Just add light and air: Structurally diverse N‐arylindoles can be prepared from readily prepared o‐styryl anilines through visible‐light photocatalysis. The reaction, which is conducted open to air, ...is mediated by Ru(bpz)3(PF6)2 (bpz=2,2′‐bipyrazine) and involves both CN bond formation and aromatization (see scheme). Using suitably substituted substrates, a 1,2‐carbon shift can be also incorporated into this cascade reaction.
•Hydrate reformation during exploitation is caused mainly by insufficient heat supply.•Hydrate reformation is likely to occur in the depressurization process.•The depressurization process should be ...combined with thermal stimulation.•A new system is designed to investigate hydrate reformation under various conditions.
Natural gas hydrates are considered as some of the most promising energy sources. However, economical, safe, and commercial exploitation of marine hydrates has not yet been achieved. One of the main obstacles is hydrate reformation in the process of hydrate exploitation, which needs to be properly addressed. The processes of hydrate dissociation and reformation under depressurization and thermal stimulation are discussed in this review. It is necessary to understand the manner in which various factors affect hydrate reformation during dissociation. Therefore, the effects of critical factors on hydrate reformation during thermal stimulation and depressurization dissociation are analysed to provide a comprehensive explanation of the hydrate reformation mechanism. In addition, several measures used to prevent hydrate reformation during the exploitation process are discussed. Finally, an experimental system was designed to investigate the exploitation methods and optimization of various hydrate-bearing sediments. Future efforts should focus on finding effective methods and evaluating the combined sequences, which can prevent hydrate reformation with acceptable exploitation efficiency.
A highly regioselective method to access 5-amido fully substituted 1,2,3-triazoles by iridium-catalyzed azide–ynamide cycloaddition under mild, air, aqueous, and bioorthogonal conditions is reported. ...The excellent regioselectivities may derive from the strong coordination between the carbonyl oxygen of ynamide and the π-acidic iridium. Since the iridium ion is insensitive to oxygen/water and exhibits low cytotoxicity, it could catalyze this reaction in both organic and biological environments efficiently. Preparation in gram-scale and application in carbohydrates highlight this method.
C−N bonds are pervasive throughout organic‐based materials, natural products, pharmaceutical compounds, and agricultural chemicals. Considering the widespread importance of C−N bonds, the development ...of greener and more convenient ways to form C−N bonds, especially in late‐stage synthesis, has become one of the hottest research goals in synthetic chemistry. Copper‐catalyzed radical reactions involving N‐centered radicals have emerged as a sustainable and promising approach to build C−N bonds. As a chemically popular and diverse radical species, N‐centered radicals have been used for all kinds of reactions for C−N bond formation by taking advantage of their inherently incredible reactive flexibility. Copper is also the most abundant and economic catalyst with the most relevant activity for facilitating the synthesis of valuable compounds. Therefore, the aim of the present Review was to illustrate recent and significant advances in C−N bond formation methods and to understand the unique advantages of copper catalysis in the generation of N‐centered radicals since 2016. To provide an ease of understanding for the readers, this Review was organized based on the types of nitrogen sources (amines, amides, sulfonamides, oximes, hydrazones, azides, and tert‐butyl nitrite).
A radical approach: Copper‐catalyzed radical reactions involving N‐centered radicals have emerged as a sustainable and promising approach to build C−N bonds. Therefore, the aim of this Review is to illustrate recent and significant advances in C−N bond formation methods and understand the unique advantages of copper catalysis in the generation of N‐centered radicals since 2016.
In this paper, we propose a methodology to design learning systems based on a memristor crossbar structure. Learning is carried out with the help of a hardware-friendly spike-timing-dependent ...plasticity learning rule. Several simplifications and adaptations are made in order to apply the learning algorithm to memristor-based neural networks. The difficulties in conducting division and representing signed weights are circumvented using the proposed techniques. In addition, different conductance-changing behaviors are considered to demonstrate the effectiveness of the proposed algorithm and architecture when applied to practical memristor devices. Furthermore, various nonidealities existing in the neural network, including variations in CMOS neurons and memristor synapses and noises associated with updating the synaptic weights, are considered. We demonstrate that the proposed learning algorithm and hardware architecture are robust against most variations and noises. This robustness of learning is promising, as variations and stochastic behaviors of memristor devices are usually substantial. Memristor-based neural networks with the proposed learning algorithm are benchmarked with the MNIST handwritten digits recognition task. A recognition accuracy as high as 97.10% is demonstrated.
Gas hydrate is a nonstoichiometric crystal compound formed from water and gas. Most nonvisual studies on gas hydrate are unable to detect how much water is converted to hydrates, and thus, the ...hydrate stoichiometry calculations are inaccurate. This study investigated the CO2 hydrate formation process in porous media directly and quantitatively. The characteristics of the time‐variable consumption of hydrate formation indicated a two‐stage formation, hydrate enclathration and continuous occupancy. The enclathration stage occurred in the first 20 min of the formation when considerable heat is released. The continuous occupancy stage lasted longer than the hydrate enclathration because the empty cages in previously formed hydrates would also be occupied. The higher formation pressures can accelerate water consumption and increase cage occupancy. The compositions of completely formed CO2 hydrates at 2.7, 3.0, and 3.3 MPa and 275.15 K were determined as CO2·6.90H2O, CO2·6.70H2O, and CO2·6.49H2O, respectively.
In this paper, we propose an online learning algorithm for supervised learning in multilayer spiking neural networks (SNNs). It is found that the spike timings of neurons in an SNN can be exploited ...to estimate the gradients that are associated with each synapse. With the proposed method of estimating gradients, learning similar to the stochastic gradient descent process employed in a conventional artificial neural network (ANN) can be achieved. In addition to the conventional layer-by-layer backpropagation, a one-pass direct backpropagation is possible using the proposed learning algorithm. Two neural networks, with one and two hidden layers, are employed as examples to demonstrate the effectiveness of the proposed learning algorithms. Several techniques for more effective learning are discussed, including utilizing a random refractory period to avoid saturation of spikes, employing a quantization noise injection technique and pseudorandom initial conditions to decorrelate spike timings, in addition to leveraging the progressive precision in an SNN to reduce the inference latency and energy. Extensive parametric simulations are conducted to examine the aforementioned techniques. The learning algorithm is developed with the considerations of ease of hardware implementation and relative compatibility with the classic ANN-based learning. Therefore, the proposed algorithm not only enjoys the high energy efficiency and good scalability of an SNN in its specialized hardware but also benefits from the well-developed theory and techniques of conventional ANN-based learning. The Modified National Institute of Standards and Technology database benchmark test is conducted to verify the newly proposed learning algorithm. Classification correct rates of 97.2% and 97.8% are achieved for the one-hidden-layer and two-hidden-layer neural networks, respectively. Moreover, a brief discussion of the hardware implementations is presented for two mainstream architectures.
Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is a poor inducer of innate antiviral immunity, and the underlying mechanism still needs further investigation. Here, we reported that ...SARS‐CoV‐2 NSP7 inhibited the production of type I and III interferons (IFNs) by targeting the RIG‐I/MDA5, Toll‐like receptor (TLR3)‐TRIF, and cGAS‐STING signaling pathways. SARS‐CoV‐2 NSP7 suppressed the expression of IFNs and IFN‐stimulated genes induced by poly (I:C) transfection and infection with Sendai virus or SARS‐CoV‐2 virus‐like particles. NSP7 impaired type I and III IFN production activated by components of the cytosolic dsRNA‐sensing pathway, including RIG‐I, MDA5, and MAVS, but not TBK1, IKKε, and IRF3‐5D, an active form of IRF3. In addition, NSP7 also suppressed TRIF‐ and STING‐induced IFN responses. Mechanistically, NSP7 associated with RIG‐I and MDA5 prevented the formation of the RIG‐I/MDA5−MAVS signalosome and interacted with TRIF and STING to inhibit TRIF‐TBK1 and STING‐TBK1 complex formation, thus reducing the subsequent IRF3 phosphorylation and nuclear translocation that are essential for IFN induction. In addition, ectopic expression of NSP7 impeded innate immune activation and facilitated virus replication. Taken together, SARS‐CoV‐2 NSP7 dampens type I and III IFN responses via disruption of the signal transduction of the RIG‐I/MDA5−MAVS, TLR3‐TRIF, and cGAS‐STING signaling pathways, thus providing novel insights into the interactions between SARS‐CoV‐2 and innate antiviral immunity.
In energy-constrained applications, SRAM systems operating in the subthreshold region are often deployed to reduce power consumption. Subthreshold SRAM designs, however, confront numerous challenges, ...such as susceptibility to process variation and reduced ON-OFF current ratio. Statistical modeling of the variation in cell stability is critical in SRAM design, especially, for designs operating in the subthreshold region, where the process and temperature variations are the most pronounced. In this paper, statistical models for estimating the static noise margins (SNMs) of SRAM cells are built from the perspective of a shifted voltage transfer characteristic. Read (hold) SNM of a subthreshold 8T cell is analyzed. It is shown that the distribution of a single-sided read SNM is a weighted sum of several normal distributions instead of a regular Gaussian distribution. The proposed statistical model is verified with simulation results in 65-nm technology. Furthermore, to mitigate performance and yield degradation, an adaptive body biasing circuit is developed. It is demonstrated through simulation that, with a negligible area and power overhead, the proposed circuit achieves a 15% improvement in the worst case read SNM.
Cascade reactions represent a class of ideal organic reactions because they empower efficiency, elegance, and novelty. However, development of cascade reactions remains a daunting task for synthetic ...chemists. Radicals are known to be well suited for cascade reactions. Compared with widely used carbon-based radicals, nitrogen-based radicals, such as neutral aminyl radicals and protonated aminyl radicals (amine radical cations), are underutilized, although they are behind some notable synthetic methods such as the Hofmann–Löffler–Freytag reaction. The constraint on their usage is generally attributed to the limited number of available stable precursors. Since amine radical cations offer increased reactivity and selectivity in chemical transformations compared with neutral aminyl radicals, their generation is of utmost importance. Recently, a surge of reports has been revealed using visible light photoredox catalysis. It has been demonstrated that amines can act as an electron donor in a reductive quenching cycle while the amine itself is oxidized to the amine radical cation. Although a number of methods exist to generate amine radical cations, the photochemical formation of these species offers many practical advantages. In this Account, we discuss our journey to the development of annulation reactions with various π-bonds and electrophilic addition reactions to alkenes using photogenerated amine radical cations. Various carbocycles and heterocycles are produced by these reactions. In our annulation work, we first show that single electron photooxidation of cyclopropylanilines to the amine radical cations triggers ring opening of the strained carbocycle, producing distonic radical cations. These odd-electron species are shown to react with alkenes and alkynes to yield the corresponding cyclopentanes and cyclopentenes in an overall redox neutral process. Further development of this annulation reaction allows us to achieve the 4 + 2 annulation of cyclobutylanilines with alkynes. In our work on electrophilic addition reactions to alkenes, we reveal that photogenerated amine radical cations are capable of undergoing the electrophilic addition reactions to alkenes to form a variety of indoles and indolines. This chemistry represents a rare oxidative C–N bond-forming reaction using visible light. Conclusions drawn from observational results and proposed mechanisms are outlined in this Account. Additionally, open discussion of our successes and deficiencies in our experiences will give readers helpful insights as to how these species tend to react. The overall utility of photogenerated amine radical cations has yet to reach its full potential. With our current results, we anticipate more new transformations can still be derived from the ring opening processes of cyclopropylanilines and cyclobutylanilines under visible light photocatalysis. Additionally, since utilizing photogenerated amine radical cations in C–N bond-forming reactions has practically been absent in literature, we are confident more new reactions have yet been exploited.
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