Cyclotrimerization acetylene

A survey of the studies of metal-catalyzed acetylene cyclotrimerization on single-crystal, supported, and bimetallic catalysts is available.539 A new study with size-selected Pdn clusters found that clusters as small as Pd7 are able to induce benzene formation at 157°C.540... 

The spectroscopic data for 9 support the proposed structure. In particular, the wSi NMR chemical shift of 5 43.19 as a triplet (Jsi-p(C/ ) = 40.11 Hz) resembles the literature value reported for the m-NiSijPC complex. 3 Compound 9 was found to be a good reactive intermediate for the double silylation reaction. The reaction of 1 with 1-phenylprop-l-yne (1 equiv) in the presence of a catalytic amount of 9 (0.03 equiv) for 6 h afforded the double-silylated producted 10 in 94 % (GC) yield. The reaction was quite sensitive to the reaction conditions. When the same reaction was carried out at higher temperature (70-75 °C), the major component was identified as the acetylene cyclotrimerization product 11 (Scheme 2), which has been characterized by spectroscopic techniques. When hex-l-yne is employed as a terminal alkyne in the reaction with 1 under the same condition, the five-membered disilyl ring compound 12 is isolated as a colorless liquid in 71 % yield. 

In addition to isolation and characterization of the ruthenacycle complexes 18 or 32, the detailed reaction mechanism of the [2 + 2 + 2] cyclotrimerization of acetylene was analyzed by means of density functional calculations with the Becke s three-parameter hybrid density functional method (B3LYP) [25, 33]. As shown in Scheme 4.12, the acetylene cyclotrimerization is expected to proceed with formal insertion/reductive elimination mechanism. The acetylene insertion starts with the formal [2 + 2] cycloaddition of the ruthenacycle 35 and acetylene via 36 with almost no activation barrier, leading to the bicydic intermediate 37. The subsequent ring-... 

The following describes results of three, relatively simple chemical reactions involving hydrocarbons on model single crystal metal catalysts that illustrate this general approach, namely, acetylene cyclotrimerization and the hydrogenation of acetylene and ethylene, all catalyzed by palladium. The selected reactions fulfdl the above conditions since they occur in ultrahigh vacuum, while the measured catalytic reaction kinetics on single crystal surfaces mimic those on reahstic supported catalysts. While these are all chemically relatively simple reactions, their apparent simplicity belies rather complex surface chemistry. 

It bonds with the C=C bond tilted somewhat with respect to the surface normal and the measured structure is in accordance with that calculated using density functional theory [55, 56]. The saturation coverage of vinylidene at 300 K is 1 ML. Furthermore, a vinylidene overlayer is found on the surface during palladium-catalyzed reactions of acetylene [13], which suggests that acetylene cyclotrimerization proceeds in the presence of a vinylidene-covered surface, rather... 

These results indicate that the origin of the acceleration in the rate of acetylene cyclotrimerization due to the addition of hydrogen measured above (Fig. 1.4) arises from a combination of the formation of a more open ethylidyne-covered surface, and possibly also the removal of the ethylidyne once it has been formed to produce regions of relatively clean palladium. 

It is evident that acetylene cyclotrimerizes rapidly on clean Pd(l 11) as described above. This, therefore, raises the question whether acetylene cyclotrimerization reactions can also occur on the ethylidyne-covered portion also. This is addressed... 

The proportion of the surface covered by ethylidyne or vinylidene species, depends on the hydrogen pressure due to the reactions depicted in Scheme 1.2. Both the coverage of carbon monoxide (Fig. 1.5b) and the rate of acetylene cyclotrimerization (Fig. 1.4) vary linearly with hydrogen pressure, as a result of the first-order hydrogen pressure dependence of vinylidene-to-ethylidyne conversion (Fig. 1.8) and ethylidyne titration from the surface (Fig. 1.7). This suggests that the number of surface sites available for reaction varies with hydrogen pressure, / (H ) and can be expressed as ... 

The step edges of these terraces are Pt-terminated, while Sn is alloyed in the center of these terraces. A local p(2x2) ordering of alloyed Sn is often observed on wider (5-7 atomic rows) terraces. This new information was important for reinterpreting some of our early chemisorption studies, especially concerning the active sites for acetylene cyclotrimerization (see discussion below). 

Abbet S, Sanchez A, Heiz U, Schneider WD, Ferrari AM, Pacchioni G, Rosch N (2000) Size-effects in the acetylene cyclotrimerization on supported size-selected Pdn clusters (l< = n< = 30). Surf Sci 454 984... 

Ferrari AM, Giordano L, Rosch N, Heiz U, Abbet S, Sanchez A, Pacchioni G (2000) Role of surface defects in the activation of supported metals A quantum-chemical study of acetylene cyclotrimerization on Pdj/MgO. J Phys Chem B 104 10612... 

The electronic structure of palladium atoms, 4d 5s , is unique and may be responsible for this specific catalytic property for the acetylene cyclotrimerization. 

The electronic structure of palladium atoms, 4d °5s°, is unique, and may be responsible for this specific catalytic property for acetylene cyclotrimerization. This raises the question of whether other transition metal atoms are also reactive for this reaction. Results are shown for deposited Rh (4c/ 5i ) and Ag (4c/ atoms. Ag atoms are almost unreactive (Fig. 3a) on supported Rh atoms, however, benzene is formed, and desorbs at around 430 K (Fig. 3a). 

Acetylene cyclotrimerization Formation of benzene from three acetylene molecules. 

Investigating the reactivity of supported size-selected metal clusters in order to contribute to the understanding of heterogeneous catalysis on a molecular level is the focus of interest in the nanocat lab. To work out the origin of size effects [23] and structure insensitivity [24] of heterogeneous catalysts, different systems and techniques have been successfully applied [20, 25-29]. Major milestones were the investigation of Pt, Au, Pd clusters towards their reactivity in the CO oxidation reaction [30-35] and the acetylene cyclotrimerization [24, 31, 36, 37] on Pd clusters. Further, in the last years successful experiments with clusters under ambient conditions have been performed [20, 38-40] and showed the possibility to use clusters for application to more realistic problems in heterogeneous catalysis. 

The cyclic process can be illustrated by acetylene cyclotrimerization to benzene by a transition metal in a low oxidation state. At the first stage, two acetylene molecules give the five-membered cycle involving the central M atom, and the insertion of the third acetylene molecule increases the number of atoms in the cycle to seven followed by the rearrangement to benzene... 

The ground spin state of chromium (acetylene) adducts is known to be of quintet, and the most plausible reaction pathway on quintet surface needs to overcome two activation barriers to finish a single catalytic cycle, as depicted in Scheme 3.4. However, the reaction on quintet surface is prohibited by presenting a ffee-energy barrier of 31.1 kcal/mol that transforms two coordinated acetylene into the key intermediate 4D. Thus, the turnover of frequency (TOP) for the catalytic cycle on the quintet surface is 1.36 X 10 h, which rules out the quintet reaction mechanism for acetylene cyclotrimerization by Cr(II)/Si02 model catalyst. 

Scheme 3.4 Catalytic cycle for acetylene cyclotrimerization by CrfliySiOj on the quintet surface.

Figure 3.12 Gibbs free energy profiles for acetylene cyclotrimerization by Crfllj/SiOj cluster model. The Gibbs free energies are calculated at 298.15 K, 1 atm as default in Gaussian09. Also shown are the total energies in parentheses. The triplet reaction pathway is depicted in gray, while the quintet parts are in black. Energies are in kcal/mol and relative to 1C plus the corresponding number of acetylenes.

Scheme 3.5 Two proposed mechanisms for acetylene cyclotrimerization by Cr(ll)/Si02 cluster model two-state reactivity versus a triplet catalytic cycle.

Liu Z, Cheng R, He X, Wu X, Liu B DFT functional benchmarking on the energy splitting of chromium spin states and mechanistic study of acetylene cyclotrimerization over the PhiUips Cr(II)/sUica catalyst, J Phys Chem A 116(28) 7538-7549, 2012. 

Martinez M, Michelini MDC, Rivalta I, Russo N, Sicilia E Acetylene cyclotrimerization by early second-row transition metals in the gas phase. A theoretical study, Inorg Chem 44(26) 9807-9816, 2005. 

Acetylene cyclotrimerization is a well-established method for the effective transformation of triple bonds to benzene rings. Acetylenic polymerization has emerged... 

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