Dinuclear titanium complexes with methylphenylsilylene bridge between cyclopentadienyl rings. Synthesis, characterization and reactivity towards ethylene

The new ansa-titanocene dichloride [{(SiMePh)(η⁵-C₅H₄)₂}TiCl₂] (1) was prepared by one pot reaction, whereas synthesis of its methylated analogue [{(SiMePh)(η⁵-C₅Me₄)₂}TiCl₂] (3) was performed in two steps with isolation of corresponding silane intermediate SiMePh(HC₅Me₄)₂ (2). The reaction of 1 and 3 with TiCl₄ afforded the dinuclear complexes [(SiMePh){(η⁵-C₅R₄)TiCl₃}₂] (R = H (4) and R = Me (5)). The catalysts formed from 4 and 5 after their activation with excess MAO exhibited a modest activity in ethylene polymerization. The polymer products consisted of high molar mass linear polyethylenes with a broad molar mass distribution. The presence of three paramagnetic titanium species in the mixture 4/MAO was revealed by EPR spectroscopy. All new prepared compounds 1-5 were characterized by multinuclear NMR, EI-MS, IR, and solid-state structures of 1, 3 and 5 were determined by X-ray single crystal diffraction.     

Simvastatin: structure solution of two new low-temperature phases from synchrotron powder diffraction and ss-NMR

Simvastatin is a substance used for the treatment of hypercholesterolemia. In addition to the already known room temperature structure of simvastatin (Čejka et al. in Acta Cryst C59:o428, 2003) two new low-temperature polymorphs were found by X-ray powder diffraction with the phase transition at 261 and 223 K (later confirmed by DSC to be 272 and 232 K). The main differences among three polymorphs consist in the side-chains conformation only and the phase changes are fully reversible. The structures of the polymorphs were studied by the powder diffraction based on synchrotron radiation as well as by solid-state NMR spectroscopy.     

On the Reliability of the AMBER Force Field and its Empirical Dispersion Contribution for the Description of Noncovalent Complexes

The reliability of the AMBER force field is tested by comparing the total interaction energy and dispersion energy with the reference data obtained at the density functional theory–symmetry‐adapted perturbation treatment (DFT–SAPT)/aug‐cc‐pVDZ level. The comparison is made for 194 different geometries of noncovalent complexes (H‐bonded, stacked, mixed, and dispersion‐bound), at the equilibrium distances as well as at longer distances (up to a relative distance of two). The total interaction energies agree very well with the reference data and only the strength of H‐bonded complexes is slightly underestimated. In the case of dispersion energy, the overall agreement is even better, with the exception of the stacked aromatic systems, where the empirical dispersion energy is overestimated. The use of AMBER interaction energy and AMBER dispersion energy for different types of noncovalent complexes at equilibrium as well as at longer distances is thus justified, except for a few cases, such as the water molecule, where the dispersion energy is highly inaccurate.     

Single source single-commodity stochastic network design

Stochastics affects the optimal design of a network. This paper examines the single-source single-commodity stochastic network design problem. We characterize the optimal designs under demand uncertainty and compare with the deterministic counterparts to outline the basic structural differences. We do this partly as a basis for developing better algorithms than are available today, partly to simply understand what constitutes robust network designs.     

Hydrogen bond dynamics in the excited states: photodissociation of phenol in clusters

We have investigated the photodynamics of phenol molecules in clusters. Possible reaction pathways following the photoexcitation of hydrogen-bonded phenol clusters have been identified theoretically using ab initio calculations. Experimentally we have studied the phenol molecules and clusters of various size distributions in a molecular beam apparatus. In particular, we have measured the H-fragment kinetic energy distributions after the excitation with 243 nm and 193 nm laser radiation. At 243 nm the KED spectra did not show any significant difference between the photodissociation of isolated molecules and phenol in larger clusters, while at 193 nm the contribution of the fast H-fragments is significantly suppressed in clusters with respect to the bare phenol molecule. We have interpreted the experimental results within the framework of the suggested reaction pathways.     

PEPPSI-type palladium complexes containing basic 1,2,3-triazolylidene ligands and their role in Suzuki-Miyaura catalysis

A series of PEPPSI-type palladium(II) complexes was synthesized that contain 3-chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki-Miyaura cross-coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50?°C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol-2-ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N-heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.     

Electron transfer reduction of carboxylic acids using SmI2-H2O-Et3N

The first general method for efficient electron transfer reduction of carboxylic acids has been developed. The protocol using SmI(2)-H(2)O-Et(3)N allows for reduction of a variety of carboxylic acids in excellent yields and provides an attractive alternative to processes mediated by reactive alkali metals, lithium aluminum hydride, and boron hydrides. Of broader significance, the method allows acyl radical equivalents to be generated from carboxylic acids under mild reaction conditions.     

Manganese(III) acetate mediated oxidative radical cyclizations. Toward vicinal all-carbon quaternary stereocenters

Manganese(III) acetate mediated oxidative radical cyclizations have been used to synthesize a range of densely functionalized and sterically congested cyclopentane-lactones. A number of the resulting lactones contain vicinal all-carbon quaternary stereocenters adjacent to a tertiary benzylic stereocenter and are formed with high levels of stereocontrol.     

Polymer Carriers for Anticancer Drugs Targeted to EGF Receptor

A novel actively targeted polymer carrier for anticancer drugs based on an N‐(2‐hydroxypropyl)methacrylamide copolymer (PHPMA) is proposed. An oligopeptide sequence GE7, attached to the polymer, is a specific ligand for the EGF receptor overexpressed on most tumor cells. Co‐attachment of selected chemotherapeutics will therefore lead to formation of tumor‐specific polymer therapeutics, further enhanced by the EPR effect. FACS measurements prove elevated binding activity of the fluorescently labeled PHPMA/GE7 conjugate in EGFR‐rich cells (FaDu, MCF‐7), compared to conjugates of scrambled peptides. Cell lines with low EGFR level (SW620, B16F10) bind the GE7 conjugate significantly less.