全文获取类型
收费全文 | 5505篇 |
免费 | 334篇 |
国内免费 | 505篇 |
专业分类
化学 | 5867篇 |
晶体学 | 7篇 |
力学 | 10篇 |
综合类 | 8篇 |
数学 | 141篇 |
物理学 | 311篇 |
出版年
2024年 | 8篇 |
2023年 | 139篇 |
2022年 | 69篇 |
2021年 | 111篇 |
2020年 | 131篇 |
2019年 | 185篇 |
2018年 | 164篇 |
2017年 | 160篇 |
2016年 | 174篇 |
2015年 | 165篇 |
2014年 | 320篇 |
2013年 | 518篇 |
2012年 | 309篇 |
2011年 | 390篇 |
2010年 | 291篇 |
2009年 | 356篇 |
2008年 | 366篇 |
2007年 | 390篇 |
2006年 | 353篇 |
2005年 | 309篇 |
2004年 | 302篇 |
2003年 | 228篇 |
2002年 | 182篇 |
2001年 | 81篇 |
2000年 | 79篇 |
1999年 | 72篇 |
1998年 | 77篇 |
1997年 | 56篇 |
1996年 | 55篇 |
1995年 | 49篇 |
1994年 | 42篇 |
1993年 | 42篇 |
1992年 | 36篇 |
1991年 | 28篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 2篇 |
1983年 | 7篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1975年 | 2篇 |
1970年 | 2篇 |
1968年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有6344条查询结果,搜索用时 15 毫秒
101.
Betaine and l-stachydrine have been tested as phase-transfer catalysts in Michael and Darzens reactions. The catalytic effect of l-stachydrine was found comparable to that of betaine in Michael addition and even higher when considering Darzens reaction.
The desired products have been obtained in higher yields in solid-liquid system than under liquid-liquid bi-phase conditions.
The influence of microwave irradiation and ultrasound on the reaction rate has been studied. 相似文献
102.
Arthur H. G. David Pablo García-Cerezo Dr. Araceli G. Campaña Prof. Francisco Santoyo-González Dr. Victor Blanco 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(24):6170-6179
We report the application of the click Michael-type addition reaction to vinyl sulfone or vinyl sulfonate groups in the synthesis of rotaxanes through the threading-and-capping method. This methodology has proven to be efficient and versatile as it allowed the preparation of rotaxanes using template approaches based on different noncovalent interactions (i.e., donor-acceptor π–π interactions or hydrogen bonding) in yields of generally 60–80 % and up to 91 % aided by the mild conditions required (room temperature or 0 °C and a mild base such as Et3N or 4-(N,N-dimethylamino)pyridine (DMAP)). Furthermore, the use of vinyl sulfonate moieties, which are suitable motifs for coupling-and-decoupling (CAD) chemistry, implies another advantage because it allows the controlled chemical disassembly of the rotaxanes into their components through nucleophilic substitution of the sulfonates resulting from the capping step with a thiol under mild conditions (Cs2CO3 and room temperature). 相似文献
103.
Marina Sicignano Dr. Rosaria Schettini Luisa Sica Giovanni Pierri Prof. Dr. Francesco De Riccardis Prof. Dr. Irene Izzo Dr. Bholanath Maity Dr. Yury Minenkov Prof. Dr. Luigi Cavallo Dr. Giorgio Della Sala 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(29):7131-7141
The first highly enantioselective arylogous Michael reaction (AMR) of 3-unsubstituted phthalides has been described. This phase-transfer methodology, which uses catalytic amounts of KOH/18-crown-6 catalyst in mesitylene in the presence of N,O-bis(trimethylsilyl)acetamide (BSA), gives access to a broad range of 3-monosubstituted phthalides with high levels of syn diastereoselectivity and good yields, starting from 3-unsubstituted derivatives and diverse α,β-unsaturated carbonyl compounds. The reaction also applies to unactivated 3-alkyl phthalides to afford 3,3-dialkyl derivatives. A plausible mechanism has been suggested. DFT analysis of possible transition states gives a rationale of the high syn diastereoselectivity observed and its correlation with the solvent's dielectric constant. 相似文献
104.
Alberto Piloni Roberto Simonutti Martina H. Stenzel 《Journal of polymer science. Part A, Polymer chemistry》2019,57(18):1994-2001
Fluorine‐19 (19F)‐based contrast agents are increasingly used for magnetic resonance imaging. Conjugated to polymers, they provide an excellent quantitative imaging tool to detect the movement of the polymeric nanoparticles in vivo as there is no background signal in tissue. One of the challenges is the decline in signal intensity when the conjugated hydrophobic fluorinated functionalities aggregate. Therefore, a new fluorinated monomer was prepared from l ‐arginine that carries a 2,2,2‐trifluoroethyl functional group for imaging. The resulting monomer, 2,2,2‐trifluoroethylamide l ‐arginine methacrylamide (3FArgMA), was copolymerized with poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), [2‐(2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranosyloxy)ethyl methacrylate or 1‐O‐methacryloyl‐2,3:4,5‐di‐O‐isopropylidene‐β‐d ‐fructopyranose, respectively, using poly(methyl methacrylate) macro‐reversible addition–fragmentation chain transfer polymerization agent. The resulting block copolymers, which varied in 3FArgMA content, were self‐assembled into micelles of hydrodynamic diameters from 25 to 60 nm. The permanently positively charged arginine functionality on the 3FArgMA displayed repulsive forces against aggregation enabling high spin–spin relaxation times (T2) in acidic as well as alkaline solutions. However, the longer poly(ethylene glycol) side functionality in PEGMEMA enabled better steric stabilization (T2~30 ms) while the short fructose side chain was not enough to maintain high T2 values, in particular when a higher 3FArgMA content was used. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1994–2001 相似文献
105.
A ligand-free, CuI-catalyzed protocol was developed for the one-step preparation of Michael adducts of aromatic thiols in high yields by reacting a mixture of an aryl halide and an electron-deficient alkene with sodium iso-propyl xanthogenate. 相似文献
106.
A novel, rational‐designed approach to access various heteroaryl‐substituted alkyl thioethers was developed via docking‐migration cascade process. By utilizing three components involving alkene, dual‐function reagent, and thioetherificating reagent, radical heteroarylalkylation of alkenes followed by thiolation of the alkyl radical intermediates proceeded smoothly, manifesting well compatibility of substrates and cascade transformations. Furthermore, this protocol also features mild conditions, broad substrate scope, and wide product diversity. 相似文献
107.
108.
109.
Yao Chen Panpan Sun Tiancheng Li Yin Zou Yiyong Huang Yongcun Shen 《Tetrahedron letters》2018,59(25):2399-2402
Asymmetric Michael addition between 3-ethyl carboxylate substituted pyrazolones and 5-alkenylthiazolones catalyzed by a series of chiral bifunctional hydrogen bonding organocatalysts was investigated. Good yields (up to 96%) and excellent enantioselectivities (up to 99% ee) were achieved by using a squaramide containing piperidine group derived from (1S, 2S)-cyclohexanediamine. This strategy provides facile access to a diverse library of thiazole-pyrazolone derivatives with potential bioactivity. 相似文献
110.
Copper(I) catalyzed radical benzylation and cyclization reaction of tertiary enamides was investigated and 3,3-disubstituted N-Alkyl isoindolin-1-ones were obtained in moderate to good yields. In this reaction, two new C–C bonds were formed in one step with high atom economy. Possible reaction pathway for the formation of the products was also discussed in this paper. 相似文献