Basische metalle : XXXVII. Zur reaktivität der ethylen(hydrido)rhodium-komplexe [C5H5RhH(C2H4)PR3]X gegenüber nucleophilen: insertion versus substitution

The complex [C₅H₅RhH(C₂H₄)PMe₃]BF₄ (I) reacts with NaF and NaCN by deprotonation to give C₅H₅Rh(PMe₃)C₂H₄ but with NaCl, NaBr and NaI the ethylrhodium compounds C₅H₅RhC₂H₅(PMe₃)X (II–IV) are obtained. The reactions of I with CO and PPrⁱ₃ yield the BF₄ salts of the cations [C₅H₅RhH(CO)PMe₃]⁺ and [C₅H₅RhH(PPrⁱ₃)PMe₃]⁺ (V, VI), respectively, from which the uncharged complexes C₅H₅Rh(CO)PMe₃ (VII) and C₅H₅Rh(PPRⁱ₃)PMe₃ (VIII) are prepared. The carbonyl compound VII is also accessible either from C₅H₅Rh(CO)₂ and PMe₃ or from C₅H₅Rh(PMe₃)₂ and CO. The reaction of I with ethylene leads to the BF₄ salt of the cation [C₅H₅RhC₂H₅(PMe₃)C₂H₄]⁺ (X) which on treatment with PMe₃ forms the complex [C₅H₅RhC₂H₅(PMe₃)C₂H₄PMe₃]BF₄ (XI). The compound [C₅H₅RhH(C₂H₄)PPrⁱ₃]BF₄ (XII) reacts with NaI by insertion to yield C₅H₅RhC₂H₅(PPrⁱ₃)I (XIII) whereas with PPrⁱ₃ the salt [C₅H₅RhH(PPrⁱ₃)₂]BF₄ (XIV) is produced. The bis(triisopropylphosphine) complex C₅H₅Rh(PPrⁱ₃)₂ (XVI) is obtained from XIV and NaH.     

Definitive Proof for the Operation of Isotopically Sensitive Branching (‘Metabolic Switching’) in FeI-Mediated C?H Bond Activation in the Gas Phase Short Communication

Rigorous regio- and stereospecific labeling experiments are performed to demonstrate the operation of the previously suggested operation of ‘isotopically sensitive branching’ in Fe^I-mediated C? H bond activation. For the hexane-1,6-diol/Fe⁺-complex, it is shown that dehydrogenation involves specifically the central C(3)/C(4) position, and the study of the stereospecifically labeled D ,L - and meso-[3,4-D₂]-isotompomers 1e and 1f demonstrates that dehydrogenation proceedes via two competing pathways (i.e. ‘anti’- vs. ‘syn’-route). The contribution of these routes to the product formation is – due to a kinetic isotope effect – controlled by the relative configuration at the labeled positions C(3)/C(4). For the D ,L -form 1e , we estimate a ratio of 49:1 in favor of the ‘anti’-route; due to an isotope effect, this ratio drops to 4.3:1 for the meso-form 1f .     

Preparation of lipids from plant and animal tissues: Pure compounds and reference mixtures

Summary The systematic analysis of complex lipid mixtures is based on the consecutive application of complementary principles of chromatography. Pure compounds, synthetic standard mixtures, and biological lipid extracts are required as reference materials. Procedures for the preparation of individual compounds and standard mixtures are outlined, and recommendations are offered regarding the choice of biological reference materials and their certification.

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Gewinnung von Lipiden aus pflanzlichen und tierischen Geweben: reine Verbindungen und Referenzgemische

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In this article, lipids have been named according to the Recommendations of the IUPAC-IUB Commission on Biochemical Nomenclature (see Eur J Biochem (1977) 79:11–21)     

Synthesen von Xyluron- und Riburonsäurederivaten

Zusammenfassung Durch acylierende Dehydratisierung von 1,2-Isopropyliden-d-xylo-und-d-ribo-pentodialdo-furanoseoxim bzw. aus Furanuronsäureestern und-amiden werden über eine Reihe von Zwischenstufen verschiedene 3-O-Acyl-pentofuranuronsäurenitrile synthetisiert und deren Reaktionen untersucht.

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Synthesis of xyluronic and riburonic acid derivatives

Various 3-O-acyl-pentofuranurononitrils were synthesized by acylating dehydration of 1,2-isopropylidene-d-xylo- andd-ribo-pentodialdo-furanose oximes, or pentofuranuronic acids via intermediates. Their reactions were investigated.

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Mit 1 Abbildung

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Aus den DissertationenH. W. Oberwalder (1970) undE. Wildschek (1967), Technische Hochschule in Graz.     

Amperometric determination of hydrogen peroxide with a manganese dioxide film-modified screen printed carbon electrode

A carbon thick film electrode modified with an MnO₂-film is investigated as an amperometric detector for hydrogen peroxide in flow-injection analysis (FIA). At an operating potential of +0.48 V vs. Ag/AgCl catalytic oxidation of the analyte is exploited for amperometric monitoring. Experimental parameters, such as pH of the carrier, working potential, flow rate and injection volume, are optimized. The amperometric signals are linearly proportional to the concentration of H₂O₂ in the range from 0.005 to 10 mg/L, showing a detection limit (3σ) of 2.3 μg/L. The method is applied to the determination of H₂O₂ in rain water and to a simple assay to quantify glucose in human plasma.     

Tetragonal low-temperature structure of LiAl

The cubic face-centered structure of LiAl (, at ) transforms into a tetragonal body-centered structure (I4₁/amd, , at ). This first-order phase transition at about during heating is probably the reason for the so-called “ anomalies” in some physical properties like specific heat, electrical resistivity and nuclear-spin lattice relaxation. This transition seems to be correlated with the composition Li:Al of the alloy and the amount of Li vacancies.     

exo/endo-Selectivity in the Reaction of ‘Bare’ FeO+ with Bicyclo[2.2.1]heptane (Norbornane)

‘Bare’ FeO⁺ reacts in the gas phase with norbornane with collision efficiency, and the most prominent cationic products correspond to [FeC₅H₆]⁺ (32%), [FeC₇H₈]⁺ (19%), [FeC₃H₆O]⁺ (19%) and [FeC₆H₆]⁺ (14%), which are structurally characterized by ligand exchange as well as collision-induced dissociation experiments. Circumstantial evidence is provided which indicates that the complexes [FeC₅H₆]⁺, [FeC₇H₈]⁺, and [FeC₆H₆]⁺ originate from an Fe(norbornene)⁺ intermediate which itself is formed by elimination of H₂O from the [FeO(norbornane)]⁺ encounter complex. Although the reactions are preceded and/or accompanied by partial H/D exchange, the isotope distribution in the productions clearly points to a preferential endo-attack of bare FeO⁺, with an endo/exo-ratio of ca. 10.3 and kinetic isotope effects k_H/k_D for the endo-abstraction of 2.4 and of 7.7 for approaching an exo-C? H bond. The preferred endo-approach of bicyclo[2.2.1]heptane by ‘bare’ FeO⁺ is in distinct contrast to the P-450-mediated or the iron(III)porphyrin-catalyzed hydroxylation of this substrate which favor reactions at the exo-face.     

13C nuclear magnetic resonance spectra—VI: Stereochemical dependence of γanti,heterosubstituent effects on 13C chemical shifts of bridgehead substituted molecules

The ¹³C NMR data of some mono- and disubstituted adamantanes, homoadamantenes and homonoradamantenes are presented. A model of dependencies of the γ_anti substituent induced shifts on torsion angles and internuclear distances between substituents and the γ carbons is proposed. Furthermore, it is shown that for the α substituent induced shifts strain within the molecular frameworks of these compounds plays no significant part.     

Head-to-head (HH) and head-to-tail (HT) conformers of cis-bis guanine ligands bound to the [Re(CO)3]+ core

We have prepared four complexes of the type [Re(guanine)(2)(X)(CO)(3)] (guanine = 9-methylguanine or 7-methylguanine, X = H(2)O or Br) in order to understand the factors determining the orientation of coordinated purine ligands around the [Re(CO)(3)](+) core. The 9-methylguanine ligand (9-MeG) was chosen as the simplest N(9) derivatized guanine, and 7-methylguanine (7-MeG) was chosen because metal binding to N(9) does not impose steric hindrance. Two types of structures have been elucidated by X-ray crystallography, an HH (head-to-head) and HT (head-to-tail) conformer for each of the guanines. All complexes crystallize in monoclinic space groups: [Re(9-MeG)(2)(H(2)O)(CO)(3)]ClO(4) (2) in P2(1)/n with a = 12.3307(10) A, b = 16.2620(14) A, c = 13.7171(11) A, and beta = 105.525(9) degrees, V = 2650.2(4) A(3), with the two bases in HT orientation and its conformer [Re(9-MeG)(2)(H(2)O)(CO)(3)]Br (3) in P2(1)/n with a = 15.626(13) A, b = 9.5269(5) A, c = 15.4078(13) A, and beta = 76.951(1) degrees, V = 2234.5(3) A(3), and the two bases in an HH orientation. Similarly, [Re(7-MeG)(2)(H(2)O)(CO)(3)]ClO(4) (4) crystallizes in P2(1)/c with a = 13.0708(9) A, b = 15.4082(7) A, c = 14.316(9) A, and beta = 117.236(7) degrees, V = 2563.5(3) A(3), and exhibits an HT orientation and [ReBr(7-MeG)(2)(CO)(3)] (5) in P2/c with a = 17.5117(9) A, b = 9.8842(7) A, c = 15.3539(1) A, and beta = 100.824(7) degrees, V = 2610.3(3) A(3), and shows an HH orientation. When crystals of any of these complex pairs are dissolved in D(2)O, the (1)H NMR spectrum shows a single peak for the H(8) resonance of the respective coordinated purine indicating a rapid equilibrium between HH and HT conformations in solution. DFT calculations simulating the rotation of one ligand around its Re-N bond showed energetic barriers of less than 8.7 kcal/mol. We find no hypochromic effect in the Raman spectrum of 3, which showed base stacking in the solid state. Neither steric interactions nor hydrogen bonding are important in determining the orientation of the ligands in the coordination sphere.