共查询到20条相似文献,搜索用时 203 毫秒
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用RHF从头计算法研究了氟代乙烷消除氟化氢反应的取代基效应.对所有构型均采用能量梯度法在3-21G基级水平上进行了优化,过渡态的构型用振动分析确认.计算结果表明,对所讨论的基团-F,-CH_3,-CN,-NH_2,在α位和β位取代,均对过渡态的结构有一定的影响,电荷分布表明,标题反应的过渡态为部分极化的四元环过渡态.-F在α位和β位的取代均使氟代烷1-2消除氟化氢反应的活化位垒升高,且前者的活化位垒高于后者;-CN在α位的取代使反应的活化位垒升高,在β位的取代使反应的活化位垒降低.-CH_3和-NH_2在α位的取代使反应的活化位垒降低,在β位的取代使反应的活化位垒升高,只是-NH_2的效应强于-CH_3.这些结果与由实验结果推测的结论一致表明,本消除反应与其它卤代烷的消除反应有显著的差别. 相似文献
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利用半经验分子轨道理论AM1方法,研究了5-亚甲基-1,3-二噁烷-4,6-二酮与亚甲基烯酮的2,3-位C=C,3,4-位C=O和1,2-位C=O三种双键位置上的环加成反应的反应机理.采用Berny梯度法优化得到反应的过渡态,并进行了振动分析确认.计算结果表明,环加成反应是按照协同的非同步途径进行的,经过一个扭曲的六员环过渡态,前线轨道分析表明反应机理为[4+2]机理.根据AM1优化得到的产物反应物及过渡态的生成热可知三个反应的活化焓分别为27.07kJ·mol-1,32.41kJ·mol-1和137.96kJ·mol-1,2,3-位C=C双键上的环加成反应的活化焓最低,这与实验中只观察到2,3-位C=C双键上环加成产物的结论是一致的. 相似文献
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介绍了一个新的多功能有机合成子环丙叉环丙烷及其衍生物的制备和反应,环 丙叉环丙烷现已可从环丙烷羧酸甲酯出发较大量地制备。其单取代衍生物亦可容易 地经对其进行去质子化-亲电取代而制备,环丙叉环丙烷的典型反应性能为:(1) 环丙烷环上四个亚甲基有增强了动力学酸度,易于去质子化;(2)中间双键对环 加成反应有很高活性;(3)环丙烷环上处于双键近端和远端的C-C单键可发生裂解 ;(4)在有些反应中,可同时表现出上述反应性能,介绍了环丙叉环丙烷及其衍 生物的一些反应:对中间C=C双键的[2+n]环加成反应、亲电和自由基加成反应、1 ,3-偶极环加成反应和过渡金属催化的反应。 相似文献
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本文采用密度泛函理论M06-2X方法计算了铑催化直接合成苯并氧杂环庚三烯和香豆素衍生物的环加成反应.通过对乙炔或一氧化碳的碳原子从六元环反应物的两个不同方向攻击和插入的两种竞争机制进行了计算研究,以阐明这种转变的主要特征.计算结果表明:(ⅰ)炔烃或一氧化碳的插入过程是反应的关键步骤;(ⅱ)对于乙炔的(5+2)环加成反应,需要更高的能量来破坏反应物的Rh-O键,反应倾向于从Rh-C键一侧完成插入;(ⅲ)对于一氧化碳的(5+1)环加成反应,两条反应路径的活化自由能都较低,两个通道会产生竞争机制. 相似文献
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Lee SI Park SY Park JH Jung IG Choi SY Chung YK Lee BY 《The Journal of organic chemistry》2006,71(1):91-96
[reactions: see text] A rhodium complex of N-heterocyclic carbene (NHC) has been developed for intra- and intermolecular [4 + 2] and intramolecular [5 + 2] cycloaddition reactions. This is the first use of a transition-metal NHC complex in a Diels-Alder-type reaction. For the intramolecular [4 + 2] cycloaddition reactions, all the dienynes studied were converted to their corresponding cycloadducts in 91-99% yields within 10 min. Moreover, up to 1900 turnovers have been obtained for the intramolecular [4 + 2] cycloaddition at 15-20 degrees C. For the intermolecular [4 + 2] cycloadditions, high yields (71-99%) of the corresponding cycloaddition products were obtained. The reaction time and yield were highly dependent upon the diene and the dienophile. For the intramolecular [5 + 2] cycloaddition reactions, all the alkyne vinylcyclopropanes studied were converted to their corresponding cycloadducts in 91-98% yields within 10 min. However, the catalytic system was not effective for an intermolecular [5 + 2] cycloaddition reaction. 相似文献
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Lewis AK Caddick S Cloke FG Billingham NC Hitchcock PB Leonard J 《Journal of the American Chemical Society》2003,125(33):10066-10073
The oxidative addition products trans-[Pd(NHC)(2)(Ar)Cl] (NHC = cyclo-C[N(t)BuCH](2); Ar = Me-4-C(6)H(4), MeO-4-C(6)H(4), CO(2)Me-4-C(6)H(4)) have been isolated in good yields from the reactions of ArCl with the amination precatalyst [Pd(NHC)(2)] and structurally characterized. The former undergo reversible dissociation of one NHC ligand at elevated temperatures, and a value of 25.57 kcal mol(-1) has been determined for the Pd-NHC dissociation enthalpy in the case where Ar = Me-4-C(6)H(4). Detailed kinetic studies have established that the oxidative addition reactions proceed by a dissociative mechanism. Rate data for the oxidation addition of Me-4-C(6)H(4)Cl to [Pd(NHC)(2)] compared to that obtained for the [Pd(NHC)(2)]-catalyzed coupling of morpholine with 4-chlorotoluene are consistent with a rate-determining oxidative addition in the catalytic amination reaction. The relative rates of oxidative addition of the three aryl chlorides to [Pd(NHC)(2)] (CO(2)Me-4-C(6)H(4)Cl > Me-4-C(6)H(4)Cl > MeO-4-C(6)H(4)Cl) reflect the electronic nature of the substituents and also parallel observed trends in coupling efficiency for these aryl halides in aminations. 相似文献
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Mechanisms of palladium-catalyzed cycloaddition reactions of methylenecyclopropane with olefins and those of oxatrimethylenemethane (OTMM) and azatrimethylenemethane (ATMM) complexes with olefins have been studied by applying the ab initio molecular orbital method. Assuming an intramolecular coupling mechanism, we have examined some model complexes, Pd(eta(2)-methylenecyclopropane)(ethylene)(PH(3)) and Pd(eta(3)-OTMM or eta(3)-ATMM)(ethylene)(PH(3)), to determine the transition state structures. The coupling of methylenecyclopropane, OTMM, and ATMM with ethylene on the metal center takes place in two steps. In the reaction of methylenecyclopropane, the first step involves an opening of the cyclopropane ring promoted by an attack of ethylene. The methylenecyclopropane moiety has a pi-allylic form at the first transition state which leads to a metallacyclic intermediate. The first transition state of the reactions of OTMM and ATMM complexes looks very similar to that of the methylenecyclopropane complex, having pi-allylic coordinations. The two paths are separated in the second step. A [3 + 2] addition product is obtained by a reductive elimination from the intermediate metallacycle, whereas a prototropic shift followed by a reductive elimination affords a [2 + 1] addition product. It is the energetics of the second stage starting from the metallacycles that differentiates the reactions of the OTMM and ATMM complexes from the reaction of the methylenecyclopropane complex. The relative stabilities of various isomeric forms of these complexes have also been studied. 相似文献
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Density functional (B3LYP) calculations using the 6-31++g basis set have been employed to study the title reaction between the cationic 1,3-dipolar 1-aza-2-azoniaallene ion (H2C=N(+)=NH) and ethene. Our calculations confirmed that [3 + 2] cycloaddition reaction takes place via a three-membered ring intermediate. In addition, solvent effects and substituent effects were also studied. For the reactions involving tetrachloroethene, there are two attacking sites. One is on the NH group in the 1-aza-2-azoniaallene ion, another is on its terminal CH2 group, and they are competitive for both approaching positions. Electron-releasing methyl substituents on ethene favor the reaction, and the potential energy surface is quite different from the previous one. 相似文献
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Katrin Schroeter Christoph A. Schalley Detlef Schrder Helmut Schwarz 《Helvetica chimica acta》1997,80(4):1205-1220
The Fe+-mediated [4+2] cycloaddition of dienes with alkynes has been examined by four-sector ion-beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me-substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e., 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but-2-yne does not affect the [4+2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety (e.g., penta-1,3-diene, liexa-2,4-diene) induce side reactions, namely double-bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4-dimethylhexa-2,4-diene. Similarly, alkynes with larger alkyl substituents (pent-1-yne, 3,3-dimethylbut-1-yne) suppress the [4 + 2] cycloaddition route. Stereochemical effects have been observed for the (E)- and (Z)-penta-1,3-diene ligands as well as for (E,E)- and (E,Z)-hexa-2,4-diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further, the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe+ and ethoxyacetylcne/isoprene/Fe+ indicate that substituents avoid proximity. 相似文献
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Possible mechanisms of the [2+2] cycloaddition reactions of ethylene (1), propylene (2), vinyl chloride (3), and styrene (4) with the Si(100)-2 x 1 surface have been investigated by theoretical calculations with the unrestricted density functional theory (DFT) and the second-order M?ller-Plesset perturbation theory (MP2). Facile occurrence of the studied reactions is supported by the low activation energies (2.45-5.76 kcal/mol) in the rate-determining steps. The buckled Si(100) surface facilitates the reactions via the low-symmetric pathways. The reactions follow the diradical mechanism of thermal [2+2] cycloaddition reactions between pi-electron donors (the ethylene derivatives) and acceptors (the Si surface) through a pi-complex precursor and a singlet diradical intermediate. The influence of substituents on the relative reactivity takes a qualitative sequence of 1 < 2 < 3 < 4. The natural bond orbital (NBO) analysis and the released heat of some model reactions suggest that the relative reactivity might be partially understood by the pi-electron-donating abilities of the substituent to stabilize the radical centers at the transition states of the rate-determining steps. 相似文献
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S. S. Rynin V. I. Faustov S. E. Boganov M. P. Egorov O. M. Nefedov 《Russian Chemical Bulletin》2010,59(6):1099-1109
The cycloaddition reactions of dichlorogermylene GeCl2 to ethylene, buta-1,3-diene, and hexa-1,3,5-triene were studied within the framework of the density functional theory (PBE
and B3LYP density functionals) and by the ab initio CBS-QB3 method. The energy characteristics of the reaction of GeCl2 with ethylene were refined and non-empirical quantum chemical calculations of reaction pathways in the GeCl2 + buta-1,3-diene and GeCl2 + hexa-1,3,5-triene systems were carried out for the first time. It was shown that the [2+1] cycloaddition reactions are
kinetically hindered and thermodynamically unfavorable, while the [4+1] and [6+1] cycloaddition reactions are characterized
by low barriers and result in thermodynamically favorable products. For the [4+1] cycloaddition to buta-1,3-diene and [6+1]
cycloaddition to hexa-1,3,5-triene, the most energetically favorable reaction pathways involve a suprafacial and antarafacial
approach of reactants, respectively. 相似文献
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Süleyman Gülcemal 《应用有机金属化学》2012,26(5):246-251
Six new [RhBr(NHC)(cod)] (NHC = N‐heterocyclic carbene; cod = 1,5‐cyclooctadiene) type rhodium complexes ( 4–6 ) have been prepared by the reaction of [Rh(μ‐OMe)(cod)]2 with a series of corresponding imidazoli(in)ium bromides ( 1–3 ) bearing mesityl (Mes) or 2,4,6‐trimethylbenzyl (CH2Mes) substituents at N1 and N3 positions. They have been fully characterized by 1 H, 13 C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)2] (NHC = imidazol‐2‐ylidene) ( 7b–9b ) were also synthesized to compare σ‐donor/π‐acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as catalysts. The symmetrically CH2Mes‐substituted rhodium complex bearing a saturated NHC ligand ( 5a ) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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Dr. Terrance J. Hadlington Dr. Arseni Kostenko Prof. Dr. Matthias Driess 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(9):1958-1962
The versatile cycloaddition chemistry of the Si−Ni multiple bond in the acyclic (amido)(chloro)silylene→Ni0 complex 1 , [(TMSL)ClSi→Ni(NHC)2] (TMSL=N(SiMe3)Dipp; Dipp=2,6-iPr2C6H4; NHC=C[(iPr)NC(Me)]2), toward unsaturated organic substrates is reported, which is both reminiscent of and expanding on the reactivity patterns of classical Fischer and Schrock carbene–metal complexes. Thus, 1:1 reaction of 1 with aldehydes, imines, alkynes, and even alkenes proceed to yield [2+2] cycloaddition products, leading to a range of four-membered metallasilacycles. This cycloaddition is in fact reversible for ethylene, whereas addition of an excess of this olefin leads to quantitative sp2-CH bond activation, via a 1-nickela-4-silacyclohexane intermediate. These results have been supported by DFT calculations giving insights into key mechanistic aspects. 相似文献
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Lu X 《Journal of the American Chemical Society》2003,125(21):6384-6385
Density functional cluster model calculations have been performed to explore the reaction mechanism for the adsorption of ethylene on Si(100). It is shown that the [2 + 2] cycloaddition of ethylene on a Si=Si dimer of Si (100) surface follows a diradical mechanism, via a pi-complex precursor and a singlet diradical intermediate, and the rate-determining step for the overall reaction is the formation of the diradical intermediate. 相似文献