The conventional oxidative Heck reaction between aryl boronic acids and alkenes typically involved the Pd
/Pd
/Pd
catalytic cycle incorporating an external oxidant and often suffered C=C bond ...isomerization for internal alkyl-substituted alkenes via chain-walking. Herein, we demonstrate that the regioselectivity (γ-selectivity vs. δ-selectivity) and pathway selectivity (hydroarylation vs. oxidative Heck coupling) of a directed Ni-catalyzed alkene arylation can be controlled by judicious tuning of the coordination environment around the nickel catalyst via optimization of an appropriate phosphine ligand and directing group. In this way, the Ni(0)-catalyzed oxidative Heck arylation that relies on transfer hydrogenation of an acceptor olefin is developed with excellent E/Z selectivity and regioselectivity. Mechanistic investigations suggest that the addition of the acceptor is crucial for lowering the energy for carbometalation and for enabling catalytic turnover.
That nucleophiles preferentially attack at the less sterically hindered carbon of epoxides under neutral and basic conditions has been generally accepted as a fundamental rule for predicting the ...regioselectivity of this type of reaction. However, this rule does not hold for perfluorinated epoxides, such as hexafluoropropylene oxide (HFPO), in which nucleophiles were found to attack at the more hindered CF
substituted β-C rather than the fluorine substituted α-C. In this contribution, we aim to shed light on the nature of this intriguing regioselectivity by density functional theory methods. Our calculations well reproduced the observed abnormal regioselectivities and revealed that the unusual regiochemical preference for the sterically hindered β-C of HFPO mainly arises from the lower destabilizing distortion energy needed to reach the corresponding ring-opening transition state. The higher distortion energy required for the attack of the less sterically hindered α-C results from a significant strengthening of the C(α)-O bond by the negative hyperconjugation between the lone pair of epoxide O atom and the antibonding C-F orbital.
Carbazole‐based donor–acceptor compounds with tunable HOMO–LUMO gaps were synthesized by Suzuki and Sonogashira cross‐coupling reactions. Their optical and electrochemical properties were fully ...characterized. The results show that materials with different emission colors ranging from blue to green to orange could be obtained. The experimental results were also supported by theoretical calculations.
Carbazole‐based donor–acceptor compounds with tunable HOMO–LUMO gaps were synthesized and fully characterized. These compounds show redshifted optoelectronic properties with different emission colors ranging from blue to green to orange.
A detailed mechanism of N-alkylpyrrole formation from 3-pyrroline and 2-phenylpropanal in the presence of a Brønsted acid catalyst was investigated in depth using the MP2 and DFT theories. The two ...mechanisms proposed earlier in recent literatures for this internal redox process were evaluated and were found not to account perfectly for the transition state and the energetic barrier of its formation. Based on the present calculations, a new mechanism was put forth.
Stable phenalenyl (PLY) radicals without sterically bulky substituents need to be synthesized for application as neutral organic molecular conductors. Verdazyl radicals, which have high stability ...even without sterically bulky substituents, were combined with PLY radicals to produce two novel radicals. The stability of the dimethylthiourea substituted PLY radical is supported by the experimental results and quantum chemical calculations.
Metal‐free synthesis of heterocycles is highly sought after in the pharmaceutical industry and has garnered widespread attention due to its environmental sustainability and cost‐effectiveness. We ...report a radical 6‐endo addition method for pyridine synthesis from cyclopropylamides and alkynes under metal‐free conditions. Various terminal and substituted alkynes are inserted as C2 units into cyclopropylamides to synthesize versatile pyridines with more than 51 examples. Mechanistic investigations and computational studies indicate the unprecedented 6‐endo‐trig addition of vinyl radicals to the imine nitrogen atom rather than the conventional 5‐exo‐trig addition to the imine carbon atom, in which the hypervalent iodine(III) plays a critical role. This reaction easily scales up with excellent functional group compatibility and suits the late‐stage pyridine installation on complex molecules.
Comprehensive Summary
Five‐coordinate neutral Cu(III) complexes have long been considered as the key intermediates in copper‐mediated/catalyzed arene trifluoromethylative reactions, but the identity ...of these complexes remains elusive. Herein, we reported, for the first time, the preparation and characterization of these putative complexes (bpy)Cu(CF3)2(C6F4R) and investigation of their C(sp2)‐CF3 bond‐forming reductive elimination. Our mechanistic studies including kinetic studies, the effect of temperature and solvents, the effect of the para‐substitution of the ligand as well as DFT calculations suggest that C(sp2)‐CF3 reductive elimination process proceeds via synchronous bond‐breaking/bond‐forming process.
Elusive key intermediates five‐coordinate neutral arylated Cu(III) complexes in copper‐mediated trifluoromethylation of arenes were characterized and Ar‐CF3 reductive elimination processes from these complexes were studied for the first time.
Abstract
Strawberry maturity detection is a key technology for automated strawberry picking and intelligent information monitoring. This paper studies strawberry maturity detection technology, and ...proposes an improved YOLOv4 convolutional neural network detection method. In view of great amount of network parameters, this paper uses MobileNetv3 backbone feature extraction network and depthwise separable convolution for lightweight improvement of the YOLOv4 network. In order to increase the model training accuracy, this paper uses the Kmeans++ clustering algorithm to calculate the prior bounding box size, and uses transfer learning and staged training methods to improve the training efficiency of the built network model. The experimental results show that the mean average precision (mAP) of the test dataset in this paper is 96.78%; the precision of mature strawberry detection is 98.72%, with recall rate 91.67% and average precision (AP) 99.56%; the precision of immature strawberry detection is 90.76%, with recall rate 83.92% and AP 94.00%. Single image detection time is 56ms, which can meet the demand for real-time high-precision detection of strawberries.
For the node localization of wireless sensor network, the cost of anchor nodes is very high. To reduce it, this paper adopts the idea of virtual anchor nodes, introduces the Gauss Marco's mobile ...model to identify the movement trajectories of virtual anchor node, and then optimize it, so that the virtual anchor node can distribute uniformly in the region detected. Combining with the centroid localization algorithm, a high coverage rate, low energy consumption, low cost and fast localization algorithm can be achieved. The simulation results show that the technique proposed in this paper is effective, and the positioning error can meet most application requirements.
Metal‐free synthesis of heterocycles is highly sought after in the pharmaceutical industry and has garnered widespread attention due to its environmental sustainability and cost‐effectiveness. We ...report a radical 6‐endo addition method for pyridine synthesis from cyclopropylamides and alkynes under metal‐free conditions. Various terminal and substituted alkynes are inserted as C2 units into cyclopropylamides to synthesize versatile pyridines with more than 51 examples. Mechanistic investigations and computational studies indicate the unprecedented 6‐endo‐trig addition of vinyl radicals to the imine nitrogen atom rather than the conventional 5‐exo‐trig addition to the imine carbon atom, in which the hypervalent iodine(III) plays a critical role. This reaction easily scales up with excellent functional group compatibility and suits the late‐stage pyridine installation on complex molecules.
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