Tetramethyldisilan-1,2-diyl-verbrückte Dicyclopentadienyl-und Diindenylmetalldichloride der 4. Gruppe – Kristallstruktur von

Tetramethyldisilane-1,2-diyl bridged Dicyclopentadienyl and Diindenyl Metal Dichlorides of the Group 4 Metals – Crystal Structure of Dicyclopentadienyl and diindenyl metal dichlorides of the type Cp′? SiMe₂SiMe₂? Cp′MCl₂ (Cp′ = C₅H₄, M = Ti ( 1 ), Zr ( 2 ), Hf ( 3 ); Cp′ = C₉H₇, M = Zr ( 4 ), Hf ( 5 )) were synthesized and characterized by means of their ¹H, ¹³C, ²⁹Si-n.m.r., MS, and IR spectra. The crystal structure of 2 was determined.     

Regiospecific alkylation of tetronic acids : Formation of 4-alkoxy-5H-furan-2-ones and 2-alkoxy-5H-furan-4-ones

2-Alkoxy-5$\text{H}$-furan-4-ones (7,8) and 4-alkoxy-5$\text{H}$-furan-2-ones (4,5) were prepared regiospecifically and in high yields from tetronic acids (4-hydroxy-5$\text{H}$-furan-2-ones) (2) in the first case by acetylating the 4-OH group and then reacting with trialkyloxonium tetrafluoroborate, and in the second case by alkylating tetrabutylammonium tetronates with dialkyl sulfate, respectively. Direct alkylation of tetronic acids with trialkyloxonium tetrafluoroborate gave in four cases regiospecific 2-O-alkylation, in one case 4-O-alkylation and in two other cases mixtures of 2- and 4-alkoxy derivatives.     

Beiträge zur Chemie der Alkylverbindungen von Übergangsmetallen. XIV. Untersuchungen über Benzylwolframverbindungen-Darstellung und Charakterisierung des Wolframtetrabenzyls

Contributions to the Chemistry of Transition Metal Alkyl Compounds. XIV. Investigations on Benzyl Tungsten Compounds. – On the Formation and Characterization of Tetrabenzyl Tungsten Unstable benzyl tungsten pentachloride is formed from tribenzyl boron and tungsten hexachloride. Other benzylating agents yield only less defined reduction products. – Reacting WCl₄ · 2 THF with dibenzyl magnesium W(CH₂C₆H₅)₄ can be obtained. The new compound is stable at room temperature and is thoroughly characterized.     

Beiträge zur Chemie der Alkylverbindungen von Übergangsmetallen. 59. Cyclopentadienyl-2-(dimethylaminomethyl)ferrocenyl-Verbindungen elektronenarmer 3 d-Elemente

Contributions to the Chemistry of Transition Metal Alkyl Compounds. 59. Cyclopentadienyl-2-(dimethylaminomethyl)ferrocenyl Compounds of Early 3 d-Elements Compounds of the type (C₅H₅)₂M(FcN) (M = Sc ( 1 ), Ti ( 2 ), V ( 3 ), Cr ( 4 ); FcN = 2-dimethylaminomethyl)ferrocenyl group), and (C₅H₅)M(FcN)₂ (M = Ti ( 5 ), Cr ( 6 ) were synthesized and investigated. A detailled characterization with respect to the existence of chelate structures was realized by the uv-vis, ¹H-n.m.r. spectroscopic measurements and determination of magnetic moments.     

Short and stereoselective synthesis of C-glycosylated glycine derivatives from glycals by radical addition and reduction

Only three steps are required for the convenient synthesis of 2-C-branched glyco-amino acids from glycals with good yields and stereoselectivities.     

Beiträge zur Chemie der Alkylverbindungen von Übergangsmetallen. 63. Darstellung und Kristallstruktur von Tetramethyltitan-tetrahydrofuran

Contributions to the Chemistry of Transition Metal Alkyl Compounds. 63. Preparation and Crystal Structure of Tetramethyltitanium Tetrahydrofuran Me₄Ti · 2THF 1 dissoziates partially in n-pentane. The formed Me₄Ti · THF 2 could be isolated. According to X-ray structure determination the molecules of 2 have a trigonalbipyramidal shape with the THF molecule in axial position.     

Ultrafast proton-coupled electron-transfer dynamics in pyrene-modified pyrimidine nucleosides: model studies towards an understanding of reductive electron transport in DNA.

5-(Pyren-1-yl)-2'-deoxyuridine (PydU) and 5-(Pyren-1-yl)-2'-deoxycytidine (PydC) were used as model nucleosides for DNA-mediated reductive electron transport (ET) in steady-state fluorescence and femtosecond time-resolved transient absorption spectroscopy studies. Excitation of the pyrene moiety in PydU and PydC leads to an intramolecular electron transfer that yields the pyrenyl radical cation and the corresponding pyrimidine radical anion (dU.- and dC.-. By comparing the excited state dynamics of PydC and PydU, we derived information about the energy difference between the two pyrimidine radical anion states. To determine the influence of protonation on the rates of photoinduced intramolecular ET, the spectroscopic investigations were performed in acetonitrile, MeCN, and in water at different pH values. The results show a significant difference in the basicity of the generated pyrimidine radical anions and imply an involvement of proton transfer during electron hopping in DNA. Our studies revealed that the radical anion dC.- is being protonated even in basic aqueous solution on a picosecond time scale (or faster). These results suggest that protonation of dC.- may also occur in DNA. In contrast, efficient ET in PydU could only be observed at low pH values (< 5). In conclusion, we propose--based on the free energy differences and the different basicities--that only dT.- but not dC.- can participate as an intermediate charge carrier for excess electron migration in DNA.     

Simultaneous assignment of all diastereotopic protons in strychnine using RDCs: PELG as alignment medium for organic molecules

The concept of using residual dipolar couplings (RDCs) for the structure determination of organic molecules is applied to the simultaneous assignment of all diastereotopic protons in strychnine. To use this important NMR parameter the molecule has to be aligned in the magnetic field. Here we present a new alignment medium for organic substrates. The optimization of the alignment properties of mixtures of poly-gamma-ethyl-L-glutamate (PELG) and CDCl(3) are described and the alignment properties of PELG at different concentrations are evaluated. A comparison of PELG with poly-gamma-benzyl-L-glutamate (PBLG) shows considerable differences in the magnitude of alignment for strychnine in the two alignment media. PELG induces a lower degree of order and makes the measurement of residual dipolar couplings (RDCs) in strychnine possible. All one-bond C-H RDCs of strychnine in PELG were determined by using 2D heteronuclear single quantum coherence (HSQC) spectroscopy. The strategy for the extraction of RDCs for methylene groups is described in detail. The RDCs and order parameters are used to assign pairs of diastereotopic protons. This methodology can distinguish not only one pair of diastereotopic protons but it can be used to assign all pairs of diastereotopic protons simultaneously. Two different calculation approaches to achieve this task are described in detail.     

Koordinationschemische Untersuchungen an Zinkdialkylen. X. Über die Reaktion von Zinkdimethyl mit sekundären Aminen

Dimethyl zinc reacts with diethyl amine, piperidine and morpholine forming definite 1:2- or 1:1-complexes. The weaker basic secondary amines pyrroline, pyrazolidine and imidazoline cause the evolution of methane and the formation of the corresponding zinc amides. In this way zinc pyrrolide, zinc pyrazolide and zinc imidazolide were obtained in form of pure substances.     

Separation of (Z)- and (E)-isomers of thioxanthene and dibenz[b,e]oxepin derivatives with calixarenes and resorcinarenes as additives in nonaqueous capillary electrophoresis

Five acidic calix[4]arenes with carboxylic or sulfonic groups at either the upper or lower rim of the cavity and one resorc[4]arene were investigated to separate three thioxanthenes (flupentixol, clopenthixol, chlorprothixene) and a dibenz[b,e]oxepin derivative (doxepin) with cis-/trans-isomerism by nonaqueous capillary electrophoresis (NACE). Partial filling of the capillary with the UV-absorbing selectors led to a low detection limit and an advantageous signal-to-noise ratio (S/N). A sufficient electrophoretic mobility of the calixarenes towards the anode was necessary to outweigh the oppositely directed electroosmotic flow (EOF). This depended from the functional groups, the dissociation and the hydrodynamic radius of the cyclophanes. In contrast, the resorcinarene was useable only by addition of sodium dodecyl sulfate (SDS) because only the complex of the two selectors had an anodic apparent electrophoretic mobility. p-Sulfonyl-calix[4]arene (ss-a1) was the most capable selector for all E/Z-isomers with maximal alpha-values ranging from 1.056 for doxepin to 1.224 for chlorprothixene. This was due to the sufficient migration in reversed direction to the EOF even at low pH* values of 3.0. Otherwise, electrostatic as well as hydrophobic interactions with the positively charged isomers seem to contribute to a superior recognition. Increasing the concentration up to 15 mM ss-a1 and using acidic media (pH* 5.0) led to high separation efficiency. Changing the organic solvent provides a powerful tool to improve selectivity with N,N-dimethylformamide-methanol (DMF-MeOH)-mixtures for thioxanthenes. Further electrophoretic parameters were optimized, such as the concentration of the electrolytes, the addition of SDS, the kind of electrolytes and the voltage. Distinct differences in selectivities were found between the derivatives with thioxanthene and dibenzo[b,e]oxepin ring system, respectively. Further, the different basic side chain was responsible for the different selectivity at higher pH* values. In contrast, the substitution at position 2 of the thioxanthenes played a secondary role. Based on the studies of single parameters a method for a simultaneous separation of the four pairs of isomers within 13 min was developed.