The first catalytic asymmetric construction of 3,3′‐bisindole skeletons bearing both axial and central chirality has been established by organocatalytic asymmetric addition reactions of 2‐substituted ...3,3′‐bisindoles with 3‐indolylmethanols (up to 98 % yield, all >95:5 d.r., >99 % ee). This reaction also represents the first highly enantioselective construction of axially chiral 3,3′‐bisindole skeletons, and utilizes the strategy of introducing a bulky group to the ortho‐position of prochiral 3,3′‐bisindoles. This reaction not only provides a good example for simultaneously controlling axial and central chirality in one operation, but also serves as a new strategy for catalytic enantioselective construction of axially chiral 3,3′‐bisindole backbones from prochiral substrates.
Bulk up: The first catalytic asymmetric construction of 3,3′‐bisindole skeletons bearing axial and central chirality has been established by organocatalytic asymmetric addition reactions of 2‐substituted 3,3′‐bisindoles with electrophiles. This reaction also represents the first highly enantioselective construction of axially chiral 3,3′‐bisindole skeletons, and utilizes the strategy of introducing a bulky group to the ortho‐position of prochiral 3,3′‐bisindoles.
Lactic acid bacteria (LAB) are the most frequently used probiotics in fermented foods and beverages and as food supplements for humans or animals, owing to their multiple beneficial features, which ...appear to be partially associated with their antioxidant properties. LAB can help improve food quality and flavor and prevent numerous disorders caused by oxidation in the host. In this review, we discuss the oxidative stress tolerance, the antioxidant capacity related herewith, and the underlying mechanisms and signaling pathways in probiotic LAB. In addition, we discuss appropriate methods used to evaluate the antioxidant capacity of probiotic LAB. The aim of the present review is to provide an overview of the current state of the research associated with the oxidative stress tolerance and antioxidant capacity of LAB.
Heterogeneous catalysis is a chemical process performed at a solid–gas or solid–liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the ...surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.
In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH
4
) under mild conditions for both chemical synthesis ...with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH
4
to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH
4
to value-added intermediate compounds including but not limited to CH
3
OH, HCHO, HCOOH, and CH
3
COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH
4
under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH
4
to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH
4
or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH
4
to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH
4
under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.
In the last few decades, scientists have been motivated by promising production of chemicals from methane under mild conditions for low energy consumption and climate remediation; significant fundamental understanding on this topic has been achieved.
Heterogeneous catalysis is one of the most important chemical processes of various industries performed on catalyst nanoparticles with different sizes or/and shapes. In the past two decades, the ...catalytic performances of different catalytic reactions on nanoparticles of metals and oxides with well controlled sizes or shapes have been extensively studied thanks to the spectacular advances in syntheses of nanomaterials of metals and oxides. This review discussed the size and shape effects of catalyst particles on catalytic activity and selectivity of reactions performed at solid-gas or solid-liquid interfaces with a purpose of establishing correlations of size- and shape-dependent chemical and structural factors of surface of a catalyst with the corresponding catalytic performances toward understanding of catalysis at a molecular level.
Heterogeneous catalysis is one of the most important chemical processes of various industries performed on catalyst nanoparticles with different sizes or/and shapes.
Chiral indole derivatives such as indolines and indolenines are important heterocyclic frameworks which constitute the core structures of many natural products and bioactive molecules. So, intensive ...attentions from chemists have been paid to the construction of such frameworks. Among different approaches, organocatalytic asymmetric dearomatization (organo-CADA) reactions of indole derivatives have become powerful methods toward this goal. Consequently, a variety of enantioenriched heterocyclic frameworks containing indoline, indolenine and the related cores have been constructed via organo-CADA reactions of indole derivatives, and a series of important natural products with structural complexity and enantiopurity have been synthesized based on these methodologies. This review summarizes the progresses in organo-CADA reactions of indole derivatives since 2004 and their applications in total synthesis of natural products, and gives some insights into challenging issues in this research field, which will enlighten the future development of this field.
An imidazolium‐type room‐temperature ionic liquid containing silsesquioxane frameworks is successfully prepared. The polar ionic liquid products are grafted onto the surface of prepared aromatic ...polyamide (PA) membranes to improve water flux of PA membranes. Membranes grafted with an ionic liquid exhibit a water flux of 1.89 L m−2 h−1 bar−1), 48% higher than that of pure PA membranes while the salt rejection still remains at 96%. This improvement can be attributed to the enhanced hydrophilicity of the membrane surface, as evidenced by the reduced water contact angle.
Imidazolium‐type ionic liquids containing silsesquioxane frameworks are prepared and grafted onto the surface of polyamide membranes to enhance the water flux of polyamide membranes. The results show that the water flux of membranes grafted with ionic liquid is increased by 48% compared with pure polyamide membranes while salt rejection is still high at 96%.
Comprehensive Summary
A new strategy for the enantioselective synthesis of axially chiral 3,3'‐bisindoles was devised by the direct coupling of two indole rings. This strategy makes use of the ...C3‐umpolung reactivity of 2‐indolylmethanols, which enables the catalytic asymmetric addition reaction of 2‐indolylmethanols with rationally designed 2‐substituted indoles, thus constructing axially chiral 3,3'‐bisindole scaffolds in overall excellent yields (up to 98%) with high enantioselectivities (up to 96 : 4 er). This approach not only has overcome the challenges in constructing axially chiral five‐five‐membered heterobiaryls, but also represents a new application of the C3‐umpolung reactivity of 2‐indolylmethanols in asymmetric catalysis. More importantly, this class of axially chiral 3,3'‐bisindoles can undergo a variety of post‐functionalizations to give axially chiral 3,3'‐bisindole‐based organocatalysts, which have found their preliminary applications in asymmetric catalysis.
A new strategy for the enantioselective synthesis of axially chiral 3,3'‐bisindoles was devised by the direct coupling of two indole rings. This strategy makes use of the C3‐umpolung reactivity of 2‐indolylmethanols to enable the reaction with rationally designed 2‐substituted indoles. This class of axially chiral 3,3'‐bisindoles can undergo post‐functionalizations to give 3,3'‐bisindole‐based chiral organocatalysts with potential applications in asymmetric catalysis.
Most materials and devices typically operate under specific environmental conditions, many of them highly reactive. Heterogeneous catalysts, for example, work under high pressure of reactants or in ...acidic solutions. The relationship between surface structure and composition of materials during operation and their chemical properties needs to be established in order to understand the mechanisms at work and to enable the design of new and better materials. Although studies of the structure, composition, chemical state, and phase transformation under working conditions are challenging, progress has been made in recent years in the development of new techniques that operate under a variety of realistic environments. With them, new chemistry and new structures of materials that are only present under reaction conditions have been uncovered.
Human-computer interaction (HCI) is an important supporting technology in the computer vision area, especially in physical education. HCI can promote the efficiency of physical education class, which ...is of great help to improve the learning efficiency. It is developing towards naturalization, intelligence, high efficiency, and materialization. Gesture recognition is very important in HCI, and plays a very important role in artistic understanding and image perception. Traditional gesture recognition methods are prone to misrecognition and result in low accuracy. In this paper, we propose a new gesture recognition method based on mask RCNN and single shot multibox detector (SSD) in HCI. Firstly, feature extraction and region segmentation are performed on the red, green, and blue (RGB) three-channel images, and the hand instance segmentation and mask are obtained. Then we modify the SSD model to obtain a new convolution layer, which can realize the fusion of shallow visual convolution layer and deep semantic convolution layer in the network structure. To solve the problem of poor classification performance caused by the imbalance of positive and negative samples, an improved loss function is proposed to improve the model ability of classifying target gestures. The experimental results show that compared with state-of-the-art methods, the proposed method has better robustness and faster detection speed while maintaining higher gesture detection accuracy.
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