Suzuki–Miyaura Cross-Coupling of Bromotryptophan Derivatives at Ambient Temperature

Mild reaction conditions are highly desirable for bio-orthogonal side chain derivatizations of amino acids, peptides or proteins due to the sensitivity of these substrates. Transition metal catalysed cross-couplings such as Suzuki–Miyaura reactions are highly versatile, but usually require unfavourable reaction conditions, in particular, when applied with aryl bromides. Ligand-free solvent-stabilised Pd-nanoparticles represent an efficient and sustainable alternative to conventional phosphine-based catalysts, because the cross-coupling can be performed at considerably lower temperature. We report on the application of such a highly reactive heterogeneous catalyst for the Suzuki–Miyaura cross-coupling of brominated tryptophan derivatives. The solvent-stabilised Pd-nanoparticles are even more efficient than the literature-known ADHP-Pd precatalyst. Interestingly, the latter also leads to the formation of quasi-homogeneous Pd-nanoparticles as the catalytic species. One advantage of our approach is the compatibility with aqueous and aerobic conditions at near-ambient temperatures and short reaction times of only 2 h. The influence of different N^α-protecting groups, boronic acids as well as the impact of different amino acid side chains in bromotryptophan-containing peptides has been studied. Notably, a surprising acceleration of the catalysis was observed when palladium-coordinating side chains were present in proximal positions.     

Luminescent Symmetrically and Unsymmetrically Substituted Diboranes(4)

A series of 4-(dimethylamino)phenyl and pentafluorophenyl-substituted 1,2-bis(dimethylamino)diboranes(4) of type A , benzo-fused cyclic 1,4-diaza-2,3-diborinanes of type B , and 1,2-diduryldiboranes(4) of type C were synthesized and structurally characterized. Spectroscopic studies revealed that the substitution pattern is a decisive factor for the observation of fluorescence in most of the derivatives A to C . For diboranes(4) of type A , unsymmetrical substitution with electron-donating and -withdrawing groups at the boron centers is crucial to invoke fluorescence, albeit weak. Substitution at the boron atoms of 1,4-diaza-2,3-diborinane species B leads to a modified skeletal structure. Finally, the grafting of 4-(dimethylamino)phenyl groups to diboranes(4) of type C results in extraordinary Stokes shifts in nonpolar solvents.     

Malaysian teachers' insights into implementing green chemistry experiments in secondary schools

1. Download : Download high-res image (174KB)
2. Download : Download full-size image

     

Effect of hydrophilicity on the properties of a degradable polylactide

A series of polylactide networks has been prepared by the copolymerization of a biodegradable oligolactide macromer with hydrophobic methyl methacrylate monomer and hydrophilic hydroxyethyl acrylate monomer, with different amounts of the hydrophilic monomer. The incorporation of the hydrophilic units into the network has been characterized with thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical spectrometry. A homogeneous material results, showing a single glass‐transition temperature and a characteristic relaxation behavior that is not the sum of those of the pure components separately. Additional hydrophilic units in the network chains lower the rubbery modulus, keeping a high modulus value at room temperature, and manifestly increase the degradation rate of the polymer. This can be attributed both to the higher water swellability of the network when hydrophilic units are present and to the higher water diffusion coefficient in a network, which has a lower crosslinking density. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 656–664, 2006     

Some platinum(II) complexes derived from aromatic alkynes

Several trans‐platinum(II) complexes, of the type R′? {Pt(PBu₃)₂}? R″? {Pt(PR₃)₂}? R′, where R′ and R″ are groups derived from a series of aromatic alkynes and diynes, have been prepared and characterized. Extensive spectroscopic data for these and other known related complexes are presented. A more precise structural study of trans‐Pt(C≡CC₆H₄C≡CPh)₂(PBu₃)₂ (cf. Z. Kristallogr. 1998; 213: 483) is reported. Copyright © 2002 John Wiley & Sons, Ltd.     

Oxidation and structural changes in NMMO-regenerated cellulose films

Carbonyl and carboxyl groups introduced by oxidative processes during production and purification of celluloses determine intra- and intermolecular interactions and thus application-related bulk and surface properties of cellulosic materials. We report a comprehensive approach to the quantification of carboxyl and carbonyl groups in cellulose films upon reconstitution from NMMO solutions. Measurements of the excess conductivity were combined with the determination of the molecular weight distribution, quantification of the carboxyl and carbonyl group content, crystallinity and film swelling in aqueous solutions. TEMPO-oxidized, NMMO-regenerated cellulose films were additionally analysed as a reference system for extensive cellulose oxidation. Our reported data demonstrate that dissolution of cellulose in NMMO results in the formation of onic acids, chain degradation, increased ionization and film swelling, whereas TEMPO-oxidation introduced carbonyl groups as well as onic and uronic acids causing a significantly increased charging, ion accumulation and swelling even at higher crystallinity.     

Switchable ionic liquids enable efficient nanofibrillation of wood pulp

Use of switchable ionic liquid (SIL) pulp offers an efficient and greener technology to produce nanofibers via ultrafine grinding. In this study, we demonstrate that SIL pulp opens up a mechanically efficient route to the nanofibrillation of wood pulp, thus providing both a low cost and chemically benign route to the production of cellulose nanofibers. The degree of fibrillation during the process was evaluated by viscosity and optical microscopy of SIL treated, bleached SIL treated and a reference pulp. Furthermore, films were prepared from the fibrillated material for characterization and tensile testing. It was observed that substantially improved mechanical properties were attained as a result of the grinding process, thus signifying nanofibrillation. Both SIL treated and bleached SIL treated pulps were fibrillated into nanofibers with fiber diameters below 15 nm thus forming networks of hydrophilic nature with an intact crystalline structure. Notably, it was found that the SIL pulp could be fibrillated more efficiently than traditional pulp since nanofibers could be produced with more than 30% less energy when compared to the reference pulp. Additionally, bleaching reduced the energy demand by further 16%. The study demonstrated that this switchable ionic liquid treatment has considerable potential in the commercial production of nanofibers due to the increased efficiency in fibrillation.     

Bestimmung des Rubidiums mittels Silicowolframs?ure; die Anwendung dieses Reagenses zur Entdeckung des Rubidiums in den Aschen von Laminaria flexicaulis

Ohne Zusammenfassung