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  • General Chemistry  (77,716)
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  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 61-71 
    ISSN: 1434-193X
    Keywords: Binaphthyls ; Chirality ; Circular dichroism ; Dendrimers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The absorption and CD spectra of a series of Fréchet (compounds 7-10) and backfolding (compounds 11-12) dendrimers derived from enantiopure (S)-2,2′-dihydroxy-1,1′-binaphthalene have been recorded (THF) in the range 200-350 nm. All the compounds examined show a positive couplet between 200 and 240 nm (1B transition of the 2-naphthol chromophore), the intensity of which (Δεmax of the low-energy branch) ranges between 100 and 40. By means of the DeVoe polarizability model the intensity of the 1B couplet has been calculated vs. the dihedral angle θ. This analysis provides θ angles of 95-110° for the Fréchet dendrimers and 100-110° for the backfolding compounds. These values clearly indicate that the torsional angle θ, defined by the two naphthalene planes, never exceeds the critical value of 110°. This investigation confirms the wide utility of CD spectroscopy to provide geometrical information that cannot be obtained by other types of structural analysis.
    Additional Material: 5 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 51-60 
    ISSN: 1434-193X
    Keywords: Dithioles ; Anthracenes ; Lithiation ; Electron donors ; Crystal structures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The synthesis of new derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene has been achieved by two different routes. Deprotonation of 8 using LDA in THF at -78 °C, followed by in situ quenching of the lithiated intermediate 9 with N,N-dimethylformamide, N-methyl isothiocyanate and methyl chloroformate gave aldehyde, thioamide and methyl ester derivatives 10-12, respectively. Sulfur insertion into the lithiated species 9 followed by reaction of the transient thiolate anion with benzoyl chloride gave the thioester derivative 13 which served as a convenient shelf-stable precursor of other mono-functionalised derivatives of 8. Debenzoylation of 13 and trapping of the transient thiolate anion with iodomethane and 6-bromohexan-1-ol yielded 14 and 15, respectively. Reaction of cation salt 17 with the anion of anthrone 18 gave compound 20, the thiolate anion of which reacted with 6-bromohexan-1-ol to afford the alcohol derivative 21. Subsequent reactions gave alcohol derivative 25 of the title system. The unexpected product 29 was obtained from reaction of 28 with triethyl phosphite. The X-ray crystal structures of compounds 12, 14, 28, and 29 are reported. The molecules adopt a saddle-like conformation; the bis(1,3-dithiole)benzoquinone system is U-shaped through an ‘accumulating bend’ comprising the boat conformation of the central (quinonoid) ring, folding of both 1,3-dithiole rings along S···S vectors, and out-of-plane tilting of the exocyclic C=C bonds, all in the same (inward) direction.
    Additional Material: 3 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 543-548 
    ISSN: 1434-193X
    Keywords: Supramolecular chemistry ; Domino reactions ; Oligopyridines ; Dihydropyridines ; Mannich bases ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---An annelation reaction is presented in which 1,3-cyclohexanedione (1) and Mannich bases 6a,b are used for the preparation of functionalized bipyridines 12a, 13a, 14a and dihydropyridine derivatives 10b, 11b. All these products possess a keto group which will allow further transformations. The same concept was applied for the synthesis of the S-shaped terpyridine 25. The reaction of the Mannich base 21 with 1,3-cyclohexanedione yielded the heptacyclic terpyridine 22, which is a key intermediate for the synthesis of torands and other tridentate clefts. The ketone 12a was used for the synthesis of the quaterpyridine 26.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 557-561 
    ISSN: 1434-193X
    Keywords: Amino ketones ; Homogeneous catalysis ; Aziridination ; Copper ; Enols ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A series of acyclic and cyclic enol derivatives 1 has been transformed into the corresponding α-amino-functionalized ketones 2 by means of enantioselective catalytic aziridination with chiral Cu complexes, prepared in situ from [Cu(MeCN)4]PF6 and the optically active ligands 3, by using (N-tosylimino)iodobenzene (PhINTs) as a nitrogen source. The best enantioselectivities (ee values of up to 52%) have been achieved for the electronically deactivated enol acetate 1aδ, but the incorporation of steric bulk and the substitution pattern at the enol double bond do not improve the ee values. The cyclic substrates react considerably less readily (only up to 45% conversion) compared to their acyclic counterparts (complete consumption). A transition structure is suggested for the asymmetric Cu-catalyzed aziridination of the enol acetate 1aδ in the presence of the chiral ligand 3b that could account for the sense of the (R)-configured product 2a.
    Additional Material: 1 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 549-555 
    ISSN: 1434-193X
    Keywords: Alkylations ; Deaminated AdoHcy ; Decarboxylated AdoHcy ; Mitsunobu reaction ; Nucleosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---5′-Acetylthio-5′-deoxy-2′,3′-O-isopropylideneadenosine (8) was directly prepared from commercially available 2′,3′-O-isopropylideneadenosine (7) and thioacetic acid under Mitsunobu conditions in almost quantitative yield. In situ cleavage of the acetylthio function of 8 followed by coupling with different alkyl bromides proceeded with high yields. Deprotection of the obtained 5′-thionucleosides yielded the S-adenosyl-L-homocysteine analogues decarboxylated AdoHcy (11), deaminated AdoHcy (14) and 5′-[3-(cyano)propylthio]-5′-deoxyadenosine (16) in good overall yields. Direct deprotection of the thionucleoside 8 delivered 5′-thio-5′-deoxyadenosine (18) in excellent yield. In addition, binding constants of these AdoHcy analogues and the DNA methyltransferase M·HhaI were determined in a fluorescence assay.
    Additional Material: 1 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 573-575 
    ISSN: 1434-193X
    Keywords: Amino acids ; Peptides ; Protecting groups ; Protecting groups ; Aluminium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The aluminium trichloride/toluene system is investigated as a novel, unusual and straightforward reagent for the removal of the 9-fluorenylmethoxycarbonyl (Fmoc) protecting group in the solution peptide synthesis. This procedure avoids any undesired side reactions, such as the frequently observed inversion of the amino acid configuration.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 611-616 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Grignard reactions ; Dynamic kinetic resolution ; Asymmetric synthesis ; Unsymmetrical 1,2-diamines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A diastereoselective synthesis of tert-butyl-1,2-diamines has been developed from the addition of tert-butylmagnesium chloride to the 1,2-bisimines derived from glyoxal and chiral amines such as 1-(S)-ethylphenylamine, 1-(S)-phenylpropylamine or 1-(S)-(p-chlorophenyl)ethylamine. The influence of solvent, temperature and chiral auxiliaries on the chemical reactivity and stereoselectivity has been fully studied. Evidence of a dynamic kinetic resolution during the bis-addition process of the organometallic, leading to the 1,2-di-tert-butylethanediamine,[1] as a single diastereomer, has been demonstrated. This resolution has been applied with high diastereoselectivities to the synthesis of unsymmetrical disubstituted 1,2-diamines, by addition of one equivalent of tert-butylmagnesium chloride, followed by one equivalent of a second Grignard reagent. Several chiral 1-tert-butyl-1,2-diamines have also been synthesized by monoaddition of tert-butylmagnesium chloride to the bisimines, followed by hydride reduction of the chiral intermediate imines.
    Additional Material: 1 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 633-638 
    ISSN: 1434-193X
    Keywords: Amide linkage ; Phosphodiester ; Hammerhead ribozyme ; Oligonucleotides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A nucleoside dimer in which the natural phosphodiester bond is replaced by an isosteric amide linkage has been prepared. This dimer analogue was subsequently incorporated chemically at the cleavage position of a hammerhead ribozyme substrate. Although the resulting substrate analogue exhibited a high affinity for the ribozyme as shown by gel retardation assays, the amide bond proved to be fully resistant to cleavage under standard conditions of ribozyme cleavage activity.
    Additional Material: 3 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 657-664 
    ISSN: 1434-193X
    Keywords: Photoinduced electron transfer ; Enantioselective syntheses ; Amino acids ; α-Hydroxy acids ; (S)-Prolinol ; Recyclable chiral auxiliary ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---PET activation of 1-(N-benzyl-N-methylglycyl)-(S)-prolinol (1) in dry acetonitrile, utilizing 1,4-dicyanonaphthalene (DCN) as a light-harvesting electron-acceptor and methyl viologen (MV++) as an electron-transfer mediator, leads to the formation of 3-[benzyl(methyl)amino]perhydropyrrolo[2,1-c][1,4]oxazin-4-one (3). When this photolysis is carried out in aqueous acetonitrile, exclusively 3-hydroxyperhydropyrrolo[2,1-c][1,4]oxazin-4-one (4) is produced. The formation of 3 can be rationalized in terms of intramolecular cyclization of the in situ generated iminium cation intermediate (2) by the OH moiety of (S)-prolinol, while 4 is generated by hydrolysis of 2 followed by acetalization. Nucleophilic alkylation of 3 and 4, using Grignard reagents and allyltrimethylsilane/TiCl4, provides 12a-d & 15 and 17a-c & 21, respectively, in a highly stereoselective manner. Hydrolysis of the resultant amides (12, 15, 17, and 21) provides α-amino acid derivatives (14) and α-hydroxy acids, respectively, in optically active form, along with the recovered (S)-prolinol chiral auxiliary in its recyclable form.
    Additional Material: 1 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 675-680 
    ISSN: 1434-193X
    Keywords: Thiadiazolidinones ; Spiro compounds ; Thiazoles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The design, synthesis and evaluation of a series of 1,2,4-thiadiazolidinones with potential muscarinic receptor binding properties has been performed. During the synthesis of the target compounds, we observed an interesting reductive cleavage of the thiadiazolidinone system which leads to the formation of the novel piperidine spiro triazine heterocycle. The synthesis, structural elucidation (NMR spectroscopy and X-ray diffraction) and biological evaluation of the new compounds are described. With the structures unequivocally established, a mechanism for the formation of the spiro compound is proposed.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/099462_s.pdf or from the author.
    Additional Material: 3 Ill.
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  • 11
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 691-693 
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---No Abstract.
    Type of Medium: Electronic Resource
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  • 12
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 681-689 
    ISSN: 1434-193X
    Keywords: Pauson-Khand reaction ; Cyclopentenones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---1-Methyl-norbornene ester 9 and 1-methyl-2,3-diazabicyclo[2.2.1]heptene ester 10 were employed in intermolecular Pauson-Khand reactions with various terminal alkynes 11a-f to give the dimethyl 1-methyltricyclo[5.2.1.05,9]dec-7-en-6-one 2,3-dicarboxylates 12 and 13, and diethyl 2,3-diaza-1-methyltricyclo[5.2.1.05,9]dec-7-en-6-one 2,3-dicarboxylates 14 and 15, respectively. Whereas the co-cyclization of norbornene 9 with alkynes 11 bearing small substituents R resulted in the preferred formation of 12 (12:13 ≤ 85:15), regioisomer 13 was obtained as the major product when sterically bulky alkynes were employed (12:13 ≥ 6:94). For 2-methyl-3-butyn-2-ol 11e a strong temperature dependency of the regioselectivity was found. The ratio of regioisomers (12e:13e) changed from 95:5 at -25 °C to 12:88 at 120 °C in toluene. In contrast, reactions with 2,3-diazanorbornene 10 showed only moderate regioselectivities in favour of 14 (14:15 ≤ 69:31), regardless of the temperature and the size of R. The observed regioselectivities support a mechanism for the Pauson-Khand reaction in which the apical rather than the basal anti oriented carbon monoxide ligand of cobalt alkyne complex 1 is replaced by the alkene.
    Additional Material: 1 Ill.
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  • 13
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 275-280 
    ISSN: 1434-193X
    Keywords: Radical reactions ; Substituent effects ; Nitrogen heterocycles ; Dihydropyridines ; Pyrrolines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A set of allene-tethered benzoyloximes (5) has been treated with nBu3SnH. Depending on their substitution pattern, a wide range of compounds has been obtained. If the stannyl radical adds on the allene, the C-centred radical thus formed undergoes either a 5-exo ring closure to give the cyclopentene derivatives 7 or a 6-endo ring closure onto the N atom to give the dihydropyridines 8. If the stannyl radical adds on the benzoyl moiety, an iminyl radical is formed which leads to the 3H-pyrroles 9 and the alkylidene-pyrrolines 10. Steric effects as well as polar effects are the factors governing the reaction course.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/99373_s.pdf or from the author.
    Additional Material: 1 Ill.
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  • 14
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 281-289 
    ISSN: 1434-193X
    Keywords: Phosphorus acid amphiphiles ; (α-Aminoalkyl)phosphonic acids ; Carboxyalkyl (α-aminoalkyl)phosphonic acid monoesters ; Spiro compounds ; Pudovik reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The addition reaction between the P-H bond of tetraoxyspirophosphoranes 1-2 and long-chain imines 3a-h (decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, and oleyl imines) occurs instantaneously at room temperature. It is diastereoselective, and quantitatively leads to the corresponding (α-aminoalkyl)spirophosphoranes 4a-h and 5e. The influence of the pentacoordinated phosphorus atom on the stereoselectivity of the Pudovik reaction might be attributed to the involvement of the rigid spirophosphoranide (PV) intermediate in the addition reaction. Selective and one-pot hydrolysis of these P-C bond spirophosphoranes readily proceeds either at room temperature in the presence of moist solvents to give the corresponding carboxyalkyl (α-aminoalkyl)phosphonic acid monoesters 6a-h and 7e, or the reaction may be carried out in the presence of 20% aqueous hydrochloric acid under reflux, to afford the free (α-aminoalkyl)phosphonic acid amphiphiles 8a-h in high yields. In contrast to their sodium salts, these single- and double-chained free and monoester phosphonic acid amphiphiles exist as zwitterions and are not soluble in water.
    Additional Material: 2 Tab.
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  • 15
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 807-811 
    ISSN: 1434-193X
    Keywords: Proline analogues ; Asymmetric synthesis ; Schöllkopf's method ; Silicon ; Amino acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The asymmetric synthesis of a new proline surrogate, incorporating the dimethylsilyl group at position 4 of proline using Schöllkopf's bis-lactim ether method, is described.
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  • 16
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 763-785 
    ISSN: 1434-193X
    Keywords: Bisdiazenes ; Homoconjugation ; Photochemistry ; Heterocycles ; Diazenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Rigid N=N/N=N (diazene/diazene) systems (F) consisting of more or less alkylated DBH and DBO chromophoric units (1, 2, X-ray structures), with very short π,π distances [d = 2.849 (1a, av.), 2.822 Å (2)] and almost perfect syn-periplanar π,π alignments [ω = 168.6 (1a), 174.2° (2)] as well as the more flexible, less “proximate” metathesis isomers (3a,c, 27a,c, d 〉4.6 Å, ω = 90-100°) have been synthesized. Homoconjugate π,π interaction (in 1, 2, not in 3, 27) is deduced from UV spectroscopic measurements [π → π* maxima at 239 (234) nm (sh, 260)], while PE analyses furnished only small interaction parameters (1a: 〈0.3 eV). The potential of the novel syn-periplanar N=N/N=N motif in 1 and 2 for the synthesis of somewhat exotic polyheterocycles has been explored by calculation (B3LYP) as well as experimentally: i.a. kinetically stabilized, all-cis-peralkylated tetrazolidines (38, 44) and perhydro-1,2,4,5-tetrazines (41, 47) have become accessible (i.a. via novel azomethine/diazene and azomethine/azomethine cycloadditions). In 1a with its unreactive DBO chromophoric subunits, in the “buttressed” derivatives 1b-d, as well as in the DBH/DBO combination 2, and likewise in more ‘distant’ 27 (differently from the analogous C=C/C=C and N=N/C=C systems), irrespective of the excitation conditions employed (light of λ ≥≥ 280, 254 nm, low temperature matrix irradiation, acetone sensitization) no [2+2]photocycloaddition was observed. Instead exclusively N2-elimination took place. It is argued that unproductive N=N/N=N photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.
    Additional Material: 9 Ill.
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  • 17
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 829-836 
    ISSN: 1434-193X
    Keywords: Astaxanthin ; Carotenoids ; Isotopic labeling ; Isotopic labeling ; Total synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The all-E isomer of (4,4′-13C2)astaxanthin (1a) has been prepared by total synthesis starting from commercially available 99% 13C enriched acetonitrile. The labelled astaxanthin was obtained in high purity and with high isotope incorporation. For this synthesis, the C15 + C10 + C15 strategy was used. The central C10-synthon, 2,7-dimethylocta-2,4,6-triene-1,8-dial (3), was coupled with 13C-enriched C15-phosphonium salt 2a. The new synthetic scheme for the preparation of the C15-phosphonium salt is discussed in this paper; the same scheme can be used to label all positions and combinations of positions of the C15-phosphonium salt.
    Additional Material: 1 Tab.
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  • 18
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 583-589 
    ISSN: 1434-193X
    Keywords: Porphyrins ; Chiral auxiliaries ; Macrocycles ; Pyrethroids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Esters, N,N-disubstituted amides, and a N-acylurea derived from the enantiopure industrial intermediate (1R)-cis-hemicaronaldehydic acid (or biocartol) are convenient synthons for the preparation of a series of chiroporphyrins by condensation with pyrrole. These chiral meso-tetracyclopropylporphyrins are obtained exclusively as the D2-symmetric α,β,α,β atropisomer, generally in low to moderate yields (2-20%), and in the urea case in excellent yield (60%). Hydrolysis of the urea substituents affords a chiroporphyrin with mono-N-substituted amide groups. 1H-NMR spectroscopy indicates that the ester, amide, and urea stereogenic groups sit on the porphyrin close to the metal binding site and restrict substrate or ligand access along a C2-symmetric groove. This structural feature of chiroporphyrins and of their metal complexes is of high potential interest in asymmetric catalysis and chiral recognition.
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  • 19
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 603-609 
    ISSN: 1434-193X
    Keywords: Benzopyranones ; (E)-2-Nonenal ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The bright orange-red color of the toadstool Tricholoma aurantium is due to the benzotropolone pigment aurantricholone (1). The compound is at least partially present as the calcium salt. Minor pigments are the yellow 2H-furo[3,2-b]benzopyran-2-one derivatives aurantricholides A (7a) and B (7b), which exhibit strong green fluorescences. Their structures have been established by total syntheses.
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  • 20
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 591-602 
    ISSN: 1434-193X
    Keywords: Cyclophanes ; Molecular recognition ; Molecular shuttles ; Rotaxanes ; Template-directed synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Two dumbbell-shaped compounds (8 and 11), each composed of two polyether chains intercepted by a 1,4-dioxybenzene recognition site, terminated by tetraarylmethane-based stoppers, and emanating from a central 9,10- or a 2,6-dioxyanthracene unit, have been synthesized. Two [2]rotaxanes (9 · 4 PF6 and 12 · 4 PF6) have been prepared by interlocking these dumbbell-shaped compounds (8 and 11) with a bipyridinium-based tetracationic cyclophane (15 · 4 PF6) - namely, cyclobis(paraquat-p-phenylene). A [3]rotaxane (10 · 8 PF6) incorporating two cyclophane components (15 · 4 PF6) was also obtained when the 9,10-dioxyanthracene-containing dumbbell-shaped compound (8) incorporating two 1,4-dioxybenzene recognition sites was employed. The 1H-NMR spectroscopic investigation of the [2]rotaxanes (9 · 4 PF6 and 12 · 4PF6) revealed that the cyclophane component encircles one of the two 1,4-dioxybenzene recognition sites in the 9,10-dioxyanthracene-containing [2]rotaxane (9 · 4 PF6) and the 2,6-dioxyanthracene unit in the other [2]rotaxane (12 · 4 PF6). These structures have been confirmed by UV/Vis and electrochemical experiments. Comparison with the spectroscopic properties of simple model compounds shows the presence of electronic interactions which lead to (i) the occurrence of very efficient energy transfer processes in the dumbbell-shaped components and (ii) perturbations in the absorption spectra with appearance of two charge-transfer absorption bands and complete luminescence quenching in the [2]rotaxanes. For the 2,6-dioxyanthracene-containing [2]rotaxane (12 · 4 PF6), it has been demonstrated that the cyclophane can be displaced from the dioxyanthracene to the 1,4-dioxybenzene station upon electrochemical oxidation.
    Additional Material: 9 Ill.
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  • 21
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 627-631 
    ISSN: 1434-193X
    Keywords: Complex nucleoside ; Antibiotics ; Radicals ; Zn-Cu couple ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Homosinefungin 5, which can be considered as an analogue of S-adenosylmethionine (SAM) and of S-adenosylhomocysteine (SAH), has been synthesized by means of a sequence in which the key step was the addition of a radical, produced by the simple treatment of an iodide precursor with a zinc-copper couple, to suitably activated olefins.
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  • 22
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 617-626 
    ISSN: 1434-193X
    Keywords: Radicals ; Photochemistry ; Aliphatic ethers ; Hydrogen abstraction ; Urazolyl radical ; α-Alkoxy-alkyl radical ; Diones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The photoaddition of 4-methyl-1,2,4-triazoline-3,5-dione (4-MTAD) with a wide variety of acyclic, cyclic and crown aliphatic ethers has been investigated. Monochromatic (λ = 514.5 nm) or polychromatic (λ ≥ 310 nm) irradiations give identical mono-urazolyl ethers as reaction products. Unsymmetrical acyclic ethers afford a mixture of the two α and α′ mono-urazolyl ethers. In the case of 12-crown-4, mono and di-substituted products are obtained. ESR experiments and quantum calculations at the AM1 and 6-31G* levels were performed and a possible reaction mechanism is proposed in which the most probable photochemical process is the H-abstraction leading to a urazolyl radical.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/099269_s.pdf or from the author.
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  • 23
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 639-643 
    ISSN: 1434-193X
    Keywords: Solid-phase synthesis ; Wang resin ; Hetero Diels-Alder reactions ; Dihydropyrans ; Reductive reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The OH groups of Wang resin were esterified with benzylidenepyruvic acid (1) to give the immobilized 1-oxabutadiene 2. The latter reacted with vinyl ethers 3a-h (dienophiles) in the presence of Eu(fod)3, and the resulting adducts 4a-h underwent reductive cleavage with LiAlH4 to afford the dihydropyrans 5a-h in high (62 to 100%) overall yields. A similar sequence carried out under conventional homogeneous liquid phase conditions led to significantly lower yields. The endo/exo selectivity of the cycloaddition reaction was the same in both cases.
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  • 24
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 645-655 
    ISSN: 1434-193X
    Keywords: INOC ; Pyrrolidines ; Piperidines ; Stereoselective synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Intramolecular nitrile oxide olefin cycloaddition (INOC) reactions of oximes 1-3 and of 24-27 derived from L-amino acids have been found to proceed stereoselectively, yielding tricyclic fused pyrrolidines and piperidines. Further manipulation led to chiral hydroxymethyl-substituted fused piperidines 33-35 and to 3-amino-4-(1-hydroxypropyl)-2-mercaptomethyl-N-methylpiperidine (36). The structures and stereochemistries of the fused systems, as well as those of the piperidines, have been established by NMR.
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  • 25
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 665-673 
    ISSN: 1434-193X
    Keywords: Cationic amphiphiles ; Transfection ; Pyridinium salts ; Vesicles ; Lipoplex ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Pyridinium amphiphiles have found practical use for the delivery of DNA into cells. Starting from 4-methylpyridine, a general synthesis has been devised for the production of pyridinium amphiphiles which allows variation in both the hydrophobic part and in the headgroup area of the compounds. By means of differential scanning microcalorimetry, zeta potential, particle size measurements and cryo electron microscopy, some characteristics of the pyridinium amphiphile/DNA complexes have been determined.
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  • 26
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1019-1025 
    ISSN: 1434-193X
    Keywords: Silver ; Methylene-oxolanes ; O-Heterocycles ; Enol ethers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Silver salts which have a basic counter-ion are efficient catalysts for the regiospecific intramolecular addition of various acetylenic alcohols. Silver carbonate in aromatic solvent proved to be the best catalyst. Alkynols in which the two reacting parts of the molecule are relatively close together in space required only catalytic amounts of silver ion, while others cyclize readily with stoichiometric quantities. This heterocyclization reaction provides a mild and convenient access to 2-methylene-oxolanes or oxanes.
    Additional Material: 3 Tab.
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  • 27
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1037-1043 
    ISSN: 1434-193X
    Keywords: Electron transfer ; Radical ions ; Oxidations ; N-Dealkylation ; Aziridines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Excess N-benzyl aziridine (1) reacts with I2 to afford dimer 2, tetramer 3, benzaldehyde (4), and iodoamine 5. The reaction is interpreted as occurring by both electron transfer (ET) and heterolytic mechanisms. An ET mechanism is substantiated for the oxidation by I2 of dimer 2 and tetramer 3, both being substrates easier to oxidise by electron abstraction than 1. Several auxiliary reactions were performed on 1 in order to firmly establish the boundaries to the competition between the ET and heterolytic mechanisms. For the reaction of 1 with 5 a reaction scheme is proposed; in a particular case, a pseudo-first order kinetic law is followed.
    Additional Material: 1 Tab.
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  • 28
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1051-1059 
    ISSN: 1434-193X
    Keywords: Chirality ; Dendrimers ; Fullerenes ; Lipofullerenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The facile synthesis of enantiomerically pure C3-symmetrical fullerene dendrimers all-R-fA-7, all-S-fC-8, all-R-fC-9, and all-S-fA-10, as well as lipofullerenes all-S-fA-11 and all-S-fC-12 involving an octahedral [3:3] addition pattern is described. The C3 symmetrical tris[bis(4-phenyl-2-oxazoline)methano]fullerenes 1, 2, 3, and 4 with known absolute configuration were used as precursors. The completion of the octahedral addition pattern was achieved by a threefold cyclopropanation with the dendritic 3,5-dihydroxybenzylic bromomalonate 5 and the lipophilic dioctadecyl bromomalonate 6. The absolute configurations of the precursor trisadducts were retained in the inherently chiral C3-symmetrical hexakisadducts 7-12. The structure and the isomeric purity of all hexakisadducts 7-12 were unambiguously proven by NMR spectroscopy and other techniques. Preliminary experiments with the dendritic hexakisadducts all-S-fC 8 and all-S-fA 10 revealed that these macromolecular ligands posess catalytic properties with respect to stereoselective cyclopropanations of styrene with ethyl diazoacetate.
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  • 29
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1081-1090 
    ISSN: 1434-193X
    Keywords: Atropisomerism ; Conformation analysis ; Liquid chromatography ; Chiral resolution ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A series of bis(oxo/thioxothiazolinyl) aromatic atropisomers with two chiral C(aryl)-N(heterocycle) axes was synthesized. The separation of antiparallel, parallel diastereomers was achieved by chromatography on silica. Conformational assignment for the bis-thione atropisomers 1a,b-4a,b and bis-one atropisomers 9a,b-12a,b was based on polarimetric results in analytical chiral chromatography on microcrystalline cellulose triacetate. The relationship conformation-1H NMR chemical shifts, found for the bis-thiones and bis-ones was successfully applied for the conformational assignment of thione-one atropisomers not assigned by chiral liquid chromatography.
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  • 30
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1065-1076 
    ISSN: 1434-193X
    Keywords: Cyclodextrins ; Inclusion compounds ; Carcinine ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A new modified β-cyclodextrin (β-CD) derivative 1 that was functionalized in position 6 with Boc-Carcinine was synthesised and its crystal structure was determined. The structure reveals a “sleeping swan”-like shape, the covalently bonded Boc-Carcinine moiety forming a folded structure with the Boc group inserted within the hydrophobic cavity of the β-cyclodextrin. The conformation of the Carcinine moiety is determined by the inclusion of the Boc group and is further stabilised by three intramolecular hydrogen bonds, two between the amide N1-H group, the carbonyl C′1=O1 group and a primary hydroxylic group of the glucose unit 5, one between the carbonyl C′0=O0 group and the primary hydroxylic group of the glucose unit 2. The β-CD macrocycle differs only slightly from unmodified β-CDs, maintaining an approximate sevenfold symmetry. The solution structure of the new β-CD derivative was investigated by NMR spectroscopy and circular dichroism (c.d.) spectroscopy. In addition to a complete (1H and 13C) assignment of the pendant Boc-Carcinine group, the NMR study allowed the assignment of all the proton resonances associated with the β-CD macrocycle. Furthermore, NMR and c.d. results indicated that the self-inclusion of the Boc group within the β-CD cavity is retained in aqueous solution. In order to estimate the strength of this self-inclusion complex a series of competition experiments with the external guest 1-adamantanol was carried out using c.d. spectroscopy.
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  • 31
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1113-1120 
    ISSN: 1434-193X
    Keywords: Peptidomimetics ; Reverse turn mimetics ; Glycomimetics ; Heterocycles ; Lactams ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -D-Glucurono-3,6-lactone and L-cysteine combine in a highly stereoselective manner to give the 7,5-bicyclic thiazolidinlactam 2. The α-hydroxy group of the D-glucurono-3,6-lactone was exchanged for an amino function (to give 13) and, after condensation with L-cysteine methyl ester, the polyol dipeptide 7 was obtained. Peptide couplings proceed without the need to protect the three secondary hydroxy groups of the seven-membered ring. The amino group of 7 was deprotected and selectively elongated to the pseudo-tripeptide 16. The depsipeptide 17 was obtained by condensation of Boc-Ala-OH with the polyol 2. Elongation at the carboxy terminus yielded 19 and 20. The bicyclic scaffold populates a well-defined solution conformation; the hydroxy groups mimic the side chains of hydrophilic amino acids and can be further functionalized.
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  • 32
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1165-1171 
    ISSN: 1434-193X
    Keywords: Porphyrinoids ; Asymmetric catalysis ; Atropisomerism ; Epoxidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -A new series of porphyrins has been synthesised, incorporating four identical chiral binaphthyl derivatives in the meso-positions. Owing to a hindered rotation around the bond between the naphthyl and the porphyrin, four atropisomers are generated, which were fully separated by preparative TLC and thoroughly characterised. The free bases were metallated with iron(III) and manganese(III) and the resulting complexes were used as catalytic precursors in styrene epoxidation. The reactions show chemoselectivity and enantioselectivity, depending on the stereochemistry of the metalloporphyrin. It is demonstrated that highest efficiency is performed by the ααββ isomer, showing C2 symmetry.
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  • 33
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1193-1197 
    ISSN: 1434-193X
    Keywords: Porphyrinoids ; Light-harvesting structures ; Energy transfer ; Nanostructures ; Photochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The synthesis of niphaphyrins, a new class of rigid, snowflake-shaped porphyrin hexamers designed to mimic the light-harvesting antenna complexes of photosynthetic purple bacteria, is described. In these D6- or D3-symmetric assemblies, six porphyrin macrocycles, as free bases or as zinc chelates, are covalently attached to the six positions of a benzene core through rigid ethynyl linkers. A very efficient singlet excited-state energy transfer has been observed from the Zn chelates to the free-base porphyrin (Fb) chromophores in a niphaphyrin in which three zinc porphyrin chelates alternate with three porphyrin free bases. The overall yield of energy transfer (ΦET) has been estimated as 98%.
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  • 34
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 963-974 
    ISSN: 1434-193X
    Keywords: Indiumallyl compounds ; Vinylcyclopropanes ; α,β-Unsaturated carbonyl compounds ; Diastereoselectivity ; Dibenzylideneacetone ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Allylindium reagents, prepared from excess allylic halide (Br or I) and indium metal, react with α,β-unsaturated ketones and aldehydes to give, after aerobic acidic workup, homoallyl-substituted vinylcyclopropanes. This process was explored and developed after a chance discovery arising from a side reaction in an attempted Pd-catalysed process. The structure of the cyclopropane arising from the reaction of bis(p-chlorobenzylidine)acetone was confirmed by X-ray crystallography. Whilst bis-α,β-unsaturated ketones give rise to a single homoallylcyclopropane species, α,β-unsaturated ketones and aldehydes give diastereomeric mixtures whose relative stereochemistry were assigned by NOE experiments. Crotylindium reagents react with good to perfect regioselectivity to afford tetrasubstituted cyclopropanes but prenylindium reagents fail to generate the analogous pentasubstituted rings.
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  • 35
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1235-1239 
    ISSN: 1434-193X
    Keywords: Nucleophilic additions ; Nucleophilic substitutions ; Azabutadienes ; Δ2-Oxazoline ; Azabutenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The reaction of some sodium alkoxides with 4,4-dichloro-1,1-diphenyl-2-azabuta-1,3-diene is described. Whereas sodium methoxide, ethoxide or isopropoxide leads to 1,3-bis(alkoxy)- and/or 1,3,4-tris(alkoxy)-2-azabut-2-enes, the sodium salt of ethyl glycolate gives a Δ2-oxazoline. Mechanisms for the formation of these products are proposed.
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  • 36
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1253-1262 
    ISSN: 1434-193X
    Keywords: Natural products ; Starfish ; LC-NMR-MS hyphenation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -LC-NMR-MS hyphenation has been applied in the screening of an asterosaponin subfraction obtained from the starfish Asterias rubens for unknown compounds. The suitability of this technique for characterizing compounds in the molecular weight range of 1200 to 1400 a.m.u. has been demonstrated. The additional information concerning the number of exchangeable protons provided by the LC-NMR-MS experiment proved to be particularly valuable for the structural elucidation. The LC-NMR-MS screening indicated the presence of hitherto unknown asterosaponins in the sample. Based on this information, four new compounds (ruberosides A-D; 2-4 and 6) have been isolated and their structures have been elucidated using one- and two-dimensional NMR techniques. A minor component has tentatively been assigned the structure 5.
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  • 37
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Coordination chemistry ; Inorganic rings ; O ligands ; Structure elucidation ; Tin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of cupferron [NH4L, L = PhN(O)NO-] with tin(IV), diphenyltin(IV) and dimethyltin(IV) halides yields the cupferronato complexes SnL4 (1), Ph2SnL2 (2) and [Me2SnL2]2 (3). All were characterized by FT-IR, FT-Raman, NMR (1H, 13C and 119Sn) spectroscopic methods and X-ray structural analysis. An almost ideal, very rare dodecahedral geometry is found in the structure of 1. The hexacoordinated tin center exhibits a bicapped tetrahedral geometry in complex 2. The tin centers are heptacoordinated in a pentagonal bipyramidal geometry in the dimeric complex 3. These complexes incorporate five-membered SnO2N2 and four-membered Sn2O2 inorganic rings. Vibrational spectral data are consistent with the structures determined. The 119Sn NMR spectra indicate that in solution 1 retains its octacoordinated nature; in compound 2 the tin atom is hexacoordinated, while in 3 the tin atom is pentacoordinated.
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  • 38
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Metallomesogens ; Liquid crystals ; Silver ; N ligands ; Amines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The liquid crystalline properties of a series of ionic silver complexes [Ag(NH2n-CnH2n+1)2]X (X = NO3, n = 6, 8, 10, 12, 14; X = BF4, n = 8, 10, 12, 14), derived from silver nitrate or tetrafluoroborate and aliphatic amines, have been investigated by optical microscopy, DSC, and X-ray diffraction. The materials exhibit a birefringent fluid phase identified as a smectic A mesophase by optical microscopy and X-ray diffraction. Some of the complexes are liquid crystalline at temperatures close to room temperature. The X-ray results are consistent with the existence in the mesophase of a bilayer organization. The silver cations adopt a U-shape, which allows for the adoption of this bilayer arrangement, with alternating ionic and apolar regions and a short-range square-planar array inside the cationic sublayers. The thermodynamic parameters suggest that the melting transition clearly involves only disorder (melting) of the chains, whereas the clearing transition implies breaking the anion-cation arrangement.
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  • 39
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Thiolato-bridged complexes ; Iron ; Ruthenium ; Mixed-valent compounds ; Mössbauer spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of [FeIIIL] with [RuIICl2(dmso)4] in methanol under argon, affords, upon addition of NaPF6, brown crystals of [LFeRuFeL](PF6)2· 3 CH3OH (1). Its one-electron oxidation product [LFeRuFeL](PF6)3 (2) [L = 1,4,7-(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane] was generated by using one equivalent of [NO]BF4. The crystal structure determination of [LFeRuFeL](BPh4)2· CH3CN · 3 (CH3)2CO shows that 1 contains a trinuclear, face-sharing thiolato-bridged dication. Mössbauer spectroscopy established that the oxidation state distribution of 1 can best be described by Fe2.5RuIIIFe2.5 and 2 by FeIIIRuIIIFeIII. Complex 1 possesses a diamagnetic (St = 0) and 2 an St = 1/2 ground state. The electronic structures of 1 and 2 are rationalized by an MO picture involving Fe-Ru metal-metal bonds of bond order 0.5 in both species. Complex 1 is formally a mixed valence compound with complete delocalization of the “excess electron” (class III).
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  • 40
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Aluminum ; Ab initio calculations ; Isomers ; Intramolecular donor-acceptor systems ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three aluminum compounds Ar*AlBr2 (1), Ar*AlBrtBu (2), and Ar*2AlBr (3), incorporating the chiral ligand Ar* = 2-[1-(S)-Me2NCH(Me)]C6H4, have been synthesized by salt metathesis reactions and have been characterized by multinuclear NMR (1H, 13C, and 27Al), mass spectrometry, and elemental analysis. The molecular structure of compound 3 has been determined by a single-crystal X-ray analysis (space group P212121). Alane 2 exists as a temperature-independent equilibrium mixture of two diastereomers in solution (ratio 45:55); results of NOE experiments on 2 are discussed. Only one isomer was found for compound 3 in solution and its dynamic behavior has been investigated by variable-temperature NMR methods; a ΔH of 56.6(4) kJ mol-1 and a ΔS of -0.79(1) J K-1mol-1 were deduced from band-shape analysis of the 1H-NMR signals of the N-bound methyl groups.
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  • 41
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Macrocycles ; Nickel ; Redox chemistry ; Schiff bases ; S ligands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A single crystal X-ray analysis of [Ni2L1](ClO4)2· MeCN · 1/4 H2O, 1a [formed directly from a mixture of nickel(II) template ions, 2,6-diformyl-4-methyl-thiophenolate, and 1,4-diaminobutane] reveals that the nickel(II) ions are in square-planar N2S2 environments and that the four “bowed” dinickel macrocycles in the asymmetric unit pack around a single central perchlorate template ion encapsulating it to form “star” clusters of stoichiometry {[Ni2L1]4(ClO4)}7+. These “stars” stack together, via π-π-stacking interactions, to form two-dimensional sheets, which are separated from one another by layers of the remaining perchlorate anions and solvent molecules. Reduction, by NaBH4, of the four imine bonds in [Ni2L2](ClO4)22a (analogous to 1a but formed from 1,3-diaminopropane not 1,4-diaminobutane) or [Ni2L2](CF3SO3)22b to amine bonds produces the corresponding tetra-amine complex, [Ni2L3](ClO4)23. These complexes are shown to contain diamagnetic nickel(II) ions by a combination of magnetic, NMR and UV/Vis spectroscopic results. The 1H NMR spectra of 1-3 run in [D3]MeNO2 and in [D3]MeCN are consistent with increasing axial binding ability in the order: 3 〈 2 〈 1. Thiocyanate ion binding studies reveal that 1 and 2 are able to coordinate two thiocyanate ions, forming [Ni2L1(NCS)2] 4 and [Ni2L2(NCS)2] 5 respectively, whereas 3 does not. Single crystal X-ray analyses of complexes 4· 2 MeCN and 5· MeCN show that adjacent square-planar and octahedral nickel(II) ions result. Two one-electron oxidations and two one-electron reductions are a feature of the electrochemistry of 1-3 in MeCN: curiously, the potentials for the oxidation processes are almost invariant whereas those for the reduction processes vary as anticipated. EPR spectroscopy shows that the first one-electron reduction process and the first one-electron oxidation process are metal centred. Spectroelectrochemical studies and redox titrations indicate that a purplish-coloured complex is produced by one-electron oxidation of 2 (λ = 870 nm, ε = 1320 L mol cm-1). The synthesis of a phenolate analogue, [Ni2L′(MeCN)4](ClO4)2 (6), of the thiophenolate complex 2a is also detailed. Complex 6 undergoes two one-electron oxidations in MeCN, but, in contrast to the thiophenolate complexes 1-3, these occur at much higher potentials. Only a single one-electron reduction process is observed and this occurs at a more negative potential than for any of 1-3.
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  • 42
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Rare earth complexes ; Naphthalocyaninato ligand ; Double-decker complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of rare earth(III) double-deckers RE[Nc(tBu)4]2 [RE = La, Ce, Pr, Nd, Eu, Gd, Tb, Y, Er; Nc(tBu)4 = dianion of tetra(tert-butyl)-2,3-naphthalocyanine] (1-9) have been prepared by treating RE(acac)3·n H2O (acac = acetylacetonate) with 6-tert-butylnaphthalonitrile in refluxing n-octanol in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). These novel sandwich-type complexes have been spectroscopically characterized. The electrochemical studies show that the first oxidation and the first reduction potentials increase with the size of the central metal ions with a relatively small separation (0.28-0.33 V), reflecting the narrow HOMO-LUMO gap.
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  • 43
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Acid derivatives ; Cyclen and cyclam derivatives ; Phosphinic acid ; Potentiometry ; Protonation and stability constants ; 31P-NMR titration ; Hydrogen bonding ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two phenylphosphinic acid derivatives of tetraaza macrocycles, (1,4,7,10-tetraazacyclododecanetetrayl)tetrakis(methylene)tetrakis(phenylphosphinic acid) [(H4L1) and (1,4,8,11-tetraazacyclotetradecanetetrayl)tetrakis(methylene)tetrakis(phenylphosphinic acid)] (H4L2), were synthesised and the crystal structures of H4L2 and its bis(1-adamantylammonium) salt were determined. In both the structures, the ring conformation is virtually the same and is stabilised by hydrogen bonds. Protonation constants of the acids and stability constants of their Zn2+ and Cd2+ complexes were determined pH-metrically at 25 °C and at an ionic strength of 0.1 mol dm-3 (KNO3). The pKA values found for both the compounds are lower than those for their carboxylic analogues and correspond to the expected electron-withdrawing ability of the -P(Ph)O2H moiety. The value pK1 of the last dissociation constant of H4L1 is surprisingly high. The complexes of Zn2+ and Cd2+ with (L1)4- and (L2)4- are less stable than those with other similar ligands, which is in accordance with lower overall basicity of H4L1 and H4L2. A pronounced drop in the stability of the complexes of H4L2 can be explained by partial coordination of the ligand to the metal ions. The solution properties of the ligands were also investigated by 31P-NMR spectroscopy. The spectra observed suggest stable (on the NMR time scale) conformations of the protonated ligands in solution. A comparison of NMR spectra in solution and CP/MAS-NMR spectra indicates that the structures of the conformers roughly correspond to the structures observed in the solid state. The conformations are stabilised by intramolecular hydrogen bonds and by hydrophobic interactions of phenylphosphinic moieties. The conformers are stable even at 90 °C, but not in the presence of complexing metal ions.Supporting information for this article is available on the WWW under -//_/_http://www.wiley-vch.de/contents/jc_2005/2000/99149_s.pdf or from the author.
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  • 44
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Fluorine ; Silicon ; Ab initio calculations ; Donor-acceptor systems ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum chemical calculations at the ab initio level indicate that Lewis donors such as amine or water form weak donor-acceptor complexes with fluoro-substituted silanes. The strength of the donor-acceptor formation increases with increasing degree of fluorine substitution. Amine and water form strong adducts only for tetrafluorosilane. The higher element homologues of Lewis donors, such as phosphane and sulfur dihydride, do not in essence coordinate. A considerable role in donor-acceptor formation is exerted by chelation effects which impose intramolecular interactions on functionalized ligands. The corresponding structures are explored.
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  • 45
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Pyridyl carboxamides ; Carbonyl complexes ; Rhenium ; Metallotropic shifts ; NMR spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of rhenium(I) tricarbonyl halide complexes of pyridyl mono- and di-carboxamides and -thioamides has been studied by dynamic NMR techniques. Oxygen coordination to ReI of the carboxamide ligand reduced the energy barrier to C-N rotation by 2-13 kJ mol-1 whereas sulfur-ReI coordination of the thioamide ligands led to a reduction of 28-33 kJ mol-1. In the pyridyl dicarboxamide and -dithioamide metal chelate complexes metallotropic shifts occur between the three donor atoms, O, N, O or S, N, S, energies [ΔG
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  • 46
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Copper ; Oxygenase ; Copper-oxygen radical species ; RPY2 ligand ; Oxygenation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We describe the reactivity of new copper AlkylPY2 type complexes which react with dioxygen and thus hydroxylate an aliphatic C-H bond of the ligand in the same manner as copper-containing monooxygenases.
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  • 47
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Relaxivity ; Nuclear magnetic relaxation dispersion ; Imaging agents ; NMR spectroscopy ; MRI contrast agents ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dipolar interaction between the metal ion and proximate water molecules represents an efficient mechanism for solvent relaxation in Gd3+ complexes currently employed as MRI contrast agents. Besides inner sphere (metal bound) and outer sphere hydration molecules, a well-defined second coordination shell may provide an additional mechanism for paramagnetic relaxation leading to a strong enhancement of the relaxivity of the complexes. Through a careful choice of hydrogen-bond-acceptor groups on the ligand we may: (1) promote the formation of a strong interaction; (2) increase the number of water molecules in the second hydration shell; (3) decrease their average distance from the paramagnetic metal center. These possibilities have been explored by considering complexes bearing phosphinate, phosphonate and carboxoamide pendant arms, by exploiting the formation of ion-pairs with cationic substrates and inclusion compounds of these adducts with -cyclodextrin. Finally, the contribution of this relaxation mechanism to the relaxivity of the commercially available MRI contrast agents is discussed and the NMRD data reevaluated and compared with crystallographic data.β
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  • 48
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Pyrazole ; Titanium ; Redox reaction ; Mercury ; Amalgams ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of [L2TiCl2] (1) (L = 3,5-tert-butylpyrazolate) with 1% Na/Hg in tetrahydrofuran (THF) affords the complex [L2TiCl(THF)2] (2), which crystallizes from hexane in moderate yield. The analogous reduction carried out in toluene gives dimeric [(L2TiCl)2] (3) in high yield. If the reduction time is extended to 3 days, the complex [L3Ti] (4) is isolated as light blue crystals in low yield. An alternative procedure for preparing compound 4 requires mixing [TiCl3(THF)3] with 3 equivalents of potassium 3,5-tert-butylpyrazolate in toluene. Crystallization from hexane affords 4 in high yield. Crystallographic data for 2, 3 and 4 are presented.
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  • 49
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Arene complexes ; Ruthenium ; Benzimidazole ; Benzothiazole ; Benzoxazole ; Cycloisomerisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fourteen ruthenium(II) complexes of the type [RuCl2(η6-arene)L], (arene = 1,4-MeC6H4Pri or C6Me6; L = benzimidazole derivative in which the NH group is substituted by N-alkyl or isoelectronic O or S atoms) have been prepared by cleavage of [RuCl2(η6-arene)]2 with the N-heterocycle L. Their spectroscopic and electrochemical properties are described. The effect of the nature of the benzazole (L) on the catalytic activity of these complexes for the intramolecular cyclization of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-dimethylfuran has also been studied; the benzimidazole complexes were found to be the most active.
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  • 50
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Palladium ; Tripodal ligands ; O ligands ; Fluxionality ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of [PdCl2(CH3CN)2] with the silver salt of the tripodal oxygen ligand L- = [(C5H5)Co{P(O)(OMe)2)}3]- yields the homoleptic oxygen ligand complex [PdL2] (1). The molecular structure of 1 was determined by single-crystal X-ray diffraction: monoclinic space group P21/n; a = 8.515(1), b = 13.627(2), c = 15.828(2) Å; β = 92.44(1)°; V = 1834.9(7) Å3; Z = 2. The complexes [Pd(PPh3)XL] [X = Cl (2a), Br (2b), I (2c)] have been prepared starting from 1. Complex 2a reacts with PPh3 to form [Pd(PPh3)2L]Cl (3a) and [Pd(PPh3)2L]L (3b). Treatment of 2a with CO in MeOH yields the carboxymethyl complex [{(PPh3)(COOMe)Pd(μ-Cl)}2] (4) and the protonated ligand HL. The carboxymethyl complex 4 shows some catalytic activity for the copolymerization of CO and ethene. Complex 4 reacts with AgL to yield [Pd(PPh3)(COOMe)L] (6). Alkylation of 2a with Me4Sn leads to the methyl complexes [Pd(PPh3)MeL] (7) and [Me2ClSnL] (8). CO insertion in the palladium-carbon bond of 7 gives the stable acetyl complex [Pd(PPh3){C(O)Me}L] (10). The palladium(II) complexes 1, 2, 3, 6, 7, and 10 of the tripodal oxygen ligand L are all fluxional molecules that have been studied by temperature-dependent 1H- and 31P-NMR spectroscopy.
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  • 51
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Cages ; Synthesis ; Lithium ; Crystal structures ; Chromophores ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three new macrocyclic cages 1, 2 and 3, able to selectively encapsulate the lithium ion, were studied. The synthesis and characterization of 2 and 3 are reported. The basicity behaviour of the three cages was investigated by spectrophotometry and by 1H and 13C NMR spectroscopy. The protonation constants were determined by potentiometric methods in aqueous solution (I = 0.15 mol dm-3 NMe4Cl, T = 298 ± 0.1 K) and were found to be logK1 = 11.67(5), 10.77(7), 11.17(7); logK2 = 7.76(5), 5.7(1), 5.6(1); logK3 = 1.3(1), 2.4(1), 1.9(1) for 1, 2, and 3, respectively. The lithium equilibrium complex formation Li+ + L = LiLH-1 + H+ was monitored by 13C and 7Li NMR spectroscopy, as well as by spectrophotometric and potentiometric techniques; log values of -9.1(1) and -8.0(1) were found for 1 and 3, respectively. For 2, precipitation of the lithium complex occurs. UV/Vis studies in aqueous solution show a significant shift towards high energy of the λmax in the lithium complex, indicating the potential for application of these ligands in colourimetric analysis. The crystal structure of 1 was examined by single crystal X-ray diffraction. Data collection was performed on two samples, namely 1a and 1b, obtained from two different crystallization procedures. Crystals of 1a are monoclinic, space group P21/n, while 1b crystallized in an orthorhombic space group Pc21b. The cell parameters are a = 12.423(3), b = 14.378(9), c = 28.436(7) Å, β = 93.74(2)° for 1a, and a = 10.674(4), b = 14.408(2), c= 17.034(2) Å for 1b. Molecular simulations were carried out on both conformational isomers 1a and 1b, with results indicating that the two isomers have essentially the same flexibility.
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  • 52
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Asymmetric synthesis ; Chirality ; Computer chemistry ; Lithium ; Dilithiomethane ; Sulfoximines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and X-ray analysis of chiral dilithiomethane analogues derived from sulfoximines are presented. The structures of the sulfoximine-stabilised mono- and dilithio salts have been compared and allow a rationalization of the second lithiation. Organophosphorus-based dilithiomethane derivatives are also structurally described. The gas-phase structures of N,S,S-trimethylsulfoximine 19 and of its mono- and dilithiated isomers 20-21 were calculated by ab initio methods employing different levels of theory. For an application in asymmetric synthesis, the dilithiation of (N-methyl)-S-ethyl-S-phenylsulfoximine (11) afforded a chiral dinucleophile that undergoes highly regio- and stereoselective alkylation reactions with electrophiles. The transmetallation of a chiral dilithiomethane reagent such as 14 with ClTi(OiPr)3 followed by addition of an aldehyde, constitutes a simple one-pot process for the formation of (E)-alkenylsulfoximines with excellent diastereoselectivity.
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  • 53
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Cooperative effects ; Coordination modes ; Nickel ; Bimetallic complexes ; π interactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Different cooperative binding modes of nitriles within the bimetallic pocket of a pyrazolate-based compartmental dinickel(II) site have been studied. The H3O2-bridged dinuclear complex 1 reacts with cyanamide to yield 4, in which a secondary hydrogencyanamido(1-) bridge spans the two metal centers at an unusually short metal-metal distance imposed by the primary ligand matrix. In 5, a single 2-cyanoguanidine (cnge) molecule is N-bound to one nickel(II) ion through its nitrile part and is coordinated to the adjacent metal site through an amido nitrogen. The characteristics of the coordination spheres of the metal centers suggest an additional side-on π-bonding interaction of the nitrile moiety with the second high-spin nickel(II) ion. This unusual interaction is corroborated by comparing the IR bands for the ν(C≡N) stretching vibration of 5 with those of complex 6, which has two end-on bound cnge molecules, and those of the related mononuclear complex 7, which lacks a second nickel(II) ion. The nature of the π-bonding interaction in 5 is further analyzed by DFT calculations on relevant model systems. Even though the π-bonding is found to be very weak, it does include some backbonding from occupied 3d MOs at the second high-spin nickel(II) ion to the π* MOs of the nitrile. Such an unconventional π-interaction is suggested to be enforced by the constrained fixation of the nitrile unit within the highly organized coordination pocket of the bimetallic framework. In contrast, the bifunctional 2-hydroxybenzonitrile is accommodated by the distinct binding of the nitrile and phenolate functions to the different metal centers in 8, which confirms that the simultaneous binding of both an OR-function and an end-on bound nitrile is indeed feasible within the active site pocket. Such a situation is reminiscent of the bimetallic effect that has been assumed to enable the cooperative hydration of nitriles at the dinickel(II) site of 1. Complexes 4·(ClO4)2, 5·(ClO4)2, 6·(ClO4)3, 7·(ClO4)(BPh4), and 8·(ClO4)2 have been characterized structurally by X-ray crystallography.
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  • 54
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Carbocycles ; Clusters ; Cobalt ; Electrochemistry ; Facial ligands ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical reduction of the cluster complexes [Co4(CO)3(μ3-CO)3(μ3-C8H8)L2] 3 (L2 = C8H8), 4 (L = CO), 5 (L2 = C6H8) and 6 (L2 = 6,6-Ph2C6H4) gives the trinuclear anion [Co3(CO)3(μ2-CO)3(μ3-C8H8)]- (7) in high yield. It is proposed that formation of 7 occurs via degradation of the radical anions [3]- - [6]-, which can be generated reversibly via cyclic voltammetry. The anion 7 is stabilised by the facial C8H8 ligand and does not degrade further. Reaction of 7 with [(C5Me5)Ru(NCMe)3][BF4] results in the formation of [Ru(C5Me5)Co3(CO)3(μ3-CO)3(μ3-C8H8)] (8). The crystal structures of [NEt4]+-7, [(C5H5)2Co]+-7, and 8 were determined. In 7 and 8, the cyclooctatetraene ligand is coordinated to the Co3 face of the cluster in the facial mode.
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  • 55
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Ammonolysis ; Chlorine ; Nitrogen ; Silicon ; Silsesquiazane ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ammonolysis of trichlorosilanes RSiCl3 [R = CH(SiMe3)2 (1a), Ph (1b), Et (1c)] leads to a series of condensed products such as six-membered rings [(Me3Si)2CHSi(OH)NH]3 (2a) and [PhSi(NH2)NH]3 (2b), disilazane [(Me3Si)2CHSi(NH2)2]2NH (3) and the cage compound (EtSi)6(NH)9 (4). The mixed Si-N-O compound 2a was obtained when liquid ammonia was not dried over sodium prior to use. The reaction with sodium gives NaCl instead of NH4Cl as a by-product which is easily removed by filtration.
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  • 56
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Thiacalixarene ; Trimetallic sandwich ; Cobalt ; Zinc ; Divergent receptors ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction between deprotonated p-tert-butyltetrathiacalix[4]arene and CoII or ZnII under strictly defined conditions leads to the formation of sandwich species in which a trimetallic unit is held between two opposed, divergently oriented, staggered O- and S-bound calixarene ligands in the cone conformation. Despite the isomorphous nature of the crystalline compounds, there is a subtle difference in the coordination of CoII and ZnII, presumably related to the differences in metal radius and electron configuration combined with structural restrictions imposed by the calixarene framework, in that whereas all three Co atoms may be considered six-coordinate (and approximately octahedral), only two of the Zn atoms appear so, the third being five-coordinate and more nearly trigonal bipyramidal in its coordination geometry.
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  • 57
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Metallo-calixarenes ; Gold complexes ; Tetraphosphane ; Stability constants ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Assemblage of the tetragold complex [L·(AuCl)4] from p-tert-butyl-calix[4]-(OCH2PPh2)4 (L) and [AuCl(tetrahydrothiophene)] was investigated by NMR and UV/Visible spectroscopic titrations. Three steps could be resolved and are assigned, respectively, to addition of one, two and four AuCl units to the macrocyclic ligand. At each stage of metallation, symmetrical species persist on the NMR time scale since the bound AuCl fragments migrate between the different phosphorus centers via both intra- and intermolecular processes. The derived stability constants (logβ1 = 4.4 ± 0.2, logβ2 = 7.0 ± 0.7, and logβ4 = 14.5 ± 2.0) indicate both positive and negative cooperativity during build-up of the tetranuclear species. An X-ray diffraction study made with crystals obtained from an L:Au (1:2) mixture revealed the formation of an unusual binuclear complex whose solid state structure comprises a polymeric chain of [L·AuCl] units linked by pseudo-tetrahedral Au+ ions, each generic unit containing a pendant linear PAuCl fragment. This polymeric species, with its two distinct binding modes, provides a simple rationale by which to explain the role of cooperativity in the overall assembly process.
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  • 58
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Triosmium cluster ; Biphosphinine ; Crystal structure ; Cyclic voltammetry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structure of the novel cluster [Os3(CO)10(4,4′,5,5′-tetramethyl-2,2′-biphosphinine)] reveals an unprecedented coordination mode of the biphosphinine ligand that occupies a doubly-bridging position spanning the open osmium triangle. The cluster is photostable, pointing to a localization of the lowest-energy electronic transition largely on the negatively charged biphosphinine ligand.
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  • 59
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Photochemistry ; Electrochemistry ; Ruthenium compounds ; α-Diimine complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photophysical, photochemical and redox properties of the title complexes were investigated. Resonance Raman measurements revealed the lowest-energy electronic transition to possess σ→π* character. At low temperatures, long-lived near-IR emission was observed. Irradiation in solution results in homolytic splitting of a Ru-Ru bond as the primary step, followed by secondary reactions of the radical fragments depending on the experimental conditions. (Spectro)electrochemical investigations of the title species proved that the axial [RuCp(CO)2] groups exert a stabilising influence on the corresponding radical cations, while destabilising the corresponding radical anions, compared to the redox behaviour of other ruthenium complexes of this type.
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  • 60
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Crystal packing ; Neural networks ; Pattern recognition ; Organometallic compounds ; Close packing ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structures of hundreds of thousands of molecular compounds have now been determined with the quantitative information being stored in large databases. Chemists are generally interested in that specific part of this information which refers to individual molecules. The packing of molecules and their spatial relation to each other is not the focus of interest, even though the material properties of a molecular crystal are determined by both the packing arrangement of the molecules and their specific properties. The lack of interest in packing has at least one obvious reason. It is difficult for the non-expert to find the appropriate packing categories when looking at a principally infinite lattice. A tool performing this search for individual categories of packing in an automatic manner would therefore be of general importance. The development and application of such a tool is described in this paper. Novel strategies to extract a set of n points forming a specific n-point polyhedron from a set of p points (p 〉 n) is at its basis. Neural networks and second moment analysis are used as the methods of pattern recognition. In order to correlate packing and shape, second moment analysis of molecular shape is also used. This novel method is applied to three classes of organometallic compounds to determine whether the crystals formed by these compounds belong to any of the three classes fcc, bcc, or hcp in an idealized sense. A strong correlation is found between molecular shape and the membership or nonmembership of these classes. The corresponding program, including appropriate tools for visual representation, is available from the authors.
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  • 61
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    Weinheim : Wiley-Blackwell
    ISSN: 1434-1948
    Keywords: Rhodium hydride complexes ; Parahydrogen ; Tin ligands ; Hydrogenations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The oxidative addition of dihydrogen to the [RhH(SnCl3)5]3-/PR3 system (PR3 = PPh3, PEtPh2, PEt3) leads to the formation of previously unknown dihydride complexes, the 1H-NMR spectra of which have been studied by means of the ParaHydrogen Induced Polarization (PHIP) method. The composition of the resulting complexes crucially depends on the type of the added phosphane. With PEt3 as the phosphane and acetonitrile as the solvent, a complex with a SnCl2L ligand (L = CD3CN) can be detected. All systems examined catalyze the hydrogenation of phenylacetylene. During these reactions, both the 1H-NMR resonances of the dihydride complexes and those of styrene, the hydrogenation product of phenylacetylene, can be detected simultaneously. In the case of SnBr3 ligands, hydrogen addition in the presence of phosphanes leads to similar dihydride complexes, which were also identified via 1H-NMR spectroscopy. Furthermore, a mixed complex of the structure [Rh(SnBr3)nBr6-n]3- has been isolated.
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  • 62
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