Conspectus Asymmetric functionalization of inert C–H bonds is undoubtedly a synthetically significant yet challenging bond-forming process, allowing for the preparation of densely functionalized ...molecules from abundantly available feedstocks. In the past decade, our group and others have found that trivalent phosphorus ligands are capable of facilitating Pd-catalyzed allylic C–H functionalization of α-alkenes upon using p-quinone as an oxidant. In these reactions, a 16-electron Pd(0) complex bearing a monodentate phosphorus ligand, a p-quinone, and an α-alkene has been identified as a key intermediate. Through a concerted proton and two-electron transfer process, electrophilic π-allylpalladium is subsequently generated and can be leveraged to forge versatile chemical bonds with a wide range of nucleophiles. This Account focuses on describing the origin, evolution, and synthetic applications of Pd-catalyzed asymmetric allylic C–H functionalization reactions, with an emphasis on the fundamental mechanism of the concerted proton and two-electron transfer process in allylic C–H activation. Enabled by the cooperative catalysis of the palladium complex of triarylphosphine, a primary amine, and a chiral phosphoric acid, an enantioselective α-allylation of aldehydes with α-alkenes is established. The combination of chiral phosphoric acid and a palladium complex of a chiral phosphoramidite ligand allows the allylic C–H alkylation of α-alkenes with pyrazol-5-ones to give excellent enantioselectivities, wherein the chiral ligand and chiral phosphoric acid synergistically control the stereoselectivity. Notably, the palladium–phosphoramidite complexes are also efficient catalysts for allylic C–H alkylation, with a wide scope of nucleophiles. In the case of 1,4-dienes, the geometry and coordination pattern of the nucleophile are able to vary the transition states of bond-forming events and thereby determine the Z/E-, regio-, and stereoselectivities. These enantioselective allylic C–H functionalization reactions are tolerant of a wide range of nucleophiles and α-alkenes, providing a large library of optically active building blocks. Based on enantioselective intramolecular allylic C–H oxidation, the formal synthesis of (+)-diversonol is accomplished, and enantioselective intramolecular allylic C–H amination can enable concise access to letermovir. In particular, the asymmetric allylic C–H alkylation of 1,4-dienes with azlactones offers highly enantioenriched α,α-disubstituted α-amino acid derivatives that are capable of serving as key building blocks for the enantioselective synthesis of lepadiformine alkaloids. In addition, a tachykinin receptor antagonist and (−)-tanikolide are also synthesized with chiral molecules generated from the corresponding allylic C–H alkylation reactions.
Magnon-polaritons are hybrid light-matter quasiparticles originating from the strong coupling between magnons and photons. They have emerged as a potential candidate for implementing quantum ...transducers and memories. Owing to the dampings of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon-polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsically nonequilibrium system may be described by a non-Hermitian Hamiltonian. Here we design a tunable cavity quantum electrodynamics system with a small ferromagnetic sphere in a microwave cavity and engineer the dissipations of photons and magnons to create cavity magnon-polaritons which have non-Hermitian spectral degeneracies. By tuning the magnon-photon coupling strength, we observe the polaritonic coherent perfect absorption and demonstrate the phase transition at the exceptional point. Our experiment offers a novel macroscopic quantum platform to explore the non-Hermitian physics of the cavity magnon-polaritons.
Effective hemostasis is vital to reduce the pain and mortality of patients, and the research and development of hemostatic materials are prerequisite for effective hemostasis. Chitosan (CS), with ...good biodegradability, biocompatibility and non-toxicity, has been widely applied in bio-medicine, the chemical industry, the food industry and cosmetics. The excellent hemostatic properties of CS have been extensively studied. As a result, chitosan-based composite hemostatic materials have been emerging. In this review, the hemostatic mechanism of chitosan is briefly discussed, and then the progress of research on chitosan-based composite hemostatic materials with multiple forms such as films, sponges, hydrogels, particles and fibers are introduced. Finally, future perspectives of chitosan-based composite hemostatic materials are given. The objective of this review is to provide a reference for further research and development of effective hemostatic materials.
This paper introduces a novel deep learning-based algorithm that integrates a long short-term memory (LSTM)-based auto-encoder (AE) network with support vector machine (SVM) for electrocardiogram ...(ECG) arrhythmias classification. The LSTM-based AE network (LSTM-AE) is used to learn the features from ECG arrhythmias signals, and the SVM is used to classify those signals from the learned features. The LSTM-AE consists of an encoder model, which extracts high-level feature information from ECG arrhythmias signals through LSTM network, and a decoder model which outputs reconstruct ECG arrhythmias signals from high-level features through LSTM network. Experiments show that the proposed method can learn better features than the traditional method without any prior knowledge, presenting a good potential for the ECG arrhythmias classification. In the classification of five heartbeats types, including normal, left bundle branch block (LBBB), right bundle branch block (RBBB), atrial premature complexes (APC), premature ventricular contractions (PVC), the proposed method achieved average accuracy, sensitivity, and specificity of 99.74%, 99.35%, and 99.84%, respectively, in the beat-based cross-validation approach, and 85.20%, 62.99%, and 90.75%, respectively, in the record-based cross-validation approach, in public MIT-BIH Arrhythmia Database. While based on the Advancement of Medical Instrumentation (AAMI) standards, the proposed method achieved average accuracy, sensitivity, and specificity of 99.45%, 98.63%, and 99.66%, respectively, in the beat-based cross-validation approach.
We consider the problem of channel estimation for millimeter wave (mmWave) systems, where, to minimize the hardware complexity and power consumption, an analog transmit beamforming and receive ...combining structure with only one radio frequency chain at the base station and mobile station is employed. Most existing works for mmWave channel estimation exploit sparse scattering characteristics of the channel. In addition to sparsity, mmWave channels may exhibit angular spreads over the angle of arrival, angle of departure, and elevation domains. In this paper, we show that angular spreads give rise to a useful low-rank structure that, along with the sparsity, can be simultaneously utilized to reduce the sample complexity, i.e., the number of samples needed to successfully recover the mmWave channel. Specifically, to effectively leverage the joint sparse and low-rank structure, we develop a two-stage compressed sensing method for mmWave channel estimation, where the sparse and low-rank properties are respectively utilized in two consecutive stages, namely, a matrix completion stage and a sparse recovery stage. Our theoretical analysis reveals that the proposed two-stage scheme can achieve a lower sample complexity than a conventional compressed sensing method that exploits only the sparse structure of the mmWave channel. Simulation results are provided to corroborate our theoretical results and to show the superiority of the proposed two-stage method.
Conspectus Understanding the structure and structure–property relationship of atomic and ligated clusters is one of the central research tasks in the field of cluster research. In chemistry, ...empirical rules such as the polyhedral skeleton electron pair theory (PSEPT) approach had been outlined to account for skeleton structures of many main-group atomic and ligand-protected transition metal clusters. Nonetheless, because of the diversity of cluster structures and compositions, no uniform structural and electronic rule is available for various cluster compounds. Exploring new cluster structures and their evolution is a hot topic in the field of cluster research for both experiment and theory. In this Account, we introduce our recent progress in the theoretical exploration of structures and evolution patterns of a class of atomically precise thiolate-protected gold nanoclusters using density functional theory computations. Unlike the conventional ligand-protected transition metal compounds, the thiolate-protected gold clusters demonstrate novel metal core/ligand shell interfacial structures in which the Au m (SR) n clusters can be divided into an ordered Au(0) core and a group of oligomeric SRAu(SR) x (x = 0, 1, 2, 3, ...) protection motifs. Guided by this “inherent structure rule”, we have devised theoretical methods to rapidly explore cluster structures that do not necessarily require laborious global potential energy surface searches. The structural predictions of Au38(SR)24, Au24(SR)20, and Au44(SR)28 nanoclusters were completely or partially verified by the later X-ray crystallography studies. On the basis of the analysis of cluster structures determined by X-ray crystallography and theoretical prediction, a structural evolution diagram for the face-centered-cubic (fcc)-type Au m (SR) n clusters with m up to 92 has been preliminarily established. The structural evolution diagram indicates some basic structural and electronic evolution patterns of thiolate-protected gold nanoclusters. The fcc Au m (SR) n clusters show a genetic structural evolution pattern in which each step of cluster size increase results in the formation of another Au4 tetrahedron or Au3 triangle unit in the Au core, and every increase of a structural unit in the Au core leads to an increase of two electrons in the whole cluster. The unique one- or two-dimensional cluster size evolution, the isomerism of the Au–S framework, and the formation of a double-helical and cyclic tetrahedron network in the fcc Au m (SR) n clusters all can be addressed from this evolution pattern. The summarized cluster structural evolution diagrams enable us to further explore more stable cluster structures and understand their structure–electronic structure–property relationships.
Improving the separation of photogenerated carriers and suppressing the rapid complication of electron–hole pairs are essential ways to improve photocatalytic hydrogen production activity. The high ...recombination rate of the photogenerated carriers is an issue encountered when developing CdS as a promising photocatalytic material. This work allowed to accelerate the separation of photogenerated electrons and holes by loading monoclinic β-AgVO
3
on hexagonal CdS nanorods to construct a one-dimensional (1D)/1D p-n heterojunction. The introduction of monoclinic β-AgVO
3
with a narrow band gap effectively improves the light absorption of CdS, which is conducive to improving the use of visible light. The integrated electric field of the p–n heterojunction can effectively transfer electrons and holes in the direction suitable to hydrogen evolution. The photoluminescence and electrochemical characterization of the catalysts showed that the p–n heterojunction formed after loading β-AgVO
3
greatly improved the separation efficiency of photocarriers. The hydrogen evolution experiments show that the composite catalyst has good photocatalytic hydrogen evolution capability and stability. The composite catalyst with the best photocatalytic performance was obtained by studying β-AgVO
3
with different loadings. The composite catalyst reached 581.5 μmol of hydrogen amount within 5 h, which is 3.8 times higher than that of CdS alone and its apparent quantum efficiency reaches 8.02%. The present work provides a possible solution for the development of perovskite and the extensiveness of CdS in photocatalytic hydrogen evolution.
Graphical abstract
We consider the problem of recovering block-sparse signals whose cluster patterns are unknown a priori. Block-sparse signals with nonzero coefficients occurring in clusters arise naturally in many ...practical scenarios. However, the knowledge of the block partition is usually unavailable in practice. In this paper, we develop a new sparse Bayesian learning method for recovery of block-sparse signals with unknown cluster patterns. A pattern-coupled hierarchical Gaussian prior is introduced to characterize the pattern dependencies among neighboring coefficients, where a set of hyperparameters are employed to control the sparsity of signal coefficients. The proposed hierarchical model is similar to that for the conventional sparse Bayesian learning. However, unlike the conventional sparse Bayesian learning framework in which each individual hyperparameter is associated independently with each coefficient, in this paper, the prior for each coefficient not only involves its own hyperparameter, but also its immediate neighbor hyperparameters. In doing this way, the sparsity patterns of neighboring coefficients are related to each other and the hierarchical model has the potential to encourage structured-sparse solutions. The hyperparameters are learned by maximizing their posterior probability. We exploit an expectation-maximization (EM) formulation to develop an iterative algorithm that treats the signal as hidden variables and iteratively maximizes a lower bound on the posterior probability. In the M-step, a simple suboptimal solution is employed to replace a gradient-based search to maximize the lower bound. Numerical results are provided to illustrate the effectiveness of the proposed algorithm.
A palladium‐catalyzed asymmetric intramolecular allylic C−H amination controlled by a chiral phosphoramidite ligand was established for the preparation of various substituted chiral ...hydropyrimidinones, the precursors of hydropyrimidines, in high yields with high enantioselectivities. In particular, dienyl sodium N‐sulfonyl amides bearing an arylethene‐1‐sulfonyl group underwent a sequential allylic C−H amination and intramolecular Diels–Alder (IMDA) reaction to produce chiral fused tricyclic tetrahydropyrimidinone frameworks in high yields and with high levels of stereoselectivity. Significantly, this method was used as the key step in an asymmetric synthesis of letermovir.
Getting to the core of it: An asymmetric palladium‐catalyzed intramolecular allylic C−H amination controlled by a chiral phosphoramidite ligand was used to provide efficient access various substituted chiral hydropyrimidines. This methodology was successfully applied to the asymmetric synthesis of letermovir (see scheme; IMDA=intramolecular Diels–Alder reaction).
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