Electronic effects on reductive elimination to form carbon-carbon and carbon-heteroatom bonds from palladium(II) complexes

The electronic properties of reactive and ancillary ligands have a large impact on the rate and scope of reductive elimination reactions. The purpose of this review is to compare and discuss published data on the effect of ligand electronic properties on the rates and scope of reductive eliminations from palladium(II). An understanding of these effects is important because reductive elimination from palladium(II) is the product-forming step of a variety of catalytic processes. The scope of this review will encompass the effect of the electron-donating abilities of alkyl, aryl, amido, alkoxo, thiolato, and phosphido groups on the rate of reductive elimination, the relative importance of inductive and resonance effects on the rate of reductive elimination, the relative sensitivity of the different classes of reductive eliminations to electronic perturbations, and the effect of the differences in electronic properties between the two aryl groups of biaryl complexes undergoing reductive elimination. In addition, this review will include the effects of electronic properties of the ancillary ligands on the rate of reductive eliminations from palladium(II). The effect of the overall electron-donating ability of ancillary ligands and the effect of the relative orientation of ancillary ligands to the two reactive ligands on the rate of reductive elimination will be discussed. Where appropriate, electronic effects on reductive elimination from complexes of other metals are described.     

Chemical vapour deposition polymerization of substituted [2.2]paracyclophanes

Chemical vapour deposition polymerisation of substituted [2.2]paracyclophanes is applied to the functionalised coating of stainless steel surfaces. Poly[o-trifluoroacetyl-p-xylylene-co-p-xylylene] ( 2a ), poly[o-hydroxymethyl-p-xylylene-co-p-xylylene] ( 2b ), poly[o-amino-p-xylylene-co-p-xylylene] ( 2c ) and poly(p-xylylene-2,3-dicarboxylic anhydride) ( 2d ) were deposited as thin layers.     

Modelling of θ-conditions for bisphenol—A polycarbonate single chains

Assessing conformational dimensions of macromolecules is a topic of long-standing interest. Because of its simplicity, it is attractive to investigate the chain properties in θ-conditions. Under these special conditions, the effects of excluded volume of the segments of the polymer chain vanish. The molecular chain is only subject to local constraints resulting from the bond structure and the hindrance to rotations about bonds. To model θ-conditions a contour length dependent cutoff is introduced ensuring that only nonbonded interactions of atoms of neighbouring monomeric units are taken into account for energy calculations. Using this energy model we will show that it is possible to model θ-conditions of a single bisphenol-A polycarbonate (BPA-PC) chain in vacuum by two different methods: (i) (Pseudo-) Langevin dynamics simulations and (ii) regular reassignment of randomly generated atom velocities during a molecular dynamics simulation. Both methods can be used to avoid oscillative dynamic behaviour of the chain. Calculations of the end-to-end vector and the radius of gyration of the equilibrium ensembles derived from simulations at different temperatures show good agreement with experimental data. Thus our model techniques are well suited to simulate θ-conditions with small computational expense.     

Statistical Investigation into the Structural Complementarity of Natural Products and Synthetic Compounds

The potential of new natural products as an important source for the exploration and development of new drugs and crop protection products is a long way from being exhausted. The statistical analysis of the structures of the natural and synthetically derived compounds has shown conspicuous variations in structural types in the natural products derived from different natural sources, which can be utilized in the search for individual active substances. The occasionally voiced prepossession that natural products have already been sufficiently examined and therefore no more innovations are to be expected can definitely be rejected.     

Effect of co-sensitization in new hybrid photo-refractive materials based on PVK polymer matrix and inorganic LiNbO<Subscript>3</Subscript> nano-crystals

A new class of photo-refractive (PR) composite materials based on poly(N-vinylcarbazole) (PVK) (co-)sensitized with nano-crystalline lithium niobate (LiNbO₃) is presented. The steady-state performance, as well as the kinetics of the photo-refractive grating formation, was investigated by degenerate four-wave mixing (DFWM) and two-beam-coupling (TBC) experiments. We found an optimum content of LiNbO₃ nano-particles of only 10⁻³ wt.%. Even at concentrations as low as 10⁻⁷ wt.% a notable effect of the nano-crystals was detected. This yields materials with improved PR performance compared to that of the standard PR polymer material sensitized by TNF only. The role of LiNbO₃ nano-particles is discussed in detail: The particles support generation and transport of the free-charge carriers. Furthermore, they increase the electron trap density.     

Safety and Quality Aspects of Acrylic Monomers

Summary : Acrylic monomers are important intermediates for the chemical industry. Especially acrylic acid (AA) is the basis for various reactions, such as polymerizations and esterifications and is, therefore, responsible for high product diversity. Spontaneous polymerization is a safety problem during the transportation and storage of acrylic monomers. In the production process, polymerization leads to blockages in the apparatus. For the prevention of these issues, special stabilizer systems are used such as hydroquinone monomethyl ether (MeHQ)/oxygen and phenothiazine (PTZ). The reactions of these stabilizer systems are not well understood at the moment. Therefore a lot of expertise and experience are necessary to guarantee safe handling. In this paper some methods for the investigation of stability related reaction kinetics are presented. A better comprehension of the mechanism of the polymerization inhibition is generated by the kinetic simulation with these data.     

Palladium-catalyzed coupling of ammonia and lithium amide with aryl halides

A mild, palladium-catalyzed coupling of aryl halides with ammonia or lithium amide to form primary arylamines as the major product is described. These reactions occurred with excellent selectivity for formation of the primary arylamine over formation of the diarylamine (9.5:1 to over 50:1 ratios of arylamine to diarylamine). In addition, the first organopalladium complex with a terminal -NH2 ligand has been isolated. This complex reductively eliminates to form arylamines.