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 232 毫秒
101.
Slow-exchange C3-symmetric inclusion complexes were successfully prepared based on a new cryptand/trispyridinium recognition motif as confirmed by proton NMR spectroscopy, electrospray ionization mass spectrometry, and molecular modeling. These inclusion complexes are the first examples of a new type of threaded structure with non-linear guests. 相似文献
102.
The acid-catalyzed benzidine rearrangement of diazo compounds is known to involve several rearrangements with the major pathway being a [5,5] sigmatropic rearrangement to provide 4,4′-diaminobiaryls. A limitation of this rearrangement has been poor conversions with pyridyl systems. Herein, we address this long standing issue to furnish hetero-biaryls via a pyridinium salt in the presence of trimethylsilyl iodide. 相似文献
103.
Concerted vs. Non‐Concerted 1,3‐Dipolar Cycloadditions of Azomethine Ylides to Electron‐Deficient Dialkyl 2,3‐Dicyanobut‐2‐enedioates
下载免费PDF全文
![点击此处可从《Helvetica chimica acta》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Alexander F. Khlebnikov Alexander S. Konev Alexander A. Virtsev Dmitry S. Yufit Grzegorz Mlostoń Heinz Heimgartner 《Helvetica chimica acta》2014,97(4):453-470
The quantum‐chemical calculations of the thermal ring opening of 1‐methyl‐2,3‐diphenyl‐ and 1,2,3‐triphenylaziridine with formation of the corresponding azomethine ylides of S‐, U‐, and W‐type as well as their cycloaddition to dimethyl acetylenedicarboxylate (DMAD) and dimethyl 2,3‐dicyanobut‐2‐enedioate, were performed at the DFT B3LYP/6‐31G(d) level of theory with the PCM solvation model. The calculations are in complete accordance with experimental results and explain the switch from the concerted to the non‐concerted pathway depending on substituents in the dipolarophile and the ylide. It was found that strong electron‐withdrawing substituents in dipolarophiles, such as in dialkyl dicyanobutenedioates, significantly reduce the barrier for the formation of zwitterionic intermediates in the reaction of azomethine ylides with such dipoles. This can render the stepwise cycloaddition competitive with the concerted one. However, the concertedness of the cycloaddition even to dipolarophiles with several electron‐withdrawing substituents is governed by a fine balance of electronic and steric effects in both ylide and dipolarophile counterparts. The hypothesis that introduction of substituents in the azomethine ylide that destabilize the positive charge in a corresponding zwitterion will favor the concerted cycloaddition even with dialkyl dicyanobutenedioates was tested theoretically and experimentally. 相似文献
104.
《Tetrahedron》2014,70(21):3377-3384
The Rh(II)-catalyzed reaction of 2-carbonyl-substituted 2H-azirines with ethyl 2-cyano-2-diazoacetate or 2-diazo-3,3,3-trifluoropropionate provides an easy access to 2H-1,3-oxazines and 1H-pyrrol-3(2H)-ones. These compounds can be selectively prepared from the same starting material using temperature as the only varied parameter. The 2-azabuta-1,3-diene intermediate, a common precursor for both heterocyclic products, isomerizes into 2H-1,3-oxazine under kinetic control, while 1H-pyrrol-3(2H)-one is the sole product of the reaction at elevated temperatures. According to DFT-calculations a one-atom oxazine ring contraction involving ring-opening to a 2-azabuta-1,3-diene intermediate, followed by a 1,5- and 1,2-prototropic shift leads to the consecutive formation of imidoylketene and azomethine ylide, which then further undergo cyclization to the pyrrole derivative. 相似文献
105.
3-Aminocoumarines and 2-iminochromen-3-amines were efficiently prepared from the Zincke-ring-opening reaction of the corresponding 2H-chromen-3-pyridinium chlorides using N-methylpiperazine. This methodology unravels the marked potential of pyridinium salts as protective groups for primary amines. 相似文献
106.
107.
Emel Ay Didier Le Nouën Hélène Chaumeil Markus Neuburger Théophile Tschamber Patrice Jacques 《Tetrahedron letters》2014
The variable-temperature («dynamic») NMR measurements of series of pyridinyl anisoles and pyridinyl phenols each of them bearing ortho, ortho′ handles of various lengths at oxygenated ring lead to the following results: (i) at room temperature, a restricted rotation around the intercyclic bond of pyridinyl anisoles and pyridinyl phenol bearing short handles formed by 9–10 atoms is underscored, (ii) all handles at the former derivatives are blocked on one side of the oxygenated ring face, and (iii) handles of 11–12 atoms long at phenol rings are able to move freely. The solid-state packing indicates that all saturated chains are substantially perpendicular to the oxygenated ring. 相似文献
108.
Issa Yavari Mehdi Adib Fatemeh Jahani-Mogaddam Mohammad Hossain Sayahi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):545-553
Crystalline phosphorus ylides are obtained in excellent yields from the addition reaction between triphenylphosphine, dimethyl acetylenedicarboxylate and strong NH-acids, such as 2-acetylpyrrole, indole, ethyl 3-indolylglyoxalate and N -benzyl-2-pyrrolylglyoxamate. Dynamic NMR effects are observed in the 1 H NMR spectra of stabilized ylides obtained from 2-acetylpyrrole and indole ( j G p = 67.1 and 68.8 kJmol m 1 respectively) and are attributed to restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. 相似文献
109.
《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2183-2187
The addition of triphenylphosphine to dialkyl acetylenedicarboxylates in the presence of trifluoroacetamide, a NH acid, leads to stable crystalline phosphorus ylides in exellent yields. These stable ylides exist as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond, resulting from conjugation of the ylide moiety with the adjacent group. 相似文献
110.
J. Fabian G. A. Rosquete L. A. Montero-Cabrera 《Journal of Molecular Structure》1999,469(1-3):163-176
The structure of a series of heterocyclic betaines was calculated by methods of density functional theory (DFT). The charge distribution and bond characteristics of these compounds were analyzed by Weinhold’s natural bond orbital analysis (NBO) and by natural resonance theory (NRT). In order to probe the aromatic character of the ring fragments, Schleyer’s nucleus-independent chemical shifts (NICSs) were calculated by GIAO-RHF. Ab initio single configuration interaction calculations (SCI) correctly predict intense π→π* transitions at low energies, but the transition energies of the color bands are overestimated. Torsion around the interfragmental bond increases the charge separation between the molecular fragments and the dipole moment. The molecular fragments become increasingly aromatic. The absorption wavelengths increase on torsion while the oscillator strengths decrease. 相似文献