Synthesis,characterization and ethylene polymerization by metallasilsesquioxane

Soluble complexes of group (IV) metallocenes anchored on a substituted polyhedral oligomeric silsesquioxane trisilanol support were prepared and characterized. These catalyst precursors formulated as [M(O^O^O)X] are found to be active in polymerization of ethylene at high temperature in combination with ethylaluminum sesquichloride (Et₃Al₂Cl_3, EASC) as co‐catalyst. The polyethylene obtained by these catalysts is linear, crystalline and displays narrow dispersity. The unique low molecular weight PE formed in this reaction exhibits properties comparable to commercial micronized PE waxes that have potential industrial applications in surface coating and ink formulations. Copyright © 2013 John Wiley & Sons, Ltd.     

NIR Mega‐Stokes Fluorophores for Bioorthogonal Labeling and Energy Transfer Systems–An Efficient Quencher for Daunomycin

A set of new azide‐ and alkyne‐bearing lepidinium‐based fluorophores were synthesized for bioorthogonal labeling schemes. These fluorescent dyes all show large Stokes‐shifts with emission maxima in the near‐infrared (NIR) region of the electromagnetic spectrum. The applicability of these dyes in the construction of energy‐transfer systems was tested using one of these new fluorescent tags and daunomycin (Dau), an anticancer drug with fluorescent features. These daunomycin conjugates are the very first examples of fluorescently modulated constructs of this anticancer agent. The dually labeled architectures proved that the applied fluorescent dye can be utilized as an efficient quencher for daunomycin. Enzymatic cleavage of a dually labeled enzyme substrate resulted in full recovery of the fluorescence of daunomycin. Such fluorescently modulated Dau conjugates can provide useful information for the mechanism of action of Dau‐regulated cell death processes.     

Synaptic plasticity modulates autonomous transitions between waking and sleep states: insights from a Morris-Lecar model

The transitions between waking and sleep states are characterized by considerable changes in neuronal firing. During waking, neurons fire tonically at irregular intervals and a desynchronized activity is observed at the electroencephalogram. This activity becomes synchronized with slow wave sleep onset when neurons start to oscillate between periods of firing (up-states) and periods of silence (down-states). Recently, it has been proposed that the connections between neurons undergo potentiation during waking, whereas they weaken during slow wave sleep. Here, we propose a dynamical model to describe basic features of the autonomous transitions between such states. We consider a network of coupled neurons in which the strength of the interactions is modulated by synaptic long term potentiation and depression, according to the spike time-dependent plasticity rule (STDP). The model shows that the enhancement of synaptic strength between neurons occurring in waking increases the propensity of the network to synchronize and, conversely, desynchronization appears when the strength of the connections become weaker. Both transitions appear spontaneously, but the transition from sleep to waking required a slight modification of the STDP rule with the introduction of a mechanism which becomes active during sleep and changes the proportion between potentiation and depression in accordance with biological data. At the neuron level, transitions from desynchronization to synchronization and vice versa can be described as a bifurcation between two different states, whose dynamical regime is modulated by synaptic strengths, thus suggesting that transition from a state to an another can be determined by quantitative differences between potentiation and depression.     

(Co)polymerization behavior of supported metallocene catalysts. I. Ligand and substituent effect

Ethylene polymerization and its copolymerization with 1‐hexene with a set of supported metallocene catalysts were studied. As a carrier, the complex of magnesium chloride with tetrahydrofuran, which was previously pretreated with a triisobutylaluminium (TIBA), was used. The investigated metallocene compound differs in the metal type (Zr or Ti), the nature of the alkyl substituent in the cyclopentadienyl ring, and the type of ligand (Cp or Ind). The effect of catalyst composition on the anchored metal content, catalyst activity, comonomer reactivity, and polymer properties was investigated. The results obtained with supported catalysts were compared with those obtained with their homogeneous counterparts under the same (co)polymerization conditions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5562–5570, 2005     

Studies of structural composition distribution heterogeneity in ethylene/1-hexene copolymers using thermal fractionation technique (SSA): Effect of catalyst structure

Investigations into the compositional heterogeneity of ethylene/1-hexene copolymers obtained with various zirconocene/MAO catalysts, either homogeneous or supported on inorganic carriers such as a complex of magnesium chloride with tetrahydrofuran or methyl alcohol, were conducted. The dependence between metallocene structure, as well as catalyst immobilization, and the compositional heterogeneity of the related products was investigated. It was found that the heterogeneity of copolymers is determined by the metallocene catalyst structure. The amount of peaks on the DSC thermograms of copolymers and their division increase with the increase of bulkiness of the ligand in the catalytic system. The immobilization of the investigated catalysts on the magnesium carrier leads to an increase of the copolymer's compositional heterogeneity. However, the modification of the MgCl₂ carrier by tetrahydrofuran or methyl alcohol seems to not have any influence on the copolymers’ CCD.     

Thermoanalytical,FTIR and X-ray studies of gemfibrozil-cyclodextrin complexes

Summary Inclusion complexation between dimethyl-β-cyclodextrin and a very poorly water-soluble serum lipid-regulating agent, gemfibrozil, was studied. Products were prepared by several methods (physical mixing, kneading, spray-drying and ultrasonic treatment) in four different molecular ratios (2:1, 1:1, 1:2 and 1:3). The possibility of complex formation between the drug and the host molecule was studied by thermal analysis. Supplementary techniques, such as Fourier transformation-infrared spectroscopy and X-ray diffractometry, were also applied to interpret the results of thermal study of the products.     

Novel 2,4-methanoadamantane-benzazepine by domino photochemistry of N-(1-adamantyl)phthalimide

The photochemical reaction of N-(1-adamantyl)phthalimide (1) gives cleanly one product, the novel hexacyclic benzazepine derivative of 2,4-methanoadamantane 2. Its structure was characterized by spectroscopic methods and X-ray analysis and represent the first example of the 2-azahexacyclo[8.7.1.1 (1,4).0 (4,9).0 (11,16).0 (12,18)]nonadeca-4,6,8-triene skeleton. The product is formed by a domino process of two consecutive excited-state intramolecular gamma-hydrogen-transfer reactions. Base hydrolysis of the benzazepine 2 gives in high yield the keto derivative of the 1,2-substituted adamantane epsilon-amino acid 3.     

Adamantane-retropeptides, new building blocks for molecular channels

Novel adamantane-oxalamide derivatives, N,N′-bis(1-adamantylglycine methyl ester)oxalamide (meso-1 and rac-1), N,N′-bis(3-aminoadamantane-1-carboxylic acid methyl ester)oxalamide (2) and N,N′-bis(3-aminoadamantane-1-carboxylic acid)oxalamide (3) were prepared and structurally characterized by spectroscopic methods and X-ray analysis. Crystal packing of the structures meso-1 and rac-1 is defined by one-dimensional α-networks of hydrogen-bonded chains. The crystal structures of 2 and 3 are characterized by two-dimensional β-networks of hydrogen bonds. The oxalamide 3 crystallizes as the solvates only. In the crystal structure of 3 the protic solvent participates in hydrogen bonding with the oxalamide moieties. However, in non-protic solvents 3 crystallizes as a solvate but the solvent does not participate in hydrogen bonding. The two-dimensional network of hydrogen bonds connecting molecules of 3 generates channels, which are filled by discrete solvent molecules.     

New Nitrogen,Sulfur-, and Selenium-Donating Ligands Derived from Chiral Pyridine Amino Alcohols. Synthesis and Catalytic Activity in Asymmetric Allylic Alkylation

Although many chiral ligands for asymmetric catalysis have been developed, there is still a need for new structures allowing the modular approach. Recently, easy synthesis of chiral pyridine-containing β-amino alcohols has been elaborated by opening respective epoxides with enantiomeric 1-phenylethylamine. This paper reports the synthetic transformation of β-amino alcohols into the new complexing pyridine-containing seleno- and thioethers. The amino alcohols were effectively converted to cyclic sulfonamidates, which were reacted with thiolates or phenyl selenide nucleophile. The reaction was diastereoselective, and its outcome depended on the configuration at the substitution center. The problem was discussed considering DFT optimized structures of both diastereomeric sulfonamidates. New amino-aldimine ligands were also synthesized from chiral pyridine-containing diamines. Nine new chiral ligands were tested in the Tsuji-Trost allylic alkylation resulting in the enantiomerically enriched product in up to 75% ee. The observed stereochemical induction agrees with the prevailing nucleophilic attack at the allylic carbon laying opposite to the complexing nitrogen of pyridine in η³-allylic intermediate complexes.     

Electro-optic and dielectric properties of new binary ferroelectric and antiferroelectric liquid crystalline mixtures

Several new binary liquid crystalline mixtures have been designed and their properties were studied by complementary methods. It has been shown that even both pure components used for the mixture design possess the ferroelectric behaviour; the induced antiferroelectric smectic phase has been detected for one of the prepared mixtures. The phase diagram has been constructed and the existence of the antiferroelectric phase was confirmed by switching time and dielectric spectroscopy measurements. Some of the resulted mixtures possess very high values of the tilt angle that reaches close below 45° degrees at saturation. Values of spontaneous polarisation were found within 50–200 nC/cm² in dependence of the mixture’s composition. Due to specific properties, the obtained mixtures might be interesting for further design of multicomponent mixtures and formulation of the advanced nanocomposite systems.