Synthesis of Gentamine C1a from Neamine Using Ionic Displacement and Radical Elimination Methods

Abstract

The conversion of tetra-N-benzyloxycarbonyl-5,6-O-cyclohexylidene neamine (8) into the corresponding olefin 13 has been investigated by three methods. Application of the Tipson-Cohen procedure via the dimesylate 10 gave a low yield (22%), as did the reaction of the diol 8 with triphenylphosphine-iodine-imidazole reagent (42%). Contrastingly, reductive radical elimination via the dixanthate 11 gave a synthetically useful yield (64%) without any chromatographic purification.     

Pressure versus Temperature Effects on Intramolecular Electron Transfer in Mixed‐Valence Complexes

Mixed‐valence trinuclear carboxylates, [M₃O(O₂CR)₆L₃] (M=metal, L=terminal ligand), have small differences in potential energy between the configurations M^IIM^IIIM^III?? M^IIIM^IIM^III??M^IIIM^IIIM^II, which means that small external changes can have large structural effects, owing to the differences in coordination geometry between M²⁺ and M³⁺ sites (e.g., about 0.2 Å for Fe? O bond lengths). It is well‐established that the electron transfer (ET) between the metal sites in these mixed‐valence molecules is strongly dependent on temperature and on the specific crystal environment; however, herein, for the first time, we examine the effect of pressure on the electron transfer. Based on single‐crystal X‐ray diffraction data that were measured at 15, 90, 100, 110, 130, 160, and 298 K on three different crystals, we first unexpectedly found that our batch of Fe₃O (O₂CC(CH₃)₃)₆(C₅H₅N)₃ ( 1 ) exhibited a different temperature dependence of the ET process than previous studies of compound 1 have shown. We observed a phase transition at around 130 K that was related to complete valence trapping and Hirshfeld surface analysis revealed that this phase transition was governed by a subtle competition between C? H???π and π???π intermolecular interactions. Subsequent high‐pressure single‐crystal X‐ray diffraction at pressures of 0.15, 0.35, 0.45, 0.74, and 0.96 GPa revealed that it was not possible to trigger the phase transition (i.e., valence trapping) by a reduction of the unit‐cell volume, owing to this external pressure. We conclude that modulation of the ET process requires anisotropic changes in the intermolecular interactions, which occur when various directional chemical bonds are affected differently by changes in temperature, but not by the application of pressure.     

Cyclisation versus 1,1‐Carboboration: Reactions of B(C6F5)3 with Propargyl Amides

A series of propargyl amides were prepared and their reactions with the Lewis acidic compound B(C₆F₅)₃ were investigated. These reactions were shown to afford novel heterocycles under mild conditions. The reaction of a variety of N‐substituted propargyl amides with B(C₆F₅)₃ led to an intramolecular oxo‐boration cyclisation reaction, which afforded the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. Secondary propargyl amides gave oxazoles in B(C₆F₅)₃ mediated (catalytic) cyclisation reactions. In the special case of disubstitution adjacent to the nitrogen atom, 1,1‐carboboration is favoured as a result of the increased steric hindrance (1,3‐allylic strain) in the 5‐alkylidene‐4,5‐dihydrooxazolium borate species.     

Numerical analysis of Ni/Al hybrid metal foams using the finite cell method

Aluminium metal foams are a new type of material that can be used in many lightweight applications. One method to improve their mechanical properties is to coat them with a thin layer of nickel by electrodeposition. A voxel representation of this hybrid foam can be obtained in a straightforward way using a CT scanner. The voxel-based geometry can then be processed and modified with respect to the thickness of the nickel layer in order to investigate its influence on the effective properties. By employing the finite cell method (FCM) we are able to automatically convert the voxel-based geometry into a finite cell grid and to directly perform a homogenization procedure. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hafnia nanoparticles – a model system for graphene growth on a dielectric

We study graphene growth on hafnia (HfO₂) nanoparticles by chemical vapour deposition using optical microscopy, high resolution transmission electron microscopy and Raman spectroscopy. We find that monoclinic HfO₂ nanoparticles neither reduce to a metal nor form a carbide while nucleating nanometer domain‐sized few layer graphene. Hence we regard this as an interesting non‐metallic catalyst model system with the potential to explore graphene growth directly on a (high‐k) dielectric.

     

Nano-energy research trends: bibliometrical analysis of nanotechnology research in the energy sector

Nano-energy, the part of nanotechnology dedicated to the study and improvement of the Energy Supply Sector, is a promising and perspective research field. A robust method to quantify international scientific activities in this field is the literature search. An evaluative bibliometric approach applied to the Science Citation Index has been done to retrieve a set of articles related to nano-energy and get knowledge of the direction and trends followed by this particular scientific topic. The resulting database showed an exponential increase of the number of publications issuing nano-based investigations in the energy sector in the last decade, accelerating to an annual growth rate of 1,100%. The most cited articles and the material-clustering protocol revealed that carbon-nanoelements and their application in solar energy harvesting and conversion, and energy storage devices have been principally investigated and represent the main focus in that continuously growing research field. The number of nanotechnology-related papers in the energy database increased monotonically for harvesting, conversion, and storage the last decade, being energy distribution and usage not affected. TiO₂ or SnO₂ nanoparticles or thin films, and nanocomposites occupied the following top positions in the investigated material ranking. This trend was constant along the decade, as confirmed by network analyses. Supported by discipline-clustering, we observed the fundamental character of the research developed between 2000 and 2009, relying mainly on material science and chemistry. Hence, further implementation of nanotechnology findings is needed to stimulate nano-based energy-focused technologies reaching widespread commercial applications.     

Unveiling the magnetic structure of graphene nanoribbons

We perform magnetotransport measurements in lithographically patterned graphene nanoribbons down to a 70 nm width. The electronic spectrum fragments into an unusual Landau levels pattern, characteristic of Dirac fermion confinement. The two-terminal magnetoresistance reveals the onset of magnetoelectronic subbands, edge currents and quantized Hall conductance. We bring evidence that the magnetic confinement at the edges unveils the valley degeneracy lifting originating from the electronic confinement. Quantum simulations suggest some disorder threshold at the origin of mixing between chiral magnetic edge states and disappearance of quantum Hall effect.     

Realization of an optomechanical interface between ultracold atoms and a membrane

We have realized a hybrid optomechanical system by coupling ultracold atoms to a micromechanical membrane. The atoms are trapped in an optical lattice, which is formed by retroreflection of a laser beam from the membrane surface. In this setup, the lattice laser light mediates an optomechanical coupling between membrane vibrations and atomic center-of-mass motion. We observe both the effect of the membrane vibrations onto the atoms as well as the backaction of the atomic motion onto the membrane. By coupling the membrane to laser-cooled atoms, we engineer the dissipation rate of the membrane. Our observations agree quantitatively with a simple model.     

Determination of local chirality in irregular single-walled carbon nanotubes based on individual hexagons

We develop a robust theoretical method for the determination of local chirality based on individual hexagons that compose single-walled carbon nanotubes (SWCNTs). The method and code are applied to various SWCNT-like irregular structures that are formed during the growth process. The local chiral index and its distribution are well defined for irregular structures as well as ideal structures of SWCNTs, and can be used to characterize any SWCNT-like irregular structures in terms of local chirality. The present method also permits monitoring the chirality of nanostructures during the growth process.