α-Chiral nitriles are prevalent structural motifs in natural products, drugs, and pharmaceutically active compounds. In addition, because the cyano group can undergo various diversifying reactions, ...they are very versatile chiral building blocks for the synthesis of optically active aldehydes, ketones, carboxylic acids, carboxamides, amines, and N-heterocycles such as tetrazolium. Accordingly, the catalytic enantioselective cyanation reaction, as one of the most important C–C bond forming reactions, has been intensely studied in the past three decades. While traditional efforts mainly focus on the addition of the cyanide to highly polarized electrophiles as well as alkene hydrocyanation, intriguing directions in enantioselective cyanations have been opened, including the development of enantioselective alkene cyanofunctionalization, cyanation via C–H bond functionalization, and C–C bond cleavage, as well as the invention of bifunctional cyanating agents to design asymmetric tandem synthesis and to exploit enantioselective cyanide-free cyanation reactions. Additionally, significant advances have also been made in developing unprecedented chiral catalysts to address a long-term challenge, the catalytic enantioselective synthesis of structurally diverse C α-tetrasubstituted or quaternary α-chiral nitriles. This perspective aims to highlight these impressive advances, illustrates their key advantages and future applications, and provides some inspiration for related research.
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•The development of anion coordination chemistry: from recognition to self-assembly.•Typical anion ligands based on hydrogen bonding.•Anion-coordination-based supramolecular ...assemblies.•Anion–π interaction and halogen bonding in designing anion ligands and assemblies.
The field of anion coordination chemistry has received more and more attention in recent years. A large number of articles, from the initial anion recognition to various applications, have been published in the past few decades. This review will focus on the recent progress in this fast-growing field, emphasizing the anion-ligand coordination properties from structural and geometrical aspects. Firstly, typical anion ligands based on hydrogen bonding are briefly summarized. This is followed by a collection of representative and the most recent examples of anion coordination compounds, whose structures are analyzed in terms of hydrogen bonding parameters and coordination number and geometry. In the third part, recent achievements in anion coordination-based supramolecular assemblies are discussed, and finally developments of two non-classical interactions (anion–π interactions and halogen bonding) in anion coordination and self-assembly processes are also described.
Covering: from 1998 to early 2014
Natural benzophenones are a class of compounds consisting of more than 300 members, which exhibit great structural diversity and bioactive properties. Many ...benzophenones have been reported from higher plants or fungi, most with polyisoprenylated benzophenone skeletons, and are mainly found in the Clusiaceae (formerly Guttiferae) family, a number from edible or medicinal species. Owing to their variable substituents and complex ring systems, many new polyisoprenylated benzophenones (PPBS), including ones with unusual skeletons, were isolated and identified. These natural benzophenones exhibit a range of biological activities including antifungal, anti-HIV, antimicrobial, antioxidant, antiviral and cytotoxic. Because of the increased numbers and biological importance of these unique natural product polyphenols, we will review natural benzophenones and provide an in-depth discussion of their structural diversity and biological activity. By focusing on these key developments in benzophenones, we will contribute a focused review, selecting examples mostly from the last 15 years, but extending our scope to other historically important benzophenones discovered prior to that time.
Natural benzophenones are a class of compounds with more than 300 members, mainly in the Clusiaceae family. We review key benzophenones, and provide an in-depth discussion of their great structural diversity and biological activity.
We present an ultra-broadband perfect absorber composed of metal-insulator composite multilayer (MICM) stacks by placing the insulator-metal-insulator (IMI) grating on the metal-insulator-metal (MIM) ...film stacks. The absorber shows over 90% absorption spanning between 570 nm and 3539 nm, with an average absorption of 97% under normal incidence. The ultra-broadband perfect absorption characteristics are achieved by the synergy of guided mode resonances (GMRs), localized surface plasmons (LSPs), propagating surface plasmons (PSPs), and cavity modes. The polarization insensitivity is demonstrated by analyzing the absorption performance over arbitrary polarization angles. The ultra-broadband absorption remains more than 80% over a wide incident angle up to 50°, for both transverse electric (TE) and transverse magnetic (TM) modes. The ultra-broadband perfect absorber has tremendous potential for various applications, such as solar thermal energy harvesting, thermoelectrics, and imaging.
We consider the problem of approximating sums of high dimensional stationary time series by Gaussian vectors, using the framework of functional dependence measure. The validity of the Gaussian ...approximation depends on the sample size n, the dimension p, the moment condition and the dependence of the underlying processes. We also consider an estimator for long-run covariance matrices and study its convergence properties. Our results allow constructing simultaneous confidence intervals for mean vectors of high-dimensional time series with asymptotically correct coverage probabilities. As an application, we propose a Kolmogorov–Smirnov-type statistic for testing distributions of high-dimensional time series.
Dietary energy available to animals is key for formulating feed as it is required for all aspects of the animal's life. In poultry, apparent (AME) and true (TME) metabolizable energy (ME) values have ...been used for feed formulation with (AMEn or TMEn) or without correction for nitrogen balance. For the past 50 yr, the accuracy of ME has been an ongoing debate, and the comparability of data produced using different bioassay systems is often questionable. Overall, the ingredient matric ME values used in feed formulation are not consistent, and to some extent, confusing. This review was to examine ME data published in the past century to elucidate the accuracy of different bioassay systems and examine the values for accuracy and useability. A variety of flaws are identified in the literature, suggesting a thorough re-thinking of feedstuff ME values currently used in feed formulation and in developing prediction equations. Two protocols, namely multiple linear regression and basal diet substitution methods, are proposed as more accurate bioassays for feedstuff ME values. AME aligns more closely with the actual energy levels of feed ingredients likely available to growing birds, which should be used for poultry feed formulations instead of AMEn. It is suggested that nutritionists need to carefully apply any reported AME values and only use those in formulation practice after careful scrutinizing. Any in vitro, NIR or table values must be calibrated or computed based on the values produced from flawless bioassays so as to apply the derived values accurately. Flaws identified in this literature review can be avoided with care to achieve more accurate AME. However, the assumption that the energy of individual ingredients is additive in a complete diet is still untrue at least under some circumstances. This may require efforts from industry and researchers to investigate relations among the main ingredients in a complete diet so that more accurate formulation can be performed based on the outcomes that may fine-tune the additivity assumption.
An innovative self‐healing polydimethylsiloxane (PDMS) elastomer, namely, PDMS‐TFB, is reported by incorporating the reversibly dynamic imine bond as the self‐healing points into the PDMS networks. ...The PDMS‐TFB elastomer features good optical transmittance (80%) in full visible light region, high stretchability (≈700%), and excellent autonomous self‐healing ability at room temperature. Surprisingly, the self‐healing behavior can take place in water and even at a temperature as low as −20 °C in air, showing a promising outlook for broader applications. As a proof‐of‐concept, this study demonstrates the use of the PDMS‐TFB elastomer for preparing anticorrosion coating and adhesive layer, and also the use of such an elastomer to be the platform for fabricating the flexible interconnector and chemical sensor. Remarkably, no significant difference is observed between the pristine and healed samples. Taking full advantage of these unique properties, it is anticipated that such a PDMS‐TFB elastomer shows wide applications in the fields of materials science, electronics, biology, optics, etc.
Autonomous self‐healing poly(dimethylsiloxane) elastomer with high optical transmittance (80%) and stretchability (≈700%) is prepared by simply mixing triformylbenzene and amine‐terminated poly(dimethylsiloxane), showing rapid and efficient self‐healing capability both in air and water over a broad temperature region. As a proof‐of‐concept, such an elastomer is used as the anticorrosion layer, reusable adhesive, and platforms for self‐healing and flexible interconnectors and chemical sensors.
The compound (Nb
6
O
19
)@Ag
34
(
t
BuC&z.tbd;C)
24
(CH
3
COO)
2
(Ag
34
) was synthesized using the solvothermal method combined with volatilization. This was the first case, to the best of our ...knowledge, of isolating a silver cluster containing a polyoxoniobate (PONb) template. The luminescence, solution behavior and solid-state stability of Ag
34
were studied in detail. Electrospray ionization mass spectrometry indicated that Ag
34
can maintain the integrity of its skeleton in solution. Detection of temperature could be a potential application of its unique luminescent behavior. We expect this work to inspire further fabrications of PONb-templated high-nuclearity silver clusters.
A polyoxoniobate-templated silver nanocluster was first obtained. The compound not only shows good chemical stability but also exhibits interesting temperature-dependent luminescent emission.
Wind speed forecasting is important for high-efficiency utilisation of wind energy and management of grid-connected power systems. Due to the noise, instability and irregularity of atmosphere system, ...the current models based on raw historical data have encountered many problems. In this study, a deep novel feature extraction approach is developed based on stacked denoising autoencoders and batch normalisation. Then the deep features extracted from raw historical data are fed to long short-term memory (LSTM) neural networks for prediction. Meanwhile, density-based spatial clustering of applications with noise is employed to process the numerical weather prediction data. By picking out the abnormal samples, the representative training samples are selected to improve the efficiency of the model. For illustration and verification purposes, the proposed model is used to predict the wind speed of Wind Atlas for South Africa (WASA). Empirical results show that deep feature extraction can improve the forecasting accuracy of LSTM 49% than feature selection, indicating that proper feature extraction is crucial to wind speed forecasting. And the proposed model outperforms other benchmark methods at least 17%. Hence, the proposed model is promising for wind speed forecasting.
Ultrasonic sensors have been extensively used in the nondestructive testing of materials for flaw detection. For polycrystalline materials, however, due to the scattering nature of the material, ...which results in strong grain noise and attenuation of the ultrasonic signal, accurate detection of flaws is particularly difficult. In this paper, a novel flaw-detection method using a simple ultrasonic sensor is proposed by exploiting time-frequency features of an ultrasonic signal. Since grain scattering mostly happens in the Rayleigh scattering region, it is possible to separate grain-scattered noise from flaw echoes in the frequency domain employing their spectral difference. We start with the spectral modeling of grain noise and flaw echo, and how the two spectra evolve with time is established. Then, a time-adaptive spectrum model for flaw echo is proposed, which serves as a template for the flaw-detection procedure. Next, a specially designed similarity measure is proposed, based on which the similarity between the template spectrum and the spectrum of the signal at each time point is evaluated sequentially, producing a series of matching coefficients termed moving window spectrum similarity (MWSS). The time-delay information of flaws is directly indicated by the peaks of MWSSs. Finally, the performance of the proposed method is validated by both simulated and experimental signals, showing satisfactory accuracy and efficiency.
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