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991.
Bruno Matos Paz Dr. Hao Jiang Prof. Dr. Karl Anker Jørgensen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(5):1846-1853
The stereochemical outcome of most aminocatalytic transformations is determined by the steric‐shielding or hydrogen‐directing approach. However, in recent years several reactions have appeared in which the stereochemical outcome is beyond these two approaches. This Concept article will highlight such reactions and postulate that the stereochemical outcome can be accounted for by electrostatic interactions between the catalyst‐bound substrate and the reagent in the transition state. 相似文献
992.
993.
Mechanistic Insights into the Mode of Action of Bifunctional Pyrrolidine‐Squaramide‐Derived Organocatalysts
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David Roca‐López Dr. Uxue Uria Dr. Efraim Reyes Dr. Luisa Carrillo Prof. Dr. Karl Anker Jørgensen Prof. Dr. Jose L. Vicario Prof. Dr. Pedro Merino 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(3):884-889
The catalytic modes of action of three squaramide‐derived bifunctional organocatalysts have been investigated using DFT methods. The [5+2] cycloaddition between oxidopyrylium ylides and enals was used as the model reaction. Two primary modes were possible for the different catalysts studied. The preference for one mode over the other was due to the possibility of additional favorable π–π interactions between the hydrogen‐bond activated pyrylium ylide and an electron‐deficient aromatic ring bonded to the squaramide NH group. The model can be extended to other reactions catalyzed by the same catalysts, such as formal [2+2] cycloadditions between nitroalkenes and α,β‐unsaturated aldehydes. The computational results were in excellent concurrence with the available experimental reports on the observed total enantioselectivity and differences in diastereoselectivity depending on the substrate and the reaction. 相似文献
994.
Donor/Acceptor‐Stabilized 1‐Silaketene: Reversible [2+2] Cycloaddition with Pyridine and Evolution by an Olefin Metathesis Reaction
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Dr. Morelia Lopez Reyes Dr. Thibault Troadec Dr. Ricardo Rodriguez Dr. Antoine Baceiredo Dr. Nathalie Saffon‐Merceron Prof. Vicenç Branchadell Dr. Tsuyoshi Kato 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(29):10247-10253
The reaction of silacyclopropylidene 1 with benzaldehyde generates a 1‐silaketene complex 2 by a formal atomic silicon insertion into the C=O bond of the aldehyde. The highly reactive 1‐silaketene 2 undergoes a reversible [2+2] cycloaddition with pyridine to give sila‐β‐lactam 3 . Of particular interest, in the presence of 4‐dimethylaminopyridine (DMAP), 1‐silaketene complex 2 evolves through an intramolecular olefin metathesis reaction, generating a new 1‐silaketene complex 8 and cis‐stilbene. Theoretical studies suggest that the reaction proceeds through the formation of a transient silacyclobutanone, a four‐membered‐ring intermediate, similar to that proposed by Chauvin and co‐workers for the transition‐metal‐based olefin metathesis. 相似文献
995.
996.
Israel Roger Montoya Matos 《高压研究》2017,37(3):296-311
Structural and spectroscopic properties of Sm3+-doped alkali silicate glasses were investigated after densification at 7.7?GPa in a large volume high pressure apparatus. The glass composition was 33M2O?+?66SiO2?+?1Sm2O3, where M?=?Li, Na or K. Raman and infrared spectroscopy revealed small changes in the vibrational modes dependent on the alkali ion. Irreversible changes were observed in the optical absorption spectra of Sm3+ ions. The Judd–Ofelt parameters were calculated to evaluate the effect of pressure on the local field. For lithium silicate, Ω2 parameter increased, suggesting the densification increased the local asymmetry of the Sm3+ environment. For sodium silicate, this parameter decreased considerably, suggesting the opposite effect on the local field, while for potassium silicate, it remained practically unchanged. The changes induced by high pressure are probably due to the irreversible changes in the distances and bond angles between the rare earth ion and the ligands. 相似文献
997.
998.
999.
Adam Slabon Eduardo Cuervo‐Reyes Oliver Niehaus Florian Winter Christian Mensing Rainer Pöttgen Reinhard Nesper 《无机化学与普通化学杂志》2014,640(10):1861-1867
The ternary Zintl phase EuMgGe was synthesized from the elements, and its structure solved by single‐crystal X‐ray diffraction. Chemical bonding is discussed, by means of electronic structure calculations at the DFT level and its physical properties characterized with respect to electronic conductivity, magnetic susceptibility, specific heat capacity, and magnetoresistivity. The compound may be interpreted according to the Zintl‐Klemm concept as (Eu2+)(Mg2+)(Ge4–) with isolated germanium anions. Resistivity measurements reveal a semimetallic character, which is consistent with the vanishing energy gap obtained from our calculations. The magnetic susceptibility and the specific heat indicate that two consecutive transitions take place, at 9 and 16 K, and they show evidence of magnetic frustration. A possible physical scenario for this magnetic behavior is discussed based on known models of partially frustrated magnets. 相似文献
1000.
Layane A. L. Silva Andr A. L. Silva Maria A. S. Rios Manoel P. Brito Alyne R. Araújo Durcilene A. Silva Ramn R. Pea-Garcia Edson C. Silva-Filho Janildo L. Magalhes Jos M. E. Matos Josy A. Osajima Eduardo R. Triboni 《Molecules (Basel, Switzerland)》2021,26(5)
Molybdates are biocidal materials that can be useful in coating surfaces that are susceptible to contamination and the spread of microorganisms. The aim of this work was to investigate the effects of copper doping of hydrated cobalt molybdate, synthesized by the co-precipitation method, on its antibacterial activity and to elucidate the structural and morphological changes caused by the dopant in the material. The synthesized materials were characterized by PXRD, Fourier Transformed Infrared (FTIR), thermogravimetric analysis/differential scanning calorimetry (TG/DSC), and SEM-Energy Dispersive Spectroscopy (SEM-EDS). The antibacterial response of the materials was verified using the Minimum Inhibitory Concentration (MIC) employing the broth microdilution method. The size of the CoMoO4·1.03H2O microparticles gradually increased as the percentage of copper increased, decreasing the energy that is needed to promote the transition from the hydrated to the beta phase and changing the color of material. CoMoO4·1.03H2O obtained better bactericidal performance against the tested strains of Staphylococcus aureus (gram-positive) than Escherichia coli (gram-negative). However, an interesting point was that the use of copper as a doping agent for hydrated cobalt molybdate caused an increase of MIC value in the presence of E. coli and S. aureus strains. The study demonstrates the need for caution in the use of copper as a doping material in biocidal matrices, such as cobalt molybdate. 相似文献