Rapid progress in the fields of organometallic chemistry and homogeneous catalysis has made it possible for synthetic chemists to consider using ubiquitous yet unreactive C–H bonds as starting points ...to construct complex organic molecules. However, a majority of the C–H functionalization reactions currently in use require noble transition metal catalysts and harsh reaction conditions, so researchers have placed a priority on the development of mild and cost-effective catalysts. Given this situation, we wondered whether earth-abundant first-row transition metals could emulate the reactivity of a noble transition metal catalyst and carry out similar C–H functionalization reactions at a lower cost and under milder conditions. We also wondered whether we could use first-row transition metals to achieve hitherto unknown, but useful, C–H functionalization reactions. This Account summarizes our research on the development of three different types of C–H functionalization reactions using low-valent cobalt catalysts: (1) hydroarylation of alkynes and olefins, (2) ortho C–H functionalization with electrophiles, and (3) addition of arylzinc reagents to alkynes involving 1,4-cobalt migration. Although synthetic chemists have previously paid little attention to cobalt in designing catalytic C–H functionalization reactions, earlier studies, particularly those on stoichiometric cyclometalation, inspired us as we developed the hydroarylation and ortho C–H functionalization reactions. In these transformations, we combined a cobalt precatalyst, a ligand (such as phosphine or N-heterocyclic carbene (NHC)), and Grignard reagent to generate low-valent cobalt catalysts. These novel catalysts promoted a series of pyridine- and imine-directed hydroarylation reactions of alkynes and olefins at mild temperatures. Notably, we observed branched-selective addition to styrenes, which highlights a distinct regioselectivity of the cobalt catalyst compared with typical rhodium and ruthenium catalysts. The combination of a cobalt–NHC catalyst and a Grignard reagent allows directed aromatic C–H functionalizations with electrophiles such as aldimines, aryl chlorides, and alkyl chlorides or bromides. This second reaction has a particularly broad scope, allowing us to introduce secondary alkyl groups at the ortho position of aryl imines, a difficult reaction to carry out by other means. Serendipitously, we found that a cobalt–Xantphos complex catalyzed the third type of C–H functionalization: the addition of an arylzinc reagent to an alkyne to afford ortho-alkenylarylzinc species through a 1,4-cobalt migration. This “migratory arylzincation” allowed us to quickly construct a diverse group of functionalized benzothiophenes and benzoselenophenes. Collectively, our studies of cobalt catalysis have provided cost-effective catalysts and milder conditions for existing C–H functionalizations and have led to some unprecedented, attractive chemical transformations.
The main factor limiting the widespread use of industrial robots in machining applications is the relatively low stiffness of the robot. Stiffness performance not only affects the quality of robotic ...milling but also affects the accuracy of the milling process. It is necessary to improve the stiffness performance in the robotic machining process. A new robot stiffness performance index is proposed to evaluate the robot stiffness in the direction normal to the surface of the workpiece to be machined. This index is based on the functional redundancy characteristics of the robot. The relationship between the redundant axis angle and the stiffness performance index at a random point in Cartesian space, and the stiffness performance index values on a random path in Cartesian space are obtained. The robot posture optimization model is established and a new robot milling posture optimization method is introduced. The experimental results prove a significant reduction in machining error and a significant increase in machining accuracy after using the proposed robotic milling posture optimization method, which proves the validity of the proposed stiffness performance index, and the robot milling posture optimization method can be widely used in the industry. Finally, the distribution of stiffness performance index is analyzed to predict the regions of better robot stiffness performance in the workbench, which can be used as a criterion before the milling operation to optimize the robot configuration to improve milling accuracy and save time.
Cobalt−phosphine and cobalt−carbene catalysts have been developed for the hydroarylation of styrenes via chelation-assisted C−H bond activation, to afford branched and linear addition products, ...respectively, in a highly regioselective fashion. Deuterium-labeling experiments suggested a mechanism involving reversible C−H bond cleavage and olefin insertion steps and reductive elimination as the rate- and regioselectivity-determining step.
Detecting epileptic seizure from EEG signals constitutes a challenging problem of significant importance. Combining adaptive optimal kernel time-frequency representation and visibility graph, we ...develop a novel method for detecting epileptic seizure from EEG signals. We construct complex networks from EEG signals recorded from healthy subjects and epilepsy patients. Then we employ clustering coefficient, clustering coefficient entropy and average degree to characterize the topological structure of the networks generated from different brain states. In addition, we combine energy deviation and network measures to recognize healthy subjects and epilepsy patients, and further distinguish brain states during seizure free interval and epileptic seizures. Three different experiments are designed to evaluate the performance of our method. The results suggest that our method allows a high-accurate classification of epileptiform EEG signals.
A cobalt-N-heterocyclic carbene catalyst, in combination with an appropriate Grignard reagent, promotes a chelation-assisted aromatic C-H functionalization reaction via addition to an aromatic ...aldimine.
A new salamandroid salamander, Qinglongtriton gangouensis (gen. et sp. nov.), is named and described based on 46 fossil specimens of juveniles and adults collected from the Upper Jurassic (Oxfordian) ...Tiaojishan Formation cropping out in Hebei Province, China. The new salamander displays several ontogenetically and taxonomically significant features, most prominently the presence of a toothed palatine, toothed coronoid, and a unique pattern of the hyobranchium in adults. Comparative study of the new salamander with previously known fossil and extant salamandroids sheds new light on the early evolution of the Salamandroidea, the most species-diverse clade in the Urodela. Cladistic analysis places the new salamander as the sister taxon to Beiyanerpeton, and the two taxa together form the basalmost clade within the Salamandroidea. Along with recently reported Beiyanerpeton from the same geological formation in the neighboring Liaoning Province, the discovery of Qinglongtriton indicates that morphological disparity had been underway for the salamandroid clade by early Late Jurassic (Oxfordian) time.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Ecological preferences and life history strategies have enormous impacts on the evolution and phenotypic diversity of salamanders, but the yet established reliable ecological indicators from bony ...skeletons hinder investigations into the paleobiology of early salamanders. Here, we statistically demonstrate by using time-calibrated cladograms and geometric morphometric analysis on 71 specimens in 36 species, that both the shape of the palate and many non-shape covariates particularly associated with vomerine teeth are ecologically informative in early stem- and basal crown-group salamanders. Disparity patterns within the morphospace of the palate in ecological preferences, life history strategies, and taxonomic affiliations were analyzed in detail, and evolutionary rates and ancestral states of the palate were reconstructed. Our results show that the palate is heavily impacted by convergence constrained by feeding mechanisms and also exhibits clear stepwise evolutionary patterns with alternative phenotypic configurations to cope with similar functional demand. Salamanders are diversified ecologically before the Middle Jurassic and achieved all their present ecological preferences in the Early Cretaceous. Our results reveal that the last common ancestor of all salamanders share with other modern amphibians a unified biphasic ecological preference, and metamorphosis is significant in the expansion of ecomorphospace of the palate in early salamanders.
•A novel convolutional neural networks has been constructed to predict irradiance.•The chaotic hybrid algorithm is applied to optimize hyperparameters of framework.•The chaotic hybrid algorithm ...alleviates the imperfect performance of framework.•The novel framework can be further applied into other areas of renewable energy.
As an important part of solar power system, photovoltaic grid-connected system and solar thermal system, solar irradiance has the inherent characteristics of variability and uncertainty. Existing data analysis methods are difficult to demonstrate better generalization. Hence, resource planners must be adaptable to accommodate these uncertainties while conducting planning. To improve the accuracy of solar energy prediction and efficiently organize the utilization of solar energy, a novel convolutional neural networks framework has been constructed. Firstly, we have established a convolutional neural network framework for solar prediction based on meteorological data from surrounding sites and different sampling times. Secondly, the chaotic GA/PSO1Genetic Algorithm/Particle Swarm Optimization.1 hybrid algorithm is applied to optimize the hyper parameters of the novel framework, which alleviates the imperfect performance caused by improper hyper parameters. At the meantime, the hybrid algorithm can reduce the manpower and resources of manual parameter adjustment. Excitability of the novel framework has been verified by benchmark tests. In the solar irradiance prediction studies, the annual average Mean Absolute Error of the proposed method is reduced by 0.1463 MJ·m-2 compared with single CNN2Convolutional neural network.2 framework. The annual average Mean Absolute Error of the proposed method is reduced by 49.47%, 47.6%, 20.34%, respectively, compared with ANN3Artificial Neural Network.3, K-means-RBF4kmeans-Radical Basis Function.4 and GBRT.5Gradient Boosted Regression Trees.5 The superiority has been fully illustrated through all the simulation test results. Therefore, the proposed method provides a basis for accurate estimation of solar power, which can promote further development of the whole power system.
In this article, a compact partial element equivalent circuit (c-PEEC) model is presented for characterizing the electromagnetic (EM) problems with finite-sized piecewise homogeneous dielectrics. ...Unlike in conventional PEEC models where dielectrics are described by massive subcircuits, the proposed c-PEEC model addresses the composite conductor-dielectric problems via circuit elements resident on conductors only. Novel null-field boundary integral equations (n-BIEs) are formulated based on the surface equivalent principle and quasi-static assumption. The elimination of the magnetic field-related equivalent sources avoids the low-frequency breakdown problem caused by the weak EM coupling at low frequencies and improves the accuracy by inhibiting the numerical errors in discretizing magnetic sources. The computational cost is significantly reduced compared with the conventional models. Furthermore, the concise configuration of the c-PEEC model extends the model-order reduction (MOR) algorithms for fundamental homogeneous models to heterogeneous integration. Three numerical examples including a resonator, a radio frequency (RF) embedded passive circuit, and an interconnection problem, are studied to validate the stability, efficiency, and accuracy of the c-PEEC model. A micro-modeling circuit is obtained from the c-PEEC model to demonstrate its compatibility with the MOR algorithms.
Mild mannered: Cobalt catalysts complexed with phenanthroline‐type ligands and activated with Grignard reagents serve as inexpensive and effective catalysts for the ortho alkylation of aromatic ...imines with a variety of olefins (see scheme). The new catalytic systems feature remarkably mild reaction conditions for CH bond activation and functionalization.
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