全文获取类型
收费全文 | 184篇 |
免费 | 5篇 |
国内免费 | 9篇 |
专业分类
化学 | 194篇 |
物理学 | 4篇 |
出版年
2023年 | 7篇 |
2022年 | 4篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 1篇 |
2015年 | 5篇 |
2014年 | 8篇 |
2013年 | 14篇 |
2012年 | 12篇 |
2011年 | 17篇 |
2010年 | 10篇 |
2009年 | 11篇 |
2008年 | 11篇 |
2007年 | 13篇 |
2006年 | 7篇 |
2005年 | 16篇 |
2004年 | 11篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 1篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1988年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有198条查询结果,搜索用时 15 毫秒
91.
Kaixuan Wang Longqing Yang Yi Li Hongye Li Zhili Liu Dr. Lichao Ning Prof. Xiaohua Liu Prof. Xiaoming Feng 《Angewandte Chemie (International ed. in English)》2023,62(32):e202307249
We realized a highly efficient formal [1,2]-sigmatropic rearrangement of ammonium ylides generated from 3-methylene-azetidines and α-diazo pyrazoamides. The employ of readily available chiral cobalt(II) complex of chiral N,N′-dioxide enabled the ring-expansion of azetidines, affording a variety of quaternary prolineamide derivatives with excellent yield (up to 99 %) and enantioselectivity (up to 99 % ee) under mild reaction condition. For the rearrangement of ammonium ylides, the installation of a pyrazoamide group as a masked brick to build chiral scaffolds proved successful. The enantioselective ring expansion process was elucidated by DFT calculations. 相似文献
92.
Sudip Baguli Subham Sarkar Soumajit Nath Dr. Dibyendu Mallick Dr. Debabrata Mukherjee 《Angewandte Chemie (International ed. in English)》2023,62(47):e202312858
Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H+ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to pyaziridines at 80 °C but unprecedentedly result N−picoCAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy, as revealed by computational analysis, plays a crucial role in this unusual 1,3-H+ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both N-picoCAAC and pyaziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3SiH. Notably, the pyaziridine-Rh(I) is superior than the N-picoCAAC-Rh(I) catalyst. 相似文献
93.
Simple one-pot syntheses allow the preparation of [2]rotaxane-based degenerate molecular shuttles featuring the recognition of pyridinium ions by BPX26C6 macrocycles. Because of the weak interactions between the BPX26C6 and pyridinium units in the [2]rotaxanes in CD3COCD3, the rates of shuttling of the BPX26C6 moieties between the pyridinium stations are rapid, relative to those of DB24C8-based shuttles, on the NMR spectroscopic time scale at ambient temperature. 相似文献
94.
Yuhua Huang Shaodong Liu Yizhou Liu Yonggang Chen Mark Weisel R. Thomas Williamson Ian W. Davies Xumu Zhang 《Tetrahedron》2018,74(17):2182-2190
NMR studies of the catalyst, deuteration experiments, mass spectrometry, and isolation and characterization of intermediates, allow us to propose an outer-sphere mechanism for the Iridium-catalyzed asymmetric hydrogenation of N-alkyl-2-arylpyridinium salts. 相似文献
95.
A range of 16 substituted benzoyl/arylcarbamoyl and benzoyl/arylthiocarbamoyl stabilised ylides have been prepared. They are found under conditions of flash vacuum pyrolysis to fragment giving a benzoyl ylide and aryl isocyanate or isothiocyanate accompanied in some cases by secondary pyrolysis products. Kinetic studies show the thiocarbamoyl ylides to react consistently faster than their carbamoyl analogues and substituent effects suggest a polar cyclic transition state, which involves attack by the benzoyl oxygen on the carbamoyl/thiocarbamoyl NH. 相似文献
96.
Shin-ichi Watanabe 《Tetrahedron letters》2004,45(40):7459-7463
A novel synthetic method of ketodiphenylselenonium ylide from alkynylselenonium salt is described. A reaction of alkynylselenonium salt, hydroxide ion, and aldehyde in the presence of silver triflate and triethylamine gave oxiranylketones just as a trans-isomer in moderate to good yields, whereas benzoyl aziridine derivatives were obtained from the reaction with sodium p-toluenesulfonamide instead of a hydroxide ion. 相似文献
97.
Isao Yamaguchi Hideo Higashi Shunsuke Kimura Moriyuki Sato 《Helvetica chimica acta》2010,93(5):819-828
Diacetylenes (DAs) having a dipolar D‐π‐A structure (D=donor: amino group; π=π‐conjugation core; A=acceptor: pyridinium (Py) and bipyridinium (BPy) groups), i.e., 4 (APBPyDA) and 5 (APPyPyDA), or an A‐π‐A structure, i.e., 7 (DBPyDA) and 8 (PyDA(Cl)), were obtained by 1 : 1 and 1 : 2 reactions of 4,4′‐(buta‐1,3‐diyne‐1,4‐diyl)bis[benzenamine] (APDA; 3 ) with 1‐(2,4‐dinitrophenyl)‐1′‐hexyl‐4,4′‐bipyridinium bromide chloride (1 : 1 : 1) ( 1 ), 1‐(2,4‐dinitrophenyl)‐4‐(pyridin‐4‐yl)pyridinium chloride ( 2 ), or 1‐(2,4‐dinitrophenyl)pyridinium chloride ( 6 ) (Schemes 1 and 2). The anion‐exchange reactions of 8 with NaI and Li(TCNQ) (TCNQ?=2,2′‐(cyclohexa‐2,5‐diene‐1,4‐diylidene)bis[propanedinitrile] radical ion (1?)) yielded the corresponding I? and TCNQ? salts 9 (PyDA(I)) and 10 (PyDA(TCNQ)). Compounds 10 and 4 exhibited a UV/VIS absorption due to a charge transfer between the TCNQ? and the pyridinium groups and a strong solute–solvent interaction of a dipolar solute molecule in the polar environment, respectively. Compounds 8 – 10 exhibited photoluminescence in solution, whereas 4 and 7 did not because of the presence of the 4,4′‐bipyridinium quenching groups. Differential‐scanning‐calorimetry (DSC) measurements suggested that the DAs obtained in this study can be converted into poly(diacetylenes) by thermal polymerization. 相似文献
98.
G. Venkateswara Rao 《Tetrahedron letters》2008,49(5):824-826
2-(N-Disubstituted amino)ethyltriphenylphosphonium bromides are prepared in quantitative yields with high purity by reacting secondary amines with 2-methoxyethyltriphenylphosphonium bromide under aqueous conditions. The differential reactivity of this reagent offers advantages for the preparation of aminoethyltriphenylphosphonium bromides possessing nucleophiles such as hydroxy groups. 相似文献
99.
100.
We report herein that, in the absence of any nucleophilic counterions, tertiary nitrogen nucleophiles such as pyridines and imidazoles can be alkylated with alcohols, by simply using their ammonium form as the acidic component of the Mitsunobu reaction. This led to efficient preparation of ionic liquids under mild conditions, avoiding the usual anion exchange step. 相似文献