全文获取类型
收费全文 | 821篇 |
免费 | 136篇 |
国内免费 | 155篇 |
专业分类
化学 | 1095篇 |
晶体学 | 1篇 |
综合类 | 1篇 |
物理学 | 15篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 8篇 |
2021年 | 22篇 |
2020年 | 39篇 |
2019年 | 22篇 |
2018年 | 26篇 |
2017年 | 31篇 |
2016年 | 62篇 |
2015年 | 39篇 |
2014年 | 57篇 |
2013年 | 110篇 |
2012年 | 56篇 |
2011年 | 60篇 |
2010年 | 55篇 |
2009年 | 52篇 |
2008年 | 65篇 |
2007年 | 58篇 |
2006年 | 52篇 |
2005年 | 38篇 |
2004年 | 34篇 |
2003年 | 55篇 |
2002年 | 26篇 |
2001年 | 14篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 20篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有1112条查询结果,搜索用时 93 毫秒
881.
Swadhin K. Mandal Setharampattu S. Krishnamurthy Thomas Stey 《Journal of organometallic chemistry》2005,690(3):742-750
Chiral “P-N-P” ligands, (C20H12O2)PN(R)PY2 [R = CHMe2, Y = C6H5 (1), OC6H5 (2), OC6H4-4-Me (3), OC6H4-4-OMe (4) or OC6H4-4-tBu (5)] bearing the axially chiral 1,1′-binaphthyl-2,2′-dioxy moiety have been synthesised. Palladium allyl chemistry of two of these chiral ligands (1 and 2) has been investigated. The structures of isomeric η3-allyl palladium complexes, (R′ = Me or Ph; Y = C6H5 or OC6H5) have been elucidated by high field two-dimensional NMR spectroscopy. The solid state structure of [Pd(η3-1,3-Ph2-C3H3){κ2-(racemic)-(C20H12O2)PN(CHMe2)PPh2}](PF6) has been determined by X-ray crystallography. Preliminary investigations show that the diphosphazanes, 1 and 2 function as efficient auxiliary ligands for catalytic allylic alkylation but give rise to only moderate levels of enantiomeric excess. 相似文献
882.
883.
Prof. Dr. Amir H. Hoveyda Yuebiao Zhou Dr. Ying Shi Prof. M. Kevin Brown Dr. Hao Wu Dr. Sebastian Torker 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(48):21488-21543
A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals. 相似文献
884.
Dr. Cheng-Che Tsai Dr. Christopher Sandford Dr. Tao Wu Buyun Chen Prof. Dr. Matthew S. Sigman Prof. Dr. F. Dean Toste 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(34):14755-14763
The mode of asymmetric induction in an enantioselective intramolecular allylic substitution reaction catalyzed by a combination of palladium and a chiral phosphoric acid was investigated by a combined experimental and statistical modeling approach. Experiments to probe nonlinear effects, the reactivity of deuterium-labeled substrates, and control experiments revealed that nucleophilic attack to the π-allylpalladium intermediate is the enantio-determining step, in which the chiral phosphate anion is involved in stereoinduction. Using multivariable linear regression analysis, we determined that multiple noncovalent interactions with the chiral environment of the phosphate anion are integral to enantiocontrol in the transition state. The synthetic protocol to form chiral pyrrolidines was further applied to the asymmetric construction of C−O bonds at fully substituted carbon centers in the synthesis of chiral 2,2-disubstituted benzomorpholines. 相似文献
885.
《Mendeleev Communications》2022,32(5):619-621
The key enantioselectivity-determining step in Pd-catalyzed asymmetric amination of 2-fluoroallylic substrates was optimized using model reaction of η3-(2-fluorocycloheptenyl)-palladium complexes bearing chiral P,P- and P,N-ligands with various amines. (S)-ButPHOX was found to be the most effective ligand allowing the formation of 2-fluoroallyl amines and anilines with high enantioselectivity. 相似文献
886.
Brian Spiegelberg Andrea Dell'Acqua Dr. Tian Xia Dr. Anke Spannenberg Dr. Sergey Tin Dr. Sandra Hinze Prof. Dr. Johannes G. de Vries 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(33):7820-7825
Catalytic isomerization of allylic alcohols in ethanol as a green solvent was achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobalt PNP pincer complex substituted with phenyl groups on the phosphorus atoms appeared to be the most active. High rates were obtained at 120 °C, even though the addition of one equivalent of base increases the speed of the reaction drastically. Although some evidence was obtained supporting a dehydrogenation–hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double-bond isomerization through alkene insertion–elimination. 相似文献
887.
Jevgenij A. Raskatov Dr. Stephanie Spiess Christian Gnamm Dr. Kerstin Brödner Frank Rominger Dr. Günter Helmchen Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(22):6601-6615
Mechanistic aspects of allylic substitutions with iridium catalysts derived from phosphoramidites by cyclometalation were investigated. The determination of resting states by 31P NMR spectroscopy led to the conclusion that the cyclometalation process is reversible. A novel, one‐pot procedure for the preparation of (π‐ allyl)Ir complexes was developed, and these complexes were characterised by X‐ray crystal structure analyses and spectral data. They are fully active catalysts of the allylic substitution reaction. DFT calculations on the allyl complexes, transition states of the allylic substitution and product olefin complexes gave further mechanistic insight. 相似文献
888.
Mahiuddin Baidya Grygoriy Y. Remennikov Dr. Peter Mayer Dr. Herbert Mayr Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(4):1365-1371
TiCl4‐induced Baylis–Hillman reactions of α,β‐unsaturated carbonyl compounds with aldehydes yield the (Z)‐2‐(chloromethyl)vinyl carbonyl compounds 5 , which react with 1,4‐diazabicyclo[2.2.2]octane (DABCO), quinuclidine, and pyridines to give the allylammonium ions 6 . Their combination with less than one equivalent of the potassium salts of stabilized carbanions (e.g. malonate) yields methylene derivatives 8 under kinetically controlled conditions (SN2’ reactions). When more than one equivalent of the carbanions is used, a second SN2’ reaction converts 8 into their thermodynamically more stable allyl isomers 9 . The second‐order rate constants for the reactions of 6 with carbanions have been determined photometrically in DMSO. With these rate constants and the previously reported nucleophile‐specific parameters N and s for the stabilized carbanions, the correlation log k (20 °C)=s(N + E) allowed us to calculate the electrophilicity parameters E for the allylammonium ions 6 (?19<E <?18). The kinetic data indicate the SN2’ reactions to proceed via an addition–elimination mechanism with a rate‐determining addition step. 相似文献
889.
Dr. Yungen Liu Prof. Dr. Chi‐Ming Che 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(34):10494-10501
[FeIII(F20‐tpp)Cl] (F20‐tpp=meso‐tetrakis(pentafluorophenyl)porphyrinato dianion) is an effective catalyst for imido/nitrene insertion reactions using sulfonyl and aryl azides as nitrogen source. Under thermal conditions, aziridination of aryl and alkyl alkenes (16 examples, 60–95 % yields), sulfimidation of sulfides (11 examples, 76–96 % yields), allylic amidation/amination of α‐methylstyrenes (15 examples, 68–83 % yields), and amination of saturated C? H bonds including that of cycloalkanes and adamantane (eight examples, 64–80 % yields) can be accomplished by using 2 mol % [FeIII(F20‐tpp)Cl] as catalyst. Under microwave irradiation conditions, the reaction time of aziridination (four examples), allylic amination (five examples), sulfimidation (two examples), and amination of saturated C? H bonds (three examples) can be reduced by up to 16‐fold (24–48 versus 1.5–6 h) without significantly affecting the product yield and substrate conversion. 相似文献
890.
以磺化聚醚砜(SPES)为基体,以不同比例的SiO2溶胶与磷钨酸(PWA)为掺杂物,制备了一种有望用于直接甲醇燃料电池(DMFC)的新型SPES/PWA/SiO2有机-无机复合膜,并经热失重分析(TGA)、差示扫描量热仪(DSC)、扫描电镜(SEM)-X射线能谱分析(EDX)等对膜的结构和性能进行了表征,探讨了复合膜用作质子交换膜的可能性.结果表明:复合膜较纯SPES膜具有更高的热稳定性、玻璃化转变温度和吸水率;虽然在室温和电池操作温度(80℃)下,复合膜的拉伸强度均低于纯SPES膜,但即使当SiO2含量高达20%(w)时,复合膜的拉伸强度仍高于Nafion112膜的;SEM图片显示SiO2和PWA在膜中分布均匀,这将有利于连续质子传输通道的形成.对于SiO2含量为15%(w),PWA含量为6%(w)的复合膜,其室温质子传导率达到了0.034S·cm-1,与Nafion112膜的相当,但其甲醇渗透率明显降低,仅为商用Nafion112膜的七分之一左右,这表明该复合膜在直接甲醇燃料电池中具有良好的应用前景. 相似文献