共查询到20条相似文献,搜索用时 31 毫秒
1.
钴原子催化活化乙烷的反应机理 总被引:1,自引:0,他引:1
采用密度泛函理论B3LYP方法分别在两种不同基组水平上, 研究了Co原子催化乙烷反应的反应机理, 优化了反应过程中各反应物、中间体、过渡态和产物的构型, 并在同一水平上计算了反应中各驻点的振动频率, 运用自然键轨道理论(NBO)方法分析了各物质的成键情况和轨道间相互作用. 在两种不同基组水平上研究所得的反应历程及相应的能量变化趋势是一致的, 其活化过程可分为C—C键活化及C—H键活化, 分别释放出CH4和H2, 反应速控步骤的活化能后者较前者低, 因此, C—H键的活化较C—C 键活化容易进行. 相似文献
2.
Roy S Davydova MP Pal R Gilmore K Tolstikov GA Vasilevsky SF Alabugin IV 《The Journal of organic chemistry》2011,76(18):7482-7490
The reaction of diaryl ketoalkynes with 1,2-diamino ethane leads to the full scission of the triple bond with the formation of acetophenone and imidazoline fragments. In this transformation, one of the alkyne carbons undergoes formal reduction with the formation of three C-H bonds, whereas the other carbon undergoes formal oxidation via the formation of three C-N bonds (one π and two σ). Computational analysis confirmed that the key fragmentation step proceeds via a six-membered TS in a concerted manner. Both amines are involved in the fragmentation: the N-H moiety of one amine transfers a proton to the developing negative charge at the enolate oxygen, while the other amine provides direct stereoelectronic assistance to the C-C bond cleavage via a hyperconjugative n(N) → σ*(C-C) interaction. 相似文献
3.
4.
Gandelman M Shimon LJ Milstein D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4295-4300
Based on the PCN ligand 2, a remarkable degree of control over C-C versus C-H bond activation and versus formation of an agostic C-C complex was demonstrated by choice of cationic [Rh(CO)(n)(C(2)H(4))(2-n)] (n=0, 1, 2) precursors. Whereas reaction of 2 with [Rh(C(2)H(4))(2)(solv)(n)]BF(4) results in exclusive C-C bond activation to yield product 5, reaction with the dicarbonyl precursor [Rh(CO)(2)(solv)(n)]BF(4) leads to formation of the C-H activated complex 9. The latter process is promoted by intramolecular deprotonation of the C-H bond by the hemilabile amine arm of the PCN ligand. The mixed monocarbonyl monoethylene Rh species [Rh(CO)(C(2)H(4))]BF(4) reacts with the PCN ligand 2 to give an agostic complex 7. The C-C activated complex 5 is easily converted to the C-H activated one (9) by reaction with CO; the reaction proceeds by a unique sequence of 1,2-metal-to-carbon methyl shift, agostic interaction, and C-H activation processes. Similarly, the C-C agostic complex 7 is converted to the same C-H activated product 9 by treatment with CO. 相似文献
5.
6.
交叉脱氢偶联反应* 总被引:2,自引:0,他引:2
发现高效高选择性的有机合成反应是有机合成化学研究中一个重要的发展方向。传统的有机合成化学是建立在官能团相互转化基础上的,又称官能团化学。非活泼化学键(如C-H键)的直接官能团化省去了一步甚至多步制备官能团化的反应底物,因此,非活泼化学键活化是提高有机合成反应效率的一个重要发展方向。交叉脱氢偶联(Cross-Dehydrogenative-Coupling,CDC)反应就是直接利用不同反应底物中的C-H键,在氧化条件下,进行脱氢偶联反应形成C-C键。交叉脱氢偶联反应实现了更短的合成路线和更高的原子利用效率,为直接利用简单的原料进行高效的复杂的有机合成任务提供了一种新的思路和手段。 相似文献
7.
8.
We have studied the oxidative addition reactions of methane and ethane C-H, ethane C-C and iodomethane C-I bonds to Pd and cis-Pd(CO)2I2 at the ZORA-BP86/TZ(2)P level of relativistic density functional theory (DFT). Our purpose, besides exploring these particular model reactions, is to understand how the mechanism of bond activation changes as the catalytically active species changes from a simple, uncoordinated metal atom to a metal-ligand coordination complex. For both Pd and cis-Pd(CO)2I2, direct oxidative insertion (OxIn) is the lowest-barrier pathway whereas nucleophilic substitution (SN2) is highly endothermic, and therefore not competitive. Introducing the ligands, i.e., going from Pd to cis-Pd(CO)2I2, causes a significant increase of the activation and reaction enthalpies for oxidative insertion and takes away the intrinsic preference of Pd for C-I over C-H activation. Obviously, cis-Pd(CO)2I2 is a poor catalyst in terms of activity as well as selectivity for one of the three bonds studied. However, its exploration sheds light on features in the process of catalytic bond activation associated with the increased structural and mechanistic complexity that arises if one goes from a monoatomic model catalysts to a more realistic transition-metal complex. First, in the transition state (TS) for oxidative insertion, the C-X bond to be activated can have, in principle, various different orientations with respect to the square-planar cis-Pd(CO)2I2 complex, e.g., C-X or X-C along an I-Pd-CO axis, or in between two I-Pd-CO axes. Second, at variance to the uncoordinated metal atom, the metal complex may be deformed due to the interaction with the substrate. This leads to a process of mutual adjustment of catalyst and substrate that we designate catalyst-substrate adaptation. The latter can be monitored by the Activation Strain model in which activation energies ΔE≠ are decomposed into the activation strain of and the stabilizing TS interaction between the reactants in the activated complex: . 相似文献
9.
Shiota Y Herrera JM Juhász G Abe T Ohzu S Ishizuka T Kojima T Yoshizawa K 《Inorganic chemistry》2011,50(13):6200-6209
The catalytic conversion of 1,2-cyclohexanediol to adipic anhydride by Ru(IV)O(tpa) (tpa ═ tris(2-pyridylmethyl)amine) is discussed using density functional theory calculations. The whole reaction is divided into three steps: (1) formation of α-hydroxy cyclohexanone by dehydrogenation of cyclohexanediol, (2) formation of 1,2-cyclohexanedione by dehydrogenation of α-hydroxy cyclohexanone, and (3) formation of adipic anhydride by oxygenation of cyclohexanedione. In each step the two-electron oxidation is performed by Ru(IV)O(tpa) active species, which is reduced to bis-aqua Ru(II)(tpa) complex. The Ru(II) complex is reactivated using Ce(IV) and water as an oxygen source. There are two different pathways of the first two steps of the conversion depending on whether the direct H-atom abstraction occurs on a C-H bond or on its adjacent oxygen O-H. In the first step, the C-H (O-H) bond dissociation occurs in TS1 (TS2-1) with an activation barrier of 21.4 (21.6) kcal/mol, which is followed by abstraction of another hydrogen with the spin transition in both pathways. The second process also bifurcates into two reaction pathways. TS3 (TS4-1) is leading to dissociation of the C-H (O-H) bond, and the activation barrier of TS3 (TS4-1) is 20.2 (20.7) kcal/mol. In the third step, oxo ligand attack on the carbonyl carbon and hydrogen migration from the water ligand occur via TS5 with an activation barrier of 17.4 kcal/mol leading to a stable tetrahedral intermediate in a triplet state. However, the slightly higher energy singlet state of this tetrahedral intermediate is unstable; therefore, a spin crossover spontaneously transforms the tetrahedral intermediate into a dione complex by a hydrogen rebound and a C-C bond cleavage. Kinetic isotope effects (k(H)/k(D)) for the electronic processes of the C-H bond dissociations calculated to be 4.9-7.4 at 300 K are in good agreement with experiment values of 2.8-9.0. 相似文献
10.
A A Fokin B A Tkachenko O I Korshunov P A Gunchenko P R Schreiner 《Journal of the American Chemical Society》2001,123(45):11248-11252
The mechanisms of C-H and C-C bond activations with dimethyldioxirane (DMD) were studied experimentally and computationally at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level for the propellanes 3,6-dehydrohomoadamantane (2) and 1,3-dehydroadamantane (3). The sigma(C-C) activation of 3 with DMD (Delta G(*) = 23.9 kcal mol(-1) and Delta G(r) = -5.4 kcal mol(-1)) is the first example of a molecule-induced homolytic C-C bond cleavage. The C-H bond hydroxylation observed for 2 is highly exergonic (Delta G(r) = -74.4 kcal mol(-1)) and follows a concerted pathway (Delta G(*) = 34.8 kcal mol(-1)), in contrast to its endergonic molecule-induced homolysis (Delta G(*) = 28.8 kcal mol(-1) and Delta G(r) = +9.2 kcal mol(-1)). The reactivities of 2 and 3 with CrO(2)Cl(2), which follow a molecule-induced homolytic activation mechanism, parallel the DMD results only for highly reactive 3, but differ considerably for more stable propellanes such as 4-phenyl-3,6-dehydrohomoadamantane (1) and 2. 相似文献
11.
[reaction: see text] Computational studies of three different reaction types involving hydrocarbons (homolytic C-C bond breaking of alkanes, progressive insertions of triplet methylene into C-H bonds of ethane, and [2+2] cyclizations of methyl-substituted alkenes to form polymethylcyclobutanes) show that the B3LYP model consistently underestimates the reaction energy, even when extremely large basis sets are employed. The error is systematic and cumulative, such that the reaction energies of reactions involving hydrocarbons with more than 4-6 C-C bonds are predicted quite poorly. Energies are underestimated for slightly and highly methyl-substituted cyclic and acyclic hydrocarbons, so the errors do not arise from structural issues such as steric repulsion or ring strain energy. We trace the error associated with the B3LYP approach to its consistent underestimation of the C-C bond energy. Other DFT models show this problem to lesser extents, while the MP2 method avoids it. As a consequence, we discourage the use of the B3LYP model for reaction energy calculations for organic compounds containing more than four carbon atoms. We advocate use of a collection of pure and hybrid DFT models (and ab initio models where possible) to provide computational "error bars". 相似文献
12.
The values of the molar standard enthalpies of formation, Delta(f)H(o)(m)(C(76), cr) = (2705.6 +/- 37.7) kJ x mol(-1), Delta(f)H(o)(m)(C(78), cr) = (2766.5 +/- 36.7) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), cr) = (2826.6 +/- 42.6) kJ x mol(-1), were determined from the energies of combustion, measured by microcombustion calorimetry on a high-purity sample of the D(2) isomer of fullerene C(76), as well as on a mixture of the two most abundant constitutional isomers of C(78) (C(2nu)-C(78) and D(3)-C(78)) and C(84) (D(2)-C(84), and D(2d)-C(84). These values, combined with the published data on the enthalpies of sublimation of each cluster, lead to the gas-phase enthalpies of formation, Delta(f)H(o)(m)(C(76), g) = (2911.6 +/- 37.9) kJ x mol(-1); Delta(f)H(o)(m)(C(78), g) = (2979.3 +/- 37.2) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), (g)) = (3051.6 +/- 43.0) kJ x mol(-1), results that were found to compare well with those reported from density functional theory calculations. Values of enthalpies of atomization, strain energies, and the average C-C bond energy were also derived for each fullerene. A decreasing trend in the gas-phase enthalpy of formation and strain energy per carbon atom as the size of the cluster increases is found. This is the first experimental evidence that these fullerenes become more stable as they become larger. The derived experimental average C-C bond energy E(C-C) = 461.04 kJ x mol(-1) for fullerenes is close to the average bond energy E(C-C) = 462.8 kJ x mol(-1) for polycyclic aromatic hydrocarbons (PAHs). 相似文献
13.
14.
Chandra AK Parveen S Zeegers-Huyskens T 《The journal of physical chemistry. A》2007,111(36):8884-8891
Quantum mechanical calculations using density functional theory with the hybrid B3LYP functional and the 6-31++G(d,p) basis set are performed on isolated triethylamine (TEA), its hydrogen-bond complex with phenol, and protonated TEA. The calculations include the optimized geometries and the results of a natural bond orbital (NBO) analysis (occupation of sigma* orbitals, hyperconjugative energies, and atomic charges). The harmonic frequencies of the C-H stretching vibrations of TEA are predicted at the same level of theory. Two stable structures are found for isolated TEA. In the most stable symmetrical structure (TEA-S), the three C-C bond lengths are equal and one of the C-H bond of each of the three CH2 groups is more elongated than the three other ones. In the asymmetrical structure (TEA-AS), one of the C-C bonds and two C-H bonds of two different CH2 groups are more elongated than the other ones. These structures result from the hyperconjugation of the N lone pair to the considered sigma*(C-H) orbitals (TEA-S) or to the sigma*(C-C) and sigma*(C-H) orbitals of the CH2 groups (TEA-AS). The formation of a OH...N hydrogen bond with phenol results in a decrease of the hyperconjugation, a contraction of the C-H bonds, and blue-shifts of 28-33 cm-1 (TEA-S) or 40-48 cm-1 (TEA-AS) of the nus(CH2) vibrations. The nu(CH3) vibrations are found to shift to a lesser extent. Cancellation of the lone pair reorganization in protonated TEA-S and TEA-AS results in large blue-shifts of the nu(CH2) vibrations, between 170 and 190 cm-1. Most importantly, in contrast with the blue-shifting hydrogen bonds involving C-H groups, the blue-shifts occurring at C-H groups not participating in hydrogen bond formation is mainly due to a reduction of the hyperconjugation and the resulting decrease in the occupation of the corresponding sigma*(C-H) orbitals. A linear correlation is established between the C-H distances and the occupation of the corresponding sigma*(C-H) orbitals in the CH2 groups. 相似文献
15.
Rybtchinski B Oevers S Montag M Vigalok A Rozenberg H Martin JM Milstein D 《Journal of the American Chemical Society》2001,123(37):9064-9077
C-H bond activation was observed in a novel PCO ligand 1 (C(6)H(CH(3))(3)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)()]BF(4) (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 degrees C, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH(3))Rh(C(6)H(CH(3))(2)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))BF(4)), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF(4)(-) anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation. 相似文献
16.
Successful dihydridocarbonyltris(triphenylphosphine)ruthenium (Ru-complex) catalyzed polymerization of 4-acetylstilbene with 1,3-divinyltetramethyldisiloxane to yield poly(3,3,5,5-tetramethyl-4-oxa-3,5-disila-1,7-heptanylene-alt-4-acetyl-3,5-stilbenylene) is reported. Polymerization results from Ru-complex catalyzed activation of aromatic C-H bonds which are ortho to the acetyl for selective anti-Markovnikov addition across the terminal C-C double bonds of 1,3-divinyltetramethyldisiloxane. The internal C-C double bond of 4-acetylstilbene does not react. 相似文献
17.
18.
Density functional PBE/TZ2p quantum chemical calculations of activated complexes and pathways of model catalytic transformations
of propane under the action of aluminum chloride-cobalt chloride ionic bimetallic complexes were carried out. The formation
of an intermediate with a broken C-C bond can occur on the cationic cluster CoAlCl4
+ characterized by the strongest coordination of propane molecule. The activation barrier to the reaction is ΔG = 25.0 kcal mol−1. Activation of alkane C-H bonds follows the alkyl pathway involving the formation of bimetallic alkyl complexes. The interaction
of activated hydrocarbon fragments bound to transition metal atoms in cobalt-chloroaluminate clusters can result in alkane
metathesis products (in this case, ethane and a polymetallic cluster containing an extendedchain alkyl radical). 相似文献
19.
Simple heteroarenes such as pyrroles and indoles undergo addition reactions to C-C triple bonds in the presence of a catalytic amount of Pd(OAc)(2) under very mild conditions, affording cis-heteroarylalkenes in most cases. The cleavage of aromatic C-H bonds is the possible rate-determining step in CH(2)Cl(2), and the addition of heteroaromatic C-H bonds to C-C triple bonds is in a trans-fashion. 相似文献
20.
Alabugin IV 《The Journal of organic chemistry》2000,65(13):3910-3919
Stereoelectronic effects proposed for C-H bonds in cyclohexane, 1, 3-dioxane, 1,3-oxathiane, and 1,3-dithiane were studied computationally. The balance of three effects, namely, sigma(C)(-)(X) --> sigma(C)(-)(H)()eq, sigma(C)(-)(H)()eq --> sigma(C)(-)(X), and n(p)(X) --> sigma(C)(-)(H)()eq interactions, was necessary to explain the relative elongation of equatorial C(5)-H bonds. The role of homoanomeric n(p) --> sigma(C(5))(-)(H)()eq interaction is especially important in dioxane. In dithiane, distortion of the ring by long C-S bonds dramatically increases overlap of sigma(C(5))(-)(H)()eq and sigma(C)(-)(S) orbitals and energy of the corresponding hyperconjugative interaction. Anomeric n(p)(X) --> sigma(C)(-)(H)()ax interactions with participation of axial C-H bonds dominate at C(2), C(4), and C(6). The balance of hyperconjugative interactions involving C-H(ax) and C-H(eq) bonds agrees well with the relative bond lengths for all C-H(ax)/C-H(eq) pairs in all studied compounds. At the same time, the order of one-bond spin-spin coupling constants does not correlate with the balance of stereoelectronic effects in dithiane and oxathiane displaying genuine reverse Perlin effect. 相似文献