On Borromean links

n-Borromean links are nontrivial links in whichn rings,n 3, are combined in such a way that any two component rings form a trivial link. The symmetry of links withn = 3 is discussed, and it is shown that such links form a variety of series whose members are different isotopy types. Examples are adduced of 3-Borromean links that are topologically chiral: Novel constructions are described ofn-Borromean links with and without at least one nontrivial sublink.     

High-sensitivity observation of dipolar exchange and NOEs between exchangeable protons in proteins by 3D solid-state NMR spectroscopy

A highly sensitive new 1H-detected 3D solid-state NMR method is described for characterizing 1H-1H spin exchange in nanocrystalline samples of 15N- and 2H-enriched protein. Long-range contacts are observed in human ubiquitin. The method is also used to show that numerous NOEs between backbone amides and crystal water protons can be observed.     

Darstellung. IR-Spektrum und Kristallstruktur von AsPh4[WSCI5]

Preparation, I.R. Spectrum, and Crystal Structure of AsPh₄[WSCl₅] The title compound is prepared by the reaction WSCl₄ with AsPh₄Cl in dichloromethane solution. It forms dark green, hygroscopic crystals. The crystal structure determination was solved by X-ray diffraction methods. (R = 3.13%, 826 independent observed reflexions). AsPh₄[WSCL⁵] crystallizes in the tetragonalic space group P4/n with two formula units per unit cell. The structure consists of AsPh₄⊕ ions and octahedral [WSCl₅]? anions with symmetry C_4v; the trans effect of the S ligand is very small. The i.r. spectrum is recorded and discussed with respect to the spectrum of WSCl₄; the latter is recorded again.     

eta 6-Arene complexes of Ni(II), efficient catalysts for 1,3-butadiene and styrene polymerization

A simple procedure for the preparation of cationic arene complexes of Ni(II) of composition [Ni(eta 6-ArX)(eta 3-C3H5)]+[BAr'4]- (X = OH, H) is reported. These compounds are shown to behave as highly active catalysts for the polymerization of 1,3-butadiene and styrene.     

Extraction inductively coupled plasma-optical emission spectrometry (ICP-OES) determination of traces of phosphorus in tungsten

A method for the separation and preconcentration of traces of phosphorus from tungsten was developed. Solid phase extraction of the phosphovanadomolybdate complex performed on a micro-column was applied. Phosphorus was determined by optical emission spectroscopy (OES) with inductively coupled plasma (ICP) excitation. A limit of detection of 0.4 g/g P with respect to the solid phase is obtained. By directly coupling the extraction/elution step to the ICP instrument a detection limit of 0.06 g/g P in W was achieved. Besides, the complexity of spectral evaluation in ICP-OES determinations of traces in spectralline-rich matrices is discussed.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Alkyl telluraalkanoates CnH2n+1Te(CH2)mCOOR (R = CH3, C2H5 OR H)

Methyl telluraalkanoates, C_nH_2n+1Te(CH₂)_mCOOR (n, m: 4, 7; 6, 4; 6, 7; 6, 9; 7, 4; 8, 7; 11, 2; 11, 5; 18, 11) were synthesized in yields ranging from 33 to 82 percent based on the quantities of the methyl ω-bromoalkanoates substrates. Disodium ditelluride was obtained from tellurium and sodium in ethylenediamine and was treated with alkyl bromides to give dialkyl ditellurides. The crude ditellurides were reduced with NaBH₄ to the alkane tellurolates which were coupled with methyl ω-bromoalkanoates to give the methyl telluraalkanoates. Ethyl 4-tellurapentadecanoate was prepared similarly from ethyl 3-bromopropanoate. The telluraalkanoates were characterized by elemental analyses, mass spectrometry, NMR spectrometry, and UV and IR spectrophotometry. Telluraalkanoates radiolabeled with ^123mTe or other radioisotopes have been reported elsewhere to be preferentially taken up by the heart and promise to be useful as myocardial imaging agents.     

Novel chiral biferrocene ligands for palladium-catalyzed allylic substitution reactions

Eleven novel aminophosphine ligands have been synthesized, all of which contain a chiral 2,2' '-bridged biferroceno unit as part of a biferrocenoazepine substructure. The efficiency of these compounds as chiral auxiliaries in palladium-mediated allylic substitution reactions has been investigated. Depending on the degree of (steric) fit between proper ligands and cyclic or noncyclic substrates, reactions with 46-87% ee were achieved. The molecular structure of a palladium dichloride complex of one of the ligands was determined by X-ray diffraction and compared to its binaphthyl analogue. In the solid state, the azepine substructure of these two complexes adopts totally different conformations with either local C(2) (binaphthyl) or local C(1) (biferrocene derivative) symmetry. These structural changes are well-reproduced by empirical force field calculations and are also reflected in significantly different behavior in asymmetric catalysis.     

Formation and decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y investigated by in situ stopped-flow MAS NMR spectroscopy

Methylation of aniline by methanol on zeolite H-Y has been investigated by in situ (13)C MAS NMR spectroscopy under flow conditions. The in situ (13)C continuous-flow (CF) MAS NMR experiments were performed at reaction temperatures between 473 and 523 K, molar methanol-to-aniline ratios of 1:1 to 4:1, and modified residence times of (13)CH(3)OH between 20 and 100 (g x h)/mol. The methylation reaction was shown to start at 473 K. N,N,N-Trimethylanilinium cations causing a (13)C NMR signal at 58 ppm constitute the major product on the catalyst surface. Small amounts of protonated N-methylaniline ([PhNH(2)CH(3)](+)) and N,N-dimethylaniline ([PhNH(CH(3))(2)](+)) were also observed at ca. 39 and 48 ppm, respectively. After increase of the temperature to 523 K, the contents of N,N-dimethylanilinium cations and ring-alkylated reaction products strongly increased, accompanied by a decrease of the amount of N,N,N-trimethylanilinium cations. With application of the in situ stopped-flow (SF) MAS NMR technique, the decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y to N,N-dimethylanilinium and N-methylanilinium cations was investigated to gain a deeper insight into the reaction mechanism. The results obtained allow the proposal of a mechanism consisting of three steps: (i) the conversion of methanol to surface methoxy groups and dimethyl ether (DME); (ii) the alkylation of aniline with methanol, methoxy groups, or DME leading to an equilibrium mixture of N,N,N-trimethylanilinium, N,N-dimethylanilinium, and N-methylanilinium cations attached to the zeolite surface; (iii) the deprotonation of N,N-dimethylanilinium and N-methylanilinium cations causing the formation of N,N-dimethylaniline (NNDMA) and N-methylaniline (NMA) in the gas phase, respectively. The chemical equilibrium between the anilinium cations carrying different numbers of methyl groups is suggested to play a key role for the products distribution in the gas phase.     

Fluordiazadiphosphetidine, 14. Mitt.: Nucleophile Substitutionsreaktionen von 1-Chlormethyl-2,2,2,4,4,4-hexafluor-3-methyl-1,3,2λ5,4λ5-diazadiphosphetidin und 1,3-Bis(chlormethyl)-2,2,2,4,4,4-hexafluor-1,3,2λ5,4λ5-diazadiphosphetidin mit ausgewählten metallorganischen Reagentien

Photochemical chlorination of (CH₃NPF₃)₂ yields ClCH₂(NPF₃)₂CH₃(I) and ClCH₂(NPF₃)₂CH₂Cl (II). By reaction with organometallics unsymmetric N-substituted hexafluorodiazadiphosphetidines are synthesized. Depending on the nucleophilic strength of the used organometallic the chlorine atom of the N-CH₂Cl group can be selectively substituted versus an organic substituent without side-reaction at the phosphorus atom.

     

Novel Acetogenins from the Leaves of Annona purpurea

The CH₂Cl₂ extract of the leaves Annona purpurea L. (Annonaceae) showed strong brine shrimp toxicity and pronounced activity against larvae of the yellow-fever mosquito Aedes aegypti. Activity-directed fractionation of the extract by a combination of column chromatography on silica gel, filtration, and high-pressure liquid chromatography led to the isolation of acetogenins 1–6 as the main active principles. The structures of two novel acetogenins named purpureacin 1( 5 ) and purpureacin 2 (6) were elucidated by spectroscopic analysis (UV, EI- and DCI-MS, EI-MS of Me₃Si derivatives,¹H- and ¹³C-NMR). The configuration of 5 and 6 was not established due to the limited sample amount. Compounds 3–6 also showed antifungal activity against Candida albicans, and 5 was slightly active against Bacillus subtilis.