全文获取类型
收费全文 | 53658篇 |
免费 | 7028篇 |
国内免费 | 4953篇 |
专业分类
化学 | 44413篇 |
晶体学 | 220篇 |
力学 | 1321篇 |
综合类 | 702篇 |
数学 | 6855篇 |
物理学 | 12128篇 |
出版年
2023年 | 709篇 |
2022年 | 875篇 |
2021年 | 1991篇 |
2020年 | 2199篇 |
2019年 | 1966篇 |
2018年 | 1729篇 |
2017年 | 1492篇 |
2016年 | 2137篇 |
2015年 | 2382篇 |
2014年 | 2928篇 |
2013年 | 4257篇 |
2012年 | 3014篇 |
2011年 | 3267篇 |
2010年 | 2823篇 |
2009年 | 3365篇 |
2008年 | 3482篇 |
2007年 | 3430篇 |
2006年 | 2851篇 |
2005年 | 2230篇 |
2004年 | 2053篇 |
2003年 | 1878篇 |
2002年 | 1396篇 |
2001年 | 1287篇 |
2000年 | 996篇 |
1999年 | 791篇 |
1998年 | 732篇 |
1997年 | 698篇 |
1996年 | 699篇 |
1995年 | 679篇 |
1994年 | 599篇 |
1993年 | 590篇 |
1992年 | 591篇 |
1991年 | 359篇 |
1990年 | 321篇 |
1989年 | 272篇 |
1988年 | 291篇 |
1987年 | 237篇 |
1986年 | 238篇 |
1985年 | 341篇 |
1984年 | 261篇 |
1983年 | 159篇 |
1982年 | 310篇 |
1981年 | 473篇 |
1980年 | 436篇 |
1979年 | 475篇 |
1978年 | 371篇 |
1977年 | 289篇 |
1976年 | 242篇 |
1974年 | 81篇 |
1973年 | 154篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Chemoselective Chromium(II)‐Catalyzed Cross‐Coupling Reactions of Dichlorinated Heteroaromatics with Functionalized Aryl Grignard Reagents 下载免费PDF全文
Dr. Andreas K. Steib Dr. Olesya M. Kuzmina Sarah Fernandez Dr. Sushant Malhotra Prof. Paul Knochel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(5):1961-1965
Chromium(II) chloride catalyzes the chemoselective cross‐coupling reaction of dichloropyridines with a range of functionalized (hetero)aromatic Grignard reagents at room temperature. Functional groups, such as esters and acetals, are well tolerated in this transformation. Previously challenging substrates, quinolines and isoquinolines, participate in the selective Cr‐catalyzed cross‐coupling in cyclopentyl methyl ether (CPME) as the solvent. The effective purging of Cr salts is demonstrated by using various solid supports. 相似文献
92.
Prof. Ian J. S. Fairlamb 《Angewandte Chemie (International ed. in English)》2015,54(36):10415-10427
This Minireview highlights the redox and non‐innocent behavior of NOx ligands (x=1, 2, or 3) in selected Pd‐mediated processes, for example, alkene and aromatic oxidation processes. A focus is placed on mechanistic understanding and linking recent transformations, such as C? H bond activation/functionalization and Wacker oxidation, with previous work on the functionalization of aromatics and alkenes by PdII salts. 相似文献
93.
Substituted Tetraaza‐ and Hexaazahexacenes and their N,N′‐Dihydro Derivatives: Syntheses,Properties, and Structures 下载免费PDF全文
Jens U. Engelhart Dr. Benjamin D. Lindner Manuel Schaffroth David Schrempp Olena Tverskoy Prof. Uwe H. F. Bunz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8121-8129
The palladium‐catalyzed coupling of a substituted o‐diaminoanthracene and a substituted o‐diaminophenazine to substituted 2,3‐dichloroquinoxalines furnishes 10 differently substituted N,N′‐dihydrotetraaza‐ or ‐hexaazahexacenes with the quinoxaline group of the azaacenes carrying fluorine, chlorine, or nitro groups. The N,N′‐dihydrotetraazahexacenes with hydrogen, chlorine, and fluorine subtituents are oxidized to azaacenes, whereas only the parent N,N′‐dihydrohexaazahexacenes, with hydrogen substituents, are oxidized by MnO2. The resultant azaacenes are characterized by their optical and spectroscopic data. In addition, single‐crystal X‐ray structures have been obtained for the parent tetraazahexacenes and their difluoro‐substituted derivatives. The di‐ and tetrachloro derivatives of the N,N′‐dihydrohexaazahexacene have also been structurally characterized. 相似文献
94.
价廉易得的L-亮氨酸先以苄基同时保护氨基及羧基得(S)-2-(二苄胺基)-4-甲基戊酸苄酯,进而在碱性条件下与乙腈发生亲核取代反应得(S)-4-(二苄胺基)-6-甲基-3-氧代庚腈,再经硼氢化钠选择性还原羰基得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚腈,用双氧水氧化得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚酸,最后在Pd(OH)2/C-H2作用下脱掉苄基得到(3S,4S)-4-氨基-3-羟基-6-甲基庚酸,即(3S,4S)-statine。整个合成路线总产率为33.6%。 相似文献
95.
Inside Back Cover: Multiple and Highly Selective Alkyne–Isonitrile C−C and C−N Couplings at Group 4 Metallocenes (Chem. Eur. J. 27/2016) 下载免费PDF全文
96.
An efficient,mild and selective Ullmann‐type N‐arylation of indoles catalysed by Pd immobilized on amidoxime‐functionalized mesoporous SBA‐15 as heterogeneous and recyclable nanocatalyst 下载免费PDF全文
A wide range of N‐arylated indoles were selectively synthesized through intermolecular C(aryl)? N bond formation from the corresponding aryl iodides and indoles through Ullmann‐type coupling reactions in the presence of a catalytic amount of Pd immobilized on amidoxime‐functionalized mesoporous SBA‐15 (SBA‐15/AO/Pd(0)) under mild reaction conditions. These cross‐coupled products were obtained in excellent yields under mild conditions at extremely low palladium loading (ca 0.3 mol%), and the heterogeneous catalyst can be readily recovered by simple filtration and reused seven times with loss in its activity. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
97.
A highly active multi‐usable palladium pyridylfluorene film‐based catalyst for C–C cross‐coupling reactions 下载免费PDF全文
The two terminal pyridyl nitrogen atoms of 2,7‐bis(4‐pyridyl)fluorene ( 1 ) were coordinated to Pd(II) ions to give self‐assembled, multilayer films using the layer‐by‐layer (LbL) method. The films were prepared by alternately dipping the substrate, pre‐coated with a polyethyleneimine layer, in aqueous solutions of PdCl2 and ethanol solutions of 1 . The resulting films were characterized using UV–visible absorption spectroscopy, atomic force microscopy (AFM), X‐ray photoelectron spectroscopy, scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP‐AES). UV–visible spectra and SEM images show almost uniform growth of the film in a near ideal LbL manner. AFM images show that nanostructured aggregates of Pd(II) complexes form on the surface. With an increase in the number of Pd(II)/ 1 bilayers, more particulate aggregates are distributed on the surface. When released from the substrate, the Pd(II) complex nanostructure shows high catalytic activity for Suzuki–Miyaura and Mizoroki–Heck cross‐coupling reactions. The catalyst loading is as low as 9.1 × 10?3 mol% Pd, as measured using ICP‐AES, and high turnover numbers of up to 1.08 × 104 are obtained. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
98.
Neal P. Mankad 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(17):5822-5829
An emerging area of homogeneous catalysis is the use of catalysts featuring two closely associated metal sites. This approach complements the traditional focus on single‐site catalysts and makes available new parameters with which to optimize catalytic behavior. Single‐site catalysts are optimized through changing 1) the identity of the metal, and 2) the steric and electronic properties of the ligands. Bimetallic catalysts introduce new optimization parameters such as 3) catalyst nuclearity (mononuclear vs. binuclear), and 4) bimetallic pairing (relative compatibility of two metal sites). In order to harness these new optimization parameters in developing systems, it is necessary to first understand the origin of bimetallic selectivity effects that already have been documented. This Concept article highlights bimetallic effects on the chemo‐, regio‐, and stereoselectivity of catalytic transformations, using selected case studies from the recent literature as illustrative examples. 相似文献
99.
Haoquan Li Dr. Helfried Neumann Prof. Dr. Matthias Beller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(29):10050-10056
A benign and efficient palladium‐catalyzed aminocarbonylation reaction of allylic alcohols is presented. The generality of this novel process is demonstrated by the synthesis of β,γ‐unsaturated amides including aliphatic, cinnamyl, and terpene derivatives. The choice of ligand is crucial for optimal carbonylation processes: Whereas in most cases the combination of PdCl2 with Xantphos ( L6 ) gave best results, sterically hindered substrates performed better in the presence of simple triphenylphosphine ( L10 ), and primary anilines gave the best results using cataCXium® PCy ( L8 ). The reactivity of the respective catalyst system is significantly enhanced by addition of small amounts of water. Mechanistic studies and control experiments revealed a tandem allylic alcohol amination/C?N bond carbonylation reaction sequence. 相似文献
100.
Development of a syn‐Selective Mannich Reaction of Aldehydes with Propargylic Imines by Dual Catalysis: Asymmetric Synthesis of Functionalized Propargylic Amines 下载免费PDF全文
Irati Lapuerta Dr. Silvia Vera Prof. Mikel Oiarbide Prof. Claudio Palomo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(21):7229-7237
Direct coupling of enolizable aldehydes with C‐alkynyl imines is realized affording the corresponding propargylic Mannich adducts of syn configuration, thus complementing previous methods that gave access to the anti‐isomers. The combination of proline and a urea Brønsted base cocatalyst is key for the reactions to proceed under very mild conditions (3–10 mol % catalyst loading, dichloromethane as solvent, ?20 °C, 1.2 molar equivalents of aldehyde) and with virtually total stereocontrol (syn/anti ratio up to 99:1; ee up to 99 %). Some possibilities of further chemical elaboration of adducts are also briefly illustrated. 相似文献