排序方式: 共有895条查询结果,搜索用时 15 毫秒
61.
Dr. Jamie Hicks Dr. Petra Vasko Andreas Heilmann Prof. Jose M. Goicoechea Prof. Simon Aldridge 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(46):20556-20560
The reactivity of the electron-rich anionic AlI aluminyl compound K2[(NON)Al]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) towards mono- and disubstituted arenes is reported. C−H activation chemistry with n-butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer-type transition state. Selectivity is therefore based on similar electronic factors to classical SNAr chemistry, which implies the destabilisation of transition states featuring electron-donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent. 相似文献
62.
Dr. Alberto Palazzolo Dr. Timothée Naret Dr. Marion Daniel-Bertrand David-Alexandre Buisson Dr. Simon Tricard Dr. Philippe Lesot Dr. Yannick Coppel Dr. Bruno Chaudret Dr. Sophie Feuillastre Dr. Grégory Pieters 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(47):21065-21070
We report the dramatic impact of the addition of N-heterocyclic carbenes (NHCs) on the reactivity and selectivity of heterogeneous Ru catalysts in the context of C−H activation reactions. Using a simple and robust method, we prepared a series of new air-stable catalysts starting from commercially available Ru on carbon (Ru/C) and differently substituted NHCs. Associated with C−H deuteration processes, depending on Ru/C-NHC ratios, the chemical outcome can be controlled to a large extent. Indeed, tuning the reactivity of the Ru catalyst with NHC enabled: 1) increased chemoselectivity and the regioselectivity for the deuteration of alcohols in organic media; 2) the synthesis of fragile pharmaceutically relevant deuterated heterocycles (azine, purine) that are otherwise completely reduced using unmodified commercial catalysts; 3) the discovery of a novel reactivity for such heterogeneous Ru catalysts, namely the selective C-1 deuteration of aldehydes. 相似文献
63.
Mei-Ling Tan Dr. Qing-Hui Guo Dr. Xue-Yuan Wang Tan-Hao Shi Dr. Qian Zhang Sheng-Kai Hou Prof. Dr. Shuo Tong Prof. Dr. Jingsong You Prof. Dr. Mei-Xiang Wang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(52):23857-23866
Despite the aesthetically appealing structures and tantalizing physical and chemical properties, zigzag hydrocarbon belts and their heteroatom-embedded analogues remain challenging synthetic targets. We report herein the synthesis of diverse O/N-doped zigzag hydrocarbon belts based on selective bridging of the fjords of resorcin[4]arene derivatives through intramolecular SNAr and palladium-catalyzed intermolecular C−N bond formation reactions. Preorganized conformations of mono-macrocyclic, half-belt and quasi-belt compounds were revealed to facilitate cyclization reactions to construct heteroatom-linked octahydrobelt[8]arenes. The acquired products had strained square-prism-shaped belt structures in which all six-membered heterocyclic rings adopted an unusual boat conformation with equatorially configured alkyl groups. The unprecedented heteroatom-bearing belts also exhibited different photophysical and redox properties to those of octahydrobelt[8]arene analogues. 相似文献
64.
Dr. Matthias Bischoff Dr. Peter Mayer Dr. Christian Meyners Prof. Dr. Felix Hausch 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(21):4677-4681
6-, 7-, and 8-membered rings are assembled from a linear precursor by successive cyclisation reactions to construct a tricyclic diazatricyclo[6.5.1.04, 9]-tetradecanedione scaffold. Advanced building blocks based on d -aspartic acid and l -pyroglutamic acid were combined by a sp3−sp2 Negishi coupling. A carbamate-guided syn-diastereoselective epoxidation followed by an intramolecular epoxide opening allowed the construction of the piperidine ring. An efficient one-pot hydroxyl-group protection twofold deprotection reaction prepared the ground for the cyclisation to the bicycle. A final deprotection of the orthogonal protecting groups and lactamisation led to the novel, sp3-rich tricycle. The final compound is a substrate mimic of peptidyl-prolyl cis-trans isomerases featuring a locked trans-amide bond. Cheminformatic analysis of 179 virtual derivatives indicates favourable physicochemical properties and drug-like characteristics. As proof of concept we, show a low micromolar activity in a fluorescence polarisation assay towards the FK506-binding protein 12. 相似文献
65.
Ömer Taspinar Tobias Wilczek Julian Erver Dr. Martin Breugst Dr. Jörg-Martin Neudörfl Prof. Dr. Hans-Günther Schmalz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(19):4256-4260
We report the first chemical synthesis of eurysterol A, a cytotoxic and antifungal marine steroidal sulfate with a unique C8−C19 oxy-bridged cholestane skeleton. After C19 hydroxylation of cholesteryl acetate, used as an inexpensive commercial starting material, the challenging oxidative functionalization of ring B was achieved by two different routes to set up a 5α-hydroxy-7-en-6-one moiety. As a key step, an intramolecular oxa-Michael addition was exploited to close the oxy-bridge (8β,19-epoxy unit). DFT calculations show this reversible transformation being exergonic by about −30 kJ mol−1. Along the optimized (scalable) synthetic sequence, the target natural product was obtained in only 11 steps in 5 % overall yield. In addition, an access to (isomeric) 7β,19-epoxy steroids with a previously unknown pentacyclic ring system was discovered. 相似文献
66.
Dr. Viktor Pfeifer Marie Certiat Dr. Donia Bouzouita Dr. Alberto Palazzolo Sébastien Garcia-Argote Elodie Marcon David-Alexandre Buisson Dr. Philippe Lesot Prof. Laurent Maron Dr. Bruno Chaudret Dr. Simon Tricard Dr. Iker del Rosal Prof. Romuald Poteau Dr. Sophie Feuillastre Dr. Grégory Pieters 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(22):4988-4996
Ruthenium nanocatalysis can provide effective deuteration and tritiation of oxazole, imidazole, triazole and carbazole substructures in complex molecules using D2 or T2 gas as isotopic sources. Depending on the substructure considered, this approach does not only represent a significant step forward in practice, with notably higher isotope uptakes, a broader substrate scope and a higher solvent applicability compared to existing procedures, but also the unique way to label important heterocycles using hydrogen isotope exchange. In terms of applications, the high incorporation of deuterium atoms, allows the synthesis of internal standards for LC-MS quantification. Moreover, the efficacy of the catalyst permits, even under subatmospheric pressure of T2 gas, the preparation of complex radiolabeled drugs owning high molar activities. From a fundamental point of view, a detailed DFT-based mechanistic study identifying undisclosed key intermediates, allowed a deeper understanding of C−H (and N−H) activation processes occurring at the surface of metallic nanoclusters. 相似文献
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68.
Dr. Caroline M. Storey Dr. Matthew R. Gyton Dr. Rhiann E. Andrew Dr. Adrian B. Chaplin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(64):14715-14723
Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands—which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C2H4)][BArF4] (n=12, 14, 16; ArF=3,5-(CF3)2C6H3)—have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr’ (Ar’=3,5-tBu2C6H3) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C−C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr’, and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2)Ar’ is encumbered relative to head-to-head coupling of HC≡CAr’ and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2H4)][BArF4] and solvent effects. 相似文献
69.
《中国化学快报》2020,31(9):2259-2262
Significance of unstable species leaching was for the first time demonstrated on MOF-derived catalysts by taking PtNi-C as an example, that was instructive for the relevant catalyst fabrication and performance study. PtNi-C catalyst was synthesized by combining Pt nanoparticles with Ni-BTC after annealing in the tube furnace and the unstable Ni species can be easily leached out in nitric acid, and the stable PtNi nanoparticles trapped in the graphite carbon layer were obtained. The greatly improved catalytic ability for alcohol fuels oxidation was verified by comparing the fresh and acid leached catalysts in terms of the high peak current density, specific and mass activity and rapid charge transfer kinetics and high catalytic stability. The current work guides the importance of unstable assistant promoter removal for the MOF derived catalysts. 相似文献
70.
Meera A. Shah Samuel Raynes Dr. David C. Apperley Dr. Russell A. Taylor 《Chemphyschem》2020,21(7):673-679
The first selective oxidation of methane to methanol is reported herein for zinc-exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM-5 both form zinc formate and methanol. Selective methane activation to form [Zn-CH3]+ species was confirmed by 13C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM-5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the 13C MAS NMR spectrum, resulting from two distinct [Zn-CH3]+ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn-CH3]+ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure–function correlations for methane conversion over zinc-exchanged zeolites. 相似文献