6539
    Keywords: dienophiles ; isomer separation ; macrocyclic ligands ; monomers ; phthalocyanines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Octa-tert-butylphthalocyanina-tonickel(II) (4) was synthesized from 3,5-di-tert-butylphthalonitrile (1) and nickel acetate. A nonstatistical mixture of the four structural isomers 4a-d was obtained. The isolation of the structural isomers 4a-d by preparative HPLC (MPLC) allowed their unequivocal identification and characterization by spectroscopic techniques. Moreover, a new family of tert-butyl-substituted phthalocya-nines 6-10 containing dienophilic functionalities were prepared by condensation of 3,5-di-tert-butylphthalonitrile (1) with 6,7-dicyano-1,4-epoxy-1,4-dihydronaphthalene (5). The separation of the phthalocyanines 6-10 and of some of their structural isomers was accomplished by normal column chromatography. The structural isomers were characterized by spectroscopic methods in terms of their symmetry.
    Additional Material: 7 Ill.
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    Reiterative Synthesis of trans-Fused Polytetrahydropyrans Using the Oxiranyl Anion (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 849-852 
    Details
    ISSN: 0947-6539
    Keywords: C-C coupling ; cyclizations ; oxiranyl anion ; synthetic methods ; tetrahydropyrans ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The oxiranyl anion is a very unstable nucleophilic epoxide. This previously uncommon nucleophile has now been used for the synthesis of polycyclic ethers. Alkylation of an oxiranyl anion, a three-carbon building block, with a triflate derived from tetrahydropyran-methanol followed by acid-catalyzed 6-endo cyclization provides a trans-fused bicyclic compound. Reiterative application of this procedure enables the rapid construction of trans-fused polytetrahydropyrans.
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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).
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    Formation of Hydrido-η3-Allyl Complexes of IrIII by Sequential Olefinic C—H Bond Activation and C—C Coupling of Alkenyl and Olefin Ligands (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 860-873 
    Details
    ISSN: 0947-6539
    Keywords: alkene complexes ; allyl complexes ; C-H activation ; C-C coupling ; iridium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bis(ethene) complex [Tp*Ir-(C2H4)2] (1*) (Tp* = tris(3,5-dimethyl-1-pyrazol-1-yl)hydroborato) undergoes thermal rearrangement to the hydrido-allyl complex [Tp*IrH(π3-C3H4Me)] (6*), through the intermediacy of the hydrido-vinyl complex [Tp*IrH(C2H3)(C2H4)] (2*). The overall conversion of 1* into 6* corresponds formally to the dimerisation of ethene by an unprecedented pathway that involves sequential C-H bond activation of a coordinated olefin molecule and C-C bond formation by coupling of the resulting vinyl and ethene moieties. Similar transformations have been observed for monosubstituted olefins like propene and 1-butene, while the internal alkene cis-2-butene experiences allylic activation of an sp3 C-H bond, which provides an alternative route to 6*. The extension of these investigations to the analogous complexes of the unsubstituted tris(pyrazolyl)hydroborato ligand Tp is also reported. Mechanistic studies on the formation of the C-C bond by coupling of the vinyl and the olefin ligands suggest the participation of a vinylidene complex (formed by α-H abstraction from the vinyl group), which then rearranges to an allene species. Evidence for the involvement of these and other key reaction intermediates is provided.
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    Enantioselective Hydrogenation of Imines with Chiral (Phosphanodihydrooxazole)iridium Catalysts (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 887-892 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric catalysis ; asymmetric hydrogenation ; iridium ; nitrogen heterocycles ; phosphorus ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cationic iridium(I) complexes of chiral phosphanodihydrooxazoles were used as catalysts for the enantioselective hydrogenation of prochiral N-alkyl and N-aryl imines. The complexes are air-stable crystalline solids that can be readily prepared and are easy to handle. The structures of two complexes were determined by X-ray analysis. For N-alkyl imines of acetophenone, enantiomeric excesses of up to 79% were obtained. Dialkyl ketimines and cyclic imines showed lower reactivity and selectivity. A remarkable dilution effect was observed for the hydrogenation of the N-phenyl imine of acetophenone: decreasing the substrate and catalyst concentration led to a significant improvement of the enantioselectivity. Thus, up to 89% ee could be achieved using 0.1 mol% of catalyst. The highest enantioselectivities were obtained in weakly coordinating solvents such as CH2Cl2. Additives such as halides, imides, or amines were found to poison the catalyst. Hydrogen pressures of 100 bar were usually employed, but in some cases identical results were achieved with only 1 bar H2.
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    Oligosilsesquioxane8-OSS-Ethyldiphenylphosphine: A New Functional Oligosilsesquioxane Ligand (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 900-903 
    Details
    ISSN: 0947-6539
    Keywords: clusters ; catalyst models ; oligo-silsesquioxanes ; silica ; supported complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligosilsesquioxanes (OSS) may be considered as molecular sections of silica. The incompletely condensed (cy-clopentyl)7T7(OH)3 (T = SiO3/2) has been used to generate a phosphine-functionalized T8-OSS system by reaction with 2-(trichlorosilyl)diphenylphosphinoethane. The resulting model molecule (cyclopentyl)7T8(CH2)2PPh2 (1) is well suited to coordinate to transition metal clusters via the phosphorous atom. We selected the tetrahedral bimetallic cluster [HFeCo3-(CO)12]. Depending on the stoichiometry, one or two phosphine ligands could be added to the cobalt atoms, with substitution of CO molecules. Owing to the cyclopentyl substituents, the resulting complexes [HFeCo3(CO)11{Ph2P(CH2)2T8-(cyclopentyl)7}] (2) and [HFeCo3-(CO)10{Ph2P(CH2)2T8(cyclopentyl)7}2] (3) are soluble in numerous organic solvents. Therefore, they could be completely characterized by 1H, 13C, 29Si, and 31P NMR spectroscopy.
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    Peptide Nucleic Acids with a Conformationally Constrained Chiral Cyclohexyl-Derived Backbone (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 912-919 
    Details
    ISSN: 0947-6539
    Keywords: DNA recognition ; helical structures ; nucleotides ; peptide nucleic acid ; thermal stability ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Peptide nucleic acid (PNA) is an achiral nucleic acid mimic with a backbone consisting of partly flexible aminoethyl glycine units. By replacing the aminoethyl portion of the backbone by an amino cyclohexyl moiety, either in the (S, S) or the (R, R) configuration, we have synthesized conformationally constrained PNA residues. PNA oligomers containing (S, S)-cyclohexyl residues were able to form hybrid complexes with DNA or RNA, with little effect on the thermal stability (Tm = 1°C per (S, S) unit, depending on their number and the sequence). In contrast, incorporation of the (R, R) isomer resulted in a drastic decrease in the stability of the PNA-DNA (or RNA) complex (Tm = -8°C per (R, R) unit). In PNA-PNA duplexes, however, the (R, R)- and (S, S)-cyclohexyl residues only exerted a minor effect on the stability, and the complexes formed with the two isomers are of opposite handedness, as evidenced from circular dichroism spectroscopy. In some cases the introduction of a single (S, S) residue in a PNA 15-mer improves its sequence specificity for DNA or RNA. From the thermal stabilities and molecular modeling based on the solution structure of a PNA-DNA duplex determined by NMR techniques, we conclude that the right-handed helix can accommodate the (S, S) isomer more easily than the (R, R) isomer. Thermodynamic measurements of H and S upon PNA-DNA duplex formation show that the introduction of an (S, S)-cyclohexyl unit in the PNA does indeed decrease the entropy loss, indicating a more conformationally constrained structure. However, the more favorable entropic contribution is balanced by a reduced enthalpic gain, indicating that the structure constrained by the cyclohexyl group is not so well suited for DNA hybridization.
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    A Comprehensive Qualitative and Quantitative Molecular Orbital Analysis of the Factors Governing the Dichotomy in the Dinorcaradiene 1,6-Methano[10]annulene system (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 958-968 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; annulenes ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interconversion between the valence tautomers 1,6-methano[10]-annulene (1) and dinorcaradiene (2) was computationally investigated by qualitative extended Hückel molecular orbital methods (with CACAO visualization) and quantitative semiempirical and ab initio methods. The fundamental bonding interactions are described in terms of perturbation theory arguments for both tautomers, the influence of π-acceptor or π-donor substituents at the C11 position is rationalized, and bonding changes during the interconversion are monitored. The electronic molecular structural preferences can be modified by through-space interactions, and residual C1-C6 σ bonding remains even at the 〉2.1 å separations of the annulenic structures. Hitherto unprecedented calculations of the geometries of C11 homo-disubstituted derivatives of 1 and 2 have been carried out at semiempirical and ab initio levels to identify the more stable tautomer and find whether two stable minima may exist for each derivative. Searches at the AM 1 level to determine transition-state structures for the interconversion of 1 to 2 and of their C11 homo-disubstituted derivatives are also reported.
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    URL: http://dx.doi.org/10.1002/chem.19970030618
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    Kinetic and thermodynamic study of the mechanism of reversible intermolecular electron-transfer reactions between horse heart cytochrome c and a series of cobalt imine complexes (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 39-46 
    Details
    ISSN: 0947-6539
    Keywords: cobalt ; cytochrome c ; electron transfer ; kinetics ; volume profile ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Kinetics and Thermodynamics of the Reversible Outer-Sphere Electron-Transfer Reactions between Horse Heart Cytochrome cII/III and [Co(phen)3]3+/2+ and [Co(bpy)3]3+/2+ were studied in detail, in particular as a function of temperature and pressure. It was possible to construct a volume profile for both reactions from the pressure data. The transition state was found to be halfway between the reactant and product states on a volume basis in all studied systems. This is in agreement with the λ≠ parameter estimated from the Marcus theory. For all the systems investigated, the differences in the activation volumes are in good agreement with the reaction volumes determined from spectrophotometric and electrochemical measurements at elevated pressure, and from the difference in the partial molar volumes of the metal complexes. The activation and reaction volumes of the bipyridine system are significantly smaller than those of the corresponding phenanthroline and terpyridine systems. A detailed mechanistic analysis is presented. The results show that the different kinetic and thermodynamic techniques employed complement one another.
    Additional Material: 7 Ill.
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    Peptide Self-Replication Via Template-Directed Ligation (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1017-1024 
    Details
    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.
    Additional Material: 11 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030706
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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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    Indium-Mediated Reactions of Enamines in the Presence of Acid (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1064-1070 
    Details
    ISSN: 0947-6539
    Keywords: allylations ; enamines ; iminium salts ; indium ; zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of enamines with allyl bromide and metallic indium in THF to afford homoallylamines was greatly accelerated by the addition of one equivalent of a suitable carboxylic acid, such as acetic acid. It was established that the likely mechanism consists of a nucleophilic addition of an indium sesquihalide to the iminium salt formed by protonation of the enamine. Substituted allyl bromides also reacted with complete allylic transposition (γ-addition). In contrast to indium-mediated allylation of carbonyl compounds in which only two of the three allyl groups of the sesquihalide are involved, all three allyl groups are involved in the reaction with enamines. As a result only 2/3 equiv of indium are required. This allylation was also performed with zinc, tin, bismuth, and aluminum in the presence of a catalytic amount of InCl3 instead of indium. However, these reactions invariably gave lower yields. The analogous reaction of methyl bromoacetate instead of allyl bromide was also studied. This “Reformatsky-type” process was also greatly accelerated by the addition of one equivalent of acetic acid. In this case, the yields remained moderate for both indium and zinc.
    Additional Material: 4 Tab.
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    Thermal Conversion of closo-1,2-(SiMe3)2-1,2-C2B4H4 to closo-1,6-(SiMe3)2-1,6-C2B4H4: Structure Determination by Ab Initio Calculations, Gas-Phase Electron Diffraction, and Low-Temperature X-Ray Diffraction (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1059-1063 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; carboranes ; electron diffraction ; rearrangements ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: closo-1,2-(SiMe3)2-1,2-C2B4H4 undergoes thermal conversion to 1,6-(SiMe3)2-1,6-C2B4H4. The reaction pathway was monitored by 11B NMR spectroscopy. The structures of the 1,2- and 1,6-isomers were optimized at the HF/6-31 G* ab initio level. Gas-phase electron diffraction studies for both isomers are reported, as well as low-temperature X-ray crystal structure determinations. Comparison of calculated structural data with the data obtained experimentally shows good agreement between theory and experiment.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970030712
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    A Comparative Study of Oxo-Ligand Effects in the Gas-Phase Chemistry of Atomic Lanthanide and Actinide Cations (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1083-1090 
    Details
    ISSN: 0947-6539
    Keywords: actinides ; C-H activation ; lanthanides ; mass spectrometry ; metal oxides ; oxygen transfer ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effects of oxo ligands on lanthanide and actinide cations have been examined for the mono- and dioxocations MO+ and MO+2 of cerium, neodymium, thorium, and uranium by probing C-H and C-C bond activation of hydrocarbons in an FT-ICR mass spectrometer. The metal monoxide cations are readily available by reaction of the “bare” metal cations with O2, CO2, N2O, or H2O. In the ensuing oxidation of MO+, UO+2 is obtained by each of these oxidants, while CeO+ could only be oxidized by N2O. NO2 was necessary for the generation of ThO+2, and NdO+2 could not be prepared at all. The monoxides are rather unreactive and only dehydrogenate reactive substrates such as 1-butene and 1,4-cyclohexadiene to generate the corresponding butadiene or benzene complexes. In contrast, CeO+2 and ThO+2 react efficiently with different substrates by abstraction of a hydrogen atom and formation of the closed-shell species OMOH+, or by oxygen atom transfer to unsaturated hydrocarbons. In marked contrast, UO+2 only undergoes very slow adduct formation with unsaturated hydrocarbons. The results are compared with the reactions of the “bare” metals with respect to the influence of the oxo ligand as well as to the underlying electronic features of the investigated complexes.
    Additional Material: 4 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19970030716
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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.
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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 
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    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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    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.
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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 
    Details
    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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    Cover page (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), 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.19970031101
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    Stereoselective Umpolung Reactions with Metalated P-Chiral Cyanohydrin Phosphates - Enantioselective Synthesis of Tertiary CyanohydrinsChiral Cyanohydrin Phosphates, part 2. Part 1: see ref. [4]. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1273-1282 
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    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; C—C bond formation ; cyanohydrins ; drug research ; umpolung ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present the first cyanohydrin derivative 4 that allows diastereoselective umpolung reactions. In 4 the OH group of the cyanohydrin is linked with a chiral phosphate, which can be removed hydrolytically after asymmetric synthesis. Cyclization of pseudoephedrine 1d with POCl3 gave 2d. This was followed by addition of racemic benzaldehyde cyanohydrin 3 to give the key intermediate 4d with complete retention of the configuration at phosphorus. Deprotonation of 4d, followed by addition of a wide variety of electrophiles afforded the crystalline products 5 with high diastereomeric excesses (de up to 94%). High asymmetric induction was also achieved for the reaction of 4-Li+ with acyl halides, α-bromoacetates, 2-cycloalkenones, α,β-unsaturated esters, and γ-bromoacetates. Lewis acid assisted hydrolysis proceeded without racemization and gave high yields of ketone cyanohydrins 6. From the ring-opened chiral auxiliary 7, optically pure pseudoephedrine 1 was readily recovered by acid hydrolysis. Optically pure (R) and (S) ketone cyanohydrins are now accessible in a very general strategy, which circumvents the substrate limitations of enzymatic synthesis.
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    Achiral Cyclodextrin AnaloguesPart 2 of a series on synthetic cyclic oligosaccharides. For Part 1, see: P. R. Ashton, C. L. Brown, S. Menzer, S. A. Nepogodiev, J. F. Stoddart, D. J. Williams, Chem. Eur. J. 1996, 2, 580-591. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1299-1314 
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    ISSN: 0947-6539
    Keywords: carbohydrates ; cyclodextrin analogues ; cyclooligomerizations ; glycosylations ; nanostructures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of a new family of cyclodextrin (CD) analogues is described. This family consists of novel cyclic oligosaccharides built from monosaccharides that possess the same relative but opposite absolute (D- and L-) configurations. The alternation of such D- and L- residues - specifically, D- and L-rhamnose or D- and L-mannose - in a macrocyclic structure results in Sn-type symmetry and, consequently, optical inactivity. The synthesis of these cyclic oligosaccharides was achieved by an economical polycondensation/cycloglycosylation approach that relies on an appropriately-derivatized disac-charide monomer and that avoids the time-consuming, and often low-yielding, stepwise growth of long linear oligosaccharide precursors. In the cases reported, the key precursors are the disaccharide monomers 1-RR and 1-MM, which bear both a glycosyl donor (cyanoethylidene function) and a glycosyl acceptor (trity-loxy group). These compounds are able to undergo Tr+-catalyzed polycondensation which, under appropriate dilution conditions, can be terminated by cycloglycosyl-ation. Thus, compound 1-RR was converted into a range of protected cyclic rhamnooligosaccharides 15-19 in 64% overall yield. All these products, including the unique cyclic dodeca- and tetradecasaccharides 18 and 19, have been isolated by preparative HPLC. Unexpectedly, treatment of the manno analogue of the disaccharide 1-RR (compound 1-MM) under the same conditions produced only the cyclic hexasaccharide 28 and numerous apparently linear oligomers. Removal of the protecting groups from 16-19 afforded the free cyclic oligosaccharides 21-24, which exhibited the predicted zero optical rotation and very simple NMR spectra, indicating highly symmetrical structures. X-ray crystallography reveals that in the solid state the cyclooctaoside 21 possesses a C2 symmetric structure, on account of a slight deformation of its cylindrical shape. The channel-type crystal packing of molecules of 21 forms nanotubes with an internal diameter of around 1 nm. Conversely, the cyclic hexasaccharide 29 possesses a Ci symmetric solid-state structure and its molecules pack to form a parquet-like superstructure.
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    Studying the Stability of a Helical β-Heptapeptide by Molecular Dynamics Simulations (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1410-1417 
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    ISSN: 0947-6539
    Keywords: helical structures ; molecular dynamics simulations ; NMR spectroscopy ; peptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: β-Peptides consisting entirely of homochiral β-amino acids R-CH(NH2)-CH2CO2H form 31-helices in solution, as shown previously by NMR analysis of pyridine and methanol solutions. The stability of the helical secondary structure of one such β-peptide (H-β-HVal-β-HAla-β-HLeu-(S,S)-β-HAla(αMe)-β-HVal-β-HAla-β-HLeu-OH, 1) has been investigated by molecular dynamics simulations using the GROMOS 96 molecular model and force field (962 methanol molecules; T = 298, 350, 400 K; with and without NOE distance restraints). The restraints derived from the NMR studies were equally well satisfied by both the restrained and the unrestrained room-temperature molecular dynamics simulations. The 31-helical conformation of 1 was shown to be so stable that it was restored spontaneously within 400 ps after unfolding had been induced by a sudden increase of the temperature from 298 to 350 K.
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    Chiral Lewis Acid Controlled Synthesis (CLAC Synthesis): Chiral Lewis Acids Influence the Reaction Course in Asymmetric Aldol Reactions for the Synthesis of Enantiomeric Dihydroxythioester Derivatives in the Presence of Chiral Diamines Derived from L-Proline (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1472-1481 
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    ISSN: 0947-6539
    Keywords: aldol reactions ; asymmetric synthesis ; diamines ; enantioselective synthesis ; Lewis acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Both enantiomers of 2,3-dihy-droxythioester derivatives were prepared with almost perfect stereochemical control by chiral Lewis acid controlled synthesis (CLAC synthesis). CLAC synthesis means synthesis of all individual diastereomers or enantiomers from the same starting materials by designing chiral Lewis acids. For example, (Z)-1-ethyl-thio-1-(trimethylsiloxy)-2-(tert-butyldi-methylsiloxy)ethene (1) reacted with aldehydes in the presence of chiral tin(II) Lewis acids using (S)-1-methyl-2-[(isoin-dolinyl)methyl]pyrrolidine (4) and (S)-1-methyl-2-[(indolinyl)methyl]pyrrolidine (5) to afford enantiomeric dihydroxy-thioester derivatives. Chiral diamines 4 and 5, which were readily prepared from L-proline, differ only in the fusion point of the benzene ring connected to the pyrrolidine moiety. The unique selectivities were ascribed to the conformational difference between the chiral tin(II) Lewis acids of chiral diamines 4 and 5, and the function of chiral sources for obtaining high selectivities has also been clarified.
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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
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031201
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    Opposite Stereochemical Effects Exerted by CeCl3 and TiCl4 on the Lewis Acid Mediated Reduction of α-Alkyl-β-ketophosphine Oxides with Metallic Hydrides: A Highly Stereoselective Protocol for the Synthesis of syn and anti α-alkyl-β-Hydroxyphosphine Oxides (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1941-1950 
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    ISSN: 0947-6539
    Keywords: alkenes ; asymmetric synthesis ; natural products ; Lewis acids ; phosphane oxides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general, highly efficient methodology for obtaining both syn and anti β-hydroxyphosphine oxides by reduction of the corresponding β-ketophosphine oxides is described. The nature of the Lewis acid was found to be pivotal in determining the outcome of these reactions. Strongly chelating TiCl4 led to the anti isomer in high diastereoisomeric excess in noncoordinating solvents (CH2Cl2) at -78°C with BH3/py as reducing agent, while nonchelating CeCl3 gave a high excess of the syn isomer in coordinating solvents (THF) at the same temperature with LiBH4 as reducing agent. In the latter case, CeCl3 is essential in achieving high yields and stereoselectivity, since it allows the reaction to be performed at low temperatures. Otherwise, higher temperatures (0°C) are required, which lower both yields and selectivities. Moreover, each step of the protocol for the synthesis of stereodefined disubstituted olefins from alkylphosphine oxides (Warren's modification of the Horner procedure) has been optimized, and the optimized procedure has been applied to the synthesis of muscalure, the pheromone of the domestic fly.
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    URL: http://dx.doi.org/10.1002/chem.19970031209
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    Total Synthesis of Oxazole- and Cyclopropane-Containing Epothilone B Analogues by the Macrolactonization Approach (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1971-1986 
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    ISSN: 0947-6539
    Keywords: epothilone ; oxazoles ; cyclopropanes ; total synthesis ; macrolactonizations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to probe structure-activity relationships in the epothilone area, two series of epothilone B analogues have been designed and synthesized. The first series containing an oxazole moiety in place of a thiazole on the side chain was constructed by utilizing key intermediates 7-9 or 10, 12, and 13 (Scheme 1), whereas the second series containing an ethano group instead of the gem-dimethyl group at position 4 was synthesized from fragments 42 and 43. A Yamaguchi-type macrolactonization reaction was used to construct the macrocycle from a secoacid, which was assembled, in both cases, by means of a) an aldol reaction, b) an Enders alkylation, and c) a Wittig-type reaction. This convergent strategy provided access to oxazole analogues 2,4,29-32 and 4,4-ethano derivatives 3,40,60-63 for biological studies.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031212
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    Oxidative Nucleophilic Substitution of Hydrogen in NitroarenesDedicated to Professor Dieter Seebach on the occasion of his 60th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 2025-2031 
    Details
    ISSN: 0947-6539
    Keywords: arenes ; carbanions ; nucleophilic aromatic substitutions ; oxidations ; reaction mechanisms ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Carbanions of 2-phenylpropionitrile were found to add to nitroarenes in liquid ammonia to form σH adducts, which were oxidized with KMnO4 to yield products of oxidative nucleophilic substitution of hydrogen (ONSH) in the position para to the nitro group. Treatment of the carbanion-nitroarene system with methyl iodide at -70°C indicated that the addition proceeds almost to completion. Thus, for the first time, the persistence of σH adducts, formed between free anions and mononitroarenes, has been demonstrated. It was also shown that KMnO4 oxidizes σH adducts to nitrobenzene faster than it does carbanions. This selective ONSH in the para positions of nitroarenes is a general process. However, some substituents hinder or inhibit the oxidation step.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19970031217
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    (CuI)2Cu3SbS3: Copper Iodide as Solid Solvent for Thiometalate ionsDedicated to Professor Hans Georg von Schnering (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 2032-2038 
    Details
    ISSN: 0947-6539
    Keywords: antimony ; chalcogens ; copper ; iodine ; solid-state structures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (CuI)2Cu3SbS3 was prepared by solid-state reaction of stoichiometric amounts of CuI, Cu, S, and Sb. The crystal structure was determined from single crystals at room temperature and - 80°C. Red-orange (CuI)2Cu3SbS3 crystallizes in the orthorhombic system, space group Pnnm (no. 58) with a = 10.488(2), b = 12.619(2), c = 7.316(1) Å, V = 968.2(2) Å3 (20°C), and Z = 4. The distribution of the copper atoms was refined by using a Gram-Charlier nonharmonic development of their atomic displacement parameters, which indicated an increasing dynamic disorder from -80°C to 20°C. Raman spectra of (CuI)2Cu3SbS3 are dominated by strong bands for [SbS3]3- units at 362 and 338 cm-1. The compound melts at 492°C (DTA, onset). Conductivity measurements reveal high ionic conductivity (6.2 × 10-5ω-1 cm-1 at 50°C, 2.9 × 10-3 ω-1 cm-1 at 200°C) with an activation energy of 0.35 eV (40-275°C).
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031218
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    The First Examples of Enantiomerically Pure Diphosphane Dioxides - (RP,RP)- and (SP,SP)-1,2-Di-ter-butyl-1,2-diphenyldiphosphane 1,2-Dioxides, and (RP)- and (SP)-1-tert-Butyl-1,2,2-triphenyldiphosphane 1,2-Dioxides (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 2052-2057 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; chiral ligands ; P ligands ; permutational isomerization ; phosphane oxides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Whereas oxidative dimerization of each of the lithiated reagents (RP)- and (SP)-tert-butylphenylphosphane oxides 1 Li and 2 Li by means of oxygen in THF at low temperature led cleanly to meso-1,2-di-tert-butyl-1,2-diphenyldiphosphane dioxide (3), the respective nucleophilic substitution reactions of 1 Li and 2 Li with the (RP)- and (SP)-tert-butylphenylphosphinic bromides (4 and 6) in THF under argon led to the corresponding (SP,SP)- and (RP,RP)-12-di-tert-butyl-1,2-diphenyldiphosphane dioxides (5 and 7) in good yields, together with the meso-diphosphane dioxide 3. The structures of 3 and 5 were established by X-ray crystallography. Similarly, the reaction of lithiated (RP)-phosphane oxide 1 Li with diphenylphosphinic bromide (8) or of achiral lithiated diphenylphosphane oxide 10 Li with (RP)-tert-butylphenylphosphinic bromide (4) provided (SP)-tert-butyltriphenyldiphosphane dioxide (9).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031221
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    Some Remarks on the Occurrence and Properties of Knödel (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 2081-2081 
    Details
    ISSN: 0947-6539
    Keywords: carbohydrates ; correspondence ; dendrimers ; micelles ; nanospheres ; 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.19970031225
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    N-Alkyloxycarbonyl-3-aryloxaziridines: Their Preparation, Structure, and Utilization As Electrophilic Amination Reagents (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1691-1709 
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    ISSN: 0947-6539
    Keywords: aminations ; electrophilic substitutions ; hydrazines ; oxaziridines ; pseudopeptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper reports the synthesis of a series of N-protected oxaziridines (N-Moc, Boc, Z or Fmoc) and discusses their ability to deliver their N-alkoxycar-bonyl fragment to amines, enolates, sulfur, and phosphorus nucleophiles (electrophilic amination). These oxaziridines are prepared by oxidation of the corresponding imines with oxone or anhydrous MCPBA lithium salt as the source of oxygen. They transfer their N-protected fragment to primary and secondary amines to give protected hydrazines in fair to excellent yield. The nitrogen transfer to free amino acids (in form of their R4N+ salts) is particularly fast, even at low temperature, providing L (or D) N-protected α-hydrazino acids. Enolates are C-aminated to give N-protected α-amino ketones, esters, or amides in modest yield, due to a side aldol reaction of the unreacted enolate with the released benzaldehyde. With tertiary amines (Et3N), sulfides (PhSMe), and phosphines (Ph3P), amination and oxidation proceed in a parallel way; the amount of amination product increases when the temperature is lowered (kinetic control). Some of the factors that can orient the oxaziridine reactivity towards amination or oxidation of nucleophiles are considered.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031019
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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.19970031102
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    Gas-Phase Reactions at Low Temperatures: Towards Absolute ZeroFrom a lecture delivered by I. W. M. S. at the meeting of the British Association for the Advancement of Science at the University of Birmingham, September 9-13, 1996. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1925-1928 
    Details
    ISSN: 0947-6539
    Keywords: gas-phase chemistry ; interstellar chemistry ; kinetics ; low-temperature chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Very recently, methods have been developed for studying the rates of chemical reactions between electrically neutral species in the gas phase at temperatures as low as 13 K. Here we review this exciting development in gasphase chemical kinetics. Emphasis is placed on the importance of the information which the experiments have provided, both from the viewpoint of posing new questions about the factors which control chemical reactivity and because of their relevance to the modelling of molecular synthesis in interstellar clouds.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031206
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    Metallic Hydrogen at High Pressures and Temperatures in JupiterFrom a lecture delivered at the meeting of the British Association for the Advancement of Science at the University of Birmingham, September 9-13, 1996. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1921-1924 
    Details
    ISSN: 0947-6539
    Keywords: high-pressure chemistry ; high-temperature chemistry ; hydrogen ; Jupiter ; metallic hydrogen ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electrical conductivities of hydrogen were measured recently at 93-180 GPa (0.93-1.8 Mbar), 0.28-0.36 mol cm-3, and 2000-4000 K. Shock temperatures were measured up to values of 5200 K at 0.83 Mbar. The conductivities indicate a continuous transition from a semiconducting to metallic fluid at 140 GPa and 3000 K. The results indicate that hydrogen becomes metallic much closer to the surface of Jupiter than thought previously and that Jupiter has no core-mantle boundary.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19970031205
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    Asymmetric Addition Reactions with Optimized Selenium Electrophiles (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1894-1902 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; chirality ; diselenides ; selenium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of various nonracemic diselenides by different methods is described. These diselenides are precursors for optically active selenium electrophiles. Their facial selectivity upon addition to styrene was investigated with respect to the chiral moiety neighboring the selenium. Diselenides 1 i, 1 n, and 1 v yielded addition products 7 with diastereomeric excesses up to 95%. Some diselenides, intermediates, and products of the addition reaction were investigated by 77Se NMR spectroscopy.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19970031123
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    Graphical abstract (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1911-1916 
    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.19970031203
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    Molecules in SpaceFrom a lecture delivered at the meeting of the British Association for the Advancement of Science at the University of Birmingham, September 9-13, 1996. (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1929-1932 
    Details
    ISSN: 0947-6539
    Keywords: gas-phase chemistry ; interstellar chemistry ; ion-molecule reactions ; star formation ; surface chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The distribution and nature of interstellar matter in the Galaxy is described. The chemical processes by which the rich variety of molecular species arise are briefly sketched. The importance of interstellar molecules in influencing the evolution of the Galaxy is emphasized.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031207
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    Oligonucleotide Analogues with 4-Hydroxy-N-Acetylprolinol as Sugar Substitute (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1997-2010 
    Details
    ISSN: 0947-6539
    Keywords: antisense agents ; chiral recognition ; oligonucleotides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Modified oligonucleotides incorporating trans-4-hydroxy-N-acetyl-L-prolinol (trans-4-HO-L-NAP) or its D-analogue as sugar substitute were synthesised with adenine and thymine as nucleobases. All-adenine oligonucleotides built from (2S,4S) or (2R,4R)-cis-4-hydroxy-N-acetylprolinol were likewise prepared. Hybridisation studies revealed that heterocomplexes formed between polyU and homochiral trans-4-hydroxy-N-acetylprolinol-based oligomers of the same as well as of opposite chirality (polyU/trans-DA*13 and polyU/trans-LA*13). The former, however, were triple-stranded. Other complexes with ribonucleic acids were polyA/trans-LT*13 and polyU/cis-LA*13. Heteroduplexes with deoxynucleic acids were formed between trans-LA*13 and oligothymidylate. Interaction was also observed for cis-LA*13 and oligothymidylate, but not with the D-hydroxyprolinol analogues. Microcalorimetry proved this interaction to be the formation of a triple-stranded complex. Two heteroduplexes, trans-LA*13/dT13 and trans-LA*13/polyU, had similar or slightly increased stability when compared to the natural dA13/dT13 or dA13/polyU systems. Microcalorimetry clearly indicated the formation of a duplex, in contrast to interactions with N-acetylprolinol oligonucleotides of different stereochemistry. Moreover, the enthalpy change was of the same magnitude but the association constant was slightly lower. Natural nucleic acids thus clearly prefer hybridisation with L-hydroxyprolinol oligomers over D-hydroxyprolinol oligomers. For the series investigated, the L-trans oligomers (Figure 1) seem best to mimic natural oligonucleotides. These modified oligonucleotides formed homocomplexes if both strands were of the same chirality, that is, homocomplexes formed between trans-LA* and trans-LT* and between trans-DA* and trans-DT*, reflecting the isochiral pu-py pairing found in natural nucleic acids. Once more, however, calorimetry proved these to be triplex interactions. Heterochiral pairing was not observed between modified oligonucleotides, but only between modified oligonucleotides and natural polyU. The thermal stabilities of these heterochiral complexes differed clearly.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031215
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    Rapid Coupling of Methyl Iodide with Aryltributylstannanes Mediated by Palladium(0) Complexes: A General Protocol for the Synthesis of 11CH3-Labeled PET Tracers (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 2039-2042 
    Details
    ISSN: 0947-6539
    Keywords: arylphosphanes ; C-C coupling ; isotopic labeling ; palladium ; tin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of methyl iodide and (excess) aryltributylstannane to give a methylarene has been studied with the focus on the realization of rapid coupling for incorporation of short-lived radionuclides into bioactive organic compounds. The coupling of methyl iodide with tributylphenylstannane (40 equiv) is accomplished in 〉90% yield within 5 min at 60°C with a tri-o-tolylphosphine-bound, coordinatively unsaturated Pd0 complex together with a CuI salt and K2CO3 in DMF. This protocol is applicable to a variety of homo- and heteroaromatic tin compounds, to give the corresponding methylated derivatives. The effects of the tri-o-tolylphosphine ligand, a Cu(I) salt, and DMF are discussed. This new protocol provides a firm chemical basis for the synthesis of 11CH3-incorporated PET tracers.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19970031219
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