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排序方式: 共有315条查询结果,搜索用时 15 毫秒
91.
Prof. Dr. A. Stephen K. Hashmi Prof. Dr. Douglas W. Stephan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(36):11928-11938
A series of propargyl amides were prepared and their reactions with the Lewis acidic compound B(C6F5)3 were investigated. These reactions were shown to afford novel heterocycles under mild conditions. The reaction of a variety of N‐substituted propargyl amides with B(C6F5)3 led to an intramolecular oxo‐boration cyclisation reaction, which afforded the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. Secondary propargyl amides gave oxazoles in B(C6F5)3 mediated (catalytic) cyclisation reactions. In the special case of disubstitution adjacent to the nitrogen atom, 1,1‐carboboration is favoured as a result of the increased steric hindrance (1,3‐allylic strain) in the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. 相似文献
92.
93.
Wang M. Y. Hashmi S. A. Sun Z. X. Guo D. L. Vita G. Yang G. W. Hemida H. 《Acta Mechanica Sinica》2021,37(7):1090-1103
Acta Mechanica Sinica - The irregularities on trains bodies are normally ignored or greatly simplified in studies concerned with aerodynamics. However, surface roughness is known to affect the flow... 相似文献
94.
Room‐Temperature Hydrohydrazination of Terminal Alkynes Catalyzed by Saturated Abnormal N‐Heterocyclic Carbene–Gold(I) Complexes 下载免费PDF全文
Dr. Rubén Manzano Thomas Wurm Frank Rominger Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(23):6844-6848
A number of saturated abnormal N‐heterocyclic carbene (NHC) complexes of gold, in combination with KBArF4 as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X‐ray crystal‐structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process. 相似文献
95.
Recently, laser welding of austenitic stainless steel has received great attention in industry. This is due to its widespread application in petroleum refinement stations, power plants, the pharmaceutical industry and also in households. Therefore, mechanical properties should be controlled to obtain good welded joints. The welding process should be optimized by the proper mathematical models. In this research, the tensile strength and impact strength along with the joint-operating cost of laser-welded butt joints made of AISI304 was investigated.Design-expert software was used to establish the design matrix and to analyze the experimental data. The relationships between the laser-welding parameters (laser power, welding speed and focal point position) and the three responses (tensile strength, impact strength and joint-operating cost) were established. Also, the optimization capabilities in design-expert software were used to optimize the welding process.The developed mathematical models were tested for adequacy using analysis of variance and other adequacy measures. In this investigation, the optimal welding conditions were identified in order to increase the productivity and minimize the total operating cost. Overlay graphs were plotted by superimposing the contours for the various response surfaces. The process parameters effect was determined and the optimal welding combinations were tabulated. 相似文献
96.
97.
An Alternative Approach to PEPPSI Catalysts: From Palladium Isonitriles to Highly Active Unsymmetrically Substituted PEPPSI Catalysts 下载免费PDF全文
Dipl.‐Chem. Anna Zeiler Dr. Matthias Rudolph Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(31):11065-11071
A series of new pyridine‐enhanced precatalyst preparation, stabilization, and initiation (PEPPSI)‐type complexes bearing different types of carbene ligands was prepared by the modular and convergent template synthesis strategy. Nitrogen acyclic carbenes, saturated and unsaturated five‐membered NHC, saturated six‐membered NHCs, and five‐membered N‐heterocyclic oxo‐carbene (NHOC) ligands on palladium were prepared this way. These new organometallic compounds then were tested in Suzuki and Negishi cross‐coupling reactions by using substrates with one or two substituents in ortho‐position of the new C?C bond being formed. Both aryl chlorides and bromides were tested as coupling partners. In some cases, the new ligands gave results similar to Organ’s successful IPr‐based and IPent‐based PEPPSI derivatives, with aryl bromides 0.05 mol % catalyst load still gave satisfactory results, with aryl chlorides 0.5 mol % were needed. 相似文献
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99.
M. Chem. Laura Nunes dos Santos Comprido Dr. Johannes E. M. N. Klein Prof. Dr. Gerald Knizia Prof. Dr. Johannes Kästner Prof. Dr. A. Stephen K. Hashmi 《Angewandte Chemie (International ed. in English)》2015,54(35):10336-10340
Bonding and stabilizing effects in gold carbene complexes are investigated by using Kohn–Sham density functional theory (DFT) and the intrinsic bond orbital (IBO) approach. The π‐stabilizing effects of organic substituents at the carbene carbon atom coordinated to the gold atom are evaluated for a series of recently isolated and characterized complexes, as well as intermediates of prototypical 1,6‐enyne cyclization reactions. The results indicate that these effects are of particular importance for gold complexes especially because of the low π‐backbonding contribution from the gold atom. 相似文献
100.
Umar Farooq Khurshid Ayub Muhammad Ali Hashmi Rizwana Sarwar Mumtaz Ali 《Natural product research》2015,29(9):813-819
A new rosane-type diterpenoid (1) has been isolated from the chloroform fraction of Stachys parviflora. Structure of 1 was proposed based on 1D and 2D NMR techniques including correlation spectroscopy, heteronuclear multiple quantum coherence, heteronuclear multiple bond correlation and nuclear Overhauser effect spectroscopy. A theoretical model for the electronic and spectroscopic properties of compound 1 is also developed. The geometries and electronic properties were modelled at B3LYP/6-31G* and the theoretical scaled spectroscopic data correlate nicely with the experimental data. 相似文献