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  • Articles: DFG German National Licenses  (1,786)
  • 1995-1999  (1,786)
  • 1935-1939
  • 1996  (1,786)
  • General Chemistry  (1,786)
  • Nuclear reactions
  • 101
    Electronic Resource
    Electronic Resource
    Fluorescent Sensors for Transition Metals Based on Electron-Transfer and Energy-Transfer Mechanisms (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 75-82 
    Details
    ISSN: 0947-6539
    Keywords: copper complexes ; electron transfer ; energy transfer ; fluorescent sensors ; nickel complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fluorescent sensors for 3d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal-receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one-electron oxidation of the metal, quenching takes place through a photoinduced metal-to-fluorophore electron-transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy-transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., CuII from NiII) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam-containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020114
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  • 102
    Electronic Resource
    Electronic Resource
    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 125-128 
    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.19960020203
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  • 103
    Electronic Resource
    Electronic Resource
    Studies on the Topography of Biomembranes: Regioselective Photolabelling in Vesicles with the Tandem Use of Cholesterol and a Photoactivable Transmembrane Phospholipidic Probe (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 129-138 
    Details
    ISSN: 0947-6539
    Keywords: bilayers ; cholesterol ; membrane ; probes ; phospholipids ; photolabelling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2H NMR, DSC and UV studies of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) vesicles have demonstrated the favourable physicochemical properties of the transmembrane phospholipid probes 1 a and 1 b for membrane topographical studies. In particular, in the presence of a physiological amount of cholesterol, only one transmembrane conformation is observed. The use of 1 a and cholesterol together for photolabelling experiments in DMPC vesicles led to a remarkable improvement in the regioselectivity of cross-linking between 1 a and DMPC, and between 1 a and cholesterol: the myristoyl chains functionalized at C11, C12 and C13 made up 95% of the total photolabelled myristates, and cholesterol was principally functionalized at the C25 position on the side-chain. This indicates the formation of a highly ordered bilayer structure and proves directly the orientation of cholesterol perpendicular to the membrane plane with its chain terminal buried in the middle of the bilayer.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020204
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  • 104
    Electronic Resource
    Electronic Resource
    The “Peroxo Perrhenic Acid” H4Re2O13: An Oxygen-Rich Metal Peroxide and Oxidation CatalystMultiple Bonds between Transition Metals and Main-Group Elements, Part 149. Part 148: W. A. Herrmann, F. E. Kühn, C. C. Romão, J. Organomet. Chem. 1995, 495, 209-213.Dedicated to Professor Herbert Roesky on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 168-173 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; crystal structure ; oxidations ; rhenium peroxo complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rhenium oxides Re2O7 and ReO3 react with hydrogen peroxide solutions yielding peroxo complexes that efficiently catalyze the oxidation of olefins, aromatics, and certain organometallic compounds. In contrast, related oxides of molybdenum (MoO3) and vanadium (V2O5) do not activate H2O2 under comparable conditions. The active rhenium peroxo complex was isolated from the system Re2O7/H2O2: the crystalline red-orange, explosive compound of formula H4Re2O13 is the most oxygen rich rhenium compound isolated to date. Its structure resembles a “peroxo perrhenic acid”. The binuclear compound could be isolated in the form of a diglyme adduct, structurally defined as two corner-sharing pentagonal bipyramids with apical oxo and aquo ligands; the equatorial positions are occupied by the bridging oxygen and by n2-peroxo groups (two [O2]2- ligands per rhenium). In contrast to the known complex [CH3ReO(O2)2].H2O, the new peroxo species [O{ReO(O2)2.H2O}2] decomposes hydrolytically during the catalytic cycle and can thus not compete in terms of catalytic activity in oxidation reactions involving H2O2. Hydrolysis yields “perrhenic acid” Re2O7.2H2O, the diglyme adduct of which compound was also characterized by means of an X-ray diffraction analysis.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020208
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  • 105
    Electronic Resource
    Electronic Resource
    The Effect of Microsolvation on E2 and SN2 Reactions: Theoretical Study of the Model System F- + C2H5F + nHF (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 196-207 
    Details
    ISSN: 0947-6539
    Keywords: eliminations ; orbital interactions ; solvent effects ; substitutions ; theoretical chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The model reaction system F- +C2H5F + nHF (n = 0-4) has been investigated by use of a density-functional method, in order to achieve a qualitative understanding of the effect of solvation on the E2 and SN2 reactions. Two characteristic effects already occur upon monosolvation: a) the activation energies of the E2 and SN2 pathways increase significantly and even become positive, because reactants are more strongly solvated than transition states; b) the SN2 transition state is stabilized much more and becomes lower in energy than the anti-E2 transition state. This agrees with general experience from gas- and condensed-phase experiments. The solvation is analyzed from two complementary viewpoints: a) as an interaction between solvent molecules and the F-/C2H5F reaction system; b) as an interaction between the [F-, nHF] solvated base and the C2H5F substrate. The extent to which condensed-phase characteristics can be modeled by this microsolvation approach is discussed.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020212
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  • 106
    Electronic Resource
    Electronic Resource
    1,2,3-Triphosphetenes, First Examples of a New Class of Phosphorus HeterocyclesReactive E=C(p-p)π Systems, Part 41. Part 40: see ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 208-213 
    Details
    ISSN: 0947-6539
    Keywords: heterocycles ; insertion reactions ; phosphaalkynes ; phosphorus ylides ; triphosphetenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hitherto unknown 1,2,3-triphosphetenes RC=P—PCF3—PCF3 (3a-d) are formed as main products in reactions of phosphaalkynes R—C≡P [R = iPr2N (1a), tBu (1b), Me2EtC (1c), 1-methylcyclohexyl (1d)] with the cyclotetraphosphane (PCF3)4 (2). According to NMR results the CF3 groups in 3a-d have a trans disposition; an X-ray diffraction study of 3a confirms this structure. The P—P bond lengths in 3a are equal [2.201 (2) and 2.204 (2) Å] and correspond to single bonds. A considerable shortening is observed for the sp2-C-N bond (1.336 Å) which, together with the elongation of the P=C bond (1.746 Å), indicates effective π donation of the lone pair on nitrogen. Surprisingly, 3a can be prepared in quantitative yields by reaction of the PP ylide Me3P=PCF3 (6) with 1 a (molar ratio: 2:1). In contrast, the corresponding reactions of 6 with the alkyl-substituted phosphaalkynes 1b-d lead to the novel phosphorus ylides Me3P=C(R)—P—PCF3—PCF3PCF3 [R = tBu (10a), Me2EtC (10b), 1-methylcyclohexyl (10c)] in good yields. In their molecular groundstate structures, determined by X-ray diffraction, the lone pair on the phosphano P atom prefers the syn position with respect to the ylidic P=C bond. An unusual lengthening of the sp2-C-C bond [1.553 (4) (10a), 1.543 (6) (10b), 1.551 (4) Å (10c)] to values typical for sp3-C/sp3-C distances is observed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020213
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  • 107
    Electronic Resource
    Electronic Resource
    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    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.19960020302
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  • 108
    Electronic Resource
    Electronic Resource
    Zintl Anions as Starting Compounds for the Synthesis of Polynuclear Transition Metal ComplexesDedicated to Professor Kurt Dehnicke on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 238-244 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; density functional calculations ; polyarsenido complexes ; polyphosphido complexes ; Zintl anions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Zintl anion P3-7 reacts with complexes of transition metal halides to form multinuclear metal phosphorus clusters. Reaction of Li3P7·3 DME with [FeCp(CO)2Br] or [NiCl2(PBu3)2] leads to the formation of [P7{FeCp(CO)2}3] (1) and [{Ni(PBu3)2}4P14] (2), respectively. X-ray structure determinations show that in 1 the P7 cage of Li3P7 remains intact but in 2 a P14 framework is formed by linkage of two norbornadiene-like P7 units. The P14 skeleton coordinates to four Ni(PBu3)2 groups. LiCp* and CoCl2 react with P7(SiMe3)3 to give [(Cp*Co)3-(P2)3] (3) with three Cp*Co groups bridged by P2 units. Reaction of FeCl2 and LiCp* with P7(SiMe3)3 yields [(Cp*Fe)3P6][FeCl3(thf)] (4) or [(Cp*Fe)3{n3-P3)Fe}P6] (5), depending on the reaction conditions. In 4 the structure of the Fe3P6 core of the [(Cp*Fe)3P6]+ cation is comparable to closo-B9H92-, while in 5 the Fe4P6 core does not obey the Wade rules. The unusual Co-As cluster [Co6As12(PEt2Ph)6] (6), prepared from K, As, and [CoCl2(PEt2-Ph)2], can be described as a Co6As6 heteroicosahedron linked to two Co3As3 octahedra by common Co3 faces. A theoretical treatment within the density functional approximation reproduces the experimental structures of 2 and 6 and allows an interpretation of molecular electronic structures. In 2 one finds P-P double bonds that are delocalized to some extent into vacant Ni AOs. For the cage compound 6 the Co 3 d AOs participate in cage bonding and 38 electrons can be assigned to cage bonds; this is in accord with the (2N + 2) rule for 18-atom cages but not with the usual electron counting rules.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020217
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  • 109
    Electronic Resource
    Electronic Resource
    Thianthrene 5-Oxide as a Mechanistic Probe in Oxygen Transfer Reactions: The Case of Carbonyl Oxides versus Dioxiranes Revisited (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 255-258 
    Details
    ISSN: 0947-6539
    Keywords: carbonyl oxides ; dioxiranes ; mechanistic studies ; oxidations ; thianthrene 5-oxide ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thianthrene 5-oxide (SSO) constitutes a useful mechanistic tool for the assessment of the electronic character of oxygen transfer agents by means of their Xso values, which reflect the extent of nucleophilic oxidation at the SO site in SSO. Treatment of dioxiranes 1 a-d with the SSO probe confirms that these are electrophilic oxidants (Xso 〈 0.15). Dioxirane sulfoxidation is sensitive to protic solvents and acids, which implies a polar mechanism (SN2) with nucleophilic attack of the sulfide electron pair on the dioxirane peroxide bond. In contrast, the carbonyl oxides 2 a and b, diethyl persulfoxide (3), and adamantylideneadamantane perepoxide (4) are nucleophilic oxidants (Xso 〉 0.85). However, the cyclopentadienone carbonyl oxides 2 c and d show low Xso values typical for electrophilic oxygen transfer agents. For these carbonyl oxides, photoisomerization to the respective dioxiranes is proposed under the photooxidation conditions of the diazoalkanes. Additionally, the trans/cis ratio of bissulfoxides (SOSO) formed provides valuable information on the stereochemical course of the oxygen-transfer process due to the steric requirements of the oxidant. Thus, the electrophilic dioxiranes preferentially attack the axial lone pair of SSO because of repulsion by the peri hydrogen atoms to afford trans-SO-SO.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020304
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  • 110
    Electronic Resource
    Electronic Resource
    Polysaccharides of Intracrystalline Glycoproteins Modulate Calcite Crystal Growth In Vitro (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 278-284 
    Details
    ISSN: 0947-6539
    Keywords: biomineralization ; calcite ; crystal morphology ; glycoproteins ; polysaccharides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Assemblies of glycoproteins from within the mineralized tissues of sea urchins and mollusks both interact in vitro in a similar manner with growing calcite crystals. A protein-rich fraction, a polysaccharide-rich fraction, and a fraction composed of densely glycosylated peptide cores were obtained by chemical and enzymatic treatment of the glycoproteins from sea-urchin spines. Each fraction was partially purified and characterized (amino acid composition, FTIR and NMR spectroscopy). A comparison of the interactions of these fractions with growing calcite crystals in vitro shows that the polysaccharide moieties of these glycoproteins are intimately involved in the interaction with growing calcite crystals on planes approximately parallel to the c crystallographic axis. Presumably the polysaccharides in the mollusk-shell glycoproteins are likewise responsible for the similar interactions of these macromolecules with calcite. We suggest that structured polysaccharide moieties of glycoproteins are important in controlling aspects of crystal growth in vivo as well.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020308
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  • 111
    Electronic Resource
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    Heterosupramolecular Chemistry: An Approach to Modulating Function in Molecular Devices (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 420-428 
    Details
    ISSN: 0947-6539
    Keywords: anthraquinones ; molecular devices ; supramolecular chemistry ; titanium dioxide ; viologens ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Linked viologen-anthraquinone molecules are attached to TiO2 nanocrystallites supported on a conducting glass substrate. The resulting assembly is incorporated as the working electrode in an electrochemical cell. Under opencircuit conditions, band-gap excitation of the semiconductor nanocrystallite results in viologen-mediated electron transfer to anthraquinone. Prior application of a negative potential step, which results in 2e-/2H+ reduction of anthraquinone, permits electron transfer only to viologen. At positive applied potentials, electron transfer following band-gap excitation is largely suppressed. Some implications of these findings for modulation of function in molecular devices are considered.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020411
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  • 112
    Electronic Resource
    Electronic Resource
    Pulled Molecular Strings and Stacked Molecular Decks: Chelate-Ring Formation vs. Metal-Metal Bridging in Dicopper(I) Complexes of 2,2′-Bipyrimidine with Diphosphine Ligands of Variable Polymethylene Chain Length (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 446-451 
    Details
    ISSN: 0947-6539
    Keywords: bridging ligands ; chelate ligands ; copper complexes ; self-assembly ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The “self-assembly” reaction between bischelating 2,2′-bipyrimidine, metallic copper, Cu2+, and two equivalents of bidentate Ph2P(CH2)nPPh2 produces crystallographically characterized dinuclear complexes with a [Cu1(μ-bpym)-Cu1]2+ core and chelating (n = 5; type I) or bridging diphosphine ligands (n = 6, 8; type II). Structures II may be designated as “inverse cryptate” arrangements. The structural data of the solids and the spectroscopic results for the dissolved species indicate a strained configuration in the case of complex [(μ-bpym)Cu2{μ-Ph2P-(CH2)n]2+ (6) with n = 6. Once the polymethylene chain length is reduced one notch further to n = 5 (5), the diphosphines can no longer bridge but help to form eight-membered chelate rings (structure I). In this complex there is a distortion towards a trigonal-pyramidal coordination geometry at the copper(I) centers, which is supported by the formation of an “organic sandwich” arrangement between the central bpym acceptor and two phosphino phenyl rings.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020414
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  • 113
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    Electronic Resource
    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020501
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  • 114
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    A Novel Route to Racemic and Nonracemic Products of the Ugi Reaction: Synthesis of Ugi′s Labile α-Adducts from Iminoaziridines and Carboxylic Acids, and their TransformationsIminoaziridines, Part 7 [1]. Part 6: ref. [37].Dedicated to Professor Ivar Ugi on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 462-469 
    Details
    ISSN: 0947-6539
    Keywords: aziridines ; imides ; isoimides ; rearrangements ; Ugi reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Iminoaziridines (11) are highly reactive synthetic equivalents for three of the four components in the Ugi four-component condensation. Thus, iminoaziridines react rapidly with carboxylic acids at temperatures as low as -20°C to afford α-amino isoimides (14), which are identical to the elusive α-adducts of isocyanides in the Ugi reaction. 1,4-Migration of the acyl group (O → α-N) in 14 furnishes the α-acylamino amides 15. Very little, if any, racemisation is observed when carboxylic acids react with nonracemic iminoaziridines [(R)-11 a,c], which are readily available. Mumm rearrangement by O → N-acyl 1,3-migration to afford α-amino imides (16 e, f) competes if the O → α-N 1,4-shift is slowed down by steric hindrance. The latter acyl shift is catalysed by carboxylic acids while the former is not. The iminoaziridines (R)-11 a,c react quantitatively and without racemisation with hydrazoic acid to produce the 5-aminoalkyltetrazoles (R)-21 a,c.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020417
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  • 115
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    Electron-poor 2,3-Dihydro-1,3-Diborolyl Complexes of Iron and Ruthenium: Synthesis, Reactivity, and Crystal and Electronic Structures of an Iron Sandwich ComplexDedicated to Professor Herbert Schumann on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 487-494 
    Details
    ISSN: 0947-6539
    Keywords: boron compounds ; diboroles ; Group 8 complexes ; sandwich complexes ; semi-empirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition product of sodium hydride and the 2,3-dihydro-1,3-diborole (CiPr)2(BEt)2CHMe (3c) reacted with [{(C5Me5)FeCl}x] to produce the green sandwich complex [(C5Me5)Fe{n5-(CiPr)2(BEt)2CMe}] (2 c), which formally contains 16 valence electrons (VE). Complex 2c has unexpected structural properties in the solid state: the 1,3-diborolyl ring is extremely folded (41°), and the Fe-C2 distance is short (1.90 Å). Analogously, violet Ru complexes 4a,c,d were obtained from 3a,c, NaH or tBuLi, and [{(C5Me5)RuCl}4]. With the less bulky heterocycles 3 b,e the new 30 VE triple-decker complexes [(C5Me5)Ru{μ,n5-(CR1)2(BR2)2CMe}RuH(C5Me5)] (5b,e) were formed, which contain a Ru-H bond. Cyclic voltammetric studies revealed the existence of stable anions 2c- and 4d- formed by reversible one-electron reduction at -1.26 and -1.40 V, respectively (vs. SCE). The red-brown anions were further characterized by ESR spectroscopy following stepwise reduction of the neutral species with potassium in THF. Addition of CO to 4a and 4d led to formation of the monocarbonyl complexes [(C5Me5)Ru(CO){n5-(CR1)2-(BR2)2CMe}] (6a,d), and 6d was characterized by X-ray structure analysis. The heterocycle in 6d is less folded (19°) than in 2 c. Its CO ligand causes a 28.5° tilt of the cyclic ligands. Reaction of CO with 2 c yielded a red product of unknown structure. The electronic structure of 2 was studied by EH-MO theory, which revealed a unique bonding in the sandwich. The s̰ electron density of the B-C bonds participates in the bonding to the iron atom; this demonstrates that the number of bonding electrons is the same as in ferrocene. Thus, the complexes 2 actually have 18 VE.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020506
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  • 116
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    Separation of Diastereomeric and Enantiomeric Alkyl Nitrates-Systematic Approach to Chiral Discrimination on Cyclodextrin LIPODEX-D (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 539-544 
    Details
    ISSN: 0947-6539
    Keywords: analytical methods ; alkyl nitrates ; chiral resolution ; cyclodextrins ; gas chromatography ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High-resolution gas chromatographic separation of all diastereomeric monomethyl-substituted cyclohexyl nitrates is shown on a nonpolar methylpolysiloxane stationary phase, and the first application of this procedure to the environmental diastereomeric analysis of alkyl nitrates is presented. Two characteristic signals in the achiral analysis of atmospheric samples could be assigned to the smallest alkyl nitrate containing two asymmetric carbon atoms, 3-methyl-2-pentyl nitrate. Retention indices in the temperature-programmed separation based on the n-alkanes were determined. The homologous series of 1-alkyl nitrates were found to be useful as ECD-visible n-alkanes. Enantiomeric separation of alkyl nitrates was achieved on heptakis(3-O-acetyl,-2,6-di-O-pentyl)-β-cyclodextrin (LIPODEX-D). The influence of the nitrooxy group and the alkyl chain length on the chiral discrimination on LIPODEX-D is discussed for 25 chiral alkyl nitrates. The absolute configurations of some alkyl nitrates were assigned by asymmetric synthesis of enantiomerically pure references. The complexity of the alkyl nitrate mixtures present in air samples does not allow a direct chiral separation as the alkyl nitrates partly coelute on the LIPODEX-D column. Column coupling of LIPODEX-D with a polar achiral stationary phase like polyalkylenglycol (PAG) was successfully applied to solve this problem, and the chiral alkyl nitrates present in a typical air sample were separated. A systematic nomenclature for alkyl nitrates is introduced to handle the steadily growing number of branched and long-chain alkyl nitrates detected in environmental analysis.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020513
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  • 117
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    The Pagodane → Dodecahedrane Concept-Shorter Routes, Higher YieldsDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 570-579 
    Details
    ISSN: 0947-6539
    Keywords: cage effects ; dodecahedranes ; organic synthesis ; pagodanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two variants of the “SN2 route” from pagodanes (A, B) to functionalized dodecahedranes (D, F) and particularly dodecahedradienes (E) offer considerable improvements in the number of operations (from nine to five to three) and yields (e.g., for diester F from 55-65 to 70-75 to 85-91%). Key steps are the regio- and stereospecific introduction of four to six bromine substituents into dimethyl pagodane-4-syn,9-syn-dicarboxylate (1 b) and a highly complex (thirteen bond-breaking/bond-forming events in four participating structures). yet very convenient (one-pot operation) and extremely efficient (nearly quantitative) transformation of secopagodane to bissecododecahedradiene with complete stereocontrol in transannular CH2 functionalizations. The prohibitively low kinetic acidity of “caged” hydrogens has so far only been overcome with the recently reported P2F reagent (Schwesinger). Further improvement of the overall economy of the pagodane → dodecahedrane scheme has been achieved by efficiently channeling a byproduct of the pagodane synthesis (ca. 10%) back into the SN2 track.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020517
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    O-Atom Transfer to Fe+n Clusters (n = 2-10) from O2, N2O and CO2: “Microoxides of Iron”This work is part of O. Gehret's dissertation (Darmstadt, D17). (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 598-603 
    Details
    ISSN: 0947-6539
    Keywords: clusters ; gas-phase chemistry ; ionmolecule reactions ; iron complexes ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the gas phase reactions of small Fe+n clusters (n = 2-10) with O2, N2O and CO2 in an FT-ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2-4) appeared to activate CO2. For all X-O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well-known X-O bond energies were used to calculate Fen-O+ bond dissociation energies (BDEs) for sizes n = 2-6; these amount to roughly 550 kJ mol-1 and thus are considerably higher than the BDE of the Fe-O+ ion. All oxidation reactions of the Fe+n clusters (n = 2-6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1-4). These “microoxides of iron” are not able to activate any further X-O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020520
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    A [3]rotaxane of the amide type (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 640-643 
    Details
    ISSN: 0947-6539
    Keywords: lactam ; macrocycles ; rotaxanes ; self-assembly ; template syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new rotaxane types, the [2]rotaxanes 12a,b and the [3]rotaxane 13, have been synthesised by treating the “axle” 7 with the stopper component 9 in the presence of the “wheel” 8. A nonionic template intermediate of type 10 is proposed. The [3]rotaxane 13 was only obtained when the chain of the “axle” had a certain length: with n = 1 only [2]rotaxane 12 a was isolated, whereas with n = 2 the [3]rotaxane 13 was formed besides the [2]rotaxane 12 b. This suggests that more extended rotaxanes and polyrotaxanes can be synthesised by the template strategy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020605
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    Bis(ZnII-cyclen) complex as a novel receptor of barbiturates in aqueous solution (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 617-623 
    Details
    ISSN: 0947-6539
    Keywords: barbiturates ; macrocycles ; molecular recognition ; receptors ; zinc complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new bis-zinc(II) receptor (Zn2L), which has two macrocyclic 12-membered tetraamine (cyclen) ZnII complexes connected through a p-xylene bridge, has been synthesized as a novel host molecule to recognize barbiturates (such as barbital (bar)) in aqueous solution. Each of the zinc(II) ions in the biszinc(II) receptor was originally intended to match the dianionic barbital anion (bar2-) with supplementary hydrogen bonds between the cyclen NH's and the three carbonyl oxygens in complementary positions to yield a 1:1 complex, Zn2L-bar2-. From an aqueous solution of equimolar Zn2L and barbital at pH 8, however, a cyclic 2:2 complex, (Zn2L-bar2-)2, was isolated and characterized by X-ray crystal analysis. The NMR study in 10% (v/v) D2O/H2O has revealed dissociation of (Zn2L-bar2-)2 solely into the original target 1:1 complex Zn2L-bar2- and established the dimerization constant for 2Zn2L-bar2- → (Zn2L-bar2-)2, Kd ( = [(Zn2L-bar2-)2]/[Zn2L-bar2-]2) to be 103.4 M-1. The thermodynamic parameters were evaluated from the NMR measurements at 25, 35, 45, and 55°C: ΔG = -1.9 × 104 J mol-1, ΔH = - 3.3 × 104 J mol-1, ΔS = - 49 J mol-1 K-1 at 25°C. Potentiometric pH titration of Zn2L (1 mM) and barbital (1 mM) disclosed extremely facile deprotonation of the two imido groups of barbital at pH less than 7 to form the dianionic barbital-bound ZnII complexes Zn2L-bar2- and (Zn2L-bar2-)2, where-by the barbital binding affinity for Zn2L was estimated to be Kbar ( = [Zn2L-bar2-]/[uncomplexed Zn2L][uncomplexed barbital]) = 105.8 M-1 at pH 8 and 25°C with I = 0.10 (NaNO3). The significance of the bis-zinc(II) receptor in stabilizing the dianionic barbital is evident by comparison with the interaction of ZnII-cyclen complex (ZnL) with barbital, which yields only a 1:1 monoanionic barbital complex, ZnL-bar- (Kbar = [ZnL-bar-]/[uncomplexed ZnL][uncomplexed barbital] = 104.2M-1 at pH 8 and 25°C with I = 0.10 (NaNO3)).
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020602
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    Dimensional reduction in II-VI materials: A2Cd3Q4 (A = K, Q = S, Se, Te; A = Rb, Q = S, Se), novel ternary low-dimensional cadmium chalcogenides produced by incorporation of A2Q in CdQ (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 656-666 
    Details
    ISSN: 0947-6539
    Keywords: cadmium compounds ; chalcogen compounds ; crystal structure ; dimensionality ; photoluminescence ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the isomorphous, layered chalcogenides K2Cd3S4 (I), Rb2Cd3S4 (II), K2Cd3Se4 (III), Rb2Cd3Se4 (IV), and K2Cd3Te4 (V) in molten A2Qx fluxes in reported (A = K, Rb; Q = S, Se, Te; x = 2 to 3). The compounds form as (Cd3Q4)n2n- layers interspersed with A + cations; the layers are composed of Cd3Q2-4 units shaped as truncated cubes. The compounds have room-temperature band gaps of 2.75, 2.92, 2.36, 2.37, and 2.26 eV for I, II, III, IV, and V, respectively, and also display strong photoluminescence. The thermal analysis data for all compounds are reported. The properties of these compounds are compared with those of the three-dimensional compounds CdS, CdSe, and CdTe, as well as those of the nanometer-sized CdQ clusters. A conceptual context is presented to connect all these different types of compounds.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020607
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    A tricyclic dehydrorubanone and new isomers of the major quinidine metabolite (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 680-684 
    Details
    ISSN: 0947-6539
    Keywords: amino alcohols ; asymmetric syntheses ; dihydroxylations ; diol cleavage Horner-Wittig reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spiroepoxide 1 was prepared from quinidine and converted into β-amino alcohol 3 (86% over two steps). Dihydroxylation of enantiopure oxazatricylic olefin (E)-4 provided diastereomeric diols 5a and 5b. Stereospecific conversion of 1,2-secondary, tertiary diol 5b into tetracyclic spiroepoxide 6 was accomplished in high yield by a one-pot tosylation-cyclization procedure. 1,2-Diol cleavage with NaIO4 in 80% acetic acid afforded the new tricyclic dehydrorubanone 7, containing the 4-oxa-7-azatricyclo[4.3.1.03,7]-decan-2-one core structure. Similarly, acetylated rubanone 9 was prepared on a 20 g scale. Reduction with NaBH4 in the presence of CeCl3 provided rubanols 10a and 10b (1:1.1). Horner-Wittig reaction of 9 with diethyl cyanomethylphosphonate was (Z)-selective, furnishing unsaturated nitrile (Z)-13. Conversion into the α,β-unsaturated aldehyde (Z)-14 and reduction afforded enartiopure allylic alcohol (Z)-12, which is a new isomer of the key quinidine metabolite 15.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020610
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    Bismuth(III) complexes of the tripeptide glutathione (γ-L-Glu-L-Cys-Gly) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 701-708 
    Details
    ISSN: 0947-6539
    Keywords: bismuth complexes ; drugs ; glutathione ; NMR spectroscopy ; red blood cells ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tripeptide glutathione (γ-L-Glu-L-Cys-Gly, GSH) is thought to play an important role in the pharmacology of bismuth drugs, but to our knowledge no chemical studies of bismuth glutathione complexes have been reported. We report here studies of interactions of the antiulcer compound ranitidine bismuth citrate (1) and [Bi(edta)]- with glutathione in aqueous solution and in intact red blood cells by NMR spectroscopy. The deprotonated thiol group is shown to be the strongest binding site for BiIII, and a complex with the stoichiometry [Bi(GS)3] is formed, as determined by 13C NMR titrations. A remarkably large lowfield shift of approximately 1.37 ppm for the β-CH2 1H NMR resonances of GSH was observed on binding to BiIII. The complex [Bi(GS)3] is stable over the pH* range 2-10 (pH* = pH meter reading in D2O solution). A formation constant log K of 29.6±0.4 (I = 0.1 M, 298 K) for [Bi(GS)3] was determined by displacement of edta by GSH. The rate of exchange of GSH between free and bound forms is pH-dependent, ranging from slow exchange (on the 1H NMR timescale) at low pH (ca. 3 s-1 at pH 4.0) to intermediate exchange at biological pH (ca. 1500 s-1). Such facile exchange may be important in the transport and delivery of BiIII in vivo. Spin-echo 1H NMR showed that 1 reacts with GSH in red cells both in vivo and in vitro. A first-order reaction of 1 with red blood cells was observed in vitro (k = 0.20±0.04 h-1, t1/2 = 3 h, 310 K), and the rate-determining step appeared to involve the passage of BiIII through the cell membrane.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020613
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    Reply to “on the definition of chirality” further comments on the chirality of large random objects (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 744-746 
    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.1460020618
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    Concepts in chemistry (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 751-751 
    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.1460020702
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    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.1460020701
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    Biomimetic ion-binding monolayers on gold and their characterization by AC-impedance spectroscopy (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 759-766 
    Details
    ISSN: 0947-6539
    Keywords: ionophores ; impedance spectroscopy ; membrane models ; monolayers ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel ion-binding monolayers on gold surfaces are presented where the molecular design is based upon the natural ion binder ferrichrome. The new ion binders possess hydroxamate coordinating groups arranged in C2 symmetry (bishydroxamate binder, BHB) or C3 symmetry (trishydroxamate binder, THB), and a separate dialkyl sulfide moiety, which serves as an anchor to the gold substrate. The separation between the ion-binding cavity and the attachment site to the gold allows each parameter to be controlled separately, namely, cavity size, its symmetry and external envelope, as well as the functional group used for immobilization. The monolayers were characterized with respect to ellipsometric thickness, wettability (advancing and receding contact angles (CAs) for water), and surface coverage; the latter is determined by metal underpotential deposition (UPD). It is shown that the introduction of hydrophobic side chains (i-butyl) improves the CAs, thickness, and surface coverage of the monolayers. A detailed analysis of the alternating-current (AC) impedance spectra is presented for THB monolayers on gold electrodes, where the impedance data are fitted to an equivalent circuit model. It is shown that the AC response in a wide frequency range can be used to probe ion binding and release in monolayer systems on electrodes.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020704
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    Synthesis, crystal structure, electronic structure, and properties of Hf2In5, a metallic hafnide with one-dimensional Hf-Hf and two-dimensional in-in bonding (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 800-804 
    Details
    ISSN: 0947-6539
    Keywords: crystal structure ; electronic structure ; hafnium compounds ; indium compounds ; metallic conductivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hf2In5 - previously reported with the tentative composition “Hf3In4” - was prepared from the elements in a tantalum tube at 970 K. The X-ray diffractional characterization by means of single-crystal refinement reveals the presence of a tetragonal structure (a = 1024.71 (9), c = 305.66 (3) pm, P4/mbm, Z = 2) of Mn2Hg5 type. Hf2In5 is Pauli-paramagnetic and a good metallic conductor. Quasi-relativistic semiempirical and scalar-relativistic ab initio band structure calculations reveal Hf2In5 to be a hafnide, composed of a two-dimensional indium network threaded by infinite hafnium chains. The amount of In-In bonding scales counterintuitively with the interatomic distances.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020709
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    Enantioselective synthesis of highly functionalized 4-piperidones by the asymmetric imino-diels-alder reaction of chiral 2-amino-1,3-butadienesAbbreviations used in this article: NaHMDS: sodium bis(trimethylsilyl)-amide; TMS: trimethylsilyl; MOM: methoxymethyl; TBDMS: tert-butyldimethylsilyl; 3-Fu: 3-furyl; p-MeOPh: 4-methoxyphenyl; o-BrPh: 2-bromophenyl; p-BrC6H4CO: 4-bromobenzoyl; (R)-MPTA: (R)-α-methoxy-α-Phenyl-α-trifluoromethylacetyl. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 805-811 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric Diels-Alder reactions ; butadienes ; Diels-Alder reactions ; piperidones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chiral 2-amino-1,3-butadienes 1 derived from commercially available (S)-2-methoxymethylpyrrolidine react with aromatic N-trimethylsilylaldimines and N-phenylaldimines in the presence of ZnCl2 to give, after the reaction workup, 4-piperidones 4 and 6, respectively, with moderate to very high enantiomeric excesses. In addition, the absolute configurations of derivatives of 4 a and 4 g were determined by circular dichroism and NMR spectroscopy on the Mosher ester, respectively.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020710
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    The β-lactone route to a totally stereoselective synthesis of carnitine derivatives (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 826-831 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric ring-opening ; carnitine ; cyclizations ; β-lactones ; nucleophilic substitutions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of the enantiomerically pure, carnitine-related β-lactones 10 and 12 starting from various carnitine precursors of opposite configuration (or carnitine itself) are described. (R)-3-Chlorocarnitine (20) has also been directly prepared from (S)-carnitine (14) and has been cyclized to 12 by a second inversion of configuration of the stereogenic centre. By nucleophilic attack at the carbonyl carbon, the β-lactone carnitine derivatives have been converted into esters, amides and guanidino congeners. Following this route, it is possible to obtain the biologically active isomer (R)-carnitine (1) starting from the otherwise useless industrial by-product (S)-carnitine (14). Nucleophilic attack by selected ambidient nucleophiles at the β-carbon of the same β-lactone derivatives results in a second inversion of configuration of the stereogenic centre. Besides aminocarnitine (3), chiral acetylcarnitine (2) and acetylthiocarnitine (5) have been synthesized in homochiral forms following this latter procedure.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020713
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    Emission from Zeolite-Occluded Manganese-Diimine Complexes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 592-597 
    Details
    ISSN: 0947-6539
    Keywords: charge transfer complexes ; diimine complexes ; emission spectroscopy ; manganese complexes ; zeolites ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Manganese complexes of 2,2′-bipyridine (bpy) and 1,10-phenantroline (phen) have been synthesised in the supercages of cubic NaX and NaY and in the hypercages of the hexagonal NaEMT faujasites. The coordination and redox chemistry were studied with ESCA, CA, FT-IR, FT-Raman, diffuse reflectance and emission techniques. FT-IR/FT-Raman shows cis coordination for all complexes and a high Mn-N stretching frequency in the phen complexes as a result of steric constraints imposed by the ligand. [Mn(bpy)2]2+ in the different zeolites shows metal-to-ligand charge transfer (MLCT; at 495 nm); for [Mn(phen)2]2+-NaY the MLCt is broadened owing to complex distortion. On MLCT excitation [Mn(bpy)2]2+ complexes show an ipsochromic shift in the emission and an increase in quantum yield with increasing steric restrictions imposed by the zeolite. The ipsochromic shift of the emission band of [Mn(phen)2]2+ in NaY results from the combined effect of the ligand field (this suggests emission from a CT state) and of coordinative distortion. The key factor influencing the emission properties is found to be the overall matrix-induced complex distortion. Cation stabilisation of the ligand anion affects emission indirectly. The decay times for [Mn-(bpy)2]2+-NaY are in the millisecond range (7.5-11.5 ms). A proposed model for excitation and emission properties of zeolite-occluded MnII complexes involves excitation of a quartet CT state, intersystem crossing and subsequent emission. The enhanced stability of the coordination sphere in the zeolite allows complexes to luminesce from a CT state, which is not detected in solution. The zeolite behaves as a supramolecular cryptating agent, protecting complexes from photodissociation.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020519
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    [Lithium tert-butylperoxide]12: Crystal Structure of an Aggregated Oxenoid (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 604-607 
    Details
    ISSN: 0947-6539
    Keywords: aggregates ; lithium compounds ; oxenoids ; peroxides ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-ray crystal structure of the dodecameric lithium tert-butylperoxide [2]12 is the first of an alkali or alkaline earth peroxide. It shows the lithium ion bridging the two oxygen atoms of the peroxide unit and a slight lenghtening of the O-O bond, in agreement with quantum-chemical calculations. A calculation for the model reaction of MeLi with LiOOH to give MeOLi and LiOH reveals the importance of Li bridging the O-O bond in the transition state of this reaction, as similarly discussed for many oxidation reactions of (transition-) metal peroxides. Preliminary theoretical studies of the O-O bond length (and thus of the oxenoid character) as a function of the aggregation of 2 disclose that increasing aggregation leads to stabilization of the charge at the anionic oxygen atom and thus to a reduction of the O-O bond length (oxenoid character). Related considerations of the effect of aggregation should also be valid for other lithium (organometallic) compounds and their structure and reactivity as well as other properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020521
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    Extended X-ray absorption fine structure (EXAFS) studies of hydroxo(oxo)iron aggregates and minerals, and a critique of their use as models for ferritin (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 634-639 
    Details
    ISSN: 0947-6539
    Keywords: biomimetic chemistry ; EXAFS spectroscopy ; ferritin ; iron homeostasis ; nanoparticles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural models for the hydroxo(oxo)iron core of ferritin include extended mineral structures and cluster systems such as [Fe11O6(OH)6(O2CPh)15] ( = Fe11) and two clusters that crystallise in the same lattice (compound 1), namely, [Fe17(m̈3-O)4(m̈3-OH) 6(m̈2-OH)10(heidi)8(H2O)12]3+and[Fe19(m̈3-O)6(m̈3-OH)6(m̈2-OH)8(heidi)10(H2O)12]1+. The suitability of these systems as models for the structure of the core of ferritin has been tested by comparing their Fe K-edge EXAFS and X-ray crystallographic results with the Fe K-edge EXAFS data on horse spleen ferritin. The interpretative procedure for the EXAFS analysis was optimised by using the X-ray crystallographic data for compound 1 as a basis. This protocol was then used to interpret the Fe K-edge EXAFS spectra of α- and γ-Fe(O)OH and to reinterpret that previously recorded for horse spleen ferritin. The published Fe K-edge EXAFS data on Fe11 were also considered. The Fe⃛Fe distances provide a clear indication of the nature of the hydroxo(oxo)iron assembly. It was found that the iron-iron interactions are the most reliable guide. Clusters are shown to be more appropriate than infinite lattices as structural models for the core of ferritin.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020604
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    Conformational studies of chiral vinylogous sulfonamidopeptides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 644-655 
    Details
    ISSN: 0947-6539
    Keywords: conformation ; crystal structure ; molecular modeling ; NMR spectroscopy ; sulfonamido-pseudopeptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational preferences of chiral vinylogous aminosulfonic acids (vs-amino acids) and of the corresponding oligomers (vs-peptides) were investigated by a combination of X-ray crystallography, variable-temperature (VT) 1H NMR spectroscopy, FT-IR spectroscopy, and NOE experiments. The major source of conformational freedom in the monomers is the rotation around the C—C bond connecting the double bond with the allylic stereocenter (N—C*—C=C). The allylic conformational perferences can be altered in the oligomers by the formation of secondary structures enforced by hydrogen bonding. Twelve-membered-ring hydrogen bonding is detected in the crystal structure of vs-dipeptide 9, while fourteen-membered-ring bydrogen bonding is the most common folding pattern for the oligomers in chloroform solution. The experimental results are complemented by computer modeling: suitable force-field (FF) parameters for the unsaturated sulfonamide group nwere develiped from ab initio calculations. A Goodman-Still systematic pseudo-Monte-Carlo search was used for the conformational search. The conformers were minimized in chloroform with the GB/SA model. The calculations correctly predicted both the size of the hydrogen-bonded ring and its relative importance, in agreement with the experimental data in solution.
    Additional Material: 16 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020606
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    Microstructured peptide-functionalised surfaces by electrochemical polymerisationDedicated to Professor Ivar Ugi on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 667-672 
    Details
    ISSN: 0947-6539
    Keywords: binding assays ; immunosensors ; electrochemical polymerisations ; peptide derivatives ; peptide immobilisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the first time, antigenic peptides have been immobilised by electrochemical polymerisation after having been modified with a polymerisable functional group. 3-Hydroxyphenylacetic acid was chosen as the novel polymerisable group. The synthetic peptides represent epitopes of the bovine foot and mouth disease virus and of the sodium channel of the cardiac muscle. The polymerisation was performed by applying a constant anodic potential or by cyclic voltammetry. A combination of these two methods was also employed, that is, cyclic voltammetry with a delay at the anodic vertex potential. No additional free phenolic monomer was required for the polymerisation. The layers formed by the polymerisation were recognised by specific antibodies. The specific binding of the antibodies to the polymer film could be demonstrated by ELISA, an enzyme-linked amperometric immunoassay, and electrochemical impedance measurements, as well as by fluorescence-labelled antibodies. A peptide derived from laminine was also immobilised by electrochemical polymerisation. It could be shown that neuroblastoma cells adhere to this layer.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020608
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  • 136
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    A bis(diimidazole)copper complex possessing a reversible CuII/CuI couple with a high redox potential (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 685-693 
    Details
    ISSN: 0947-6539
    Keywords: copper complexes ; imidazole ligands ; redox systems ; structure elucidation ; enzyme models ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The new diimidazole ligand, bis (1-methyl-4,5-diphenylimidazol-2-yl)ke-tone (BIMDPK), has been synthesised, characterised and shown to form four-coordinate bis(diimidazole) CuII and CuI complexes in the salts [Cu(bimdpk)2]-[BF4]2 and [Cu(bimdpk)2][PF6], the structures of which have been determined by X-ray crystallography. The cations of these salts have a very similar geometry with CuII-Nav = 1.949 and CuI-Nav = 1.999 Å; the N-Cu-N interbond angles are constrained by 1) the bite angle of the BIMDPK ligand to 94 ± 2° and 2) the interligand steric interactions, which lead to the dihedral angle of the intraligand CuN2 planes of 68.2° for CuII and 74.9° for CuI-that is, a CuN4 geometry intermediate between tetrahedral and square planar. The X-band EPR spectrum for the powdered CuII compound is typical of an approximately D2 CuN4 centre possessing a dxy ground state (gx = 2.080, gy = 2.075, gz = 2.291; Az = 112.3 × 10-4 cm-1). The UV/vis spectra are dominated by charge-transfer bands, and both the CuII and CuI systems are intensely coloured. The EPR and electronic spectra indicate that these cations have a very similar structure in the solid state and in solution; the potential of the [Cu(bimdpk)2]2+/[Cu(bimdpk)2]+ couple is 0.59 V vs. SCE in MeCN and 0.80 V vs. SCE in CH2Cl2, and the electron self-exchange constant in MeCN is 1.9 × 104M-1S-1. Comparisons are made between the properties of the [Cu(bimdpk)2]2+/+ centres and related Cu centres in chmical and biological systems; the results of this study reinforce the view that a [Cu(His)4] centre should not be precluded from consideration in biological electron transport.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020611
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  • 137
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    The aminium salt and photoinduced electron transfer initiated diels-alder cycloaddition of electron-rich allenes: Evidence for a stepwise mechanism and the importance of steric and electronic effects for the reactivity of distonic radical cation intermediatesRadical-Cation-Catalyzed Reactions, Part 8. Part 7: see ref. [1].Dedicated to Prof. Dr. S. Hünig on the occasion of his 75th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1031-1040 
    Details
    ISSN: 0947-6539
    Keywords: allenes ; cycloadditions ; Diels-Alder reactions ; mechanistic studies ; radical cations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several novel electron-rich aryl-allenes have been synthesized, characterized, and used as dienophiles in the radical-cation-catalyzed cycloaddition with 1,2,3,4,5-pentamethylcyclopentadiene, which affords, in most cases, the Diels-Alder products in 5 min at 0°C with a high peri-, chemo-, facial, and stereoselectivity. In line with oxidation-potential considerations it is concluded that the electron-transfer-induced reaction proceeds along a [3 + 2] pathway by cycloaddition of the diene radical cation to a neutral allene with a rather short chain length. The low cycloaddition yields from some of the allenes are interpreted as evidence for a stepwise mechanism involving distonic radical cations as key intermediates. We discuss the tendency of the distonic radical cations to undergo ring closure to the Diels-Alder-product radical cations in terms of enthalpy considerations, which, we suggest, offer a novel criterion for the design of stepwise radical cation reactions.
    Additional Material: 5 Tab.
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    URL: http://dx.doi.org/10.1002/chem.1460020818
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    Oxidation products of organic trisulfanes: Molecular structures and energies of various isomers of the dimethyltrisulfane oxides Me2S3O and Me2S3O and of 1,3-tBu2S3O2Sulfur Compounds, Part 193; for Part 192 see R. Steudel, Ind. Eng. Chem. Res. 1996, 35, 1417. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1060-1067 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; rotamers ; stereoisomers ; sulfane oxides ; torsional potentials ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio MO calculations (MP 2/6-311 G**//HF/6-311 G**) have been performed for several isomers (including rotamers) of Me2S3O and Me2S3O2. MeS(O)SSMe exists as five rotamers; the most stable form (1a) has a helical backbone CSSSC with S-S bond lengths of 206.0 (SIISII) and 212.4 pm (SIISII). The most stable rotamer of MeS-S(O)SMe (2a), is less stable than 1a by 10.7 kJ mol-1; it is of Cs, symmetry, while a rotamer of Cs symmetry (2b) is less stable than 2a by only 1.4 kJ mol-1. Both 2a and 2b are stabilized by O ⃛ H hydrogen bonds. The S-S bond lengths of 2a are 210.0 and 212.0 pm; the CSSSC chain is not helical (CSSS torsion angles 166.3 and -75.4°). The 1,3-dioxide MeS(O)-SS(O)Me (3) has two equivalent chiral centers and exists as diastereomers. The most stable isomer 3a (RR)/(SS) is of C2 symmetry with methyl groups trans to each other; the SO bonds form an angle of about 90°. The meso form 3b is less stable than 3a by 17.2 kJ mol-1 and the rotamers 3c and 3d are less stable by 25.6 kJ mol-1 and 28.4 kJ mol-1, respectively. The trisulfane-1.2-dioxide MeS(O)-S(O)SMe has two nonequivalent chiral centers and exists as five isomers. The most stable form, the (RS)/(SR) form 4a, is less stable than 3a by 21.4 kJ mol-1 and is characterized by SS bonds of 220.9 (SIIISIII) and 208.3 pm (SIIISII). The rotamer 4b is less stable by 5.9 kJ mol-1. The isomers 4c, 4d, and 4e are all of (SS)/(RR) configuration and are less stable than 4a by 6.3, 12.7, and 12.0 kJ mol-1. For comparison, ab initio MO calculations (HF/6-311 G*) for tBuS(O)SS(O)-tBu yielded two diastereomers of practically identically energy that both contain helical CSSSC backbones. The (RS) form is less stable than the (RR)/(SS) form by 1.8 kJ mol-1.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020903
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    Protonation and alkylation of the thiolato donors in [Fe(CO)(“NHS4”)]: Effects on the structural, electronic, and redox properties of metal-sulfur complexesTransition Metal Complexes with Sulfur Ligands, Part 120. Part 119: D. Sellmann, T. Becker, F. Knoch, Chem. Ber. 1996, 119, 509-519. “NHS4”2- = 2,2′-Bis(2-mercaptophenylthio)diethylamine(2-).Dedicated to Professor Rudolf Taube on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1092-1098 
    Details
    ISSN: 0947-6539
    Keywords: cyclic voltammetry ; iron complexes ; redox systems ; structure elucidation ; sulfur ligands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effects of protonation and alkylation at the sulfur donors in metal-sulfur complexes have been investigated by taking the specific example of [Fe(CO)(“NHS4”)] (1). The 18 valence electron (VE) complex 1 consists of a low-spin FeII center and the dithioether thiolato amine ligand “NHS4”2-- ( = 2,2′-bis(2-mercaptophenylthio)diethylamine(2-)). Complex 1 can be reversibly protonated at the two thiolato donors; this results in an increase in ν(CO) of 35 cm-1 after the first protonation and 45 cm-1 after the second. Alkylation of 1 with one or two equivalents of the oxonium salts R3OBF4 (R = Me, Et) yields [Fe(CO)(“NHS4”-R)]BF4 (R = Me: 4, Et: 5), [Fe(CO)(“NHS4” -R2)](BF4)2 (R = Me: 6, Et: 7), and the methyl ethyl derivative [Fe(CO)(“NHS4” -Me-Et)](BF4)2 (8). An increase in ν(CO) of 31-32 cm-1 is observed for each successive alkylation. Due to the C1 symmetry of 1, complexes 4, 5, and 8 are formed as 1 : 1 mixtures of two diastereomers, whereas 6 and 7 are present as only one stereoisomer. Acidic hydrolysis of 4, 6, and 7 liberates the corresponding ligands [“NHS4” -Rn](BF4)n (9-11; n = 1,2), which were isolated as the ammonium tetrafluoroborate salts. The molecular structure of 8a has been elucidated by X-ray structure analysis. This shows that the Fe-N and Fe-S bonds in the [FeNS4] core do not change after alkylation of the thiolato donors despite the Δν(CO) of ca. 60 cm-1 between 1 and 8, which indicates a distinct decrease in electron density at the Fe center. This decrease can be rationalized by increased π-acceptor character of the sulfur donors upon protonation or alkylation. The change in electron density at the [Fe(“NS4”)] core is further corroborated by cyclic voltammetry. For each successive protonation or alkylation of 1, the redox couple potentials shift by 600-800 mV. As a consequence, the diethyl derivative, for example, gives rise to reduced species (19 or 20 VE) which are not observed for 1. The implications of these results for the reduction of N2 within the coordination sphere of the FeMo cofactor of nitrogenases are discussed.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020907
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    Peptide helices as rigid molecular rulers: A conformational study of isotactic homopeptides from α-methyl-α-isopropylglycine, [L-(αMe)Val]nLinear Oligopeptides, Part 360. Part 359: A. Bianco, M. Maggini, G. Scorrano, C. Toniolo, G. Marconi, C. Villani, M. Prato, J. Am. Chem. Soc. 1996, 118, 4072-4080. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1104-1111 
    Details
    ISSN: 0947-6539
    Keywords: amino acids ; conformation ; helices ; molecular rulers ; oligopeptides ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Terminally blocked, isotactic homopeptides from the sterically demanding α-methylvaline of general formula Y-[L-(αMe)Val]n-OtBu (Y = Z, pBrBz, Ac; n = 2-8) have been prepared step-by-step in solution and fully characterized. The conformations preferred in solution (β-turn and right-handed 310-helix) have been assessed by FT-IR, 1H NMR and CD spectroscopy. The molecular and crystal structures of the Z-protected trimer, hexamer, heptamer and octamer have been determined by X-ray diffraction. In the crystal state, while the trimer is folded in a type III β-turn conformation, the longest homopeptides form well-developed, regular, right-handed 310-helices. The screw sense in the helix of the pBrBz-blocked octamer has been confirmed to be right-handed by solid-state and solution CD spectroscopy. The possible exploitation of these peptide helices as rigid and precise molecular rulers is discussed.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020909
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  • 141
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    Thermal and light-induced spin transition in [Fe(bpen)X2] (bpen = 1,6-Bis(2-pyridyl)-2,5-diazahexane, X = NCS-, NCSe-)Dedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1129-1133 
    Details
    ISSN: 0947-6539
    Keywords: iron complexes ; kinetics ; magnetic susceptibility ; Mössbauer spectroscopy ; spin crossover ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal and light-induced spin transitions in [Fe(bpen)(NCS)2] and [Fe(bpen)(NCSe)2] have been investigated by means of magnetic susceptibility and Mössbauer measurements. The HS → LS transition in [Fe(bpen)(NCS)2] is detected at temperatures below 120 K (T1/2 [when the fraction of high-spin species γHS = 0.5] = 68 K) with a residual HS fraction of 8% at 20 K. The transition is fairly abrupt and no hysteresis is observed, in contrast to earlier studies (ref. [8]). The generation of metastable HS states in [Fe(bpen)(NCS)2] is achieved by rapid cooling of the sample and by irradiating the compound at low temperatures with light of appropriate wavelengths (LIESST). The HS → LS relaxation of the metastable spin states after rapid cooling is observed at temperatures between 35 and 42.5 K and shows strong deviations from single exponential behavior. The spin-transition behavior changes drastically on substitution of the thiocyanate ions by NCSe-. In [Fe(bpen)(NCSe)2] a gradual and complete spin transition between 140 and 250 K is observed (T1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long-lived metastable HS states. The spintransition curves derived from magnetic susceptibility and Mössbauer studies show good agreement for each compound, and no hints of significantly different Lamb-Mössbauer factors for the HS and LS states are observed.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020912
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  • 142
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    Porphyrin Synthesis by the “3+1” Approach: New Applications for an Old Methodology (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1197-1200 
    Details
    ISSN: 0947-6539
    Keywords: aromaticity ; MacDonald condensation ; porphyrinoids ; pyrroledialdehydes ; tripyrranes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acid-catalyzed condensation of tripyrranes with pyrrole-2,5-dicarboxaldehydes, followed by oxidation with an electron-deficient quinone, affords porphyrin products in excellent yields. This previously little used methodology has now been exploited in the synthesis of novel porphyrin structures, including tetrapyrrolic compounds with fused aromatic rings. By utilizing other aromatic or unsaturated dialdehydes, the “3+1” approach also allows the synthesis of new aromatic porphyrinoid systems, including benzene- and pyridine-containing macrocycles and carbaporphyrins.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021004
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  • 143
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    The [Fe(etz)6](BF4)2 Spin-Crossover System - Part One: High-Spin ⇌ Low-Spin Transition in Two Lattice SitesDedicated to Professor Hans Georg von Schnering on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1427-1434 
    Details
    ISSN: 0947-6539
    Keywords: iron complexes ; LIESST ; spin crossover ; tetrazoles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The [Fe(etz),](BF,), spin-cross-over system (etz = 1-ethyl-1 H-tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K: a 10.419(3), b=15.709(1), c = 18.890(2) Å = α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) Å3 and Z = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sites nA:nB is 2:l. Iron(II) on site A undergoes a thermal low-spin (LS) → high-spin (HS) transition with T1/2I, = 105 K. whereas that on site B remains in the high-spin state down to cryogenic temperatures. Application of external pressure of up to 1200 bar between 200 and 60 K does not cause formation of the low-spin state on site B. On site A the high-spin state can be populated as a metastable state at 20 K by irradiating the sample with λ = 514.5 nm; on site B a light-induced population of the low-spin state can be achieved with λ = 820 nm.
    Additional Material: 12 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021115
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    The [Fe(etz)6](BF4)2 Spin-Crossover System - Part Two: Hysteresis in the LIESST RegimeDedicated to Professor Karl Heinrich Lieser on the occasion of his 75th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1435-1439 
    Details
    ISSN: 0947-6539
    Keywords: hysteresis ; iron complexes ; LIESST ; spin crossover ; tetrazoles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the [Fe(etz)6](BF4)2 spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high-spin (HS) → low-spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light-induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS ⇌ LS conversion on site B reveal a light-induced HS⇌LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B-site complexes with a critical temperature T↑c≍77 K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first-order kinetics (self-acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self-retardation.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021116
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    A General Scheme Based on Empirical Increments for the Prediction of Hydrogen-Bond Associations of Nucleobases and of Synthetic Host-Guest complexes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1446-1452 
    Details
    ISSN: 0947-6539
    Keywords: hydrogen bonds ; LFER ; molecular recognition ; NMR spectroscopy ; nucleobases ; thermodynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Association energies ΔGt in chloroform, in part also in carbon tetrachloride, were determined by NMR titrations of suitably substituted nucleosides and several synthetic analogues. Based on these and on many literature data, two simple free energy increments were derived describing the ΔGt values of 58 complexes within 1.8 kJ mol-1. With chloroform as solvent the increment for the primary interaction between donor and acceptor is 7.9 kJ mol-1, for the secondary one 2.9 kJ mol-1, irrespective of whether the latter is attractive or repulsive. Addition of only 1% methanol to CCl4 led to a decrease in association constants by a factor of 25. Calorimetric titrations of G-C nucleoside derivatives in CCl4 showed substantial contributions from G dimers, in line with NMR titrations, and surprisingly small decreases in entropy. Preliminary NOE measurements allowed us to single out some of the possible association modes; they are also in line with expected self- and triple-association modes of the nucleobases. These modes are generally in accord with nucleobase associations predicted by MM calculations in the literature, which in turn agree with predictions based solely on the increments derived in the present work.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960021118
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    N-heterocyclic carbenesCarbenes may be defined as “divalent” carbon compounds. More precisely, the carbene carbon is linked to two adjacent groups by covalent bonds and possesses two nonbonding electrons, which may have antiparallel spins (singlet state) or parallel spins (triplet state). In other compounds with formally “divalent” carbons (e.g., carbon monoxide, isocyanides), resonance stabilization of the “divalent” carbon by neighbouring groups is extremely strong. The term carbene, conceived by Doering, Winstein and Woodward, seems well suited to describe the class of compounds under discussion, but the carbene nomenclature conflicts with established rules of the International Union of Pure and Applied Chemistry (IUPAC). The nomenclature for metal-carbene complexes was introduced by E. O. Fischer [2a-d] and became common in the organometallic literature.: Generation under Mild Conditions and Formation of Group 8-10 Transition Metal Complexes Relevant to CatalysisHeterocyclic Carbenes, Part 5. Part 4: see ref. [20].Dedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 772-780 
    Details
    ISSN: 0947-6539
    Keywords: carbenes ; Group 8-10 complexes ; heterocycles ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stable mono- and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloroand acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene). A general novel method for the deprotonation of N-substituted azolium salts in liquid ammonia-the liquid ammonia route-is described. This method yields not only the known free monimeric 1,3-dimethylimidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3-dimethyl-imidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of RuII, RhI PdII, OsII and IrI are presented. Reaction of the potentially chelating ligand 1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diylidene with [(COD)-RhCl]2 yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metalcarbene complexes (Rh, Os) are reported. N-Heterocyclic carbene complexes of Group 8-10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal-carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis-basic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.
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    URL: http://dx.doi.org/10.1002/chem.1460020706
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  • 147
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    Dynamic behavior in solution of the trans-hydridodihydrogen complex [OsHCl(n2-H2)(CO)(PiPr3)2]: Ab Initio and NMR studies (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 815-825 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; dihydrogen complexes ; isotope effects ; NMR spectroscopy ; osmium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of complex [OsHCl-(CO)(PiPr3)2] (1) with hydrogen gives the trans-hydridodihydrogen complex [OsH-Cl(n2-H2)(CO)(PiPr3)2] (2). The H-H distance in the dihydrogen ligand, determined by variable-temperature 1H T1 measurements, is 0.8 Å. The fast-spinning nature of the dihydrogen ligand of 2 has been deduced by evaluating the deuterium quadrupole coupling constant for the n2-D2 ligand of [OsDCl(n2-D2)(CO)(PiPr3)2] ([D3]2). Measurements of the equilibrium constants for formation of 2 (K) give ΔH0 = - 14.1 ± 0.5 kcal mol-1 and ΔS0 = - 30 ±1 e.u. An equilibrium isotope effect Kd/K of 2.8 is found for this reaction. The activation parameters for the H2 loss from 2 are ΔH
    Additional Material: 11 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020712
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  • 148
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    Conformation and self-association of a hybrid peptide of cecropin a and melittin with improved antibiotic activity (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 838-846 
    Details
    ISSN: 0947-6539
    Keywords: aggregation ; antibiotics ; circular dichroism ; helices ; peptides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 15-residue hybrid peptide containing residues 1-7 from cecropin A and residues 2-9 from melittin, CA-(1-7)M(2-9), is a potent antibiotic with broader activity than cecropin A, but without the cytotoxic character of melittin. The conformational behaviour of CA(1-7)M(2-9) including the formation of multimeric species in solution has been investigated by circular dichroism, ultracentrifugation, electrospray mass spectrometry, NMR and energy calculations. Addition of hexafluoroisopropanol or liposomes causes the appearance of a CD spectrum characteristic of a helical structure that changes with pH, buffer and peptide concentration. The concentration dependence is atypical, as the ellipticity at 222 nm decreases with peptide concentration and is not correlated with a correponding decrease in helix content as measured from the NMR spectra. The presence of aggregated structures is demonstrated by ultracentrifugation and ES-MS experiments, which also provide an indication of the stoichiometry. Longrange NOEs suggest a model of aggregation with neighbouring molecules packed antiparallel. Aggregation causes very slow proton-deuterium exchange in some amide protons in the C-terminal region and provides a method for estimating a very large association constant (ca. 106M-1) as well as the stoichiometry of the aggregates. The tendency to aggregate seems to be an inherited feature from melittin and may enhance the antibiotic activity either by faciliting the incorporation of the peptide into the membrane in large quantities or by promoting the disruption of the membrane.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020715
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  • 149
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 752-755 
    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.1460020720
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  • 150
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    A stereoselective synthesis of (E)-α, β-unsaturated ketones involving the reactions of organocerium reagents with secondary β-enamino ketones (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 913-918 
    Details
    ISSN: 0947-6539
    Keywords: assymetric substitution ; cerium reagents ; enamino ketones ; regioselectivity ; synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective construction of trisubstituted alkenes has wide applicability to the synthesis of many natual products Specifically, β-disubstituted enones are important functionalized trisubstituted alkene targets. The reaction of organocerium reagents with secondary β-enamino ketones affords β=disubsituted α,β-unsaturated ketones in fairly good yields. This process shows considerable stereoselectivity, and α,β-unsaturated ketones of (E) configuration are predominantly observed. Organolithium-derived cerium reagents display better stereoselectivity than organomagnesium-based ones. The mechanism of the reaction varies with nitrogen substitution: N-phenyl groups give 1,2-addition products, whereas substitution products are observed with N-alkyl groups. When organocerium reagents were used with β-enamino ketones bearing secondary alkyl groups at the nitrogen atom, a lack of reactivity was observed.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/chem.1460020802
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  • 151
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    Synthesis of 4-silylcyclobut-2-enethiones and their use in cyclobutadiene generation (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 949-956 
    Details
    ISSN: 0947-6539
    Keywords: alkynyl sulfides ; cyclobutadienes ; cyclobutenethiones ; desilylation ; ynamines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alkynyl silyl sulfides 2 reacted with ynamines 3 to give 1:1 adducts. The structure of 4-silylcyclobut-2-enethiones 4 was confirmed by X-ray analysis of 4a. A cyclobutadiene intermediate is probably not involved in this reaction; we think that it is initiated by a silyl transfer from 2 to 3, and that this is followed by a combination of the resulting ions to give the unsaturated thioketene 7, which undergoes an electrocyclization to give products 4. In the reaction of thiones 4 with trimethyloxonium tetrafluoroborate (Meerrwein salt) selective methylation at sulfur was observed to give cyclobutenethionium ions 16. Ions 16 underwent fluoride-induced desilylation with various fluoride sources to give cyclobutadienes 17, which could not be isolated, but trapping of 17 a-c was possible with dimethyl acetylenedicarboxylate yielding regioisomeric benzene derivatives 20-23. Similarly, 17 a and bis(methylthio)methylene malononitrile (24) led to hexatriene derivative 27 by a sequence of cycloaddition and two ring-opening reactions. In contrast, silyl-substituted cyclobutadienes 17d,e dimerized even in the presence of trapping agents to anti-tricyclo-[4.2.0.02,5]octadienes 29; this suggests that a two-step cycloaddition is taking place, rather than a concerted Diels-Alder reaction. Attempts to intercept 17d, e, generated from 16d,e with cyclopentadiene (30), gave deprotonation of 30 leading to substitution of the methylthio group in 16 and finally to formation of cyclobutenes 33 through a hydrogen shift.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020807
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  • 152
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    X-ray structural studies of highly enantioselective Mn(salen) epoxidation catalysts (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 974-980 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric eponidations ; Cstalysis ; manganese complexes ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(sa1en) complexes (1 - 10) was examined. The X-ray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6′= -tBu; 4,4 = -tBu]+ClO4-), and 10 (6,6′= -tBu; 4,4′=-Br) were determined. Catalysts 1 - 9 were derived from (R,R)-1, tdiaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethyIenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the orfho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five- and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1 - 10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6′ = -H; 4,4′ = -H),2(6,6′ = -CH3; 4,4′ = -CH3),3(6,6′=-tBu;4,4′=-H),4(6,6′=-tBu; 4,4′ =-CH3), 5 (6,6′=-tBu; 4,4′=-tBu), and 6 (6,6′=-tBu; 4,4′= -trityl), and electronically modifying the para substituents in 7 (6,6 = -tBu; 4,4 =-OMe) and 8 (6.6′ = -tBu; 4,4′=-OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020810
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  • 153
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    Effects of nitrogen substitution in poly(pyrazolyl)borato ligands: From orbital energy levels to C-H ⃛ O hydrogen bondingDedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 992-1000 
    Details
    ISSN: 0947-6539
    Keywords: cyclovoltammetry ; electronic effects ; photoelectron spectroscopy ; pyrazole ligands ; Mössbauer spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic effect of substituting CH with N in poly(pyrazolyl)-borato ligands and their transition-metal complexes is shown to be a decrease in energy of the filled metal and ligand orbitals. This conclusion is based on the cyclovoltammograms and photoelectron spectra of bis(hydrotris(azolyl)borato)-iron(II) and -cobalt(II) complexes (azolyl = pyrazolyl and 1,2,4-triazolyl) and on MO calculations. 57Fe Mössbauer spectra of bis(hydrotris(1,2,4-triazolyl)-borato)iron(II) show that there is a fine-tuning of the HOMO-LUMO gap by a shift in transition temperature for the spin equilibrium. 15N NMR spectroscopy supports the assignment of a higher negative charge to the exodentate N-4 nitrogen than to the endodentate N-2 or N-3 positions in the poly(azolyl)borato anions, where azolyl is 1,2,4-triazolyl or tetrazolyl. The electron withdrawing effect of the additional nitrogen atoms and the in-corporation of water of crystallization by O-H ⃛ N bonding both assist in the formation of (azolyl)C-H ⃛ O bonds.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020813
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  • 154
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 909-912 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020819
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  • 155
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020820
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  • 156
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    A novel catalytic cycle for the synthesis of epoxides using sulfur ylides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1024-1030 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; diazo compounds ; epoxidations ; sulfur ylides ; synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel, neutral catalytic cycle for the synthesis of epoxides from carbonyl compounds and diazo compounds using catalytic quantities of transition metal salts and sulfides has been developed. In this catalytic cycle, the diazo compound is decomposed by the transition metal salt to give a metallocarbene, and this is picked up by the sulfide to give a sulfur ylide, which then reacts with the aldehyde to give an epoxide and returns the sulfide back into the catalytic cycle. To obtain good yields of epoxides it is necessary to maintain a low concentration of the diazo compound (by slow addition), otherwise dimerisation of the diazo compound is the dominant reaction. Factors affecting the outcome of the reaction were studied. The reactions are relatively insensitive to solvent, but are sensitive to the structure of the sulfide, the metal salt and the concentration. Unhindered sulfides give good yields of epoxides with any metal salt, but with hindered sulfides higher yields are obtained with Cu(acac)2 than with Rh2(OAc)4. The yields of epoxides are sensitive to sulfide concentration especially when using substoichiometric amounts of sulfides. Higher concentration leads to faster rates of formation and subsequent reaction of the sulfur ylides, and consequently to higher yields. This novel catalytic cycle has also been applied to base-sensitive aldehydes. We found that our new catalytic cycle for epoxidation gives much improved yields of epoxides compared to those obtained by traditional sulfur ylide chemistry and is tolerant to a wide variety of sensitive functional groups. Ketones also participate in the catalytic cycle, although they give reduced yields of epoxides compared to aldehydes and require a slightly elevated temperature.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020817
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  • 157
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    TiIV-BINOLate-catalyzed highly enantioselective additions of β-substituted allylstannanes to aldehydesFirst presented at the „15th Regiosymposium of the Universities Basel, Freiburg, and Mulhouse“ in Falkau, September 27-29, 1995. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1077-1084 
    Details
    ISSN: 0947-6539
    Keywords: allylstannanes ; asymmetric allylations ; catalysis ; C-C bond formation ; homoallylic alcohols ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantiomerically pure homoallyl alcohols were prepared from aldehydes R1-CH=O (R1 = Ph, pentyl, Ph-CH=CH-, iPr) and β-substituted allyl-stannanes H2C=CR2-CH2-SnBu3 (R2 = pentyl, tBuPh2SiO-CH2-, tBuPh2SiO-CH2-CH2-, PhS-CH2-CH2-). These reactions were catalyzed by the same additives - Ti(OR)4 (10 mol%) and (R)-BINOL (20 mol%) - that Keck et al. used in analogous reactions with methallyl-and allyltributylstannane. To attain optimum ee values (96.4-99.2% in the reaction with hexanal) these additives had to be premixed for 2 h at room temperature. Ti(OEt)4 and Ti(OiPr)4 gave equally good results, while Ti(OMe)4, Ti(OCHEt2)4, and Ti(OtBu)4 were inferior. Our procedure works in the absence of molecular sieves [which were previously found to give rather unreliable results in reactions catalyzed by Ti(OiPr)4/(R)-BINOL] and can be extended to enantioselective addition reactions with allyl- and methallyltributylstannane, too (+ hexanal: 97.4 and 97.0% ee, respectively).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020905
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  • 158
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    Catalytic properties of layered gold-palladium colloidsDedicated to Professor Max Herberhold on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1099-1103 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; colloids ; gold-palladium colloids ; HRTEM ; hydrogenations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Layered bimetallic gold-palladium colloids in the size range of 20-56 nm have been synthesized by the seed=growth method: gold seeds were convered by palladium layers of various thickness and vice vaers. The outer metal was coordinated by trisulfonated triphenylphosphine and sodium sulfanilate ligands to stabilize the bimetallic particles to such an extent that they could be isolated in the solid state. Owing to the hydrophilic ligand shell, redispersion in water was posible in any concentration. High-resolution transmission electronn microscopy and energy-dispersive X-ray analysis was used to characterized the colloids. Stabilized and nonstabilized gold-palladium and palladium-gold systems on a TiO2 support were used as heterogeneous catalysts for the hydrogenation of hex-2-yne to cis-hex-2-ene. Both the palladium-plated gold seeds and the gold-plated palladium particles showed considerably increased activities compared with the pure metals. The ligand shell seems not to influence the catalytic behavior because protected and unprotected colloids behave very similarly.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020908
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  • 159
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    A Convergent synthesis of carbohydrate-containing dendrimersThis paper should be regarded as Part 1 of a series on Synthetic Carbohydrate Dendrimers. A list of abbreviations used in this paper is given in ref. [40]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1115-1128 
    Details
    ISSN: 0947-6539
    Keywords: carbohydrates ; cluster glucosides ; convergent syntheses ; dendrimers ; neoglycoconjugates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of carbohydratecontaining dendrimers has been achieved by a convergent growth approach. The synthetic strategy involves: 1) the synthesis of the triglucosylated derivative of tris(hydroxymethyl)methylamine (TRIS), 2) the introduction of a glycine-derived spacer and 3,3′-iminodipropionic acid derived branching units on to the TRIS derivative by amide bond formation, 3) condensation of the above saccharidecontaining dendrons with a trifunctional 1,3,5-benzenetricarbonyl derivative, used as the core, by formation of amide bonds, and 4) deprotection of the saccharide units. A 9-mer and an 18-mer, carrying nine and eighteen saccharide units at the periphery, respectively, have been synthesized, in high yields at each step, by this synthetic strategy. By a variety of chromatographic and spectroscopic techniques, the dendrimers were shown to be structurally homogeneous, monodisperse, and error-free at all steps in their growth. These investigations were complemented by molecular modeling studies on the dendrimers. The presence of slightly distorted C3 symmetry was noted in both the 9-mer and the 18-mer.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020911
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  • 160
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    The unimolecular chemistry of protonated glycinamide and the proton affinity of glycinamid - mass spectrometric experiments and theoretical model (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1143-1149 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; glycinamide ; mass spectrometry ; protonations ; quantum chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential energy hypersurface of protonated glycinamide (GAH+) has been investigated experimentally and theoretically. The calculated G2(MP2) value for the proton affinity of glycinamide, PAcalcd = 919 kJ mol-1, is in good agreement with the measured value of 908PAexp〈914kJ mol-1. The fact that the amide group is a better hydrogenbond acceptor explains why glycinamide has a higher PA than glycine. Proton transfer experiments with glycinamide performed in a Fourier transform mass spectrometer and analysis of metastable GAH+ ions in a four-sector mass spectrometer show that the lowest-energy unimolecular reactions are two distinct processes: 1) loss of CO, which has a substantial barrier for the reverse reaction, and 2) loss of CO plus NH3, which has no barrier for the reverse reaction. Ab initio quantum chemical calculations give a reaction model that is consistent with the observed fragmentation pattern.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020915
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  • 161
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    Molecular structure of the chloroiron(III) derivative of the meso-unsubstituted 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin and weak ferromagnetic exchange interactions in the Alu oxoiron(iv) porphyrin π radical cation complex (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1159-1163 
    Details
    ISSN: 0947-6539
    Keywords: electronic structure ; exchange coupling ; heme proteins ; iron complexes ; porhyrins ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The molecular structure of the chloroiron(III) porphyrinate [FeIIICl- (tmtmp)] (1) is described. The doming off the bifacially encumbered tmtmp dianionic ligand present in 1 is characterized by a separation of 0.09 (1) Å between the mean plane of the four pyrrole nitrogens (4NP) and that of the porphyrin core. The coordination polyhedron of the five-coordinate iron atom is a square pyramid of C4v symmetry. The metal is displaced by 0.44 (1) Å from the 4Np mean plane towards the axial chloro ligand. The Fe-Np and Fe-Cl bond lengths are 2.057 (6) and 2.223(5) Å, respectively. Upon treatment of [FeIIITf(tmtmp)] (2) with m-chloroperoxybenzoic acid, the green oxoferryl π radical cation complex [FeIV=O(tmtmp)]+ (3) is obtained. It has been studied by EPR, Mössbauer, and resonance Raman spectroscopy. It exhibits an EPR spectrum strikingly similar to those of compounds I of Micrococcus lysodeikticus catalase (APX-I) and ascorbate peroxidase (APX-I) The exchange interactions between the spins of the radical cation and the ferryl iron are of the ferromagnetic type and are the weakest ever found for a synthetic compound I model. The shift of + 22 cm-1 in the radical marker band v2 in the resonance Raman spectra upon oxidation of 2 to 3 confirms that the electronic state of the porphyrin π radical cation is predominantly 2Alu in this tmtmp compound 1 model.
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    URL: http://dx.doi.org/10.1002/chem.1460020917
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  • 162
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021001
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  • 163
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    Homochiral and heterochiral dimers of the methylzinc alkoxide formed from dimethylzinc and enantiomeric 3-exo-(dimethylamino)isoborneol - origin of the distinct differences in solution-phase behavior and crystal structures (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1173-1181 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; asymmetric alkylations ; catalysis ; structure ; elucidation ; zinc complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dimethylzinc reacts with (2S)-or (2R)-3-exo-(dimethylamino)isoborneol [(2S)- or (2R)-DAIB] to eliminate methane and produce a tricoordinate methylzinc aminoalkoxide, which forms a dimeric structure. The homochiral dimerization of the enantiomeric compound leads to the chiral, (S,S) or (R,R) dinuclear Zn complex, while the heterochiral interaction forms the meso (S,R) dinuclear compound. In both solution and crystalline state, the heterochiral dimer is more stable than the homochiral dimer. This stability difference in solution is the origin of the chirality amplification observed in the amino alcohol promoted asymmetric addition of dimethylzinc to benzaldehyde. In toluene, the homochiral dimer dissociates more readily into the monomer than the heterchiral isomer and also undergoes dissociation of the N-Zn dative bond making the two N-methyl groups equivalent. The differences in solution behavior between the diastereomers can be understood by comparing their crystal structures. X-ray analysis indicates that the labile Zn-O and Zn-N bonds in the (S,S) dimer are longer than those in the (S,R) isomer. Skeletal congestion caused by the polycyclic framework is the prime factor determining the properties of the dinuclear Zn complexes, with both steric and electronic factors governing their geometries. The distances between the C-2 proton and N-CH3 of the other DAIB moiety in the homochiral dimer are close to the sum of the van der Waals radii. A significant nuclear Overhauser effect is seen between these protons in the homochiral dimer. The tetrahedral Zn atoms in the dinuclear complexes are linked covalently to the methyl group, to two oxygen atoms through covalent/electrostatic hybrid bonds, and to the dimethylamino group through electrostatic interaction. The repulsive interaction of the 1, 3-synoriented Zn-CH3 bonds significantly contributes to the lower stability of the homochiral dimeric complex. The N-Zn interaction in the homochiral dimer is labile, owing to the increase in the electrostatic interaction between the Zn atom and the neighboring oxygen atoms. This view is supported by the ab initio molecular orbital calculations of the model systems.
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    URL: http://dx.doi.org/10.1002/chem.1460020919
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  • 164
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    Stereoselective Preparation of Polyfunctional Cyclopentane Derivatives by Radical Nickel- or Palladium-Catalyzed Carbozincations (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1204-1220 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric cyclization ; catalysis ; cyclopentane ; natural products ; palladium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of 5-hexenyl iodides with diethylzinc (2 equiv) and catalytic amounts of a PdII or NiII complex like PdCl2(dppf), PdCl2(MeCN)2, or Ni(acac)2 results in an efficient ring closure (THF, RT, 2-12 h) affording cyclopentylmethylzinc iodides, which, after transmetalation with CuCN·2LiCl, can be further functionalized by treatment with a range of electrophiles like allylic halides, acyl chlorides, enones, nitroolefins, ethyl propynoate, and alkynyl halides to yield polyfunctional cyclopentane derivatives. The ring closures occur via radical intermediates, and the stereochemistry of the products can be explained according to the rules for radical cyclizations developed by Beckwith. The preparation of several di- and trisubstituted cyclopentanes has been achieved with high stereoselectivity. Tandem ring closures can be performed to construct bicyclic or tricyclic ring systems. Cyclizations of iodo-ethylenic and acetylenic esters and ketones can be accomplished, although the high reactivity of acetylenic ketones leads to unexpected cyclization products. The synthetic utility of this method has been demonstrated by an enantioselective synthesis of (+)-methyl epijasmonate and (-)-methyl cucurbate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021006
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    Gas-Phase Experiments Aimed at Probing the Existence of the Elusive Water Oxide MoleculeFirst reported at the 44th Conference on Mass Spectrometry and Allied Topics, Portland (Oregon, USA), 13 May, 1996.Dedicated to Professor Gerhard Ertl on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1235-1242 
    Details
    ISSN: 0947-6539
    Keywords: computer chemistry ; electron transfer ; hydrogen peroxide ; mass spectrometry ; water oxide ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Collisional activation, charge reversal, and six different neutralization - reionization mass spectrometric experiments with [H2,O2]·+ radical cations and [H2,O2]·- radical anions were performed in order to probe the predicated existence of neutral water oxide, H2OO, the long sought after tautomer of hydrogen peroxide, HOOH. The experiments together with ab initio calculations indicate that H2OO is a local minimum on the [H2,O2] potential-energy surface, and the elusive molecule seems to be formed as a transient upon neutralization of the corresponding radical cation H2OO·+ in the gas phase.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021009
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  • 166
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    Preparation, Characterization, and Reactivity of a Diamagnetic Vanadium Nitride (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1258-1263 
    Details
    ISSN: 0947-6539
    Keywords: dinitrogen activation ; nitrides ; structure elucidation ; vanadium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of (Ph2N)3V-(THF) (1) with [(CH3)3Si]N3 and [(CH3)3Si]CHN2 proceed at room temperature to afford the corresponding derivatives (Ph2N)3V[N-Si(CH3)3] (2a) and (Ph2N)3V[N2CHSi(CH3)3] (3). Reactions of both 2a and 3 with lithium isopropylamide provide the unprecedented anionic vanadium nitride species [(Ph2N)3V(μ-N)Li(THF)3]·(THF)0.5 (4), which displays moderate nucleophilicity. It reacts with (CH3)3SiCl and Mel to reform the complex (Ph2N)3V(N-R) [R = Si(CH3)3 (2a), Me (2b)]. Reactions with Cp2ZrCl2 and CpTiCl3 lead to the formation of the heterodinuclear bridged nitride complexes [(Ph2N)3V(μ-N)ZrClCp]2] [Cp = Cp]=Cp (6a), CH3C5H4 (6b)] and [{(Ph2N)3V(μ-N)}2TiClCp] (8).
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021012
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  • 167
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    Kinetic Analysis of the Stepwise Platination of Single- and Double-Stranded GG Oligonucleotides with Cisplatin and cis-[PtCl(H2O)(NH3)2]+ (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1283-1291 
    Details
    ISSN: 0947-6539
    Keywords: antitumour agents ; DNA ; kinetics ; nucleotides ; platinum complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report the first direct comparison of the kinetics of platination of defined single- and double-stranded DNA with the anticancer drug cisplatin. The courses of the reactions of the 14-mer duplex d(A-T-A-C-A-T-G-G-T-A-C-A-T-A)·d(T-A-T-G-T-A-C-C-A-T-G-T-A-T) with [15N]cisplatin and cis-[PtCl(H2O)-(15NH3)2]+ and of each of the single strands with [15N]cisplatin have been studied at 298 K, pH 6, by [1H, 15N] NMR spectroscopy. As expected the reactions of cisplatin proceed via cis-[PtCl(H2O)(NH3)2]+, and lead to two monofunctional adducts on the duplex and two on the GG single strand. In both the GG single strand and the duplex, one of the two G's is platinated faster than the other (by a factor of ca. 4). Remarkably, ring closure on the duplex to form the GG chelate occurs about an order of magnitude faster for one monofunctional adduct than for the other. The latter monofunctional adduct has distinctive 1H and 15N NMR chemical shifts for Pt-NH3, and is very long-lived (persists for 〉5 d). The Pt-Cl bond in this monofunctional adduct is protected from hydrolysis by the duplex. In contrast, the two monofunctional adducts on the GG single strand undergo ring closure at about the same rate. Equilibria between kinked and distorted forms of the GG platinated duplex, the platination of G's on the complementary strand, and the potential biological significance of long-lived monofunctional adducts of platinum drugs with DNA are discussed.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021016
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  • 168
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    Polystyrene-Poly(propylene imine) Dendrimers: Synthesis, Characterization, and Association Behavior of a New Class of Amphiphiles (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1616-1626 
    Details
    ISSN: 0947-6539
    Keywords: amphiphiles ; block copolymers ; dendrimers ; micelles ; surfactants ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new class of amphiphilic macromolecules has successfully been synthesized by creating well-defined diblock copolymers of polystyrene (PS, Mn=3.2 × 103, Mw/Mn=1.04) with poly-(propylene imine) dendrimers. A poly-styrene core molecule with a primary amine end-group was prepared by a quantitative three-step modification procedure of acid-functionalized polystyrene. On this core molecule, five different generations from PS-dendr-NH2 up to PS-dendr-(NH2)32 were constructed in high yields. The molecular structure of the block co-polymers was analyzed in detail with NMR and IR spectroscopy and electrospray mass spectrometry. With conductivity measurements and monolayer pressure-area isotherm determinations, we observed generation-dependent amphiphilic behavior. Dynamic light scattering and transmission electron microscopy showed that amphiphile geometry had a distinct effect on aggregation behavior; this is in qualitative agreement with Israelachvili's theory. Critical association concentrations determined with the pyrene probe luminescence technique were as low as 5 × 10-7 M. The amphiphiles presented here, therefore, show a resemblance to traditional block copolymers with regard to size and stability, and are similar to surfactants with respect to tuning of the shape.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021221
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  • 169
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    Novel Heterocumulenes: Bisiminopropadienes and Linear Ketenimines (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1318-1329 
    Details
    ISSN: 0947-6539
    Keywords: cumulenes ; heterocumulenes ; ketenimines ; matrix isolation ; thermolysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Flash vacuum thermolysis (FVT) of suitably substituted isoxazol-5(4H)-ones 7-9 leads to three different types of ketenimines, namely, the isoxazolonoketenimines 2, the novel bisiminopropadienes RN=C=C=C=NR (5), and the C-cyanoketenimines 14, all characterized by a combination of FVT/matrix isolation/IR spectroscopy and FVT/MS. An unusual, linear C=C=N-C backbone in ketenimines 2g and 2h is revealed by their exceptional spectroscopic properties as well as an X-ray crystal structure of 2g, and confirmed by density functional calculations (B3LYP/6-31 G*); these compounds are best described as resonance hybrids of ketenimines and isonitrile ylides R2C-C≡N-R'. The identification of the highly reactive bisiminopropadienes 5 is supported by the observed shifts in the IR bands of the 15N and 13C isotopomers as well as theoretical calculations, tert-Butyl-substituted isoxazolones 7e and 7f, and 8i form the expected ketenimines 2, which then undergo elimination of isobutene and CO2 to generate C-cyanoketenimines 14 and 14i. N-Phenyl-dicyanoketenimine 32 is also described.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021020
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  • 170
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    Multi-mode switching based on dihydroazulene/vinylheptafulvene photochromism: Synergism of photochromism and redox switching in heteroaryl-functionalized systemsFor the preceeding publication. see ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1150-1158 
    Details
    ISSN: 0947-6539
    Keywords: electrochromes ; cyclic voltammetry ; heterocycles ; optical memory ; photochromes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of the dihydroazulenes (DHAs) DHA-a-DHA-f containing covalently linked heteroaromatic subunits derived from dibenzodioxin, thianthrene, phenoxathiine, N-methylphenothiazine, N-methylphenoxazine, and N,N′-dimethylphenazine groups are described, and their spectroscopic and analytical data are reported. The dihydroazulene/vinylheptafulvene (DHA/VHF) photochromism (photochemical forward and thermal back reaction) depends with high sensitivity on the electronic properties of the functional groups. Whereas the dimethylphenazine derivative DHA-f is photochemically inactive towards rearrangement, all other DHAs (DHA-a-DHA-e) were found to isomerize to the corresponding vinylheptafulvene forms under irradiation. Cyclic voltammetry revealed that the DHA and the VHP forms have significantly different oxidation and reduction waves. The products of the-oxidative one-electron transfer are characterized by UV/Vis/NIR spectroelectrochemistry. Those DHAs having weaker donor substituents (DHA-a-DHA-c) undergo oxidative dimerization whereas DHAs with stronger donating heterocyclic subunits (DHA-d-DHA-f) form stable radical cations.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020916
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  • 171
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020921
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  • 172
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1047-1051 
    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.1460020920
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  • 173
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1189-1194 
    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.19960021003
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  • 174
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    Photochemistry of N-Phthaloylcysteine Derivatives: Multiplicity-Directed Regioselective CH Activation (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1388-1394 
    Details
    ISSN: 0947-6539
    Keywords: amino acids ; C-H activation ; electron transfer ; photochemistry ; state selectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photochemistry of the methyl esters of N-phthaloylcysteine derivatives 1b-5b was studied. The results are remarkable, because they prove a pronounced, multiplicity-controlled regioselectivity of the initial CH activation step. From substrates 1b-4b the benzazepine-1,5-dione 6 was produced exclusively by the singlet path. The formation of compound 6 is initiated by a γ-H abstraction; this was demonstrated by deuterium labeling experiments. The penicillamine derivative 5b was unreactive in the singlet manifold. From substrates 1b-3b and 5b the thiazinoisoindoles 7-9 and 11 were produced exclusively by the triplet path. The sterically hindered S-isopropylcysteine derivative 4b also furnished this product type (10) as a proportion of the products in the singlet manifold. These annulation products result from a primary photoinduced electron-transfer (PET) step followed by heterolytic ∊-H activation. The mechanistic scenario was elucidated by quenching and sensitization experiments. An interplay of CH activation steps and electron back-transfer is probably responsible for this type of spin selectivity.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021110
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  • 175
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    Synthesis, Crystal Structure, Magnetism, and Magnetic Anisotropy of Cyclic Clusters Comprising six Iron(III) Ions and Entrapping Alkaline Ions (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1379-1387 
    Details
    ISSN: 0947-6539
    Keywords: aggregates ; clusters ; host-guest chemistry ; iron complexes ; magnetic anisotropy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of ferric chloride and β-diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6-(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single-crystal X-ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in organic solution, as demonstrated by 23Na and 7Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high-spin ferric ions. The energy of the excited states was studied in detail by high-field DC and pulsed-field differential magnetization experiments at 0.7 and 1.5 K. Single-crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single-ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic-type behavior in transition-metal clusters.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021109
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  • 176
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    Photoswitched and Functionalized Oligothiophenes: Synthesis and Photochemical and Electrochemical Properties (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1399-1406 
    Details
    ISSN: 0947-6539
    Keywords: electrochromes ; fluorescence ; molecular devices ; oligothiophenes ; photochromes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The extended dithienylethene compounds 1-3 have been synthesized. They undergo photochromic and electrochemical changes by photoinduced interconversion between open and closed forms of type A and B. In addition, both forms of the N-methylated derivatives 1b and 2b have absorption bands in which excitation results in very large differences in fluorescence between the two forms with very little effect on the opening/closing state, a feature of interest for optical memory data systems. The compounds 2 and 3 contain six and eight conjugated thiophene units in the closed forms, respectively, and thus represent oligothiophenes endowed with a photoactivated switch; they are of special interest in this respect in view of the potential use of switched oligothiophenes in molecular electronic devices.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021112
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  • 177
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    [Cu24S12(PMeiPr2)12], [Cu28S14(PtBu2Me)12], [Cu50S25(PtBu2 Me)16], [Cu70Se35(PtBu2Me)21], [Cu31Se15(SeSiMe3)(PtBu2Me)12] and [Cu48Se24(PMe2Ph)20]: New Sulfur- and Selenium-Bridged Copper ClustersDedicated to Professor Hans Georg von Schnering on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1407-1416 
    Details
    ISSN: 0947-6539
    Keywords: clusters ; copper complexes ; selenium compounds ; structure elucidation ; sulfur compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of copper(I) acetate with bis(trimethylsilyl)chalcogenides E-(SiMe3)2 (E = S, Se) in the presence of tertiary phosphines in organic solvents leads to the formation of chalcogenbridged copper clusters in crystalline form. In recent investigations, the following structures were isolated and characterised by X-ray crystallography: [Cu24S12-(PMeiPr2)12] (1), [Cu28S14(PtBu2Me)12] (2), [Cu50S25(PtBu2Me)16] (3), [Cu70Se35-(PtBu2Me)21] (4), [Cu31Se15(SeSiMe3)-(PtBu2Me)12] (5) and [Cu48Se24(PMe2-Ph)20] (6). The cluster cores of 1-6 have new structures or new structural characteristics and go some way towards providing an insight into the mechanisms of cluster formation reactions. With the help of the aforementioned compounds a number of structure principles that govern sulfur-and selenium-bridged copper clusters can be formulated.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021113
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  • 178
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    Aryloxycarbonylcarbene Complexes of Bis(oxazolinyl)pyridineruthenium as Active Intermediates in Asymmetric Catalytic CyclopropanationsDedicated to Professor Henri Brunner on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 303-306 
    Details
    ISSN: 0947-6539
    Keywords: asymmetric syntheses ; carbene complexes ; cyclopropanations ; diazoacetates ; ruthenium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of [RuCl2(pybox)-(C2H4)] (1) (pybox = 2,6-bis[4′S)-isopropyloxazolin-2′-yl]pyridine) and 2,6-di-tert-butyltolyl diazoacetate (4) (DBT-DA) in benzene at 50°C gave a stable 2,6-di-tert-butyltolylcarbonylcarbene-ruthenium complex 5 in 94% yield. The structure of 5 was characterized by NMR spectroscopy. 2,6-Diisopropylphenyl diazoacetate (6) and 2,4,6-trimethylphenyl diazoacetate (7) also gave the corresponding carbene complexes 9 and 10, respectively. Asymmetric carbene transfer from the carbene complexes to styrene resulted in formation of the trans isomer of phenylcyclopropanecarboxylates 2 with high enantioselectivity: 2 was obtained as the sole product in 80% yield (55% ee) from 5 at 80 °C and in 82% yield (97% ee) from 9 at 60°C; from 10 at 40°C, a mixture of 2 and 3 in a ratio of 97:3 was formed in 91% yield (97% ee for 2 and 99% ee for 3). After the carbene transfer reaction, the ethylene complex 1 could be regenerated and isolated by treatment of the reaction mixture under an ethylene atmosphere. The carbene complexes 9 and 10 (2 mol%) exhibited catalytic activity in the asymmetric cyclopropanation of styrene with the corresponding diazoacetates.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19960020311
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  • 179
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    The photochemical Formation of Fulvene from Benzene via Prefulvene-a Theoretical StudyDedicated to Professor Roy McWeeny on the occasion of his 70th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 335-341 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; benzene ; fulvene ; isomerization ; photochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of semiempirical MN-DOC-CI and ab initio CASSCF calculations reveal that fulvene is a primary product of the photolysis of benzene. The photochemical step is assumed to lead to prefulvene, and both a synchronous conversion and a two-step process via bicyclo[3.1.0]hexa-1,3-diene (isofulvene) could be ruled out as possible pathways from prefulvene to benzene. The most probable mechanism for the photochemical isomerization of benzene to fulvene involves the intermediate structures prefulvene and 1,3-cyclopentadienylcarbene and has to compete with the almost barrierless formation of benzvalene and rearomatization to benzene. The short-lived intermediates are rather flexible structures with negligible barriers to inversion at the radical center in the threemembered ring of prefulvene and prebenzvalene and to rotation around the exocyclic single bond in 1,3-cyclopentadienylcarbene.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020315
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020401
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  • 181
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    NMR Chemical-Shift Anomaly and Bonding in Piano-Stool Carbonyl and Related Complexes-an Ab Initio ECP/DFT StudyDedicated to Professor Hans Georg von Schnering on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 348-358 
    Details
    ISSN: 0947-6539
    Keywords: carbonyl complexes ; density-functional theory ; NMR chemical shifts ; pseudopotentials ; transition-metal complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The origin of the unusually large carbonyl 13C shifts and of unusual periodic trends in four-legged piano-stool complexes [M(n5-C5H5)(CO)4]- (M=Ti, Zr, Hf) and in related species has been investigated by using a combination of ab initio effective-core potentials (ECPs) and density-functional theory (DFT). The ECP/SOS-DFPT(IGLO) calculations indicate a considerable reduction in the anisotropy of the 13C(CO) chemical shift tensors compared to terminal carbonyl ligands in “normal” complexes. This is due to large paramagnetic contributions from metal d AO type (dz2, dxy) orbitals to the parallel component, σ33, of the shielding tensors of the carbonyl carbon atoms. The neutral d4 Group 5 and 6 complexes [M(n5-C5H5)(CO)4] (M=V, Nb, Ta) and [M(n5-C5H5)(CO)3CH3] (M=Cr, Mo, W) exhibit successively smaller but still significant paramagnetic d-orbital contributions to σ33, consistent with the observed less dramatic deshielding. The three-legged d6 piano-stool complexes [M(n5-C5H5)(CO)3] (M=Mn, Tc, Re) do not exhibit these reductions of the shielding anisotropy, but have carbonyl 13C shift tensors comparable to regular octahedral carbonyl complexes. The special situation for the four-legged complexes is related to the presence of high-lying occupied metal d orbitals, and particularly to the favorable spatial arrangement of these d orbitals with respect to the carbonyl ligands. Bent-sandwich d2 complexes like [Zr(n5-C5H5)2(CO)2] exhibit comparable deshielding contributions from an occupied metal d orbital. For similar reasons, the 17O resonances for these piano-stool and bent-sandwich complexes are also predicted to be at unusually high frequencies, with low shift anisotropy. NMR shifts for the (n5-C5H5)-ligand atoms and the structures of the complexes are also discussed.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020317
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    Highly Selective Formation of tert-Butyl Hydroperoxide from the Reaction of Isobutane and O2 in a Zeolite under Visible Light (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 385-389 
    Details
    ISSN: 0947-6539
    Keywords: hydroperoxides ; isobutane ; oxygen ; photooxidations ; zeolites ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Isobutane and oxygen gas loaded into zeolite BaY react upon irradiation with green or blue light to yield tert-butyl hydroperoxide in a single-photon process. This was discovered when monitoring the reaction at room temperature in situ by FT-infrared spectroscopy. Selectivity was 98%, even upon conversion of more than half of the reactants loaded into the zeolite. Diffuse reflectance spectra revealed a visible absorption tail which originates from an isobutane · O2 collision complex. It is attributed to the isobutane · O2 contact charge-transfer absorption, whose onset is shifted from the UV into the visible region by the high electrostatic field of the zeolite.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020406
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  • 183
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    Structural Principles of the Coordination Number Eight: WF2-8, ReF2-8, and XeF2-8 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 398-402 
    Details
    ISSN: 0947-6539
    Keywords: coordination geometry ; fluorides ; square antiprisms ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: WF6, ReFn (n = 6 and 7), and XeF6 combined with NOF to give (NO+2) WF2-8, (NO+)2ReF2-8, and (NO+)2XeF2-8, respectively. Also NO2F reacted with ReF6 to form (No2+)ReF2-8. Cs2XeF8·4BrF5 crystallized from a solution of Cs2XeF8 in BrF5. These five compounds were subjected to X-ray structure determinations. The structure of [NO(NOF)2]+IF-8 was used as a standard for comparison. All anions exhibit square-antiprismatic geometry, independent of their electronic configurations. Bond lenghts in ReF2-8 and XeF2-8 are larger than in WF2-8 and IF-8, owing to the presence of nonbonding electrons. Deviations from the ideal structure in XeF2-8 are attributed to cation-anion interactions.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020408
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  • 184
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    An Ab Initio Study of Structures and Energetics of Copper Sulfide Clusters (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 429-435 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; chalcogen compounds ; copper clusters ; ligand stabilisation ; thermodynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio calculations for the sulfur-bridged copper clusters [Cu2nSn(PR3)m] (n = 1-4, 6; m = 0, 2, 4, 6, 8; R=H, CH3) were compared with those of theoretical investigations of the selenium-containing analogues that have been recently reported. The theoretical work was carried out in order to find a possible explanation for the experimentally observed-different-properties of both cluster types. The structural principles turned out to be independent of the chalcogen, except the necessary shortening of the copper-chalcogen atomic distance that results from the formal substitution of sulfur for selenium. Comparison of the energetics of the sulfur- and selenium-containing compounds showed that it is necessary to discuss the influence of the tertiary phosphine ligands that protect the existing clusters from reaction to give the solids Cu2S and Cu2Se. However the results of the calculations suggest that the different thermodynamic data of the cluster core and the Cu—P bonds-at least if PH3 or PMe3 are taken into account-cannot be the only cause for the differences in the experimental behaviour. The responsibility rests rather with kinetic effects, such as the steric demand of the actual ligands or the activation energy of decomposition of the ligand shell during the cluster-forming reaction.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020412
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  • 185
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    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 476-479 
    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.19960020503
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  • 186
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    Ionic Fluorination of Carbon Monoxide as a Route to Gasphase Carbonylation of Inert C—H and N—H Bonds (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 495-501 
    Details
    ISSN: 0947-6539
    Keywords: carbon monoxide ; carbonylations ; fluorine compounds ; gas-phase chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Gaseous FCO+ ions from the ionization of mixtures of nitrogen trifluoride and carbon monoxide execute selective and efficient CO-functionalization of the C-H bonds of benzene and toluene and of the N-H bond of ammonia. The occurrence of these carbonylation reactions has been unambiguously ascertained by Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry, and the details of the structure and the mechanism of formation of the precursor FCO+ ions have been investigated. FT-ICR experiments show that these ions, structurally assigned as F-C-O+ by collisionally activated dissociation (CAD) spectrometry, arise from the reaction of CO.+ with NF3 and of NF+2 with CO. Combining the latter F+ transfer with the independently observed fluoride-ion abstraction by FCO+ from NF3 results in a catalytic cycle in which gaseous NF+2 ions promote the conversion of carbon monoxide into carbonic difluoride, F2CO, with nitrogen trifluoride as the source of F.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020507
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  • 187
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    Conformational Preference of 2-(Halomethyl)- and 2-(Oxymethyl)Pyridines: Microwave Spectrum, Ab Initio, and MM3 Studies of 2-(Fluoromethyl)Pyridine (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 516-522 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; conformation ; microwave spectroscopy ; pyridines ; stereoelectronic effect ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One single conformer was assigned from the microwave spectrum of 2-(fluoromethyl)pyridine, investigated in the gas phase in the 26.0-39.0 GHz spectral region at about -10°C. Its Cα-F bond was found to be coplanar with the ring and anti to the N-C2 bond (syn to the C2-C3 bond). There was no indication in the microwave spectrum of the presence of other rotameric forms of the molecule.The results of the spectroscopic study were backed up by ab initio calculations at the MP2/6-31 G** (frozen core) level. These calculations predict that the assigned conformer is the only stable form of the molecule. The transition state was calculated to have the CH2F group 180° from the stable anti conformation. The energy of the transition state was computed to be 20.2 kJ mol-1 higher than the energy of the anti rotamer.The results are interpreted in terms of a stereoelectronic effect, and the orbital overlaps responsible for the observed effect are discussed. It is shown that 2-(fluoromethyl)pyridine serves as a good model for 2-(alkoxymethyl)pyridines, previously found to show the same conformational preference.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020510
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  • 188
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    Synthesis of Methyl 5′-Thio-α-isomaltoside via an Acyclic Monothioacetal and its Behavior toward Glucoamylase (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 556-560 
    Details
    ISSN: 0947-6539
    Keywords: binding studies ; glucoamylase ; isomaltoses ; oligosaccharides ; thiosugars ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methyl 5′-thio-α-isomaltoside (1), which contains the ring-sulfur analogue of the nonreducing glucoside of isomaltose, was synthesized from gentiobiose through a novel ring opening-recyclization approach. The nonreducing glucoside of per-O-benzylated phenyl 1-thio-β-gentiobioside underwent O-5′-C-1′ bond cleavage with dimethyl-boron bromide and thiolacetic acid to give the acyclic monothioacetal 4 with the 1-thioglucopyranoside at the reducing end intact. The HO-5′ group in 4 was inverted by a standard oxidation-reduction process with good efficiency. Recyclization under Mitsunobu condition allowed C-5′-S-1′ bond formation with inversion of configuration at C-5′, to give 1 after functional group interconversion. TLC analysis showed that 1, unlike isomaltose, was not hydrolyzed by glucoamylase from Rhizopus niveus. A fluorometric assay confirmed that the dissociation constant (Kd) for 1 with the enzyme was 39 mM at 20°C, which is comparable with that for isomaltose. A binding assay involving fluorescence titration of the enzyme-1 complex with gluconolactone indicated that the disaccharide 1 was bound to the catalytic and noncatalytic subsites. Since isomaltose is known to bind only to the noncatalytic subsites, this result indicates a relatively high affinity of the 5-thioglucose moiety for the catalytic subsite.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chem.19960020515
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  • 189
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    Versatile Synthesis of Enantiomerically Pure 2-Alkoxy-1-Ethynylcyclopropanes and their Application in the Synthesis of Enantiomerically Pure Bicyclo-[3.3.0]oct-1-en-3-onesCyclopropyl Building Blocks for Organic Synthesis, Part 31. Part 30: T. Heiner, S. Michalski, K. Gerke, G. Kuchta, M. Buback, A. de Meijere, Synlett 1995, 335; Part 29: J. L. Coudret, K. Ernst, A. de Meijere, B. Waegell, Synthesis 1994, 920.Dedicated to Professor Manfred Regitz on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 545-555 
    Details
    ISSN: 0947-6539
    Keywords: alkynes ; asymmetric syntheses ; cyclopropanes ; spiro compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of chiral, nonracemic 2-alkoxy-1-alkynylcyclopropanes 7 were synthesized in good to very good yields from enantiomerically pure glycidol derivatives (glycidol tosylate, epichlorohydrin) by boron trifluoride promoted addition of lithium trimethylsilylacetylide followed by protection of the secondary hydroxyl group and finally a diastereoselective γ-elimination. The 2-ethoxy derivative (S,R)-7 b was deprotonated with n-butyllithium, and the resulting 1-lithio-2-ethoxy derivative (S,R)-20 functionalized by treatment with oxygen followed by tosyl chloride. Protodesilylation and catalytic hydrogenation smoothly furnished 1-ethenylcyclopropyl sulfonates, which underwent a clean Pd0-catalyzed SN2′-type substitution with dimethyl propargylsodiummalonate to give the (E)-configurated enyne (R,E)-26 with a methylenecyclopropane end group. A diastereoselective Pauson-Khand reaction completed the sequence to give the enantiomerically pure spirocyclopropaneannelated bicyclo[3.3.0]octane derivative 31.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/chem.19960020514
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  • 190
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    Total synthesis of Swinholide A, Preswinholide A, and Hemiswinholide A (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 847-868 
    Details
    ISSN: 0947-6539
    Keywords: natural products ; swinholide A ; total syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total synthesis of synthesis of swinholide A (1) has been accomplished via key intermediate aldehyde 12 (Fig. 3), whose construction started from L-rhamnose (18), epoxide 21, and phenylsulfone orthoester 22, and proceeded through an Enders asymmetric alkylation (16 + 17 → 15), a Ghosez cyclization (21 + 22 → 20), and a Corey-Sharpless coupling reaction (13 + 14 → 12). Elaboration of compound 12 along slightly different pathways culminated in the synthesis of carboxylic acid 10 and hydroxy compound 11, whose union by an esterification reaction, followed by ring closure of the subsequently derived hydroxy acid under Yamaguchi conditions, led to swinholide A (1) upon deprotection. The chemistry developed also allowed the total synthesis of prewinholide A methyl ester (7), preswinholide A (8), and hemiswinholide A (78).
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chem.1460020716
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  • 191
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    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020721
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  • 192
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    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.1460020801
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  • 193
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    Observing a homogeneous ziegler catalyst precursor at work: Insertion reactions into the zirconium - carbon bond of the (butadiene)ZrCp2/B(C6F5)3 addition product (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 919-924 
    Details
    ISSN: 0947-6539
    Keywords: homogeneous catalysis ; metallocenes ; polymerization ; Ziegler catalysts ; zirconium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (Butadiene)zirconocene adds to B(C6F5)3 to form the metallacyclic metallocene-boron betaine system 1, which contains a C-F → Zr interaction. Addition of tert-butylisocyanide or tert-butyl-cyanide leads to cleavage of the zirconium - fluorine linkage and formation of the adducts 4 and 7, respectively. With additional tert-butylisocyanide, 4 reacts further to yield the corresponding η2-imino-acyl betaines 5 and 6 (2 stereoisomers), whereas 7 inserts additional tert-butyl-cyanide into the Zr-C bond to give a chiral metallacyclic N-zirconaketimine complex that is probably stabilized by an internal borate → Zr interaction. Ethene and propene insertion reactions into the Zr - C bond of 1 follow a similar course: the mono-insertion products 9 and 10, respectively, generated in situ and characterized by NMR spectroscopy at - 35°C in [D8]toluene solution, are chiral and are also stabilized by internal borate → Zr coordination. Complexes 9 and 10 are likely to be intermediates in the formation of active homogeneous Ziegler polymerization catalysts.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020803
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  • 194
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    Oxypyriporphyrin, the first fully aromatic porphyrinoid macrocycle with a pyridine subunitConjugated Macrocycles Related to the Porphyrins, Part 6. Part 5: ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 944-948 
    Details
    ISSN: 0947-6539
    Keywords: aromaticity ; porphyrins ; pyridones ; MacDonald condensation ; tautomerizations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the first example of an aromatic pyridine-containing porphyrinoid 10 has been accomplished in excellent yields by the acid-catalyzed “3 + 1” condensation of 3-hydroxy-pyridinedicarboxaldehyde (8) with tri-pyrrane 9. The key intermediate 8 was obtained by the selenium dioxide oxidation of the known biscarbinol 7. The aromaticity of “oxypyriporphyrin” 10 has been confirmed by MS, NMR, IR, and UV/Vis spectroscopy. This system afforded a monocation in 0.2% TFA-chloroform, and a dication was observed in 2% TFA-chloroform; these species also retained macrocyclic aromaticity. Oxypyriporphyrin readily formed the corresponding metal chelates 14a-c by reaction with zinc, copper(II), or nickel(II) acetate, and this observation suggests that there are extensive possibilities for the use of 10 in coordination chemistry. Oxypyriporphyrin and the related semiquinone system oxybenziporphyrin represent the first two members of a new class of aromatic porphyrinoids.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020806
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  • 195
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    Oxidative addition of aryl halides to transient anionic à-aryl-palladium(0) intermediates - application to palladium-catalyzed reductive coupling of aryl halides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 957-966 
    Details
    ISSN: 0947-6539
    Keywords: catalysis ; C-C coupling ; mechanistic studies ; oxidative addition ; palladium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rates and mechanism of the reactions of a series of aryl-ligated, anionic palladium(0) complexes Ar-Pd0-(PPh3)-2 with para-substituted iodobenzenes were investigated by means of transient electrochemistry. The reaction was found to be first order in each reactant and to proceed similarly to oxidative addition of aryl halides to the halide-ligated species X-Pd0(PPh3)-2, although much faster and less sensitive to electronic factors. Owing to the short lifetime (t1/2 ≍ 1 - 5 ms) of the product of this reaction, it could not be characterized in detail. However, based on kinetic results, this transient species is thought to be an anionic pentacoordinated bisarylpalladium(II) complex, which undergoes rapid loss of halide ligand to yield, most probably, a bisarylpalladium(II) neutral species. Based on the study of this reaction and on previously reported results, we propose a mechanism for the palladium-catalyzed homocoupling of aryl halides consisting of a catalytic cycle initiated by oxidative addition of an aryl halide to a zerovalent tris-ligated palladium center. Two-electron reduction of the pentacoordinated arylpalladium(II) anionic species thus formed gives a tris-ligated anionic arylpalladium(0) center, which undergoes oxidative addition with a second aryl halide molecule to eventually lead to a bisaryl-palladium(II) neutral species. Reductive elimination of a bisaryl molecule from this center closes the catalytic cycle by regenerating the initial zerovalent palladium complex. The application of this sequence to the catalytic heterocoupling of aryl halides is discussed, and it is concluded, on the basis of Hammett correlations, that statistical yields should be observed, in agreement with the results obtained for preparative reactions in DMF.
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    URL: http://dx.doi.org/10.1002/chem.1460020808
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  • 196
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    Structural motifs of the dimeric lewis glycolipids as determined by NMR spectroscopy and molecular dynamics simulations (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 981-988 
    Details
    ISSN: 0947-6539
    Keywords: computer simulations ; conformations ; Lewis glycolipids ; molecular dynamics ; thioglycosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several monomeric and dimeric Lewis glycolipids have been investigated by NMR spectroscopy, and structural aspects were modelled by computer. From the pseudo-C2-symmetric tetrasaccharide unit that forms the recognition domain of the Lewis Y and Lewis b antigens, a totally C2-symmetric tetrasaccharide was designed that contains the structural element common to all Lewis antigens. Finally, a model for the presentation of dimeric Lewis antigens at membrane surfaces was derived. The overall shapes of the dimeric Lewis oligosaccharides are defined by the connectivity of the sugar residues within rigid tri- and tetrasaccharide building blocks.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020811
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  • 197
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    Synthesis of a conformationally constrained heparin-like pentasaccharide (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1007-1013 
    Details
    ISSN: 0947-6539
    Keywords: antithrombotics ; conformation ; molecular recognition ; oligosaccharides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An octasulfated pentasaccharide 1 having an L-iduronic acid moiety in a fixed 1C4 conformation was synthesized by the coupling of a triosyl donor 3 with a disaccharide acceptor 4 followed by deprotection and O-sulfation. The acceptor 4 was prepared from the fully acetylated 5-C-allyl-β-D-glucose building block 7 by means of a TMSOTf-promoted glycosylation, intramolecular substitution and ozonolysis of the olefinic bond as the key reactions. Compound 1 showed very low activity in an antithrombin 111-mediated anti-Xa assay; this reflects the importance of the presence of a flexible L-iduronic acid moiety in heparin-like antithrombotics.
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    URL: http://dx.doi.org/10.1002/chem.1460020815
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  • 198
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    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    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.1460020901
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  • 199
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    On the definition of chirality (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 743-744 
    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.1460020617
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  • 200
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    Self-assembling [2]- and [3]rotaxanes from secondary dialkylammonium salts and crown ethersMolecular Meccano, Part 7: for Part 6; see the preceding paper in this issue (ref. [14]). (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 729-736 
    Details
    ISSN: 0947-6539
    Keywords: interlocking moleucles ; molecular recognition ; pseudorotaxanes ; rotaxanes ; template syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of three new rotaxanes - two [2]rotaxanes and a [3]rotaxane - formed by a “threading followed by stoppering” approach is described. These template-directed syntheses rely on the formation of pseudorotaxane intermediates, which self-assemble in solution from functionalized secondary dialkylammonium hexafluorophosphate threads and macrocyclic polyether rings (either dibenzo-[24]crown-8 or its asymmetric constitutional isomer). The stoppers - substituted 1,2,3-triazoles-were created by thermally allowed 1,3-dipolar cycloadditions between azido groups, which terminate the threads, and di-tert-butyl acetylenedicarboxylate.
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    URL: http://dx.doi.org/10.1002/chem.1460020615
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