807 research outputs found
Catalytic asymmetric aziridination of α,β-unsaturated aldehydes
No full textThe development, scope, and application of the highly enantioselective organocatalytic aziridination of α,β-unsaturated aldehydes is presented. The aminocatalytic azirdination of α,β-unsaturated aldehydes enables the asymmetric formation of β-formyl aziridines with up to >19:1 d.r. and 99% ee. The aminocatalytic aziridination of α-monosubstituted enals gives access to terminal α-substituted-α-formyl aziridines in high yields and up to 99% ee. In the case of the organocatalytic aziridination of disubstituted α,β-unsaturated aldehydes, the transformations were highly diastereo- and enantioselective and give nearly enantiomerically pure β-formyl-functionalized aziridine products (99% ee). A highly enantioselective one-pot cascade sequence based on the combination of asymmetric amine and N-heterocyclic carbene catalysis (AHCC) is also disclosed. This one-pot three-component co-catalytic transformation between α,β-unsaturated aldehydes, hydroxylamine derivatives, and alcohols gives the corresponding N-tert-butoxycarbonyl and N-carbobenzyloxy-protected β-amino acid esters with ee values ranging from 92-99%. The mechanisms and stereochemistry of all these catalytic transformations are also discussed
Recent Advances in the Synthesis and Application of Two-Dimensional Zeolites
No full textTwo-dimensional (2D) zeolites, originating from lamellar precursors, are a special kind of porous materials, in which crystalline zeolite nanolayers are weakly assembled in one particular direction. As there is no covalent-bond between layers, the stacking sequences could be manipulated and possibly controlled to derive a family of zeolitic materials with structural diversity. The research on 2D zeolites arises from the primary casual discovery during traditional hydrothermal synthesis, which is followed by appropriate postsynthetic modifications to produce hierarchical lamellar zeolites. Recently, this area has experienced a remarkable evolution, which is highlighted by the rational synthesis of zeolite nanosheets by adopting a family of bifunctional surfactants and novel 3D zeolites through the Assembly-Disassembly-Organization-Reassembly (ADOR) mechanism. Particularly, the direct construction of hierarchical lamellar zeolites with bifunctional surfactant is promising for industrial catalysis. Besides, an innovative top-down method has demonstrated to be effective to produce novel types of 2D zeolites. Novel or even unfeasible zeolite topologies could be constructed by organizing and reassembling of the resulting layers. Here, the most attractive points achieved in the field of 2D zeolites are summarized, and the challenges and opportunities of zeolite chemistry associated with this type of porous materials are also proposed.NSFC of China [21471009]; National Major Scientific Instruments Development Project [21527803]; State Key Development Program of Basic Research of China [2013CB933402]; China Postdoctoral Science Foundation [2015M580012]SCI(E)[email protected],SI
On the group of zero-cycles of holomorphic symplectic varieties
No full textFor a moduli space of Bridgeland-stable objects on a K3 surface, we show that
the Chow class of a point is determined by the Chern class of the corresponding
object on the surface. This establishes a conjecture of Junliang Shen, Qizheng
Yin, and the second author
Zeolite A synthesized from alkaline assisted pre-activated halloysite for efficient heavy metal removal in polluted river water and industrial wastewater
No full textHigh quality zeolite A was synthesized through a hydrothermal process using alkaline-assisted pre-activated halloysite mineral as the alumina and silica source. The synthesis conditions employed in this study were finely tuned by varying the activating temperature, sodium hydroxide content, water content and Si/Al ratio. The obtained zeolite A showed excellent adsorption properties for both single metal cation solutions and mixed cation solutions when the concentrations of the mixed cations were comparable with those in polluted natural river water and industrial wastewater. High adsorptive capacities for Ag+ (123.05 mg/g) and Pb2+ (227.70 mg/g) were achieved using the synthesized zeolite A. This observation indicates that the zeolite A synthesized from alkaline-assisted pre-activated halloysite can be used as a low-cost and relatively effective adsorbent to purify heavy metal cation polluted natural river water and industrial wastewater. (C) 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.Swedish Research Council (VR); Swedish Governmental Agency for Innovation Systems (VINNOVA) through the Berzelii EXSELENT; National Natural Science Foundation of China [21477129, 21273237]SCI(E)ARTICLE254-2625
Electron Crystallography Reveals Atomic Structures of Metal-Organic Nanoplates with M-12(mu(3)-O)(8)(mu(3)-OH)(8)(mu(2)-OH)(6) (M = Zr, Hf) Secondary Building Units
No full textNanoscale metal organic frameworks (nMOFs) have shown tremendous potential in cancer therapy and biomedical imaging. However, their small dimensions present a significant challenge in structure determination by single-crystal X-ray crystallography. We report here the structural determination of nMOFs by rotation electron diffraction (RED). Two isostructural Zr- and Hf-based nMOFs with linear biphenyldicarboxylate (BPDC) or bipyridinedicarboxylate (BPYDC) linkers are stable under intense electron beams to allow the collection of high-quality RED data, which reveal a MOF structure with M-12(mu(3)-O)(8)(mu(3)-OH)(8)(mu(2)-OH)(6) (M = Zr, Hf) secondary building units (SBUs). The nMOF structures differ significantly from their UiO bulk counterparts with M-6(mu(3)-O)(4)(mu(3)-OH)(4) SBUs and provide the foundation for clarifying the structures of a series of previously reported nMOFs with significant potential in cancer therapy and biological imaging. Our work clearly demonstrates the power of RED in determining nMOF structures and elucidating the formation mechanism of distinct nMOF morphologies.National Natural Science Foundation; Ministry of Science and Technology of the People's Republic of China [NSFC21671162, NSFC21471126, 2016YFA0200702]; National Thousand Talents Program of the People's Republic of China; 985 Program of Chemistry and Chemical Engineering Disciplines of Xiamen University; U.S. National Science Foundation [DMR-1308229]SCI(E)ARTICLE148128-81345
A Transfer Report on the Development of a Framework to Evaluate Search Interfaces for their Support of Different User Types and Search Tactics
No full textAs the understanding of search systems, user needs and seeking strategies is developing, the design of search user interfaces is evolving to support more complicated and exploratory forms of search. With the design of new search features that enable these richer modes of exploration, comes the need to better understand the support they provide. In this report a new evaluation framework is presented that analyses search features for how they a) contribute to an overall interface, b) allow users to carry out different search tactics, and c) support different types of users and their needs. The novel contributions of the framework improve on some of the limitations of typical user studies, and allow search systems to be systematically analysed in much more detail and in much less time. The presented evaluation framework is then validated in three ways. First the validity of the models used as the building blocks of the framework are investigated through related work. Second the method of integrating these building-block models is validated and strengthened by consensus of expert opinion. Third, the overall approach is validated by comparing its analyses to the results of previously carried out user studies. The validation process has shown both the value of the framework and identified areas of future work that should be addressed for the framework to be completed. This report concludes with the set of contributions that the framework makes, and why the remaining work will be challenging, but critical to the final design
Magnetically driven formation of 3D freestanding soft bioscaffolds.
No full text3D soft bioscaffolds have great promise in tissue engineering, biohybrid robotics, and organ-on-a-chip engineering applications. Though emerging three-dimensional (3D) printing techniques offer versatility for assembling soft biomaterials, challenges persist in overcoming the deformation or collapse of delicate 3D structures during fabrication, especially for overhanging or thin features. This study introduces a magnet-assisted fabrication strategy that uses a magnetic field to trigger shape morphing and provide remote temporary support, enabling the straightforward creation of soft bioscaffolds with overhangs and thin-walled structures in 3D. We demonstrate the versatility and effectiveness of our strategy through the fabrication of bioscaffolds that replicate the complex 3D topology of branching vascular systems. Furthermore, we engineered hydrogel-based bioscaffolds to support biohybrid soft actuators capable of walking motion triggered by cardiomyocytes. This approach opens new possibilities for shaping hydrogel materials into complex 3D morphologies, which will further empower a broad range of biomedical applications
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