Phosphorus heterocycle synthesis by RPX2·AlX3 addition to [1,n]dienes VI : RPX2·AlX3 phosphorylations in the presence and absence of water

The electrophilic addition of RPX₂·AlX₃ (1) (R=Ph or Me; X=Cl or Br) to [1,n]dienes was found to be an efficient and useful reaction for the synthesis of a variety of new phospha-heterocycles such as substituted phospholanes, dihalophosphorinanes, phosphorinenes, 3-phosphabicyclo[3.1.0]hexanes, 2-phosphabicyclo[2.2.1] heptanes, 2-phosphabicyclo[3.2.1]octanes and substituted phosphetanes. An ionic multistep mechanism, governed by the stability of the intermediate carbonium ions, is suggested. The phosphorylation reaction was found to be most sensitive to water and in its presence (even 0.1 eq.) it takes a different course. The products, results from hydrogen and RP(O)X addition to the double bond. The phosphorylation is initiated by an unusual protonation process which employs the specific catalytic properties of the AlCl₃·H₂O complex (HX + RPX₂ in the presence or absence of AlCl₃ do not give any defined phosphorylation products but polymers). Some of the examined dienes give with water-doped RPX₂·AlX₃ phosphorylations and/or HX additions, involving in several cases participation of both double bonds to give intramolecular cyclisation products. These products may explain the substances obtained following phosphorylation of simple n-alkenes. Compound 16, one of the new addition compounds, was found to cyclise to the 2-oxaphospholane system (19).     

The 13C-NMR spectrum and stereochemistry of heteronemin

The ¹³C-NMR spectrum of heteronemin (1), a new sesterterpene from marine origin is reported. Assignment of most of the signals was accomplished by a combination of off-resonance decoupling, PRFT measurement, comparison with suitable known model compounds and LIS measurements. An all trand-anti-trans configuration is suggested from 1 according to the ¹³C-NMR data.     

Neue Wege der serologischen HIV-Diagnostik

Ohne Zusammenfassung

---

New approaches to serological HIV diagnostics

     

Long-range many-body polyelectrolyte bridging interactions

We investigate polyelectrolyte bridging interactions mediated by charged, flexible, polyelectrolyte chains between fixed cylindrical macroions of opposite charge in a two-dimensional hexagonal crystalline array. We show that in the asymptotic regime of small macroion density, the polyelectrolyte-mediated attraction is long range, falling off approximately linearly with the macroion array density. We investigate the polyelectrolyte free energy as a function of the macroion density and derive several analytic limiting laws valid in different regimes of the parameter space.     

An amorphous magnetically ordered ultra-thin film of Fe2O3 on graphite: Mössbauer spectroscopy

An amorphous ultra-thin film of Fe₂O₃ was prepared on partially oriented graphite (Papyex) by surface oxidation of an adsorbed monolayer of Fe(CO)₅. Mössbauer spectra exhibit super-paramagnetism below 79 K. The Mössbauer fraction parallel to the film exceeds that perpendicular by 16% at room temperature.     

Segregation instabilities of moving interfaces

The segregation of solute particles on a moving interface leads to the appearance of two types of instabilities near competing velocity thresholds. This behavior is shown to occur in a variety of exactly solvable models where the interface motion is coupled to a diffusion process of the solute particles. These models directly apply to the propagation of internal domain walls, but can also be generalized to surfaces of growing crystals in the kinetics-limited regime.     

A new enantiomerization mechanism for tripodal penta-coordinate Zn(II)(nta) complexes. Theoretical clarification of the observed 1H NMR spectrum

Based on DFT calculations (RB3LYP/LANL2DZp), the unexpected single-line 1H NMR spectrum of Zn(II)(nta), nta = 2,2',2'-nitrilotriacetate, can be ascribed to a non-dissociative enantiomerization process (deltadeltadelta<=>lambdalambdalambda) from C3viaC3v to C3 symmetry. The energy barrier is rather low and depends to a lesser extent on the nature of the co-ligand in [Zn(nta)X]2- (X: H-, CH3- NH2-, OCH3-, F-, Cl-, Br-, I-) and [Zn(nta)Y]- (Y: NCH, CO, N2, O(CH3)2), but more so on the overall charge of the complex. The energy barrier for enantiomerization of [Zn(nta)X]2- is between 5.7 and 6.7 kcal mol-1, and for [Zn(nta)Y]- between 2.2 and 3.1 kcal mol-1.     

Kinetics and mechanism of the reactions of Pd(II) complexes with azoles and diazines. Crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O

Substitution reactions of the complexes [Pd(bpma)(H2O)]2+, [Pd(bpma)Cl]+, [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+, where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO4 at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H2O)]2+ and [Pd(bpma)Cl]+, are more reactive than the complexes of dien, viz. [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+. Also, the aqua complexes, [Pd(bpma)(H2O)]2+ and [Pd(dien)(H2O)]2+, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (Delta S++, Delta V++) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O was determined by X-ray diffraction. Crystals are monoclinic with the space group P2(1)/c. The coordination geometry of [Pd(bpma)(H2O)]2+ is distorted square-planar. The Pd-N (central) bond distance, 1.958(5) A, is shorter than the other two Pd-N distances, 2.007(5) and 2.009(5) A. The Pd-O distance is 2.043(5) A.     

A note on certain de Bruijn sequences with forbidden subsequences

We consider words over a finite alphabet with certain uniqueness properties (a subsequence of length k does not occur more than once) and distance properties (at least j other symbols separate the occurrence of the same symbol). The maximal length of these words is realised by linear de Bruijn sequences with certain forbidden subsequences. We prove the existence of these maximal sequences.