The development of efficient methods, particularly catalytic and enantioselective processes, for the construction of all-carbon quaternary stereocentres is an important (and difficult) challenge in ...organic synthesis due to the occurrence of this motif in a range of bioactive molecules. One conceptually straightforward and potentially versatile approach is the catalytic enantioconvergent substitution reaction of a readily available racemic tertiary alkyl electrophile by an organometallic nucleophile; however, examples of such processes are rare. Here we demonstrate that a nickel-based chiral catalyst achieves enantioconvergent couplings of a variety of tertiary electrophiles (cyclic and acyclic α-halocarbonyl compounds) with alkenylmetal nucleophiles to form quaternary stereocentres with good yield and enantioselectivity under mild conditions in the presence of a range of functional groups. These couplings, which probably proceed via a radical pathway, provide access to an array of useful families of organic compounds, including intermediates in the total synthesis of two natural products, (-)-eburnamonine and madindoline A.
Solar photocatalysis is a potential solution to satisfying energy demand and its resulting environmental impact. However, the low electron–hole separation efficiency in semiconductors has slowed the ...development of this technology. The effect of defects on electron–hole separation is not always clear. A model atomically thin structure of single‐unit‐cell Bi3O4Br nanosheets with surface defects is proposed to boost photocatalytic efficiency by simultaneously promoting bulk‐ and surface‐charge separation. Defect‐rich single‐unit‐cell Bi3O4Br displays 4.9 and 30.9 times enhanced photocatalytic hydrogen evolution and nitrogen fixation activity, respectively, than bulk Bi3O4Br. After the preparation of single‐unit‐cell structure, the bismuth defects are controlled to tune the oxygen defects. Benefiting from the unique single‐unit‐cell architecture and defects, the local atomic arrangement and electronic structure are tuned so as to greatly increase the charge separation efficiency and subsequently boost photocatalytic activity. This strategy provides an accessible pathway for next‐generation photocatalysts.
Single‐unit‐cell Bi3O4Br nanosheets with both bismuth and oxygen defects are controllably prepared. These engineered surface defects can effectively tune the local atomic arrangement and electronic structure so as to greatly increase the charge‐separation efficiency. Benefiting from these features, defect‐rich single‐unit‐cell Bi3O4Br displays 4.9 and 30.9 times enhanced photocatalytic hydrogen evolution and nitrogen fixation activity, respectively, over bulk Bi3O4Br.
The development of stable and efficient hydrogen evolution reaction (HER) catalysts is essential for the production of hydrogen as a clean energy resource. A combination of experiment and theory ...demonstrates that the normally inert basal planes of 2D layers of MoS2 can be made highly catalytically active for the HER when alloyed with rhenium (Re). The presence of Re at the ≈50% level converts the material to a stable distorted tetragonal (DT) structure that shows enhanced HER activity as compared to most of the MoS2‐based catalysts reported in the literature. More importantly, this new alloy catalyst shows much better stability over time and cycling than lithiated 1T‐MoS2. Density functional theory calculations find that the role of Re is only to stabilize the DT structure, while catalysis occurs primarily in local Mo‐rich DT configurations, where the HER catalytic activity is very close to that in Pt. The study provides a new strategy to improve the overall HER performance of MoS2‐based materials via chemical doping.
Re‐doped MoS2 atomic layers in the distorted tetragonal structure show excellent activity and stability for electrocatalytic hydrogen production. Atomic‐level scanning transmission electron microscopy combined with density functional theory calculations reveal active local Mo‐rich structures and explain the best performance in Re0.55Mo0.45S2. The study provides a new catalyst design strategy through chemical doping.
Single‐atom catalysts (SACs) are the smallest entities for catalytic reactions with projected high atomic efficiency, superior activity, and selectivity; however, practical applications of SACs ...suffer from a very low metal loading of 1–2 wt%. Here, a class of SACs based on atomically dispersed transition metals on nitrogen‐doped carbon nanotubes (MSA‐N‐CNTs, where M = Ni, Co, NiCo, CoFe, and NiPt) is synthesized with an extraordinarily high metal loading, e.g., 20 wt% in the case of NiSA‐N‐CNTs, using a new multistep pyrolysis process. Among these materials, NiSA‐N‐CNTs show an excellent selectivity and activity for the electrochemical reduction of CO2 to CO, achieving a turnover frequency (TOF) of 11.7 s−1 at −0.55 V (vs reversible hydrogen electrode (RHE)), two orders of magnitude higher than Ni nanoparticles supported on CNTs.
A novel atomically dispersed transition‐metal single‐atom catalyst supported on carbon nanotubes is synthesized with atomic loading as high as 20 wt%, excellent selectivity, and activity for the electrochemical reduction of carbon dioxide.
Exploring new materials is essential in the field of material science. Especially, searching for optimal materials with utmost atomic utilization, ideal activities and desirable stability for ...catalytic applications requires smart design of materials' structures. Herein, we report iridium metallene oxide: 1 T phase-iridium dioxide (IrO
) by a synthetic strategy combining mechanochemistry and thermal treatment in a strong alkaline medium. This material demonstrates high activity for oxygen evolution reaction with a low overpotential of 197 millivolt in acidic electrolyte at 10 milliamperes per geometric square centimeter (mA cm
). Together, it achieves high turnover frequencies of 4.2 s
(3.0 s
) at 1.50 V vs. reversible hydrogen electrode. Furthermore, 1T-IrO
also shows little degradation after 126 hours chronopotentiometry measurement under the high current density of 250 mA cm
in proton exchange membrane device. Theoretical calculations reveal that the active site of Ir in 1T-IrO
provides an optimal free energy uphill in *OH formation, leading to the enhanced performance. The discovery of this 1T-metallene oxide material will provide new opportunities for catalysis and other applications.
Exosomes contain cell-specific collections of bioactive materials including proteins, lipids, and RNAs that are transported to recipient cells to exert their impacts. MicroRNAs (miRNAs) can function ...as tumor suppressor or oncogenic genes and miR-21 is one of the most frequently up-regulated miRNAs in solid tumors including colon cancer. The aim of this study was to investigate the role of miR-21, secreted from exosomes, in proliferation and invasion of colon cancer, along with the mechanistic details. We used a variety of biochemical techniques including ultracentrifugation-based exosome purification, electron transmission microscopy, western blot and RT-qPCR to detect the expression levels of miR-21 in exosomes purified from culture media of human colonic adenocarcinoma cell lines. We then performed functional and mechanistic studies using three colon cancer cell lines HT29, T84 and LS174 as well as the normal colon epithelial cells CRL1831. miR-21 target PDCD4 was investigated for its role in mediating miR-21 effects. Expression of miR-21 was significantly up-regulated in exosomes of colon cancer cells, compared to the normal human colon epithelial cells. Treatment of colon cancer cells with isolated exosomes or miR-21 led to an increased expression of genes involved in cell proliferation, invasion and extracellular matrix formation. miR-21 targets PDCD4, TPM1 and PTEN were down-regulated by exosomes and silencing of PDCD4 mimicked miR-21 functional effects, even the induced resistance against 5-FU. Our study suggests that targeted inhibition of exosomes, particularly those carrying miR-21, may represent a novel approach for treatment of colorectal cancer.
Because chiral dialkyl carbinols, as well as their derived esters, are significant as intermediates and end points in fields such as organic, pharmaceutical, and biological chemistry, the development ...of efficient approaches to their asymmetric synthesis is an important endeavor. In this report, we describe a method for the direct catalytic enantioselective synthesis of such esters, beginning with an alkyl halide (derived from an aldehyde and an acyl bromide), an olefin, and a hydrosilane, catalyzed by nickel, an earth-abundant metal. The method is versatile, tolerating substituents that vary in size and that bear a range of functional groups. We further describe a four-component variant of this process, wherein the alkyl halide is generated in situ, thus obviating the need to isolate either an alkyl electrophile or an alkylmetal, while still effecting an alkyl–alkyl coupling. Finally, we apply our convergent method to the efficient catalytic enantioselective synthesis of three esters that are bioactive themselves or that have been utilized in the synthesis of bioactive compounds.
Herein we report a versatile Mizoroki–Heck‐type photoinduced C(sp3)−N bond cleavage reaction. Under visible‐light irradiation (455 nm, blue LEDs) at room temperature, alkyl Katritzky salts react ...smoothly with alkenes in a 1:1 molar ratio in the presence of 1.0 mol % of commercially available photoredox catalyst without the need for any base, affording the corresponding alkyl‐substituted alkenes in good yields with broad functional‐group compatibility. Notably, the E/Z‐selectivity of the alkene products can be controlled by an appropriate choice of photoredox catalyst.
Under visible‐light irradiation (455 nm, blue LEDs) at room temperature, alkyl Katritzky salts react smoothly with alkenes in a 1:1 molar ratio in the presence of 1.0 mol % of commercially available photoredox catalyst without the need for any base, affording the corresponding alkyl‐substituted alkenes in good yields with broad functional‐group compatibility (see scheme).
Single‐atom catalysts (SACs) are attracting significant attention due to their exceptional catalytic performance and stability. However, the controllable, scalable, and efficient synthesis of SACs ...remains a significant challenge. Herein, a new and versatile seeding approach is reported to synthesize SACs supported on different 2D materials such as graphene, boron nitride (BN), and molybdenum disulfide (MoS2). This method is demonstrated on the synthesis of Ni, Co, Fe, Cu, Ag, Pd single atoms as well as binary atoms of Ni and Cu codoped on 2D support materials with the mass loading of single atoms in the range of 2.8–7.9 wt%. In particular, the applicability of the new seeding strategy in electrocatalysis is demonstrate on nickel SACs supported on graphene oxide (SANi‐GO), exhibiting excellent catalytic performance for electrochemical CO2 reduction reaction with a turnover frequency of 325.9 h−1 at a low overpotential of 0.63 V and high selectivity of 96.5% for CO production. The facile, controllable, and scalable nature of this approach in the synthesis of SACs is expected to open new research avenues for the practical applications of SACs.
A facile, controllable, and scalable method is developed for the fabrication of single‐atom catalysts (SACs) on various 2D material supports with high loading and activities via a new seeding approach, significantly accelerating the practical application of SACs for the areas of electrocatalysis and catalysis.
Modern object detectors rely heavily on rectangular bounding boxes, such as anchors, proposals and the final predictions, to represent objects at various recognition stages. The bounding box is ...convenient to use but provides only a coarse localization of objects and leads to a correspondingly coarse extraction of object features. In this paper, we present RepPoints (representative points), a new finer representation of objects as a set of sample points useful for both localization and recognition. Given ground truth localization and recognition targets for training, RepPoints learn to automatically arrange themselves in a manner that bounds the spatial extent of an object and indicates semantically significant local areas. They furthermore do not require the use of anchors to sample a space of bounding boxes. We show that an anchor-free object detector based on RepPoints can be as effective as the state-of-the-art anchor-based detection methods, with 46.5 AP and 67.4 AP50 on the COCO test-dev detection benchmark, using ResNet-101 model. Code is available at https://github.com/microsoft/RepPoints.
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