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  • General Chemistry  (1.786)
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  • 1995-1999  (3.772)
  • 1945-1949
  • 1996  (3.772)
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  • 1
    Digitale Medien
    Digitale Medien
    Ion exchange purification of G6PDH from unclarified yeast cell homogenates using expanded bed adsorption (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 204-216 
    Details
    ISSN: 0006-3592
    Schlagwort(e): expanded bed adsorption ; bakers' yeast ; G6PDH ; STREAMLINE ion exchange adsorbents ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The use of expanded beds of STREAMLINE ion exchange adsorbents for the direct extraction of an intracellular enzyme glucose-6-phosphate dehydrogenase (G6PDH) from unclarified yeast cell homogenates has been investigated. It has been demonstrated that such crude feedstocks can be applied to the bed without prior clarification steps. The purification of G6PDH from an unclarified yeast homogenate was chosen as a model system containing the typical features of a direct extraction technique. Optimal conditions for the purification were determined in small scale, packed bed experiments conducted with clarified homogenates. Results from these experiments were used to develop a preparative scale separation of G6PDH in a STREAMLINE 50 EBA apparatus. The use of an on-line rotameter for measuring and controlling the height of the expanded bed when operated in highly turbid feedstocks was demonstrated. STREAMLINE DEAE has been shown to be successful in achieving isolation of G6PDH from an unclarified homogenate with a purification factor of 12 and yield of 98% in a single step process. This ion exchange adsorbent is readily cleaned using simple cleaning-in-place procedures without affecting either adsorption or the bed expansion properties of the adsorbent after many cycles of operation. The ability of combining clarification, capture, and purification in a single step will greatly simplify downstream processing flowsheets and reduce the costs of protein purification. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    Rat hepatocyte morphology and function on lactose-derivatized polystyrene surfaces (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 259-265 
    Details
    ISSN: 0006-3592
    Schlagwort(e): hepatocytes ; lactose-derivatized polystyrene ; polystyrene ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Hepatocytes isolated from male Fisher 344VF rats were cultured on two substrates, collagen I and a lactose-derivatized polystyrene (PS-lactose), to compare morphological and functional differences. Hepatocyte morphology changed dramatically depending upon the substrate, shown through actin cytoskeletal staining and scanning electron microscopy. Functional assays performed included albumin secretion, reduced glutathione content, UDP-glucuronosyl transferase, and cytochrome P4501A1 activity. The presence of dexamethasone and dimethylsulfoxide (DMSO) in the media was required for the maintenance of several differentiated functions for cells cultured on collagen. In general, cells cultured on the PS-lactose substrate showed a much slower loss of function over the same period of time. The maintenance of differentiated function of cells on PS-lactose was enhanced with the addition of dexamethasone and DMSO. This is the first report of a culture system in which hepatocytes, cultured on a polymer substrate without additional protein coatings or media additives, have been able to maintain differentiated functions for up to 1 week. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    Conservative chemical modification of proteins to study the effects of a single protein property on partitioning in aqueous two-phase systems (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 290-299 
    Details
    ISSN: 0006-3592
    Schlagwort(e): proteins, modified ; partitioning in aqueous system ; thaumatin ; β-lactoglobulin ; BSA ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Relatively conservative modifications of three proteins were carried out to alter their surface properties. The protein properties modified were hydrophobicity and charge. This was done by acylation of amino groups with anhydrides. For the hydrophobic modification experiments, two proteins (β-lactoglobulin and bovine serum albumin [BSA]) and four anhydrides (hexanoic, butyric, succinic, acetic) were used. For the modification of surface charge the protein thaumatin was selected and various proportions of the free amino groups were blocked with acetic anhydride to give a series of proteins with differing isoelectric points. Detailed characterization and purification of selected modified proteins was carried out including molecular weight measurements and conformational analysis. The criteria used for selecting the modified proteins for subsequent investigation of their partitioning in aqueous two-phase systems (ATPS) is described. With a judicious choice of starting material it was found that limited chemical modifications to proteins could effectively alter surface hydrophobicity or charge almost independently, with little effect on other molecular properties. It appears, however, that the method for chemical modification and the reaction conditions must also be carefully controlled. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    Use of chemically modified proteins to study the effect of a single protein property on partitioning in aqueous two-phase systems: Effect of surface charge (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 309-315 
    Details
    ISSN: 0006-3592
    Schlagwort(e): surface charge ; proteins, modified ; partitioning in aqueous system ; thaumatin ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: A series of charge-modified thaumatins with different values of surface charge were partitioned in aqueous two-phase systems (ATPS) to study the effect of surface charge as a single property on partitioning. Electrophoretic mobility of the proteins in titration curves was used as a measure of surface charge. Four modified proteins derived from thaumatin with the following values of isoelectric point: 8.70, 8.15, 5.60, and 4.50 were used for partitioning. The resolution of the systems in terms of protein surface charge was calculated. Partitioning of modified thaumatins in PEG 4000/dextran systems with phosphate buffer, Tris buffer, NaCl, KCl, and sulfate salts was carried out. Among the sulfate salts tested, the addition of 50 mM Li2SO4 to the system buffered with phosphate gave the highest value of resolution for differences in surface protein charge (RSPC). It shows a decrease in the value of K (partition coefficient) with an increase in the protein's charge. The addition of 100 mM KCl to the system promoted the opposite effect on the RSPC value. Charge-modified proteins were partitioned in PEG/salt systems to investigate the ability of these systems for resolving differences in surface charge. The PEG/citrate system seemed to have almost no ability for resolving proteins on the basis of surface charge differences; PEG/phosphate systems had some capability for resolving differently charged proteins. The more negative proteins tended to have higher values of K than the more positively charged fractions. The use of charge-modified proteins allowed the investigation of the effect of protein surface charge on partitioning in aqueous two-phase systems independently from other protein parameters as they were prepared from a common parent protein thaumatin. This technique provides an interesting novel tool to investigate the effect of protein surface charge on partitioning in ATPS taking protein charge as an independent parameter. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    A versatile oxygenator and perfusion system for magnetic resonance studies (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 348-354 
    Details
    ISSN: 0006-3592
    Schlagwort(e): oxygenator ; NMR spectroscopy ; organ perfusion ; mammalian cell culture ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: A compact, reusable membrane oxygenator has been constructed for the perfusion of cultured cells and isolated organs. While the oxygenator was designed to be compatible with nuclear magnetic resonance (NMR) spectroscopy studies, it can also be used for any experiment which requires warming and oxygenation of perfusates. For the NMR studies, the oxygenator can be positioned at the opening of the magnet bore which allows oxygenation and warming of the perfusate immediately prior to delivery to the tissue, therefore eliminating problems with heat or oxygen loss which may occur with the long perfusion lines. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bit.260490302
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  • 6
    Digitale Medien
    Digitale Medien
    Fluid shear stress induction of the transcriptional activator c-fos in human and bovine endothelial cells, HeLa, and Chinese hamster ovary cells (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 383-390 
    Details
    ISSN: 0006-3592
    Schlagwort(e): c-fos protein ; endothelium ; hemodynamics ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The c-fos protein belongs to a family of transcriptional cofactors that can complex with proteins of the Jun family and activate mRNA transcription from gene promoters containing an activator protein 1 (AP-1) binding element. The shear stress inducibility of the c-fos protein was studied in human and animal cell lines of vastly different origins. Primary human umbilical vein endothelial cells (HUVEC), bovine aortic endothelial cells (BAEC, passage 2-14), HeLa cells, and Chinese hamster ovary (CHO) cells were subjected to steady laminar shear stress using a parallel plate flow apparatus. After 1 h of flow exposure at 25 dyn/cm2, the c-fos levels in nuclei of shear stress HUVEC, BAEC, HeLa, and CHO were 5.4 ± 2.0 (n = 3), 2.25 ± 1.38 (n = 6), 2.14 ± 0.07 (n = 8), 1.92 ± 0.58 (n = 2) times higher, respectively, than in matched stationary controls. Flow exposure at 4 dyn/cm2 caused no enhancement of c-fos levels in any of the cell lines tested, but caused significant reduction in c-fos expression in the HeLa cells. The c-fos induction by shear stress could be blocked by pharmacological agents. For example, the flow induction of the c-fos protein levels was blocked by 50% with the preincubation of HUVEC with a protein kinase C inhibitor, H7 (10 μM) and blocked completely in HeLa cells preincubated with the phospholipase C inhibitor, neomycin (5 mM). The minimum time of shear stress exposure required to induce the c-fos protein expression in HeLa cells was found to be as low as 1 min. By Northern analysis, the c-fos mRNA levels were found to be elevated in BAEC, CHO, and HeLa cells exposed to 25 dyn/cm2 for 30 min. These studies indicate that c-fos induction is a consistent genetic response in a variety of mammalian cells that may alter cellular phenotype in mechanical environments. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    Vancomycin production in batch and continuous culture (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 49 (1996), S. 412-420 
    Details
    ISSN: 0006-3592
    Schlagwort(e): Amycolatopsis orientalis ; vancomycin production ; chemostat culture ; phosphate inhibition ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Production of the glycopeptide antibiotic vancomycin by two Amycolatopsis orientalis strains was examined in batch shake flask culture in a semidefined medium with peptone as the nitrogen source. Different growth and production profiles were observed with the two strains; specific production (Yp/x) was threefold higher with strain ATCC 19795 than with strain NCIMB 12945. A defined medium with amino acids as the nitrogen source was developed by use of the Plackett-Burman statistical screening method. This technique identified certain amino acids (glycine, phenylalanine, tyrosine, and arginine) that gave significant increased specific production, whereas phosphate was identified as inhibitory for high specific vancomycin production. Experiments made with the improved medium and strain ATCC 19795 showed that vancomycin production kinetics were either growth dissociated or growth associated, depending on the amino acid concentration. In chemostat culture at a constant dilution rate (0.087 h-1), specific vancomycin production rate (qvancomycin) decreased linearly as the medium phosphate concentration was increased from 2 to 8 mM. In both phosphate and glucose limited chemostats, qvancomycin was a function of specific growth rate; the maximum value was observed at D = 0.087 h-1 (52% of the maximum specific growth rate). Under phosphate limited growth conditions, qvancomycin was threefold higher (0.37 mg/g dry weight/h) than under glucose limitation (0.12 mg/g dry weight/h). © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    On-line monitoring of respiration in recombinant-baculovirus infected and uninfected insect cell bioreactor cultures (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996), S. 36-48 
    Details
    ISSN: 0006-3592
    Schlagwort(e): insect cell culture ; Sf-9 cells ; respiration ; bioreactor ; on-line monitoring ; baculovirus expression vector system ; recombinant proteins ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Respiration rates in Spodoptera frugiperda (Sf-9) cell bioreactor cultures were successfully measured on-line using two methods: The O2 uptake rate (OUR) was determined using gas phase pO2 values imposed by a dissolved oxygen controller and the CO2 evolution rate (CER) was measured using an infrared detector. The measurement methods were accurate, reliable, and relatively inexpensive. The CER was routinely determined in bioreactor cultures used for the production of several recombinant proteins. Simple linear relationships between viable cell densities and both OUR and CER in exponentially growing cultures were used to predict viable cell density. Respiration measurements were also used to follow the progress of baculoviral infections in Sf-9 cultures. Infection led to increases in volumetric and per-cell respiration rates. The relationships between respiration and several other culture parameters, including viable cell density, cell protein, cell volume, glucose consumption, lactate production, viral titer, and recombinant β-galactosidase accumulation, were examined. The extent of the increase in CER following infection and the time postinfection at which maximum CER was attained were negatively correlated with the multiplicity of infection (MOI) at multiplicities below the level required to infect all the cells in a culture. Delays in the respiration peak related to the MOI employed were correlated with delays in the peak in recombinant protein accumulation. DO levels in the range 5-100% did not exert any major effects on viable cell densities, CER, or product titer in cultures infected with a baculovirus expressing recombinant β-galactosidase. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996) 
    Details
    ISSN: 0006-3592
    Schlagwort(e): Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bit.260500101
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  • 10
    Digitale Medien
    Digitale Medien
    Aggregation of ligand-modified liposomes by specific interactions with proteins. II: Biotinylated liposomes and antibiotin antibody (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996), S. 169-183 
    Details
    ISSN: 0006-3592
    Schlagwort(e): liposomes ; biotin ; aggregation kinetics ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The aggregation of biotinylated phospholipid vesicles (liposomes) cross-linked by antibiotin IgG was studied experimentally and theoretically. The liposomes were either low density liposomes that contained 0.4 mol% biotinylated phospholipid (≈100 exposed biotin molecules per liposome), or high density liposomes that contained 2.7 mol% biotinylated phospholipid (≈1000 exposed biotin molecules per liposome). The solution turbidity and mean particle size measured by quasi-elastic light scattering (QLS) were monitored throughout the aggregation. Three different lots of antibiotin antibodies, each with different association constants and binding heterogeneities, were used. The antibody binding characteristics affected the aggregation rates. The aggregation kinetics were analyzed using a model based on the Smoluchowski theory of aggregation, fractal concepts of aggregate microstructure, and Rayleigh and Mie light scattering theory. The experimental conditions of liposome concentration, protein concentration, and ligand density under which aggregation occurred correlated well with calculated sticking probabilities based on isotherms describing the adsorption of antibiotin antibody to the liposomes. These results are compared with prior observations made when avidin was used as the cross-linking protein. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 11 Ill.
    Materialart: Digitale Medien
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  • 11
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996) 
    Details
    ISSN: 0006-3592
    Schlagwort(e): Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bit.260500201
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  • 12
    Digitale Medien
    Digitale Medien
    Collagenase digestion of bone fragments in microgravity (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996), S. 211-216 
    Details
    ISSN: 0006-3592
    Schlagwort(e): microgravity ; bioprocessing ; sedimentation ; turbulence ; collagenase ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The effect of a quiescent microgravity fluid environment on the activity of collagenase directed at demineralized bone fragments was investigated over a period of 10 days. Enzyme treatment resulted in greater mass loss in microgravity, with nearly three times the loss of mass during Space Shuttle mission STS-62 compared to the stationary ground control. Clinorotation enhanced the loss of mass relative to a stationary control, but this increase was still significantly less than the increase with exposure to microgravity. This suggests the detrimental influence of turbulence on the enzyme function and the benefit of using microgravity to provide both low turbulence and uniformity of unequally dense materials within the reaction chamber. The results are considered for their general applicability to a variety of bioprocessing applications that may be enhanced in microgravity. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bit.260500203
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  • 13
    Digitale Medien
    Digitale Medien
    Mini-review: Mechanical factors affecting cartilage regeneration in vitro (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996), S. 430-437 
    Details
    ISSN: 0006-3592
    Schlagwort(e): cartilage ; tissue regeneration ; chondrocytes ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: In the last 5 to 10 years, tissue engineering has revolutionized the way in which medical researchers and clinicians are thinking of and, in some cases, actually treating diseases involving tissue damage and destruction. One such disease, osteoarthritis, results from progressive degeneration of articular cartilage, which has a limited ability to repair itself. With tissue engineering, scientists are now able to regenerate cartilage in vitro from isolated mature chondrocytes. While the regeneration process is still not fully understood, enough has been learned that physicians are already implanting cultured chondrocytes into humans and other animals in the hopes of effecting joint repair. One aspect which has not been fully explored is the effect of mechanical stress on developing and implanted cartilage, especially over the long term. This article will review in brief what is now known about the mechanical factors affecting cartilage regeneration in vitro and what still remains to be determined for optimum tissue engineering of cartilage constructs. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 14
    Digitale Medien
    Digitale Medien
    Osteoblast migration on poly(α-hydroxy esters) (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996), S. 443-451 
    Details
    ISSN: 0006-3592
    Schlagwort(e): osteoblast ; migration ; poly(αhydroxy esters) ; poly(DL-lactic-co-glycolic acid) ; PLGA ; biodegradable polymers ; tissue engineering ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: We investigated the migration of rat calvaria osteoblast populations on poly(α-hydroxy ester) films for up to 14 days to determine effects of substrate composition and culture conditions on the migratory characteristics of osteoblasts. Initial osteoblast culture conditions included cell colonies formed by seeding a high (84,000 cells/cm2) or low (42,000 cells/cm2) density of isolated osteoblasts on the polymer films, and bone tissue cultures formed by plating bone chips directly on the substrates. High density osteoblast colonies cultured and allowed to migrate and proliferate radially on 85:15 poly(DL-lactic-co-glycolic acid) (PLGA) films, 75:25 PLGA films, and tissue culture polystyrene controls demonstrated that the copolymer ratio in the polymer films did not affect the rate of increase in substrate surface area (or culture area) covered by the growing cell colony. However, the rate of increase in culture area was dependent on the initial osteoblast seeding density. Initial cell colonies formed with a lower osteoblast seeding density on 75:25 PLGA resulted in a lower rate of increase in culture area, specifically 4.9 ± 0.3 mm2/day, versus 14.1 ± 0.7 mm2/day for colonies seeded with a higher density of cells on the same polymer films. The proliferation rate for osteoblasts in the high and low density seeded osteoblast colonies did not differ, whereas the proliferation rate for the osteoblasts arising from the bone chips was lower than either of these isolated cell colonies. Confocal and light microscopy revealed that the osteoblast migration occurred as a monolayer of individual osteoblasts and not a calcified tissue front. These results demonstrated that cell seeding conditions strongly affect the rates of osteoblast migration and proliferation on biodegradable poly(α-hydroxy esters). © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
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  • 15
    Digitale Medien
    Digitale Medien
    Different measures of human hematopoietic cell culture performance are optimized under vastly different conditions (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996), S. 505-513 
    Details
    ISSN: 0006-3592
    Schlagwort(e): bone marrow ; hematopoiesis ; perfusion ; culture optimization ; stroma ; stem cells ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Hematopoiesis, the formation of mature blood cells from stem (LTC-IC) and progenitor (CFU-GM) cells in the bone marrow, is a complex tissue-forming process that leads to many important physiological functionalities. Consequently, a functioning ex vivo hematopoietic system has a variety of basic scientific and clinical uses. The design and operation of such a system presents the tissue engineer with challenges and choices. In this study, three culture variables were used to control ex vivo human hematopoiesis. Systematic variation of inoculum density (ID), medium exchange interval (MEI), and the use of preformed stroma (PFS) showed that (1) all three variables significantly influenced culture performance, (2) the three variables interacted strongly, and (3) the variables could be manipulated to achieve the optimization of different performance criteria. Donor-to-donor variability in culture performance was great at low ID but was minimized at higher ID. PFS had a large positive effect on cell and CFU-GM output at low ID, but had minimal effect at higher ID. In fact, PFS caused a decrease in LTC-IC output at high ID. The effects of PFS indicated that stromal cell elements became more limiting than proliferative cell elements as ID was reduced.In cultures without PFS, maximum cell output was obtained with high ID using a short MEI, whereas the greatest cell expansion ratio was obtained at low ID with an intermediate MEI. Maximum CFU-GM output was obtained from cultures with high ID using a short to intermediate MEI, whereas the greatest CFU-GM expansion ratio was obtained at intermediate ID with an intermediate MEI. The addition of PFS altered the locations of these maxima. In general, PFS moved the maxima to lower ID, and culture output became more sensitive to MEI. Therefore, the optimization of one performance criterion always resulted in a decline of the others. This study demonstrates that ex vivo tissue function is sensitive to many culture variables in an interactive fashion and that systematic multivariable studies are required to characterize tissue function. Once the effects of individual variables and their interactions are known, this knowledge can be used to optimize tissue performance with respect to desired criteria. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 16
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 50 (1996) 
    Details
    ISSN: 0006-3592
    Schlagwort(e): Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/bit.260500501
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  • 17
    Digitale Medien
    Digitale Medien
    Thermal stability studies of a globular protein in aqueous poly(ethylene glycol) by 1H NMR (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 410-421 
    Details
    ISSN: 0006-3592
    Schlagwort(e): lysozyme ; thermal stability ; 1H NMR ; conformational flexibility ; melting temperature ; PEG ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The reversible folding destabilization of hen lysozyme has been confirmed by a melting temperature (Tm) decrease in aqueous poly(ethylene glycol) (PEG). The percent denatured, extracted from the histidine 15 C2H (H15 C2H) native and denatured peak areas from 500-MHz one-dimensional proton nuclear magnetic resonance (1D 1H NMR) spectra in D2O, was analyzed through denaturation temperatures at 0% and 20% (w/w) PEG 1000. The lysozyme (3.5 mM) Tm decreased by 4.2°C and 7.1°C in 20% (w/w) PEG 1000 at pH 3.8 and 3.0, respectively. The Tm decreased with increasing lysozyme concentration. Additionally, the temperature-induced resonance migrations of 17 protons from 8 residues indicate that the native lysozyme structure undergoes temperature-induced conformational changes. The changes were essentially identical in both 0% and 20% (w/w) PEG 1000 at both pH 3.0 and 3.8. This small, local restructuring of the hydrophobic box region may be a manifestation of temperature-dependent solution hydrophobicity, whereas active-site cleft fluctuations may be due to the inherent active-site flexibility. The lysozyme structure in PEG at 35°C was determined to be essentially native from the 1H nuclear Overhauser effect spectroscopy (NOESY) fingerprint regions. Additionally, lysozyme chemical shifts, from 1D spectra, in PEG 200, 300, and 1000 at 35°C and various concentrations were essentially identical, further confirming that the conformation remains native in various PEG solutions. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
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  • 18
    Digitale Medien
    Digitale Medien
    Factors affecting cellulose hydrolysis and the potential of enzyme recycle to enhance the efficiency of an integrated wood to ethanol process (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 375-383 
    Details
    ISSN: 0006-3592
    Schlagwort(e): cellulase ; enzyme recycling ; enzyme adsorption ; lignocellulosic hydrolysis ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Past technoeconomic modeling work has identified the relatively large contribution that enzymatic hydrolysis adds to the total cost of producing ethanol from lignocellulosic substrates. This cost was primarily due to the high concentration of enzyme and long incubation time that was required to obtain complete hydrolysis. Although enzyme and substrate concentration and end-product inhibition influenced the rate of hydrolysis, the effect was less pronounced during the initial stages of hydrolysis. During this time most of the cellulases were adsorbed onto the unhydrolyzed residue. By recycling the cellulases adsorbed to the residual substrate remaining after an initial 24 h, a high rate of hydrolysis, with low overall residence time and minimal cellulase input, could be achieved for several rounds of enzyme recycle. A comparison of the front end (pretreatment, fractionation, and hydrolysis) of a softwood/hardwood to ethanol process indicated that the lignin associated with the softwood-derived cellulose stream limited the number of times the cellulose containing residue could be recycled. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 19
    Digitale Medien
    Digitale Medien
    Sparging and agitation-induced injury of cultured animals cells: Do cell-to-bubble interactions in the bulk liquid injure cells? (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 399-409 
    Details
    ISSN: 0006-3592
    Schlagwort(e): cell damage ; cell culture ; bubble aeration ; agitation ; bubble coalescence and breakup ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: It has been established that the forces resulting from bubbles rupturing at the free air (gas)/liquid surface injure animal cells in agitated and/or sparged bioreactors. Although it has been suggested that bubble coalescence and breakup within agitated and sparged bioreactors (i.e., away from the free liquid surface) can be a source of cell injury as well, the evidence has been indirect. We have carried out experiments to examine this issue. The free air/liquid surface in a sparged and agitated bioractor was eliminated by completely filling the 2-L reactor and allowing sparged bubbles to escape through an outlet tube. Two identical bioreactors were run in parallel to make comparisons between cultures that were oxygenated via direct air sparging and the control culture in which silicone tubing was used for bubble-free oxygenation. Thus, cell damage from cell-to-bubble interactions due to processes (bubble coalescence and breakup) occurring in the bulk liquid could be isolated by eliminating damage due to bubbles rupturing at the free air/liquid surface of the bioreactor. We found that Chinese hamster ovary (CHO) cells grown in medium that does not contain shear-protecting additives can be agitated at rates up to 600 rpm without being damaged extensively by cell-to bubble interactions in the bulk of the bioreactor. We verified this using both batch and high-density perfusion cultures. We tested two impeller designs (pitched blade and Rushton) and found them not to affect cell damage under similar operational conditions. Sparger location (above vs. below the impeller) had no effect on cell damage at higher agitation rates but may affect the injury process at lower agitation intensities (here, below 250 rpm). In the absence of a headspace, we found less cell damage at higher agitation intensities (400 and 600 rpm), and we suggest that this nonintuitive finding derives from the important effect of bubble size and foam stability on the cell damage process. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 20
    Digitale Medien
    Digitale Medien
    Modulation of the phosphate-starvation response in Escherichia coli by genetic manipulation of the polyphosphate pathways (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 434-438 
    Details
    ISSN: 0006-3592
    Schlagwort(e): polyphosphate ; Escherichia coli ; phosphate starvation ; gene expression ; heterologous ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The effect of intracellular polyphosphate on the phosphate-starvation response in Escherichia coli was studied by genetically manipulating the intracellular polyphosphate levels and by performing phosphate shifts on the genetically engineered strains. Strains that produced large quantities of polyphosphate and were able to degrade it induced the phosphate-starvation response to a lesser extent than wild-type strains, whereas strains that were unable to degrade a large intracellular polyphosphate pool induced the phosphate-starvation response to a greater extent than wild-type strains. These results have important implications for expression of heterologous genes under control of the phoA promoter. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 21
    Digitale Medien
    Digitale Medien
    Effect of high shear on proteins (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 458-465 
    Details
    ISSN: 0006-3592
    Schlagwort(e): concentric-cylinder shear device ; rotor/stator homogenization ; shear ; shear rate ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Shear is present in almost all bioprocesses and high shear is associated with processes involving agitation and emulsification. The purpose of this study is to investigate the effect of high shear and high shear rate on proteins. Two concentric cylinder-based shear systems were used. One was a closed concentric-cylinder shear device (CCSD) and the other was a homogenizer with a rotor/stator assembly. Mathematical modeling of these systems allowed calculation of the shear rate and shear. The CCSD generated low shear rates (a few hundred s-1), whereas the homogenizer could generate very high shear rates (〉 105 s-1). High shear could be achieved in both systems by increasing the processing time. Recombinant human growth hormone (rhGH) and recombinant human deoxyribonuclease (rhDNase) were used as the model proteins in this study. It was found that neither high shear nor high shear rate had a significant effect on protein aggregation. However, a lower melting temperature and enthalpy were detected for highly sheared rhGH by using scanning microcalorimetry, presumably due to some changes in protein's conformation. Also, SDS-PAGE indicated the presence of low molecular-weight fragments, suggesting that peptide bond breakage occurred due to high shear. rhDNase was relatively more stable than rhGH under high shear. No conformational changes and protein fragments were observed. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 9 Ill.
    Materialart: Digitale Medien
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  • 22
    Digitale Medien
    Digitale Medien
    Properties of two insect cell lines useful for the baculovirus expression system in serum-free culture (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 494-499 
    Details
    ISSN: 0006-3592
    Schlagwort(e): cell metabolism ; baculovirus ; insect cells ; recombinant protein OSF-2 ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: The properties of Sf9 and Tn5 insect cells were analyzed comparatively under serum-free culture conditions. Sf9 cells in SF900II medium apparently utilized sucrose as a primary nutrient both before and after virus infection, yielding small amounts of lactate and ammonia. Tn5 cells in Excell 401 medium consumed all the nutrients examined, including sucrose. The productivity of a recombinant glycoprotein, OSF-2, by Tn5 cells, was moderate in both monolayer and spinner cultures, but the ability to secrete it was compromised in the former case. Relative to the Tn5 cultures, Sf9 produced 30-fold more OSF-2 in either culture mode. © 1996 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 23
    Digitale Medien
    Digitale Medien
    NMR imaging of heavy metal absorption in alginate, immobilized cells, and kombu algal biosorbents (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 538-543 
    Details
    ISSN: 0006-3592
    Schlagwort(e): NMR imaging ; biosorption ; alginate ; shrinking core model ; Laminaria ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: In this contribution, an NMR imaging study of heavy metal absorption in alginate, immobilized-cell biosorbents, and kombu (Laminaria japonica) algal biomass is presented. This method provides the good possibility of directly monitoring the time evolution of the spatial distribution of the ions in the materials. From these results, we demonstrate that rare earth ions are absorbed with a steep reaction front that can be described very well with a modified shrinking core model, while copper ions are absorbed with a more diffuse front.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 24
    Digitale Medien
    Digitale Medien
    Enzymatic resolution of 1-phenyl-1,2-ethanediol by enantioselective oxidation: Overcoming product inhibition by continuous extraction (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 544-550 
    Details
    ISSN: 0006-3592
    Schlagwort(e): oxidoreductase ; chiral alcohol ; racemic resolution ; membrane reactor ; continuous extraction ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Oxidations of alcohols by alcohol dehydrogenases often suffer from low conversions and slow reaction rates due to severe product inhibition. This can be overcome by continuous product extraction, because only the concentrations, but not the kinetic parameters, can be changed. As a consequence, it is favorable to apply a differential circulation reactor with continuous product extraction, where only a small amount of product is formed per cycle. The product is then directly extracted using a microporous hydrophobic hollow fiber membrane. This results in an increase of the relative activity of the dehydrogenase at a given conversion. The reaction investigated is the kinetic resolution of racemic 1-phenyl-1,2-ethanediol by glycerol dehydrogenase (GDH). The resulting oxidation product, 2-hydroxyacetophenone, causes a strong product inhibition. Additionally, it reacts in a chemical reaction with the cofactor lowering its active concentration. Because the GDH needs β-nicotinamide adenine dinucleotide (NAD+) as a cofactor, lactate dehydrogenase is used to regenerate NAD+ from NADH by reducing pyruvate to (L)-lactate. A conversion of 50% with respect to the racemate and an enantiomeric excess 〉99% of the (S)-enantiomer was reached.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 25
    Digitale Medien
    Digitale Medien
    Immobilization of DNA onto a polymer support and its potentiality as immunoadsorbent (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 51 (1996), S. 581-590 
    Details
    ISSN: 0006-3592
    Schlagwort(e): microfiber ; graft polymerization ; DNA immobilization ; immunoadsorbent ; DNA ; anti-DNA antibody ; systemic lupus erythematosus ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Biologie , Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
    Notizen: Immobilization of DNA to the surface of poly(ethylene terephthalate) (PET) microfibers with a high specific surface area of 0.83 m2/g was carried out to give the fiber surface an affinity for anti-DNA antibody. Following ozone oxidation, the microfibers were subjected to graft polymerization of monomers including acrylic acid, methacryloyloxyethyl phosphate, N,N-dimethylaminoethyl methacrylate, N-vinylformamide, and glycidyl methacrylate. Calf thymus DNA was immobilized to the grafted fiber surface through either covalent binding or polyion complexation with the grafted polymer chains. The highest surface density of DNA immobilized (0.6 μg/cm2) was obtained when DNA was immobilized through formation of phosphodiester linkage between the hydroxyl group of DNA and the phosphate group in grafted poly(methacryloyloxyethyl phosphate) using 1,1-carbonyldiimidazole, or through polyion complexation between the anionic DNA and the cationic grafted poly(N,N-dimethylaminoethyl methacrylate) chains. Batch adsorption of anti-DNA antibody to the grafted PET fibers with and without DNA immobilized on their surface was conducted with serum obtained from systemic lupus erythematosus model mice. The DNA-immobilized PET fibers exhibited a higher adsorption capacity and specificity than the others. In addition, the DNA-immobilized fibers effectively adsorbed human anti-DNA antibody.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
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  • 26
    Digitale Medien
    Digitale Medien
    Alkenyl Fischer Carbene Complexes and α,β-Unsaturated Imine Derivatives: Synthesis of Azepines and Mechanistic NMR StudiesDedicated to Professor Heinz Hoberg on the occasion of his 70th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 88-97 
    Details
    ISSN: 0947-6539
    Schlagwort(e): azadienes ; azepines ; cycloadditions ; Fischer carbenes complexes ; reaction mechanisms ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: 4-Amino-1-azadienes 1 react with α,β-unsaturated Fischer carbene complexes at -40°C to give stereoselectively a variety of substituted 3H-4,5-dihydroazepines 3; similarly, 1-hydroxy-1-azadienes (α,β-unsaturated oximes) 6 afforded the corresponding azepine derivatives 7. Chiral, nonracemic carbene complexes 11 gave azepines 12-13 (d.e. = 40-44%) upon reaction with oxime 6a; the major isomers were obtained in a diastereomerically and enantiomerically pure form (45-50% overall yield) after crystallization. An X-ray structure of 12a allowed assignment of the absolute stereochemistry. The acid hydrolysis of azepines synthesized provided racemic and enantiomerically pure 1,6-dicarbonyl compounds (±)-5, (±)-9, and (-)-14, as well as diol (-)-15. The mechanism of the reaction of 1 and 2 was investigated by multinuclear (1H, 13C, 15N, and 183W) NMR characterization of four intermediates (A, B, C, and D) at low temperature. The experimental sequence of events involves: i) 1,2-nucleophilic addition of the unsubstituted imine nitrogen of 1 to the metal carbene function (zwitterion A, -60°C), ii) cyclization to the seven-membered ring with 1,2-migration of the pentacarbonyl metal (zwitterion B, -40°C), iii) reductive elimination and coordination of the metal to the amine nitrogen (intermediate C, -40°C), and iv) thermal decomplexation and tautomerization (intermediate D and compound 3, above -20°C).
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020116
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  • 27
    Digitale Medien
    Digitale Medien
    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020201
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  • 28
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020202
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  • 29
    Digitale Medien
    Digitale Medien
    Mechanism of an Alkyl-Dependent Photochemical Homolysis of the Re-Alkyl Bond in [Re(R)(CO)3(α-diimine)] Complexes via a Reactive σπ* Excited State (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 228-237 
    Details
    ISSN: 0947-6539
    Schlagwort(e): homolytic cleavage ; organometallic compounds ; photochemistry ; rhenium complexes ; time-resolved spectroscopy ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: MLCT excitation of the complexes [Re(R)(CO)3(α-diimine)] (R = Me, Et, benzyl (Bz); α-diimine = iPr-PyCa, R′-DAB) results in the homolysis of the Re-R bond leading to the formation of radicals R. and [Re(CO)3(α-diimine)]. as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low (10-2) and depends on the temperature and excitation wave-length, whereas for R = Et and Bz the quantum yield is near unity and independent of T and λexc. The reaction is shown to proceed via a σ(Re-R)π* excited state that is rapidly (〈 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time-resolved IR and UV/Vis absorption spectra studied in the ns-μs and ps-μs time domains, respectively, show that the σπ* excited state is rather long-lived (τ ≈ 250 ns) in noncoordinating solvents; the dissociation of the Re-R bond from this state is strongly accelerated by polar or coordinating solvents (τσπ*〈 20 ps). The σπ* excited state is spectroscopically characterized by a (presumably σπ* → MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground-state values. The relative energies of the MLCT and reactive σπ* states, controlled by the nature of the alkyl lig-and, determine the photoreactivity of the complexes.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020216
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  • 30
    Digitale Medien
    Digitale Medien
    Design and Synthesis of a “Starburst”-Type Nonadiazo Compound and Magnetic Characterization of Its Photoproduct (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 259-264 
    Details
    ISSN: 0947-6539
    Schlagwort(e): dendrimers ; high-spin molecules ; photochemistry ; polycarbenes ; solid solutions ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A “starburst”-type nonadiazo compound was designed and synthesized by using the cyclotrimerization of 3,5-dibenzylphenyl ethynyl ketone as a key step. The diazo compound was photolyzed in methyltetrahydrofuran solid solution at cryogenic temperatures and analyzed by means of Faraday magnetometry and EPR spectroscopy. While the m-phenylene-connected nonacarbene was predicted to have a nonadecet (S = 9) ground state, the magnetic data of the photoproduct was more consistent with a pentadecet (S = 7) species. The result was interpreted in terms of the intramolecular cross-linking between the carbene centers by the determination of both the amount and the multiplicity of the spin. In the “starburst”-type polycarbenes with extended branching, the reactive carbene centers are able to approach one another and thus appear to readily recombine. This study highlights one of the limitations of the extension of the carbene network by way of flexible “starburst”-type structures.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020305
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  • 31
    Digitale Medien
    Digitale Medien
    Synthesis, Structure, and Stereoselective Reaction of a Chiral Hydroxy-Stabilized Metal-Free Enolate (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 382-384 
    Details
    ISSN: 0947-6539
    Schlagwort(e): asymmetric synthesis ; chirality ; enolates ; hydrogen bonds ; structure elucidation ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The reaction of acetophenone with tetrabutylammonium hydroxide affords the tetrabutylammonium enolate of phenyl (2-hydroxy-2-phenyl)propyl ketone. The crystal structure of this chiral enolate shows intramolecular hydrogen bonding between the hydroxyl group and the enolate oxygen atom. Furthermore, the α-methylene units of the ammonium counterion form hydrogen bonds to the basic enolate C and O atoms and to the O atom of the hydroxy group. This three-point bonding occurs selectively on the Re,Re side, a phenomenon which may be responsible for the direction of diastereo-selectivity in the epoxide-forming reaction of the enolate with N-bromosuccinimide.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020405
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  • 32
    Digitale Medien
    Digitale Medien
    A 2,3-Connected Tellurium Net and the Cs3 Te22 Phase (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 390-397 
    Details
    ISSN: 0947-6539
    Schlagwort(e): band structures ; hypervalent bonding ; semiempirical calculations ; tellurium compounds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The bonding in the recently reported Cs3 Te22 phase, which contains both Te8 rings and remarkable Te6 sheets, is studied by approximate molecular orbital theory. Our focus is on the geometric and electronic features of the unique 2,3-connected Te net found as a substructure in this phase. The calculations show that both the linear and T-shaped Te geometries in the 2,3-connected Te net of Cs3 Te22 are determined by their particular electron count. Both types of tellurium atoms are hypervalent; we make connections to other well known hypervalent molecules, such as XeF2, I3-, and BrF3. Several possible variations and distortions of this net are discussed, all of which are found to be less stable. The discrete crown-shaped Te8 units that appear in the phase show normal covalent bonding and should occur in smaller molecular entities, too. According to our computations, Cs3 Te22 should be metallic. Two structurally related phases, CsTe7 and Cs2 Te15, are suggested.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020407
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  • 33
    Digitale Medien
    Digitale Medien
    Zirconium(II)- and Hafnium(II)-Assisted Reductive Coupling of Coordinated Carbonyl Groups Leading to Ketenylidene Complexes of Zirconium(IV) and Hafnium(IV) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 412-419 
    Details
    ISSN: 0947-6539
    Schlagwort(e): carbon-carbon coupling ; carbonyl complexes ; hafnium complexes ; ketenylidene complexes ; zirconium complexes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The biscyclopentadienyldicarbonyl derivatives of zirconium(II) and hafnium(II) ([MCp2(CO)2]; M=Zr, Hf) promote the reductive coupling of coordinated carbon monoxide to give, in the presence of N,N-dialkylcarbamtes of the tetravalent metals [M(O2CNR2)4], the ketenylidene complexes [M3Cp2(μ2-CCO)-(μ3-O)(O2CNR2)6] (1 a: M=Zr, R=Et; 1 b: M=Zr, R=iPr; 2: M=Hf, R=iPr). The yields of the isolated zirconium complexes are as high as 60%, while that of the hafnium derivative is 40%. The X-ray crystal structure analysis of 1 b shows that it consists of trinuclear molecules, with the three zirconium atoms held together by the bidentate C2O ligand, by the tridentate bridging oxide and by the bidentate carbamato groups. Labelling experiments with [ZrCp2(13CO)2] show that the ketenylidene ligand originates from the coordinated CO groups. These ketenylidene complexes, which are rare examples of compounds containing a CCO but no CO ligand, are characterized by an intense IR band at about 2015 cm-1, associated with the bridging C2O ligand. The ketenylidene group of 1 b was readily removed and replaced by a bidentate μ-oxo ligand of the same hapticity by reaction with carbon dioxide or acetone or by thermal decomposition. The resulting product [Zr3Cp2(μ-O)(μ3-O)-(O2CNiPr2)6] (3) crystallizes in the same space group as 1 b and with similar cell constants and bond parameters.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020410
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  • 34
    Digitale Medien
    Digitale Medien
    9-Borabarbaralanes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 458-461 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; barbaralanes ; borabarbaralanes ; Cope rearrangement ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The reaction of MgCOT(thf)x with tBuBF2 or PhBCl2 affords the first 9-borabarbaralanes 2 (C8H8BR, a: R = tBu; b: R=Ph). With the aminoboron dihalides BCl2NiPr2 and BCl2N(SiMe3)tBu 9-borabicyclo[4.2.1]-nona-2,4,7-trienes 3 (a: R=NiPr2, b: R=N(SiMe3)tBu) and the trans-9-borabicyclo[4.3.0]nona-2,4,7-triene 4 are obtained. The bicyclic compounds 3a and 3 b are converted into 9-borabarbaralanes 2c and 2 d, respectively, by irradiation in solution as well as by heating. All 9-borabarbaralanes 2 are fluxional in solution. In the crystalline state, the B-phenyl derivative 2b displays a well-ordered van der Waals crystal structure. The theoretical prediction that the degenerate Cope rearrangement in barbaralanes will be retarded by π-acceptor groups in the 9 position has been verified. Quantum chemical calculations employing density functional theory support and help interpret the experimental findings. The isoelectronic 9-barbaralyl cations, in contrast, have such high Cope barriers that other rearrangement pathways are followed instead.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020416
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  • 35
    Digitale Medien
    Digitale Medien
    Solvent Effect on the NMR Chemical Shieldings in Water Calculated by a Combination of Molecular Dynamics and Density Functional Theory (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 452-457 
    Details
    ISSN: 0947-6539
    Schlagwort(e): density-functional theory ; liquid water ; molecular dynamics simulations ; NMR chemical shifts ; solvent effects ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The solvent effect on the NMR chemical shielding in liquid water is calculated from a combination of molecular dynamics simulations and quantum chemical calculations for protons and 17O. The simulations are performed with three different potentials, ab initio as well as empirical ones, to study the influence of the force field. From the liquid configurations obtained in these simulations, molecules are randomly chosen together with neighbouring molecules to give clusters of water typical for the liquid at the selected temperature and density. Different cluster sizes are studied. The clusters are treated as supermolecules in quantum chemical calculations of chemical shifts by sum-over-states density functional perturbation theory with individual gauge for localised orbitals. The influence of the quantum chemical method is studied with an ab initio coupled Hartree-Fock gauge including atomic orbitals calculations with different basis sets for a selected cluster. An average over clusters yields the chemical shielding in the liquid at the selected temperature and density. The calculated values for the gas-liquid shift, which are in best agreement with experiment, are -3.2 ppm (exp. -4.26 ppm) for the proton and -37.6 ppm (exp. -36.1 ppm) for 17O, but the results depend strongly on the chosen interatomic potential.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020415
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  • 36
    Digitale Medien
    Digitale Medien
    Anion-cation redox competition and the formtion of new compounds in highly covalent systems (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1053-1059 
    Details
    ISSN: 0947-6539
    Schlagwort(e): chalcogenides ; periodic trends ; redox reactions ; soft chemistry ; transition metals ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The increase in energy of the sp anionic band on going from oxides to the less electronegative sulfides, selenides, or tellurides (with a similar trend in neighboring columns) facilitates anion-cation redox interactions involving the d levels of transition metal cations and the sp levels of anionic species. The interactions can induce phase transitions or gradual change within a given structural model. When a cation is reduced by electron transfer to its d levels from the sp band, holes appear at the top of the latter. Interesting soft chemistry can be carried out based on redox processes that neutralize the holes with electrons. This approach also allows particular structural types to be stabilized. Three structural domains can be recognized amongst the transition elements: 1) on the left-hand side of the periodic table layered structures are observed involving M4+ and (chalcogen)2- ions; 2) formation of sets of metal-metal bonds is then observed, the geometry of which depends on both the initial electron population and the electron transfer to the metal; 3) on the right-hand side the metals in their highest oxidation state are no longer active, and the holes at the top of the sp band are taken up by a catenation of the anions, which can lead to full polymerization of the anionic sublattice.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020902
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  • 37
    Digitale Medien
    Digitale Medien
    Dendritic models of redox proteins: X-ray photoelectron spectroscopy and cyclic voltammetry studies of dendritic bis(terpyridine) iron(II) complexes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1085-1091 
    Details
    ISSN: 0947-6539
    Schlagwort(e): cyclic voltammetry ; dendrimers ; metallodendrimers ; terpyridine ; X-ray photoelectron spectroscopy ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A series of iron-containing metallodendrimers from the zeroth to the third generation was synthesized and characterized. The iron(II) ion of these metal complexes is encapsulated within a hydrophobic polyether dendritic envelope. X-ray photoelectron spectroscopy analysis revealed that the bonding environment of the iron atom was essentially ronment of the iron atom was essentially the same in dendrimers of different generations. However, cyclic voltammetry studies indicated that the reversibility of the metal redox process decreased in the higher-generation dendrimers. These observations were similar to those of a related electrochemical study conducted on the redox protein cytochrome C, in which the decrease in electron transfer reversibility was explained as a result of the increasing remoteness of the metal ion of the higher-generation dendrimers from the electrode surface.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020906
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  • 38
    Digitale Medien
    Digitale Medien
    An unexpected structural principle for crystalline MgBr2·OEt2: Chains of trigonal bipyramids (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1112-1114 
    Details
    ISSN: 0947-6539
    Schlagwort(e): coordination ; crystal structure ; halogen compounds ; magnesium compounds ; structure elucidation ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: MgBr2·OEt2 (1), which is commercially available and has been examined by different methods in the past, was obtained in crystalline form as a by-product of the reaction of amine-stabilised A1C1 solution with mesitylene magnesium bromide. The following unexpected structure was obtained by X-ray techniques: five-coordinate magnesium centres form trigonal bipyramids, which are connected by two common bromine atoms into chains. Structural details and especially the relationship with solid MgBr2, MgBr2·(OEt2)2 and MgI2·(OEt2)2 (2) are discussed in order to illuminate the unusual bonding in 1.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020910
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  • 39
    Digitale Medien
    Digitale Medien
    High-spin → low-spin relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and thermal spin-state trapping - dynamics of spin transition versus dynamics of phase transitionDedicated to Professor Gerhard Herberich on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1134-1138 
    Details
    ISSN: 0947-6539
    Schlagwort(e): iron complexes ; magnetic susceptibility ; Mössbauer spectroscopy ; phasetransitions ; spin crossover ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≍285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mössbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal spin-state trapping was monitored at temperatures between 104 and 118 K by magnetic susceptibility measurements. The relaxation mechanisms of the HS states generated by LIESST and thermal spinstate trapping are completely different. We suggest that the HS → LS relaxation after thermal spin-state trapping is triggered by an additional structural phase transition of the system.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020913
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  • 40
    Digitale Medien
    Digitale Medien
    Synthesis of enantiopure homoallylic alcohols and ethers by diastereoselective allylation of aldehydes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1164-1172 
    Details
    ISSN: 0947-6539
    Schlagwort(e): allylations ; allylsilanes ; double stereodifferentiation ; ephedrine ; homoallylic alcohols ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Enantiopure homoallylic alcohols 5, which are important building blocks in organic synthesis, are obtained with an ee of greater than 99% and a yield of 75-95% by cleavage of the secondary homoallylic ethers 4 using sodium in liquid ammonia. The ethers 4 are formed with excellent diastereoselectivity and in 52-89% yield by treatment of the aldehydes 1 with the trimethylsilyl ether of N-trifluoroacetylnorpseudoephedrine (2) in the presence of a catalytic amount of TMS triflate or TMS borontriflate, followed by addition of allylsilane 3. Nearly all achiral aliphatic aldehydes employed gave a diastereoselectivity of over 99:1. With the chiral aldehydes 24, the difference between matched and mismatched pairs was low; this reveals that there is strong reagent control.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020918
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  • 41
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021002
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  • 42
    Digitale Medien
    Digitale Medien
    Hydrogen, the First Alkali Metal (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1201-1203 
    Details
    ISSN: 0947-6539
    Schlagwort(e): alkali metals ; hydrogen ; metallic hydrogen ; metallization densities ; periodicity ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We examine the density-dependent metal-nonmetal transition (MNMT) for the Group 1 elements hydrogen, rubidium and caesium. Remarkably, the experimentally observed metallization densities agree closely with those predicted from the 1927 classical theory of K. F. Herzfeld, which outlines the critical conditions necessary for the metallization of any element of the periodic system. The metallization densities of the alkali metal elements are also consistent with the quantum mechanical theory of the MNMT, as first set out by Sir Nevill Mott.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021005
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  • 43
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020102
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  • 44
    Digitale Medien
    Digitale Medien
    Thermochemistry of Metal Nitrides in the Ca/Zn/N System (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1514-1517 
    Details
    ISSN: 0947-6539
    Schlagwort(e): calcium compounds ; calorimetry ; nitrides ; ternary metal nitrides ; zinc compounds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The energetics of ternary oxide formation is reasonably well understood, but little is known of the energetics of ternary nitride formation. We have developed new procedures for high-temperature oxide melt solution calorimetry on metal nitrides and employed them to study compounds in the Ca/Zn/N system. The calorimetry utilized the easy reduction of MoO3 in the solvent to supply a pathway for rapid oxidation of “N3-” to N2 gas. These experiments represent the first high-temperature solution calorimetry of ternary metal nitrides. The results indicate that Ca3N2 acts as a basic nitride analogous to CaO in oxide systems, while Zn3N2 acts as an acidic nitride (“N3-” acceptor) in this ternary nitride system.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021207
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  • 45
    Digitale Medien
    Digitale Medien
    Oxygen and Nitrogen in Competitive Situations: Which is the Hydrogen-Bond Acceptor? (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1509-1513 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; binding studies ; crystal packing ; drug design ; hydrogen bonds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: In the design of novel protein ligands one of the major challenges is the replacement of functional groups to modify and improve the binding characteristics. Often nitrogen- and oxygen-containing groups are exchanged, or both atoms occur in a competitive situation. We have investigated the hydrogen-bonding abilities of oxygen atoms covalently bound to two non-hydrogen atoms of which at least one is formally assigned to an sp2-type hybridization. In particular, examples in which such oxygen atoms compete with nitrogen atoms in the same molecular segment have been studied. Based on interaction energies obtained from ab initio calculations for complexes of these molecules with water, the oxygen atoms can be classified as rather weak hydrogen-bond acceptors; nitrogen atoms present in the same fragment exhibit much stronger interaction energies. The ab initio results are confirmed by the relative frequencies with which oxygen and nitrogen atoms are found to be involved in hydrogen bonding in the crystal structures of organic molecules containing the fragments of interest.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021206
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  • 46
    Digitale Medien
    Digitale Medien
    Ruthenium(II)-Catalyzed Oppenauer-Type Oxidation of Secondary Alcohols (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1533-1536 
    Details
    ISSN: 0947-6539
    Schlagwort(e): catalysis ; dehydrogenations ; oxidations ; ruthenium complexes ; secondary alcohols ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Highly efficient ruthenium-catalyzed Oppenauer-type oxidations of secondary alcohols to ketones have been developed. The catalytic system consists of [(PPh3)3RuCl2] (1) and K2CO3 or [(C4Ph4COHOCC4Ph4)(μ-H)(CO)4Ru2] (2) in refluxing acetone. The catalytic reaction is of high efficiency and permits a catalyst: substrate ratio of 1:1000 at 56 °C. In some cases the initial turnover rate exceeds 1500 h-1. The reaction was found to be general and compatible with double bonds and oxidation-sensitive aromatics. With some allylic alcohols, isomerization to saturated ketones took place. The reaction proceeds by ruthenium-catalyzed dehydrogenation of the alcohol and subsequent hydrogen transfer to acetone. No primary kinetic isotope effect was observed for the catalytic reaction when α-deuterated 1-phenylethanol was employed as substrate (kH/kD=1.1); this shows that β-hydride elimination from a ruthenium alkoxide intermediate is not the rate-limiting step.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021210
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  • 47
    Digitale Medien
    Digitale Medien
    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 4-6 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020103
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  • 48
    Digitale Medien
    Digitale Medien
    Notice to Authors 1996 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 7-8 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020105
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  • 49
    Digitale Medien
    Digitale Medien
    Synthesis and Molecular Structure of the First Neutral Transition-Metal Complex Containing a Linear M=C=C=C=C=CR2 chainDedicated to Professor Max Schmidt on the occasion of his 70th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 19-23 
    Details
    ISSN: 0947-6539
    Schlagwort(e): carbon ligands ; diynes ; iridium compounds ; metallacumulenes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The reaction of [IrH2Cl(PiPr3)2] (2) with the pentadiyne derivative HC≡C—C≡C—CPh2OH (1) yields the five-coordinate compound [IrH(C≡C—C≡CPh2OH)Cl(PiPr3)2] (3), which photochemically rearranges to give the vinylidene-substituted isomer trans-[IrCl(=C=CH-C≡CPh2OH)(PiPr3)2] (4). On treatment of 3 with pyridine, the octahedral complex [IrH (C≡C—C≡CPh2OH)Cl(py)(PiPr3)2] (5) is formed. While attempts to eliminate water from 4 (or 5) failed, the reaction of 3 with one equivalent of trifluorosulfonic acid anhydride and two equivalents of triethylamine led to the formation of trans-[IrCl(=C=C=C=C=CPh2)(PiPr3)2] (6), which is the first neutral transition-metal complex containing a C5R2 ligand. The X-ray structural analysis of 6 reveals a perfect square-planar coordination geometry around the metal centre and an almost linear IrC5 chain with Ir-C—C and C—C—C bond angles of 174-179°.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020107
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  • 50
    Digitale Medien
    Digitale Medien
    Isolation of Selected Exchange Processes in Nuclear Magnetic Resonance (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 45-49 
    Details
    ISSN: 0947-6539
    Schlagwort(e): exchange processes ; kinetics ; NMR spectroscopy ; zirconium complexes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We have shown that it is possible to inhibit the transfer of magnetization in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis- and trans-ZrCl4L2 with excess free ligand L (L = (CH3O)3PO). The forward and backward reaction rates involving two selected sites can be studied while the effect of competing exchange processes is “quenched”. This can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval, or alternatively by inversion of the magnetization of all other sites in the exchange network. The rate of exchange from the free to the cis site was determined to be kcis←free = 0.018 s-1. In the usual methods, this process would tend to be overshadowed by the almost two hundred times faster competing exchange process from the cis to the trans site (Ktrans←cis = 3.32 s-1).
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020110
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  • 51
    Digitale Medien
    Digitale Medien
    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020101
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  • 52
    Digitale Medien
    Digitale Medien
    What Do Ab Initio Calculations Predict for the Structure of Lithiated Azaenolates of Peptides? (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 9-18 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; azaenolates ; NMR chemical shifts ; peptides ; Ramachandran maps ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Structures and conformations of the azaenolate lithium salts of amides (formamide, acetamide, and N-methylacetamide) and of the dipeptide model N-formylalaninamide were investigated by means of ab initio MO theory. Four possible structures of the lithiated C-enolates of acetamide were also included in the study. All structures were calculated at the HF/6-31+G(d) and MP2(fc)/6-31 + G(d)/HF/6-31 + G(d) levels; the lithiated azaenolates of formamide were also investigated at higher theoretical levels (up to MP4(fc)/6-311 + G(d,p)/MP2(fc)/6-311 + G(d,p)). For the lithiated azaenolates of all amides investigated, the most stable structure contains a four-membered ring in which the lithium ion is complexed by the oxygen and nitrogen atoms; the substituents attached to the carbon and nitrogen atoms of the azaenolate are in a cis arrangement. The lithiated azaenolates of acetamide are predicted to be more stable than the corresponding C-enolates. To simulate solvation, calculations on complexes of the lithiated azaenolates of formamide with up to three molecules dimethyl ether were also performed, and all azaenolates of amides were also reoptimized by ab initio reaction-field calculations. Both solvation models reduce the preference for lithium-chelated cis structures. The Ramachandran maps of the dilithiated bis(azaenolate) of N-formylalaninamide (having cis or trans arrangements of the azaenolate substituents) were scanned by MNDO calculations for conformational accessible regions. Thirteen stable structures were subsequently optimized at the HF/6-31 + G(d) ab initio level. The global minimum resembles a peptide in C7 conformation, but other conformations, not known for peptides, are close in energy. The structures of dimers of the lithiated azaenolates of N-methylacetamide and of glycinaldehyde were also calculated. The NMR chemical shielding of carbon, nitrogen, and oxygen atoms in all structures were predicted ab initio by using the gauge-including atomic orbital (GIAO) method.
    Zusätzliches Material: 11 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020106
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  • 53
    Digitale Medien
    Digitale Medien
    Ruthenium-Catalyzed cis-Dihydroxylation of Alkenes: Scope and Limitations (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 50-57 
    Details
    ISSN: 0947-6539
    Schlagwort(e): alkenes ; catalysis ; dihydroxylations ; electrophilicity ; ruthenium compounds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Oxidative ruthenium catalysis (0.07 molequiv RuCl3·(H2o)3, 1.5 molequiv NaIO4, EtOAc/CH3CH3CN/H2O 3:3:1), beyond the usual C—C bond cleavage to give dicarbonyls, has been shown to syn-dihydroxylate a wide range of alkenes (except for strained bicyclic alkenes, sterically hindered trisubstituted alkenes, and most tetrasubstituted alkenes) to give vicinal diols rapidly (within minutes) and efficiently. The minor products are the usual oxidative fission products, namely, ketones and aldehydes or carboxylic acids, and sometimes ketols. Longer reaction times lower the yields of most diols, probably owing to oxidative glycol cleavage. Reactions with substrates containing one or more electron-with-drawing groups in conjugation with or adjacent to the alkene moiety are generally slower but give better yields. The diastereoselectivity of the present “flash” dihydroxylation, anti to the existing α-stereogenic center, with cycloalkenes is excellent whereas that with acyclic alkenes is moderate to poor. Sodium metaperiodate is still the best co-oxidant for the catalytic reaction. Aqueous acetonitrile (approximately 86%) as an alternative solvent system was found to give better yields of 1,2-diols than the original solvent system in some cases.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020111
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  • 54
    Digitale Medien
    Digitale Medien
    Multinational Papers in the Front Row (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 475-475 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020504
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  • 55
    Digitale Medien
    Digitale Medien
    C-Disaccharides of Ketoses (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 502-510 
    Details
    ISSN: 0947-6539
    Schlagwort(e): alkynes ; C-glycosides ; cobalt complexes ; cyclizations ; enzyme inhibitors ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Reaction of gluconolactone 2 with allylmagnesium bromide at low temperatures afforded ketopyranose 3, which could easily be converted into open-chain ketoses (R)-6 and (S)-6. Their reaction with lithioacetylide 9 afforded propargylic alcohol derivatives (R)-10 and (S)-10, which could not be cyclized directly to the desired C-ketosides. They were converted by standard procedures into (R)-14 and (S)-14 and then into dicobalthexacarbonyl complexes (R)-16 and (S)-16. A facile acid-catalyzed ring closure gave the desired C-ketosides (R)-18 α/β and (S)-18α/β, respectively, in different ratios. In order to demonstrate that removal of the protective groups and hydrogenation of the CC triple bond proceed smoothly, (R)-18 α was transformed into the deprotected target molecule (R)-1 α. For the assignment of the new chiral centers at C-2/2′ and at C-8, (S)-18α was transformed into azido derivative (S)-22α, which underwent intramolecular cycloaddition to afford the spiro derivative (S)-25α. Because of the conformational constraints in this molecule, unequivocal configurational assignment was possible with the help of NMR data.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020508
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  • 56
    Digitale Medien
    Digitale Medien
    Nucleophilic Addition of Secondary Phosphines to Cationic Dienyl Tricarbonyliron Complexes: A novel Route to Optically Active PhosphinesOptically Active Transition Metal Complexes, Part 6. Part 5: U. Englert, M. Käser, A. Salzer, Inorg. Chem. 1995, 34, 6231-6232. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 523-528 
    Details
    ISSN: 0947-6539
    Schlagwort(e): carbonyl complexes ; chiral ligands ; iron complexes ; phosphorus ligands ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Secondary phosphines such as HPPh2 and to the cationic iron dienyl complex [η5-(1R)-ethylnopadienyl)Fe-(CO)3]+ (1) by nucleophilic addition. The phosphonium salt initially formed is readily deprotonated to yield an optically active tertiary phosphine [(n4-(1 R)-ethylnopadienePPh2)Fe(CO)3] (2b). A similar reaction also occurs with [C6H7Fe-(CO)3]+ (3) and [C7H9Fe(CO)3]+ (4) to give [(C6H7PPh2)Fe(CO)3] (5) and [(C7H9PPh2)Fe(CO)3] (6) in good yields. The mechanism of formation of these novel phosphines is discussed. Complex 2 b crystallizes in the space group P212121 (no. 19); 5 crystallizes in the space group P21/c (no. 14). Like other monodentate optically active phosphines, 2 b is capable of coordinating to transition metal complexes. It forms palladium complexes on reaction with [{μ-chloro(allyl)palladium}2] as well as with [{μ-chloro[(N,N-dimethylamino-kN-2-methyl)phenyl-kC]palladium}2] (11). The latter reaction product crystallizes in the space group P31 (no. 144).
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020511
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  • 57
    Digitale Medien
    Digitale Medien
    Synthetic Cyclic Oligosaccharides-Syntheses and Structural Properties of a Cyclo[(1 → 4)-α-L-rhamnopyranosyl-(1 → 4)-α-D-mannopyranosyl]trioside and -tetraosideSynthetic Cyclic Oligosaccharides, Part 1. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 580-591 
    Details
    ISSN: 0947-6539
    Schlagwort(e): carbohydrates ; cyclodextrin analogues ; cyclooligomerizations ; glycosylations ; nanotubes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An efficient polycondensation-cyclization approach to the synthesis of cyclodextrin analogues is demonstrated by the preparation of cyclohexaoside 1 and cyclooctaoside 2. The key intermediate, disaccharide 3, bearing the cyanoethylidene group as a glycosyl donor function and the trityloxy group as a glycosyl acceptor function was prepared in 15 steps starting from L-rhamnose and D-mannose. The crucial cyclooligomerization of the disaccharide monomer 3 was carried out in the presence of TrClO4 as a promoter with the use of ultra-dry conditions at normal concentrations. This reaction led to formation of the cyclic oligosaccharides 28 and 29 (in 34 and 31% yield, respectively), which were deprotected to afford 1 and 2, respectively. The X-ray crystal structural analysis of the cyclooctaoside 2 reveals a cylindrical shape for the cyclic oligosaccharide with C4 symmetry. Individual molecules of 2 are arranged perfectly in stacks that form nanotubes in the solid state.
    Zusätzliches Material: 14 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020518
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  • 58
    Digitale Medien
    Digitale Medien
    Masthead (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996) 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020601
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  • 59
    Digitale Medien
    Digitale Medien
    Self-assembling tetrathiafulvalene-based rotaxanes and catenanes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 624-633 
    Details
    ISSN: 0947-6539
    Schlagwort(e): catenanes ; macrocycles ; rotaxanes ; self-assembly ; tetrathiafulvalenes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A general stepwise approach is described for the preparation of tetrathiafulvalene (TTF)-based linear and monoand dimacrocyclic compounds incorporating one or two 1,4-dioxyphenylene, 9,10-dioxyanthrylene, or 1,5- or 2,6-dioxynaphthylene units from readily available starting materials. By utilizing the π-π stacking interactions of the TTF unit with the dipyridinium dication of 1,1′-[1,4-phenylenebis (methylene)] bis-4,4′-bipyridinium bis(hexafluorophosphate), a rotaxane and two [2]catenanes were synthesized starting from the linear and monomacrocyclic compounds, respectively. From the dioxyphenylene-based dimacrocycle, three [3]pseudocatenanes (trans, cis, and a mixture of cis/trans isomers) were obtained with the trans compound as the major product. From the dioxyanthrylene dimacrocycle, only the trans-[3]pseudocatenane was obtained. Catenane products were formed quantitatively from the 1,5-dioxynaphthylene dimacrocycle in a template-directed reaction, affording a trans-[3]pseudo-catenane together with a [4]pseudocatenane (mixture of cis/trans isomers). From the 2,6-dioxynaphthylene dimacrocycle, a cis-[3]pseudocatenane was obtained as the major product and a trans-[3]pseudocatenane as the minor one. For the [3]pseudocatenanes (i.e., both the cis and trans catenanes), in which the TTF units were clamped by the tetracationic macrocycle, isomerizations were completely prevented even in the presence of trifluoroacetic acid. All new rotaxanes and catenanes were characterized by electrospray mass spectrometry, and the cis- and trans- [3]pseudocatenanes were additionally investigated by 1H NMR spectroscopy. The electrochemical and spectral properties of the rotaxane and the catenanes are reported. Catenane formation increases the redox potentials of the TTF unit. The results demonstrate the versatility of TTF as a building block in the construction of supramolecular structures.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020603
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  • 60
    Digitale Medien
    Digitale Medien
    From quinidine to new enantiopure materials-tricyclic allylic N,O-acetals and a stereospecific, one-pot conversion of 1,2-secondary, tertiary diols into spiroepoxides (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 673-679 
    Details
    ISSN: 0947-6539
    Schlagwort(e): chemoselectivity ; cinchona alkaloids ; clathrates ; osmium tetroxide ; spiro compounds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Hydrobromination of quinidine (1) with fuming HBr furnished diastereomeric secondary bromides 2a and 2b in 82% yield. After acetylation the resulting bromides 2a-Ac and 2b-Ac could be separated na dconverted stereospecifically into ethylidene rubanes Z)-4 and (E)-4, respectively. cis-Dihydroxylation of (Z)-olefin 4 with OsO4 was shown to be feasible by two catalytic variants, giving the two diastereomeric diols 5a and 5b, separable by chromatography. A simple onepot procedure was developed for converting the sterically hindered 1,2-secondary, tertiary diols stereospecifically into spiroepoxides (5a 6a-Ac; 5b 6b-Ac). Our procedure involves overall inversion of configuration. The procedure complements the Kolb-Sharpless route to epoxides from 1,2-disecondary diols with overall retention of configuration. The other two diastereomeric spiroepoxides 6c and 6d were prepared in one pot under different conditions (chloramine T, then alkali). Two unprecedented tricyclic allylic N,O-acetals (Z)-7 and (E)-7 were also obtained. The structure of spiroepoxide 6c (as a CH2Cl2 monosolvate) and of tricyclic olefinic N,O-acetal (E)-7 was corroborated by X-ray crystallography.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020609
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  • 61
    Digitale Medien
    Digitale Medien
    Stereoselective transformations with configurationally labile α-phenylselenoalkyllithium compoundsChiral Organometallic Reagents, Part XIX. For Part XVIII, see ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 694-700 
    Details
    ISSN: 0947-6539
    Schlagwort(e): asymmetric alkylations ; chiral auxiliaries ; kinetics ; organolithium compounds ; stereoselective syntheses ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Complexation of the configurationally labile α-phenylselenoalkyllithium compound 8 with 1,2-bisdimethylaminocyclohexane 15 led to two diastereomeric complexes 13 and 14 in a 7:3 ratio. Owing to ligand acceleration the complexes 13 and 14 added more rapidly to benzaldehyde than the uncomplexed organolithium compound 8. Trapping of complexes 13 and 14 by benzaldehyde was shown to occur more rapidly than their equilibration. This corresponds to non-Curtin-Hammett kinetics, in which enantiomeric enrichment in the products reflects the equilibrium ratio of the complexes 13 and 14.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020612
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  • 62
    Digitale Medien
    Digitale Medien
    Di(polyprenyl) phosphates as models for primitive membrane constituents: Synthesis and phase properties (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 789-799 
    Details
    ISSN: 0947-6539
    Schlagwort(e): amphiphiles ; cytomimetic chemistry ; phosphitylation ; polyprenols ; vesicles ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Identical- and mixed-chain di(polyprenyl) phosphates, derived from 3-methyl-2-buten-1-ol, (E)-geraniol, (E,E)-farnesol, (E,E,E)-geranylgeraniol, (R)-citronellol, or (R)-tetrahydrogeraniol have been synthesized by phosphitylation and oxidation with iodine/water. These phosphates gave organized systems in water, in particular vesicles, the physical properties of which were studied by photon correlation spectroscopy, electron microscopy, entrapment of a fluorescent dye (calcein), 31P NMR, stopped-flow/light-scattering studies, and fluorescence polarization with lipophilic probes. Interdigitation of mixed-length phosphates was not found to have any favorable effect on the stability of the vesicles. The C〉15 vesicles were formed despite the fact that they are composed of only one molecular species and are devoid of any reinforcer, but those with two geranyl chains appeared to be rather fragile. Phosphates containing C5 chains are soluble in water. The results obtained lend weight to the hypothesis that polyprenyl phosphates might have been primitive membrane constituents, and stimulate the search for appropriate reinforcers.
    Zusätzliches Material: 9 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020708
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  • 63
    Digitale Medien
    Digitale Medien
    Novel sulfur-containing inorganic aprotic superacids as effective initiators of low-temperature cracking and isomerization of alkanes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 812-814 
    Details
    ISSN: 0947-6539
    Schlagwort(e): alkanes ; cracking ; isomerizations ; sulfur compounds ; superacids ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A novel family of inorganic aprotic superacids has been found. Oxochlorides and chloride of sulfur combined with aluminium halides (SOCl2·n AlX3, SO2Cl2·n AlX3, MeSO2Cl·n AlX3, CF3SO2Cl·n AlX3, S2Cl2·n AlX3; n = 2 or 3, X = Br or Cl) are found to be effective promoters for the cracking and isomerization of alkanes at room temperature.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020711
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  • 64
    Digitale Medien
    Digitale Medien
    Nanometre-scale molecular ribbonsDedicated to Professor Heinz A. Staab on the occasion of his 70th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 832-837 
    Details
    ISSN: 0947-6539
    Schlagwort(e): cyclophanes ; macrocyclisations ; molecular ribbons ; nanostructures ; repetitive syntheses ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The longest molecular ribbons known to date (3-10), composed of a series of [3.3]metacyclophane units, have been synthesised by means of a repetitive synthetic strategy. These multiple ring systems with up to nine bridged benzene rings in a row are the longest structurally perfect cyclophane sequences known to date. The synthetic strategy comprises three steps: ester reduction to a tetrakis(hydroxymethyl) compound, derivatisation to the corresponding tetrakis(bromomethyl) derivative, and double cyclisation with the new key building block 14, which was especially developed for this sequence. Single-crystal X-ray analyses (3 a-7 a) and 1H NMR spectroscopy (3-10) show that, regardless of their length, these molecules adopt zigzag folded all-syn conformations. π-Stacks of nanometre dimensions are thus formed.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020714
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  • 65
    Digitale Medien
    Digitale Medien
    The silaguanidinium cation and the search for a stable silylium cation in condensed phases (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 869-876 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; silaguanidinium cations ; silylium cations ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Quantium mechanical calculations at the MP2/6-31 G(d) level are reported for the silaguanidinium cation Si(NH2)3+ (1) and derivatives thereof. The equilibrium structure 1a has D3 symmetry with planar amino groups rotated out of the SiN3 plane by 19.60. The Si-N bond length of 1 a (1.658 Å) is intermediate between a single and a double bond. Isodesmic reactions show that the stabilization of the silylium cation 1 a by the amino groups (63.5 kcal mol-1) is about 40% of the resonance stabilization of the guanidinium cation (159.3 kcal mol-1), but 1 a is clearly better stabilized than alkyl-substituted silylium cations. The electronic stabilization of 1 a by the amino groups is also made obvious by the calculated complexation energy with one molecule of water. The calculated stabilization through complexation of water at HF/6- 31 G(d) is markedly lower for Si(NH2)3-(H2O)+ (6) (28.8 kcal mol-1) than for SiMe3(H2O)+ (40.6 kcal mol-1). The tris(dimethylamino) silylium cation Si(N-Me2)3+ (8) is even more stable than 1 a. The complexation energy of Si(NMe2)3-(H2O)+ (10) is only 17.3 kcal mol-1. IGLO calculations of the 29Si N M R chemical shifts predict that 1 a and 8 should not show the same extremely low shielding that is calculated for alkyl-sub-stituted silylium ions. The calculated 29Si resonances for 8 are in reasonable agreement with the experimental N M R spectrum of (Me2N)3 SiB(C6F5)4. AM 1 calculations predict that the substituted tripyrrolidino silylium cation 12 would be an even better candidate for a stable tricoordinate silylium cation in condensed phases. One of the pyrrolidine rings of 12 has tert-butyl groups in the 2 and 5 positions, which serve as a steric fence around the silicon atom.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020717
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  • 66
    Digitale Medien
    Digitale Medien
    Diastereoselective zwitterionic aza-claisen rearrangement: Synthesis of nine-membered ring lactams and transannular ring contraction (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 894-900 
    Details
    ISSN: 0947-6539
    Schlagwort(e): aza-Claisen rearrangement ; azoniones ; indolizidinones ; ring contractions ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The zwitterionic aza-Claisen rearrangement of optically active 3-pyrrolidine acryl esters and various acid chlorides to generate optically active azoninones proceeds with high simple diastereoselectivity (internal asymmetric induction) and a complete 1,3-chirality transfer. The reaction path observed depends on the subsitution pattern of the allylic system: while the more electron-rich alkylated allyl amine fromed predominantly von Braun type products, the α,β-unsaturated esters could be rearranged with high yields. The azoniones thus obtained were treated with electrophiles, inducing regio- and diastereoselective transannular ring contractions. The resulting indolizidinones should be useful key intermediates in alkaloid synthesis.
    Zusätzliches Material: 1 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020719
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  • 67
    Digitale Medien
    Digitale Medien
    Photoreactions of phenyl-substituted N-(pent-4-enyl-1-oxy)pyridine-2(1 H)-thiones (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1014-1023 
    Details
    ISSN: 0947-6539
    Schlagwort(e): alkoxy radicals ; cyclizations ; pyridinethiones ; radicals ; tetrahydrofurans ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A series of hitherto unknown N-(pent-4-enyl-1-oxy)pyridine-2(1 H)-thiones (6) were prepared from substituted pent-4-enyl tosylates or benzylic chlorides. On irradiation with incandescent light heterocycles 6 liberated alkoxy radicals 2, which were studied for rearrangement reactions. Surprisingly, all transformations involving the 1-phenylpent-4-enyl-1-oxy radical (2a), for example, to give the substituted thioether 8, 2-bro-momethyl-5-phenyltetrahydrofuran (11), or the tetrahydrofuran 14a, were not stereoselective. On the other hand 2-, 3- mono-, and 1,5-disubstituted pent-4-enyl-1-oxy radicals 2d-e and 2g cyclized in good yields and with good to excellent stereoselectivities to give the corresponding 2,4-cis- and 2,3-trans-phenyltetra-hydrofurfuryl radicals 3d-e, and the trans-2-benzyl-5-methyl substituted intermediate 3g. The major reaction mode of the 4-phenylpent-4-enyl-1-oxy radical (2f) was the 6-endo cyclization, which afforded 3-phenyltetrahydropyran (13f) as the major product (endo:exo = 93:7) after trapping with hydrogen donors. According to the experimental data of the present study, the unusual reactivity of the 1-phenylpent-4-enyl-1-oxy radical (2a) in 5-exo-trig ring closures could be caused by a coplanar arrangement of the benzyloxy moiety in the transition state of the cyclization. This interaction would lock the radical center in 2a in a preferred conformation, which would result in similar steric effects for both cis- and trans- cyclizations.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020816
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  • 68
    Digitale Medien
    Digitale Medien
    Redox Properties of the Diatomic Bare Iron Chalcogenides FeO and FeS in the Gas PhaseDedicated to Professor Helmut Baumgartel on the occasion of his 60th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1230-1234 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; iron oxide ; iron sulfide ; mass spectrometry ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The electron-transfer behavior of the binary iron chalcogenides FeO and FeS has been examined by means of mass spectrometry and ab initio calculations using the averaged coupled-pair functional (ACPF) method. The experimental and theoretical results are in good agreement with each other and also with previous studies. The ionization energies (IE) of the diatomic species are found to be IE(FeO) = 8.8±0.2 eV, IE(FeO+) = 17.9±0.4 eV, IE(FeS) = 8.3±0.3 eV, and IE(FeS+) = 16.3±0.5 eV. Two new diatomic dications, FeS2+ and FeO2+, are shown to exist as metastable minima on the corresponding potential-energy surfaces. The data enable an evaluation of the intrinsic gas-phase redox properties of FeS and FeO, and the comparison demonstrates that iron sulfide is more prone to undergo facile reduction and oxidation than iron oxide.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021008
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  • 69
    Digitale Medien
    Digitale Medien
    A Theoretical Study of the Additivity of Proton Affinities in Aromatics: Polysubstituted Benzenes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1251-1257 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; benzenes ; electrophilic substitutions ; proton affinities ; QSAR ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: An additivity rule for proton affinities (PAS) in polysubstituted benzenes is derived from the MP2(fc)/6-31 G**//HF/6-31 G* + ZPE(HF/6-31 G*) theoretical model by use of the concepts of homodesmic reactions and independent substituents. The performance of the additivity rule of thumb is very good; this is evidenced by the excellent agreement of the estimated PAS with the latest experimental data. We believe that the additivity should work for larger aromatic compounds too. The PA increments, which characterize the influence of each substituent on a particular site of the benzene ring undergoing electrophilic substitution, proved useful in discussing various chemical properties of this family of compounds.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021011
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  • 70
    Digitale Medien
    Digitale Medien
    Processing of Blue Boron Nitride Thin Films with a Solid-Gas ReactionDedicated to Professor Rudolf Taube on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1269-1274 
    Details
    ISSN: 0947-6539
    Schlagwort(e): borazine ; boron nitride ; materials science ; thin films ; titanium complexes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: For the first time, solid-gas reaction techniques have been used for the synthesis and processing of thin films of boron nitride. Clear to intensely colored blue BN films were grown on Si(100) substrates by the transformation of borazine (B3N3H6) with a titanium complex as initiator under flowing nitrogen gas. The thickness of the films ranged from 70 to 100 nm, as determined by Rutherford backscattering (RBS) and atomic force microscopy (AFM) analyses. The intensity of the blue color of the thin film can be correlated to its thickness. The composition of the film determined by RBS studies corresponds to the stoichiometric formula B0.49N0.45O0.06, and N/B and O/B ratios are found to be 0.92 and 0.12, respectively. Nitrogen contents determined by nuclear reaction analysis agree well with the RBS results. Moreover, Auger electron spectroscopy (AES) measurements show that no titanium is present in the films and confirm the composition determined by RBS studies. X-ray photoelectron spectroscopy (XPS) shows the presence of boron and nitrogen in the blue BN film. Electron spin resonance (ESR) experiments at 293 and 12 K indicate a single broad signal with a g value (g = 2.005) close to that of a free electron. This synthetic approach provides opportunities for the preparation of new thin-film materials and for the fundamental study of solid-gas reactions.
    Zusätzliches Material: 9 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021014
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  • 71
    Digitale Medien
    Digitale Medien
    Coordination Arrays: Synthesis and Characterisation of Rack-Type Dinuclear Complexes (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1292-1302 
    Details
    ISSN: 0947-6539
    Schlagwort(e): complexes with nitrogen ligands ; ruthenium complexes ; self-assembly ; supramolecular chemistry ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A series of mono- and dimetallic complexes of rack type containing a dinucleating bis-tridentate ligand and RuII-2,2′:6′,2′-terpyridyl (tpy) sites were synthesised and characterised. The 1H NMR spectra of the dimetallic complexes were correlated to structural features, and the crystal structures of the dimetallic complexes were determined. They provide information about the way in which the central substitutent affects the overall shape of the racks and the relative disposition of the metal centres, measured by the pinching angle of the bis-tridentate ligand and the convergence angle of the ancillary tpy units. The latter demonstrates that a CH3 group yields the least bent complex and indicates how parallel the tpy units are. The dimetallic racks exhibit metal-metal interactions mediated by the bis-tridentate ligands as indicated by electrochemical and spectroscopic methods. The bridging-ligand π* orbital and the dπ metal orbital are stabilised by complexation of a second metal. The results obtained provide guidelines for the design of extended racks bearing several metal centres in a linear arrangement, which also represent potential components of molecular electronic devices.
    Zusätzliches Material: 14 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021017
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  • 72
    Digitale Medien
    Digitale Medien
    Organometallic Half-Sandwich Complexes Promote the Formation of Linear Oligopeptides from Amino Acid EstersMetal complexes of biologically important ligands, Part LXXXVIII. Part LXXXVII: R. Urban, R. Krämer, S. Mihan, K. Polborn, B. Wagner, W. Beck, J. Organomet. Chem. 1996, 517, 191.Dedicated to Professor Erwin Weiss on the occasion of his 70th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1518-1526 
    Details
    ISSN: 0947-6539
    Schlagwort(e): half-sandwich complexes ; peptide syntheses ; peptides ; rhodium complexes ; ruthenium complexes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Organometallic dipeptide ester complexes of the general formula [(L)M(Cl)(K2-NH2CH2CONCH2CO2R)] (1: L=Cp*, M=Rh, 2: L=Cp*, M=Ir, 3: L=η6-C6Me6, M=Ru) react smoothly with various α-L-amino acid esters in the presence of NEt3 to yield the tripeptide ester complexes [(L)M(Cl)(K2-NH2CHR'CONCH2CONHCH2CO2R)] (5-7). In the same fashion chloro K2-tetrapeptide ester complexes 10 and 11 are obtained either from tripeptide ester complexes or by subsequent addition of two equivalents of amino acid ester to a dipeptide ester complex. When the strong base NaOMe is used in the reaction of the diglycine ester compounds with amino acid esters. K3-tripeptide ester complexes 12 and 13 are produced, in which one of the two coordinated peptide nitrogen atoms is pyramidal. The hexamethylbenzene ruthenium complexes 13 with tripeptide ligands are formed with very high diastereoselectivity. A plausible reaction mechanism for the metal-promoted peptide synthesis is presented. Synthesis and isolation of the peptide esters proceeds without racemization.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021208
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  • 73
    Digitale Medien
    Digitale Medien
    Regiochemistry of Twofold Additions to [6,6] Bonds in C60: Influence of the Addend-Independent Cage Distortion in 1,2-MonoadductsDedicated to Professor Michael Hanack on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1537-1547 
    Details
    ISSN: 0947-6539
    Schlagwort(e): additions ; fullerenes ; regioselectivity ; semiempirical calculations ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2-(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2-monoadducts C61(COOEt)2 (1), C61-(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62-(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021211
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  • 74
    Digitale Medien
    Digitale Medien
    Electron-Transfer Properties and Active-Site Structure of the Type 1 (Blue) Copper Protein Umecyanin (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 104-109 
    Details
    ISSN: 0947-6539
    Schlagwort(e): copper proteins ; cross-reactions ; electron-transfer reactions ; kinetics ; metalloproteins ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The electron self-exchange rate constant for the Type 1 blue copper protein umecyanin from horseradish roots has been determined as 6.1 × 103 M-1 S-1 at pH 7.5, I = 0.100 M, 25°C by an NMR line-broadening method. The value obtained is one of the lower self-exchange rate constants determined for this class of protein; this is attributed to the presence of positively charged residues near to the electron-transfer site. The self-exchange rate constants calculated by means of a Marcus analysis of data for the cross-reactions (25°C) of umecyanin with azurin and cytochrome c551 (both from Pseudomonas aeruginosa) are substantially less at 8.0M-1 S-1 and 13.9M-1S-1, respectively, and are independent of pH in the range 7.0-8.0, I = 0.100M. The discrepancy between the self-exchange rate constants obtained by these two different methods can be rationalised if it is assumed that umecyanin reacts with the two proteins employed in the cross-reaction studies through the same site, but that this site is different from that used for the self-exchange process. A comparison of the primary structure of umecyanin with those of other Type 1 copper proteins has revealed that a glutamine rather than a methionine is likely as the fourth ligand of Cu at the active site. Other comparisons are made with stellacyanin, and the electron-transfer reactivity of the two proteins is discussed.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020118
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  • 75
    Digitale Medien
    Digitale Medien
    Substrate Uptake and Activation by Dimolybdenum and Ditungsten Hexaalkoxides-Factors Influencing the Cleavage of C-X Multiple Bonds in the Reactions Between [Mo2(OR)6] (R = tBu or CH2tBu) and Ar2C=S, Et2NC≡N, and P(nBu)3Dedicated to Professor R. R. Schrock at the start of his sixth decade (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 110-117 
    Details
    ISSN: 0947-6539
    Schlagwort(e): kinetics ; metal-metal bonds ; molybdenum complexes ; multiple bonds ; tungsten complexes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The reaction between diarylthiones and [Mo2(OCH2tBu)6] in hydrocarbon solvents yielded [Mo2(OCH2tBu)6-(μ-S)(=CAr2)] by cleavage of the C=S double bond. The Lewis base adduct [Mo2 (OCH2tBu)6(μ-S) (=CPh2) (PMe3)] (2) has been crystallographically characterized; it contains six- and five-coordinate Mo atoms linked through μ-S and μ-OR groups. The rate of the cleavage of the C=S bond has been studied by variable-temperature 1H NMR in [D8]toluene. A Hammett plot shows that both electron-donating and electron-releasing substituents in the aryl groups enhance the rate relative to Ph2C=S. The activation parameters for cleavage of the C=S bonds in Ph2C=S, (p-MeOC6H4)2C=S, and (m-CF3C6H4)2C=S exhibit essentially identical values for ΔS
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020119
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  • 76
    Digitale Medien
    Digitale Medien
    Mineralization of Gold Nanoparticles in a Block Copolymer Microemulsion (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1552-1555 
    Details
    ISSN: 0947-6539
    Schlagwort(e): block copolymers ; micelles ; mineralization ; nanostructures ; thin films ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Controlled mineralization of gold nanoparticles has been performed in a microemulsion of polystyrene-block-poly(2-vinylpyridine). The starting point was the formation of a thermodynamically stable dispersion of HAuCl4 in inverse micelles of the block copolymer in toluene, which became metastable when the gold was reduced. Kinetic control of the transformation allowed the following stages of the mineralization/coagulation process to be stabilized: 1) one gold particle per micelle, 2) aggregated micelles containing two or three gold particles, and 3) a state in which empty micelles coexist with larger polymer-stabilized gold particles. Distinctive variations in the spectra were observed depending on the particle size and whether two particles had formed a couple with orientation-dependent dipolar interactions.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021213
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  • 77
    Digitale Medien
    Digitale Medien
    Asymmetric Control in the Pictet-Spengler Reaction by Means of N-Protected Amino Acids as Chiral Auxiliary Groups (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1566-1571 
    Details
    ISSN: 0947-6539
    Schlagwort(e): amino acids ; asymmetric syntheses ; carbolines ; chiral auxiliaries ; Pictet-Spengler reaction ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Aromatic and aliphatic Schiff bases of tryptamine react with Fmoc- or phthaloyl-protected amino acid chlorides to form N-acyliminium intermediates, which, in the presence of titanium alkoxides at room temperature, undergo Pictet-Spengler reactions to give tetrahydro-β-carbolines with diastereomeric ratios of up to 99:1. The chiral auxiliary can be removed from the Pictet-Spengler adducts by means of a simple reduction. To rationalize the observed stereoselectivity a transition-state model is proposed in which the titanium atom coordinates both the carbonyl group of the N-acyliminium ion and the amino acid protecting group.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021215
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  • 78
    Digitale Medien
    Digitale Medien
    A Combined Spectroscopic, Photophysical and Theoretical (DFT) Study of the Electronically Excited Inorganometallic Complexes [Ru(E)(E′)(CO)2(iPr-DAB)] (E=Cl, Me, SnPh3, PbPh3; E′=GePh3, SnR3, PbR3 (R=Me, Ph); iPr-DAB=N,N'-diisopropyl-1,4-diaza-1,3-butadiene): Evidence of an Exceptionally Long-Lived 3σπ* Excited State for [Ru(SnPh3)2(CO)2(iPr-DAB)] (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1556-1565 
    Details
    ISSN: 0947-6539
    Schlagwort(e): density functional calculations ; IR spectroscopy ; ruthenium complexes ; time-resolved spectroscopy ; UV ; vis spectroscopy ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The photophysical properties of the metal-metal bonded complexes [Ru-(E)(E′)(CO)2(iPr-DAB)] (E=Cl, E′=SnPh3, PbPh3; E=Me, E′=SnPh3, PbPh3; E=SnPh3, E′=SnMe3, SnPh3, GePh3; E=PbPh3, E′=PbMe3, PbPh3, GePh3; iPr-DAB=N,N'-diisopropyl-1,4-diaza-1,3-butadiene) have been studied. According to time-resolved emission, UV/vis and IR spectra, combined with density functional (DFT) MO calculations, the lowest excited state has triplet metal-to-ligand charge-transfer (3MLCT), triplet halide-to-ligand charge-transfer (3XLCT) or 3σ(E-Ru-E)π* character, depending on the nature and combination of the ligands E and E′. The 3σ(E-Ru-E′)π* state is a bound state whose lifetime is strongly influenced by the ligands E and E′. An exceptionally long lifetime (264 μs at 80 K) is observed for the 3σ(Sn-Ru-Sn)π* state of the symmetrically substituted [Ru(SnPh3)2(CO)2(iPr-DAB)] complex. The reason for this long lifetime is the fact that the excited state of this complex is hardly distorted with respect to the ground state, owing to the delocalised character of the σ(Sn-Ru-Sn) bonding orbital, which mixes strongly with the π* orbital of the iPr-DAB ligand. This delocalisation is also responsible for the unusually high oscillator strength of the σ → π* electronic transition in the visible spectral region.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021214
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  • 79
    Digitale Medien
    Digitale Medien
    Synthesis of a Pentasaccharide-Oligonucleotide Conjugate: A Novel Antithrombotic Agent (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1572-1577 
    Details
    ISSN: 0947-6539
    Schlagwort(e): antithrombotics ; enzyme inhibitors ; oligonucleotides ; oligosaccharides ; protecting groups ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Derivatization of the octadecathymidylate derivative 18 (T18 ODN) containing a free amine function with sulfo-SIAB® gave the corresponding iodoacetyl ODN 21. Conjugation of the latter with the thiol-containing pentasaccharide 17c gave pentasaccharide-ODN conjugate III, which exhibited anti-Xa and antithrombin activities of 173 U mg-1 and 5 U mg-1, respectively.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021216
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  • 80
    Digitale Medien
    Digitale Medien
    Concave Hydrocarbons (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1585-1595 
    Details
    ISSN: 0947-6539
    Schlagwort(e): concave hydrocarbons ; cyclophanes ; host/guest chemistry ; ion-selective electrodes ; prismands ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Concave hydrocarbons, such as 1 (C36H36), 4 (C60H60), 5 (C54H48), and 6 (C60H52), represent three-dimensionally clamped analogues of π-prismands. They encapsulate small metal ions and accomplish metal-ion extraction from aqueous solution. Their remarkable selectivity allows applications such as incorporation in ion-selective electrodes. The synthetic route is based on well-established cyclophane methodology and, thus, offers a general approach to a whole family of concave hydrocarbons.
    Zusätzliches Material: 11 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021218
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  • 81
    Digitale Medien
    Digitale Medien
    Stereodivergent Hetero-Diels-Alder Reactions of Chiral 1-Oxa-1,3-butadienes through a Conformational Switch Induced by Lewis Acids (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 139-148 
    Details
    ISSN: 0947-6539
    Schlagwort(e): asymmetric ; syntheses ; Diels-Alder reactions ; dihydropyrans ; Lewis acids ; oxabutadienes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The stereodivergent asymmetric hetero-Diels-Alder reaction of achiral and chiral 1-oxa-1,3-butadienes carrying an oxazolidine moiety with various enol ethers in the presence of different Lewis acids is described as a highly stereoselective and efficient approach to dihydropyrans, which can be used for the synthesis of carbohydrates. In the cycloaddition of the achiral oxabutadiene very good endo/exo selectivity was possible, and with the chiral oxabutadienes excellent 1,6-asymmetric induction was additionally observed. In the processes a reversal of facial selectivity occurs by changing the Lewis acid, allowing the synthesis of both enantiomers of the dihydropyrans with the same auxiliary. Thus, cycloaddition of 1 to 2 in the presence of Me2 AlCl gives predominantly the endo product 3 (3:4 = 10:1), whereas with SnCl4 the exo product 4 is obtained (3:4 = 1:15). The reaction of 7 and 1 a in the presence of Me2AlCl as promoter nearly exclusively yields the endo-I adduct 16 a (16 a+17 a:18 a+19 a = 〉 50:1; 16 a:17 a = 60:1), whereas with TMS-OTf the endo-II-product 17 a was obtained as the main component (16 a+17 a:18 a+19 a = 〉 50:1; 16 a:17 a = 1:7.9). The use of SnCl4 leads to a mixture of endo and exo, again, however, with excellent induced selectivity. A similarly good induction was obtained with the oxabutadiene 9 containing the new auxiliary 8. Also, other enol others 1 b-g were used, some of which afforded excellent induction. Mechanistic considerations are used to explain the results.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020205
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  • 82
    Digitale Medien
    Digitale Medien
    Synthesis and Properties of 4,4,9,9-Tetramethyl-1-oxa-cycloundecane-5,6,7,8-tetrone and 5,5,10,10-Tetramethyl-1-oxa-cyclotridecane-6,7,8,9-tetroneDedicated to Professor Richard Neidlein on the occasion of his 65th birthday (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 271-277 
    Details
    ISSN: 0947-6539
    Schlagwort(e): biomineralization ; medium-sized rings ; calcite ; crystal morphology ; glycoproteins ; polysaccharides ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The synthesis of 4,4,9,9-tetramethyl-1-oxa-cycloundecane-5,6,7,8-tetrone (9) and 5,5,10,10-tetramethyl-1-oxa-cyclotridecane-6,7,8,9-tetrone (10) has been achieved in a multistep procedure. The key steps in this synthesis were the ring closure of 23 and 24 to 25 and 26, respectively, and the oxidation of the triple bond with RuO2/NaIO4 to the dihydroxydiketones 31 and 32. Compound 9 is the first cyclic tetraketone for which an intramolecular donor-acceptor stabilization has been found. A strong transannular interaction between the ether oxygen and the C4O4 unit in 9 was detected by X-ray studies on single crystals of 9. The transannular distance is 2.7-2.8 Å. Further evidence for a strong transannular interaction was obtained from the comparison of the reduction potential and the first band in the UV/Vis spectrum with the corresponding values from other openchain tetraketones. These findings were substantiated by PE investigations on 9. The crystal structures of the dihydroxy-diketones 31 a and 32 a showed that, in the case of the eleven-membered ring (31 a), there are also short transannular distances between the ether oxygen and the C2O2 moiety (2.5 Å and 2.9 Å). In the case of the thirteen-membered ring (32 a), no transannular interactions were found in the solid state.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020307
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  • 83
    Digitale Medien
    Digitale Medien
    Mechanisms of Water Exchange between Lanthanide(III) Aqua Ions [Ln(H2O)n]3+ and bulk water: A Molecular Dynamics Simulation Approach Including High-Pressure Effects (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 285-294 
    Details
    ISSN: 0947-6539
    Schlagwort(e): computer simulations ; high-pressure chemistry ; lanthanide complexes ; ligand exchange ; mechanistic studies ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: We studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(III) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell (τres (1st shell) = 1577, 170 and 410 ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12-18 ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H2O)9]3+ aqua ion follow a dissociatively activated Id mechanism via an eightfold-coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and 10 ps. The assignment of an Id mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (1800) and by the fact that the calculated average activation volume ΔV≠ = + 4.5 cm3 mol-1 is clearly smaller than the estimated activation volume ΔV≠lim ≈ΔV0 = + 7.2 cm3 mol-1 for a limiting D process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the bulk and maintains the coordination equilibrium between a [Sm(H2O)8]3+ and a [Sm(H2O)9]3+ aqua ion. The resulting trajectory pattern of incoming and leaving water molecules is an alternation of elimination and addition reactions and cannot be classified into the scheme of D, I or A mechanisms for substitution processes. The reaction volume ΔV0 for the coordination equilibrium [Sm(H2O)8]3+ + H2O → [Sm(H2O)9]3+ can be evaluated consistently both by a thermodynamic and a geometric approach. The observed exchange events for [Yb(H2O)8]3+ exhibit the characteristics of an Ia mechanism. The water exchange takes place via a transition-state geometry close to that of a tricapped trigonal prism and involves a slightly negative activation volume.
    Zusätzliches Material: 13 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020309
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  • 84
    Digitale Medien
    Digitale Medien
    Gas-Phase Acid-Induced SN2′ versus SN2 Mechanism in Allylic AlcoholsGas-Phase Acid-Induced Nucleophilic Displacement Reactions, Part 10. Part 9: ref. [1]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 316-322 
    Details
    ISSN: 0947-6539
    Schlagwort(e): allylic alcohols ; gas-phase chemistry ; ions ; mechanistic studies ; nucleophilic substitutions ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A first demonstration of the existence of the concerted SN2′ mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA+ acids (GA+=CnH+5 (n = 1, 2), iC3H+7, and (CH3)2F+) with trans- (1) and cis-2-buten-1-ol (2) as well as with 1-buten-3-ol (3). Firm evidence in favor of the concerted SN2′ pathway accompanying the classical SN2 one in these systems was obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA+ attack on 1-3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3-butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive SN2′ and SN2 pathways in the oxonium ions from 1 (SN2′ (57 ± 2%) and SN2 (43 ± 2%)) and 3 (SN2′ (54 ± 2%) and SN2 (46 ± 2%)), whereas, with 2, the SN2 (66 ± 2%) reaction prevailed over the SN2′ one (34 ± 2%). The role of intrinsic structural factors in determining the SN2′/SN2 branching in the selected oxonium ions is discussed.
    Zusätzliches Material: 3 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020313
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  • 85
    Digitale Medien
    Digitale Medien
    Regioselectivity in the Gas-Phase Nucleophilic Attack on O-Protonated 3-Methyl-2-cyclohexen-1-ol and 1-Methyl-2-cyclohexen-1-olGas-Phase Acid-Induced Nucleophilic Displacement Reactions, Part 11, Part 10: G. Renzi, A. Lombardozzi, E. Dezi, A. Pizzabiocca, M. Speranza, preceding paper in this issue. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 323-334 
    Details
    ISSN: 0947-6539
    Schlagwort(e): cyclohexenols ; elimination reactions ; gas-phase chemistry ; mechanistic studies ; nucleophilic substitutions ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Our radiolytic study of the occurrence in the gas phase of concerted SN2′ reactions on several open-chain allylic oxonium ions generated in the gas phase from the attack of gaseous GA+ acids (GA+=CnH+5 (n = 1, 2), iC3H+7, and (CH3)2F+) on suitable substrates is now continued with cyclic allylic alcohols, such as 3-methyl-2-cyclohexen-1-ol (1) and 1-methyl-2-cyclohexen-1-ol (2), with both MeOH and NMe3 as neutral nucleophiles. With MeOH as the nucleophile, the substitution reaction exclusively takes place on 1 as the starting compounds, whereas when the substrate is 2 it is accompanied by extensive elimination. With NMe3, only the elimination reaction is observed in the same systems. The analysis of the isomeric distribution of the substitution and elimination products allows definition of the corresponding reaction patterns. As for open-chain oxonium ions, the nucleophilic attack on O-protonated 1 and 2 is preceded by significant intramolecular interconversion. Partial unimolecular dissociation of the same ionic intermediates also takes place. After careful evaluation of the extent of these side processes, it is demonstrated that the O-protonated 1 undergoes the concerted SN2 process with MeOH almost exclusively (≥ 99%). With O-protonated 2, how ever, the concerted SN2′ pathway (84-95%) prevails over the classical SN2 one (6-17%). Concomitant [1,2] (E2) and [1,4] elimination (E2′) pathways involve attack of the selected nucleophiles on the oxonium ions from 1 and 2. Their relative extent (E2′/E2:1.78-1.96 (1); 1.43-1 70 (2)) appears only slightly dependent on the nature of the ionic substrate, the nucleophile (whether MeOH or NMe3), and the leaving group (whether H2O or MeOH). The effects of both intrinsic structural factors and experimental conditions in determining the SN2′/SN2 and E2′/E2 branchings in the selected oxonium ions is discussed and compared with related gas-phase data.
    Zusätzliches Material: 9 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020314
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  • 86
    Digitale Medien
    Digitale Medien
    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 369-372 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960020403
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  • 87
    Digitale Medien
    Digitale Medien
    Pseudorotaxanes formed between secondary dialkylammonium salts and crown ethers“Molecular Meccano”, Part 6: for Part 5, see ref. [45e]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 709-728 
    Details
    ISSN: 0947-6539
    Schlagwort(e): crown ethers ; dialkylammonium salts ; hydrogen bonding ; molecular recognition ; pseudorotaxanes ; self-assembly ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A very simple self-assembling system, which produces inclusion complexes with pseudorotaxane geometries, is described. The self-assembly of eight pseudorotaxanes with a range of stoichiometries-1:1, 1:2, 2:1, and 2:2 (host:guest)-has been achieved. These pseudorotaxanes self-assemble from readily available components-well-known crown ethers, such as dibenzo[24]crown-8 and bis-p-phenylene[34]crown-10, and secondary dialkylammonium hexafluorophosphate salts, such as (PhCH2)2NH+2PF-6 and (nBu)2NH+2PF-6-and have been characterized not only in the solid state, but also in solution and in the “gas phase”. The pseudorotaxanes are stabilized largely by hydrogen-bonding interactions and, in some instances, by aryl-aryl interactions.
    Zusätzliches Material: 40 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020614
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  • 88
    Digitale Medien
    Digitale Medien
    A new nickel(III) oxide family: MSr3NiO6 (M = Sc, In, Tm, Yb and Lu) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 737-741 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Jahn-Teller distortions ; magnetic properties ; neutron powder diffraction ; nickel oxides ; X-ray powder diffraction ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The new phases MIIISr3NiIIIO6 have been prepared for M = Sc, In, Tm, Yb and Lu. Thermogravimetric analysis indicates that these phases are stoichiometric nickel(III) oxides. Rietveld refinement of their crystal structures from powder X-ray diffraction data confirms that they adopt the rhombohedral K4CdCl6- type structure (space group R\documentclass{article}\pagestyle{empty}\begin{document}$ \bar 3 $\end{document}c, a = 9.6595 (2) and c = 10.8546 (3) Å for ScSr3NiO6). The M site is fully occupied for M = Sc and In, but a deficiency of scattering for M = Tm, Yb and Lu is shown to be due to Ni substitution through a simultaneous refinement of the YbSr3NiO6 structure using X-ray and time-of-flight neutron diffraction data. The refined composition is (Yb0.83Ni0.17)-Sr3NiO6. The magnetic suseptibilities of the M = Sc, In and Lu samples show Curie-Weiss behaviour down to 6K; however, ScSr3NiO6 shows a broad transition between 250 and 290 K, with Curie-Weiss behaviour above and below this anomaly. This transition is thought to be between the statically and dynamically Jahn-Teller distorted regimes of octahedrally coordinated, low-spin Ni3+.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020616
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  • 89
    Digitale Medien
    Digitale Medien
    Cover page (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. cpi 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.1460020620
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  • 90
    Digitale Medien
    Digitale Medien
    Repetitive Strategy for Exponential Growth of Hydroxy-Functionalized Dendrons (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1330-1334 
    Details
    ISSN: 0947-6539
    Schlagwort(e): dendrimers ; exponential growth ; repetitive syntheses ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The synthesis of dendrons with orthogonally protected functional groups, which allows the construction of dendrons in an exponential fashion, is described. The sequence has been carried out on a gram scale for the first, second, and fourth generation. The dendrons have a relatively loose and flexible structure, which should make further growth feasible. Owing to the dendrons' adjustable and broadly applicable substitution pattern, they are the first representatives of a future construction kit for the systematic synthesis of layered dendrimers. The coupling step involves urethane formation.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021021
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  • 91
    Digitale Medien
    Digitale Medien
    Graphical abstract (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1347-1351 
    Details
    ISSN: 0947-6539
    Schlagwort(e): Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021103
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  • 92
    Digitale Medien
    Digitale Medien
    A Paradigm Case for the Merging of Glycal and Enzymatic Assembly Methods in Glycoconjugate Synthesis: A Highly Efficient Chemo-Enzymatic Synthesis of GM3 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1359-1362 
    Details
    ISSN: 0947-6539
    Schlagwort(e): anhydrosugars ; enzymatic catalysis ; gangliosides ; glycoconjugates ; glycosidations ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A concise, regio- and stereoselective synthesis of the ganglioside GM3 has been achieved. Use of an α-1,2-oxirane derived from lactal facilitated installation of the anomeric β-ceramide side chain. Introduction of the sialic acid residue at C3, of the lactose was accomplished regiospecifically by enzymatically mediated sialyl transfer from CMP-Neu-5-Ac.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021105
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  • 93
    Digitale Medien
    Digitale Medien
    Synthesis and Structure of a Bis(Double Helicate) and its Cryptatoclathrate“Adamantanoid Chelate Complexes”, Part 6; for Part 5, see ref. [4]. (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1363-1367 
    Details
    ISSN: 0947-6539
    Schlagwort(e): clathrates ; cryptates ; helicates ; selfassembly ; zinc complexes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The structures of the hitherto unknown bis(double helicate) 10 and its cryptatoclathrate (10)2·2THF were unequivocally determined by X-ray diffraction. Bis(double helicate) 10 is formed in a one-pot synthesis starting from CH-acidic bis (tetrazolylmethyl ketone) 9 and Zn(OAc)2. The formation of racemic, homochiral 10 from [Zn2L32] fragments, which are formed in a self-assembly process, is governed by chiral self-recognition. According to NMR studies only 10 is present in solution. 13C CP/MAS NMR spectroscopy and X-ray analysis confirm aggregation of 10 with two molecules of THF to yield the inclusion compound (10)2·2 THF in the solid state.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021106
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  • 94
    Digitale Medien
    Digitale Medien
    Se2NBr3, Se2NCl5, Se2NCl-6: New Nitride Halides of Selenium(III) and Selenium(IV) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1373-1378 
    Details
    ISSN: 0947-6539
    Schlagwort(e): ab initio calculations ; crystal structure ; halogen compounds ; nitrides ; selenium compounds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Se2NBr3, a planar molecule shaped liked dividers, was formed by the reaction of SeBr4 with N(SiMe3)3, which reduced the selenium to the trivalent state. Under the same conditions, Se2NCl5 was obtained by the reaction of SeCl4 with N(SiMe3)3, retaining the tetravalent state of selenium. The molecule is formed by two edge-sharing pseudotrigonal bipyramids with selenium as their central atoms. Se2NCl5 reacts with PPh4Cl to form the Se2NCl-6 ion by the addition of a Cl- ion to only one of the two selenium atoms, resulting in pseudooctahedral coordination of this atom. Ab initio calculations for all the compounds reported confirm the experimentally determined structures.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021108
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  • 95
    Digitale Medien
    Digitale Medien
    Investigation of the Self-Assembly Pathway of Pentanuclear Helicates by Electrospray Mass Spectrometry (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1395-1398 
    Details
    ISSN: 0947-6539
    Schlagwort(e): bipyridine ligands ; helicates ; kinetics ; mass spectrometry ; self-assembly ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The self-assembly of the pentanuclear double helicates Hh, Ha and He from the corresponding oligobipyridine strands Lh, La and Le and Cu1 ions has been investigated by NMR and electrospray mass spectrometry (ESMS). Where as Hh is assembled rapidly (in less than 20 min), He (about 20 h) and especially Ha (about 60 h) form much more slowly. The rate decreases strongly with increasing steric bulk of the substituents in the 4,4′-positions on the bipyridine units; this indicates that the search processes (wrapping, unwrapping) that lead to the final helicate are strongly hindered by the size of the substituents. The ESMS data give information about the species present in solution under different conditions and allow the formulation of possible formation pathways, which may involve, in particular, helicates of hairpin type.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021111
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  • 96
    Digitale Medien
    Digitale Medien
    Phosphorus-Containing Dendrimers: Synthesis of Macromolecules with Multiple Tri- and Tetrafunctionalization (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1417-1426 
    Details
    ISSN: 0947-6539
    Schlagwort(e): dendrimers ; macromolecules ; phosphorus compounds ; substitutions ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A variety of dendrimers up to generation 7 possessing terminal P(S)Cl2, (3-[G1]-3-[G7]) or P(O)Cl2, (7-[G1]-7-[G3], 7-[G3, 7-[G7) have been tri- and tetrafunctionalized. Selective monosubstitution of P(X)Cl2 (X = S, 0) termini with allylamine or propargylamine gave the trifunctionalized dendrimers 5-[G1], 5-[G7], 8-[G1], 8-[G3], 10[G1], 11-[G4], ll-[G4], 12-[G1]-12-[G3], 12-[G5], and 12-[G7]. Reaction of dendrimers 4-[G1] and 4-[G4], possessing terminal P(S)[N-allyl) fragments, with propargylamine afforded trifunctionalized dendrimers l4-[G1 and l4-[G4]. Multiply trifunctionalized macromolecules 13-[G2] and 13-[G3] with P(S)(NH-allyl)(NH-propargyl) moieties at the surface were prepared by treatment of 8-[G2] and 8-[G3] with propargylamine. Dendrimers 15-[G1]-l5-[G3] and l6-[G1]-l6-[G3] with P(O)(NH-allyl)(OC6H4CHO) and P(O)(NH-propargyl)(OC6H4CHO) termini were also synthesized. Reaction of hydrazine or cyanomethylenetriphenylphosphorane (17) with compounds 15-[G1], 15-[G3], and 16-[G1]-16-[G3] led to the multiply tetrafunctionalized dendrimers 18-[G1], 18-[G3], 19-[G1], 20-[G1]-20-[G3], 21-[G1], and 21-[G2].
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021114
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  • 97
    Digitale Medien
    Digitale Medien
    New Ruthenium(II) Complexes of Functionalized Monoanionic Aryldiamine N,C,N'-Terdentate Ligands: Syntheses of [RuII{2,6-(Me2NCH2)2-4-R-C6H2}-(terpy)]+Cl-; X-ray Structure of a Dimeric Organolithium Compound, [Li{2,6-(Me2NCH2)2-4-Ph-C6H2}]2 (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1440-1445 
    Details
    ISSN: 0947-6539
    Schlagwort(e): aryldiamines ; chelate ligands ; organometallic compounds ; ruthenium complexes ; structure elucidation ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The new anionic functionalized aryldiamine ligands [2,6-(Me2NCH2)2-4-R-C6H2]- (R = Me3SiC≡C, C6H5, Me3-Si), formally derived from [2,6-(Me2-NCH2)2C6H3]-, have been prepared as their lithium compounds. The compound [Li{2,6-(Me2NCH2)2-4-Ph-C6H2}]2 crystallizes in the monoclinic space group C2/c (no. 15) with a = 13.1225(5), b = 13.5844(7), c = 18.9859(12) Å, β = 105.329(5)°, V = 3264.0(3) Å3. Z = 4. The structure refinement converged to R1 = 0.0374 for 2037 observed reflections [Fo〉4σ(Fo)] and wR2 = 0.0922 for 2560 unique data. The organolithium compounds have been used in transmetalation reactions to give the corresponding functionalized organoruthenium(II) complexes [RuII{2,6-(Me2NCH2)2-4-R-C6H2}-(terpy)]+Cl- (terpy = 2,2′;6′,2′-terpyridine). The RuII species with R = HC°C has also been synthesized.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021117
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  • 98
    Digitale Medien
    Digitale Medien
    Studies Towards the Total Synthesis of Epothilones: Asymmetric Synthesis of the Key Fragments (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1477-1482 
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    ISSN: 0947-6539
    Schlagwort(e): asymmetric syntheses ; epothilones ; macrolides ; natural products ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Members of a new class of macrolide - the so-called epothilones (1) - showing a taxol-like biological activity have recently been isolated. A convergent approach to 1 is presented, and the asymmetric syntheses of the three key intermediates 3, 4, and 8 are reported.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021121
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  • 99
    Digitale Medien
    Digitale Medien
    A Novel Type of Resonance-Stabilised Dicobalt Complex with a Cyclopentadienylidene Bridge - Synthesis, Structure and Electronic Properties of [(C5R5)Co(μ-C5H4)Co(L)(C5R5)'] (L = C2H4, CO, CNtBu, PR3, P(OMe)3; R = H, Me) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1453-1465 
    Details
    ISSN: 0947-6539
    Schlagwort(e): carbenes ; CH activation ; dinuclear complexes ; sandwich complexes ; spin equilibria ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The dinuclear μ-cyclopentadienylidene complexes [(C5R5)Co(μ-C5H4)-Co(C2H4)(C5R'5)] (5) [5aa (R = R' = H), 5ab (R = H, R' = Me), 5ac (R = H, R'5 = H4Me), 5ba (R = Me, R' = H), 5bb (R = R' = Me) and 5da (R5 = Me4Et, R' = H)] were synthesised from [(C5R5)Co(η4-C5H6)] (4) [4a (R = H), 4b (R = Me), 4d (R5 = Me4Et)] and [(C5R'5)Co(C2H4)2] (1) [1a (R' = H), 1b (R' = Me) and 1c (R'5 = H4Me)]. In these reactions, both CH bonds of the methylene group of coordinated cyclopentadiene are activated under mild conditions. Substitution of the ethylene ligand in 5 by L leads to the carbonyl, isocyanide, phosphine, and phosphite derivatives [(C5R5)Co(μ-C5H4)Co(L)-(C5R'5)] [6aa, 6ab, 6ba, 6bb (L = CO), 7aa (L = tBuNC), 8aa, 8ab (L = PMe3), 9aa (L = PMe2Ph), 10aa (L = PMePh2) and 11aa (L = P(OMe)3)]. The crystal structures of 5aa, 5ab, 5ba, 6aa and 8aa have been determined. The experimental geometry is rationalised in terms of two limiting structures with μ-η4:η1 and μ-η5:η1 coordination of the bridging cyclopentadienylidene ligand. On the basis of the 18 valence electron rule, zwitterionic character is assigned to latter. This structure is preferred when L is an acceptor ligand, as in 6. In solution, 5-11 are fluxional with rapid rotation about the very short cobalt-carbene C bond. In addition the hindered rotation of the ethylene ligand in 5ab was studied by DNMR spectroscopy. In solution, a singlet-triplet equilibrium was established by variable-temperature NMR spectroscopy for 8aa. The temperature-dependent 1H NMR line shifts were analysed by means of an isotropic shift model to give 24≤ΔH° ≤ 32 kJ mol-1 and 45≤ΔS° ≤74 J mol-1 K-1 with the triplet state being preferred by entropy at higher temperatures.
    Zusätzliches Material: 9 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021119
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  • 100
    Digitale Medien
    Digitale Medien
    First Observation of a [1,7] Boron Shift - Three Independent Fluxional Processes in the Tricarbonyliron Complex of Cycloheptatrienyl(dipropyl)borane (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1483-1488 
    Details
    ISSN: 0947-6539
    Schlagwort(e): boron compounds ; fluxionality ; iron complexes ; NMR spectroscopy ; rearrangements ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The fluxional behavior of [1-4-η4 - exo - 7 - dipropylborylcyclohepta - 1,3,5-triene]tricarbonyliron (14) has been studied by dynamic NMR. The 2 D 1H and 13C EXSY NMR experiments carried out at various temperatures revealed that three independent fluxional processes occur in 14. The fastest rearrangement is a [1,7] B sigmatropic shift accompanied by [1,2] Fe migration (EA = 71.2±2.3 kJ mol-1, ln A = 32±1). Two other rearrangements also take place: [1,3] B sigmatropic and [1,3] Fe haptotropic migrations. The analysis of the observed selectivity towards [1,j] B sigmatropic shifts ([1,3] and [1,7] B shifts are observed, but [1,5] B shifts are not) in terms of theoretical predictions gives reason to assume that the [1,j] boron migrations occur with inversion of configuration at the migrating atom.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/chem.19960021122
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