Reactions involving transition metals: XX. A comparison of the reactions of 1,2,3,4,7,7-hexafluorobicyclo[2.2.1]heptadiene,and 2,3-bis(trimethyltin)- and 2,3-dichloro-1,4,5,6,7,7-hexafluorobicyclo[2.2.1]hepta-2,5-dienes with low valent transition metal complexes

1,2,3,4,7,7-Hexafluorobicyclo[2.2.1]heptadiene (1) and 2,3-bis(trimethyltin)-1,4,5,6,7,7-hexafluorobicyclo[2.2.1]hepta-2,5-diene (2) react with [M(Ph₃P)₄] (M = Pt, Pd) to afford air-stable adducts. 2,3-Dichloro-1,4,5,6,7,7-hexafluorobicyclo[2.2.1]hepta-2,5-diene (3) gives only [PtCl₂(PPh₃)₂] with [Pt(Ph₃P)₄], but a low yield of an adduct was obtained with [Pd(PPh₃)₄]. The diene 1 also reacts with Fe(CO)₅ to form the complex [(C₇H₂F₆)Fe(CO)₄], and with [Rh(C₂H₄)₂(acac)] to give [(C₇H₂F₆)Rh(acac)] in which the diene acts as a bidentate ligand. Similar products could not be isolated from the reactions of 2 and 3. A stable adduct, believed to be [{C₇F₆(SnMe₃)₂}Rh(CO)₂(μ-Cl)₂Rh(CO)₂] has been isolated from the reaction between 2 and [Rh(CO)₂Cl]₂. This adduct reacts with PPh₃ to give the bridge-cleavage product [{C₇F₆(SnMe₃)₂}RhCl(CO)(PPh₃)₂]. Reaction of 1 with [Rh(CO)₂Cl]₂ gives an unstable adduct which could not be isolated, and 2 does not react at room temperature. The chloro derivative 3 reacts with [PdCl₂(PhCN)₂] to give the adduct [(C₇F₆Cl₂)PdCl(PhCN)], but 1 and 2 do not react under similar conditions. Stable substitution products [(C₇F₆R₂)M] (R = H, M = Fe(CO)₂(η-C₅H₅); R = SnMe₃, M = Fe(CO)₂(η-C₅H₅), Mn(CO)₅, Ir(CO)₂(PPh₃)₂, Rh(CO)₂(PPh₃)₂; R = Cl, M = Ir(CO)₂(PPh₃)₂, Rh(CO)₂(PPh₃)₂) have been isolated from the reactions of the dienes with carbonylmetal anions. Insertion of the CHCH bond occurs when 1 is heated with [MnMe(CO)₅] to give [{C₇F₆H₂C($\text{O)Me}M}$n(CO)₄], and this, on reaction with either PPh₃ or [Pt(PPh₃)₄], gives [(C₇F₆H₂COMe)Mn(CO)₄PPh₃].     

Use of a gas-sensor array for detecting volatile organic compounds (VOC) in chemically induced cells

An application of gas sensors for rapid bioanalysis is presented. An array of temperature-modulated semiconductor sensors was used to characterize the headspace above a cell culture. Recombinant Saccharomyces cerevisiae yeast cells, able to respond to 17-estradiol by producing a reporter protein, were used as a model system. Yeast cells had the DNA sequence of the human estrogen receptor stably integrated into the genome, and contained expression plasmids carrying estrogen-responsive sequences and the reporter gene lac-Z, encoding the enzyme -galactosidase. The sensor-response profiles showed small but noticeable discrimination between cell samples induced with 17-estradiol and non-induced cell samples. The sensor array was capable of detecting changes in the volatile organic compound composition of the headspace above the cultured cells, which can be associated with metabolic changes induced by a chemical compound. This finding suggests the possibility of using cross-selective gas-sensor arrays for analysis of drugs or bioactive molecules through their interaction with cell systems, with the advantage of providing information on their bioavailability.     

New palladium(II)-catalyzed asymmetric 1,2-dibromo synthesis

[reaction: see text] The oxidation of olefins by chiral monometallic and bimetallic Pd(II)-Cu(II) catalysts in bromide-containing aqueous-THF reaction mixtures produced chiral 1,2-dibromides. With alpha-olefins, the ee's were about 95% while most of the internal alkenes gave somewhat lower enantioselectivities ( approximately 80%).     

A superposition principle in quantum logics

A new definition of the superposition principle in quantum logics is given, which enables us to define the sectors. It is shown that the superposition principle holds only in the irreducible quantum logics.     

How many data clusters are in the Galaxy data set?

Advances in Data Analysis and Classification - In model-based clustering, the Galaxy data set is often used as a benchmark data set to study the performance of different modeling approaches. Aitkin...     

Synthesis and conformational properties of 2,6-bis-anilino-3-nitropyridines

The synthesis of 2,6-bis-anilino-3-nitropyridines that are alkylated or acylated at the anilino nitrogen atoms is described. These derivatives show characteristic differences in the 1H-NMR spectra compared with the unsubstituted parent compound. These differences are used to determine structure-conformation relationships of this type of compounds. The conclusions drawn from the 1H-NMR spectra in this respect are supported by X-ray crystallographic data and by 1H-NMR data of conformationally restricted analogues. Preliminary investigations indicate that these relationships can in principle be extended to other diarylamines.     

Fluorescence quenching studies on poly(methyl methacrylate) particles : Matching of the refraction index of the stabilizer phase to that of the solvent with added carbon disulphide

Detailed information about the interior morphology of sterically stabilized colloid particles ought to be accessible from light-scattering measurements when the index of refraction of the stabilizer layer is matched to that of the diluent. When carbon disulphide is added to dispersions of poly(methyl methacrylate) (PMMA) latex particlesstabilized with poly(12-hydroxystearic acid) (PHSA) is dodecane, the scattered light intensity varies continuously with time; a permanent match of indices of refraction in this system is not achieved. Fluorescence quenching experiments with CS₂ as quencher were performed on similar particles containing 0.02 mol% anthracene groups covalently bound to the PMMA chains in order to obtain information about CS₂ swelling of these particles. Time-dependent quenching experiments indicate rapid panetration of CS₂ (< 10 min) into the 150-nm diameter particles. Stern-Volmer analysis of steady-state CS₂ quenching data yielded results typical for polymer systems containing fluorophores in a distribution of environments; the particle-bound anthracenes are not all equally accessible to quencher. Nevertheless, all the anthracene labels are quenched to some extent. It is concluded that within 10 min after exposure, CS₂ penetrates throughout the entire volume of the particles.     

Oligo(phenylene vinylene)-poly(methylstyrene) hybrids: controlled step-wise molecular wiring of oligo(phenylene vinylene)

Well defined oligo(phenylene vinylene) grafted polymers known as oligo(phenylene vinylene)-poly(methylstyrene) hybrids have been developed using a step-wise synthetic protocol, where the length of the OPV can be controlled systematically to achieve specific optoelectronic properties. The process allows the structural modification of attached OPV at a molecular level either by varying the chain length or by changing functionalities. The step-wise generation of OPV chains on the backbone of a highly soluble polymer ensures solubilization in a variety of solvents and also the exhibition of interesting optical properties.     

Construction of giant dendrimers using a tripodal building block

Giant pentane-soluble organo-silicon dendrimers have been synthesized using a triallylphenol brick according to a new divergent construction that uses a hydrosilylation-nucleophilic substitution sequence up to the ninth generation (G(9)). All the reactions were monitored by (1)H, (13)C, and (29)Si NMR until G(9), indicating that they were clean at the NMR accuracy until this last generation. MALDI TOF mass spectra were recorded for G(1) to G(4) and show the nature and amounts of defects that are intrinsic to the divergent construction. This technique and SEC (recorded up to G(5)) confirm the monodispersity (1.00 to 1.02) from G(1) to G(5). HRTEM and AFM images recorded for the high generations disclose the expected smooth dendrimer size progression and the globular shape. At G(9), the theoretical number of termini (TNT) is 177 407 branches (abbreviation: G(9)-177 047). It is estimated that more than 10(5) terminal branches are actually present in the G(9) dendrimer, far beyond the De Gennes "dense-packing" limit (6000 branches), and it is believed that the branch termini turn inside the dendrimer toward the core. This is corroborated by lower reaction rates and yields for the highest generation numbers presumably due to intradendritic reactions. It is probable that the dendritic construction is limited by the density of branches inside the dendrimer, i.e., far beyond the dense-packing limit.