Cyclotrimerization of acetylene

Mesitylene can be synthesized from acetone by catalytic dehydrocyclization (17). Similarly, cyclotrimerization of acetylenes has produced PMBs such as hexamethylbenzene (18). Durene has been recovered from Methanex s methanol-to-gasoline (MTG) plant in New Zealand (19). 

By analogy with the cyclotrimerization of acetylenes into arenes, and with the cocyclooligomerization of nitriles and acetylenes into pyridines (see Scheme 131 in Section V,A,1), the cyclization of benzonitrile into 2,4,6-triphenyl-s-triazine can be achieved by means of Fe(CO)5 or Fe2(CO)9.241... 

Supercritical water was recently used as solvent of cyclotrimerization of acetylenes catalyzed by [CpCo(CO)2] [59] the reaction has some early precedents [55-57]. 

It appears difficult to propose a unified mechanism to explain all experimental observations of the cyclotrimerization of acetylene. The most common pathway, studied mainly with cobalt complexes,72 73 involves a metallacyclopentadiene intermediate ... 

Cyclotrimerization of acetylenes to form benzene derivatives is studied intensively mainly because of the challenge of chemo- and regioselectivities. A regiose-lective and highly chemoselective method for preparing substituted benzenes was described via PdCl2-catalyzed cyclotrimerization in the presence of CuCl2 537... 

Reinhard, S., Soba, P., Rominger, F., and Bluemel, J. (2003) New silica-immobilized nickel catalysts for cyclotrimerizations of acetylenes. Advanced Synthesis and Catalysis, 345, 589-602. 

Complex 224 is also obtained by cyclotrimerization of acetylene promoted by complex 223, with displacement of the bicyclotriene ligand. It is noteworthy that complex 223 is obtained from 200 by simple cycloaddition of alkyne to the cyclooctatriene ligand (139) [Eq. (24)]. 

Fig. 4. Multi-collision CID results for (C2H2)3Fe+ and (C6H6)Fe+ in the laboratory energy scale with He as the collision gas. (C2H2)3Fe+ is produced by sequential addition of C2H2 to Fe+ and (C6H6)Fe+ is produced by direct addition of C6H6 to Fe+. The similarity in the threshold for dissociation of (C2H2)3Fe+ and (C6H6)Fe+ suggests that Fe+ has mediated the cyclotrimerization of acetylene to benzene [35]...

Figure 1 shows the Temperature Programmed Reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 < n 5 30) clusters. Pdi, Pd2, and Pds, form exclusively benzene at temperatures around 300 K, while for cluster sizes up to Pdg a broad feature between 400 K and 700 K is observed in the TPR, Figure 1 [17]. For Pd7, an...

Figure 4 illustrates the probable mechanism of polymerization and cyclotrimerization of acetylenes. These reactions compete with each other, and the suppression of cyclotrimerization is necessary to achieve selective polymerization. An important... 

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-... 

An examination of the photodecoloration of bis-[4-(dimethylamino)dithio-benzyl]nickel has shown it to proceed in two stages, the first of which increases with increasing concentration of dissolved oxygen.Rate enhancements of thirty- to forty-fold are reported for the cyclotrimerization of acetylene under u.v. irradiation in the presence of a Ni"-Si02 catalyst pretreated with... 

A new experimental setup has recently been designed to study the chemical properties of size-selected metal clusters deposited on oxide substrates [210,211], Pd clusters have been produced by a laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [210-212], The monodispersed clusters have been deposited with low kinetic energy (0,l-2eV) onto an MgO thin-film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [224], Figure 6 shows the temperature-programmed reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 30)... 

Effects of donor molecules on the palladium-catalyzed cyclotrimerization of acetylenes with olefins were investigated by Ibers et a/. 

The transition metal-catalyzed cyclotrimerization of acetylene (eq. (1)) was discovered by Berthelot [1] back in the mid-19th century using heterogeneous systems. 

Yamazaki s complex (Structure 5) contains two alkyne molecules linked together to form a five-membered metallacycle. Arene-solvated cobalt atoms, obtained by reacting cobalt vapor and arenes, have been used by Italian workers to promote the conversion of a,w-dialkynes and nitriles giving alkynyl-substituted pyridines [20]. -Tolueneiron(0) complexes have also been utilized for the co-cyclotrimerization of acetylene and alkyl cyanides or benzonitrile giving a-substituted pyridine derivatives. However, the catalytic transformation to the industrially important 2-vinylpyridine fails in this case acrylonitrile cannot be co-cyclotrimerized with acetylene at the iron catalyst [17]. 

Jonas and Tadic [71] have investigated the homogeneous cobalt-catalyzed co-cyclotrimerization of acetylene and olefins. The reaction with -Ind-cobalt bis(ethylene) as the catalyst was carried out with ethylene, a-olefins and 2-butene as well as cyclohexene and cyclooctene (eq. (25)). 

Supercritical water exhibits better solvent properties for apolar organic compounds than water itself and was applied by Jerome and Parsons [79] as well as Dinjus and co-workers [80] as the solvent for the Co-mediated cyclotrimeriza-tion of monosubstituted acetylenes to benzene derivatives. Eaton et al. published the cyclotrimerization of acetylenes bearing functional groups in a water/methanol (80 20) mixture using an R-Cp cobalt cod complex as the catalyst. The water solubility of the Co complex was achieved by the special substituent R=C0(CH2)2CH20H on the Cp ligand [81]. 

From a mechanistic viewpoint, the formation of the cyclo-P6 ligand from P4 has not yet been completely clarified. Nevertheless, suggestive hypotheses based on the dimerization of P3 units or the trimerization of P2 units have been proposed [70,102,106]. The latter mechanism is particularly intriguing as it parallels the well-known metal-mediated cyclotrimerization of acetylenes in the coordination sphere of cobaltocenes [107]. 

The cyclotrimerization of acetylene to benzene has been studied by Rucker et al. (8J) over Pd(lll), (100), and (110) at pressure near 1 atm. The (110) surface was four-fold less active than Pd(lll) or (100), which contrasts with their relative selectivities during TDS under UHV conditions. The activity at high pressures was correlated with the fraction of the various surfaces that exposed clean Pd atoms, as probed by postreaction CO adsorption-desorption. In all cases, most of the surface was covered with a carbonaceous residue. The authors stated that the reaction rate is first-order in acetylene pressure for all three surfaces. The extensive data on the Pd(lll) surface clearly indicate first-order kinetics in that case. However, the limited data presented for Pd(110) seem (to the present author) to be better fitted by an order of —2.5, which is closer to the value of three suggested by the overall stoichiometry. 

The influence of adsorbed Si, P, S, and Cl on the medium-pressure cyclotrimerization of acetylene to benzene over Pd(l 11), (100), and (110) has been studied by Logan et al. (113). Whereas both sulfur and chlorine decrease the activity, silicon increases the activity. The effect of phosphorous depends on the crystal face. According to their work function measurements, sulfur withdraws electron density from the Pd surface (as is also expected for chlorine), whereas Si donates electron density, and P has the least effect on the work function. Thus, the qualitative influences on catalytic activities correlate with the influences of the additives on the electronic character of the surface. In addition, Si decreases the carbon coverage seen in postreaction AES from —82 to —70% of a monolayer, whereas sulfur and chlorine increase the amount of carbonaceous residue. The authors interpreted these results by suggesting that the electron-donating ligands keep the Pd surface cleaner for the desirable reaction by... 

Cyclotrimerization of acetylene to form benzene is a thermally allowed and highly exothermic pericyclic reaction, but no such reaction occurs, because of its high reaction barrier. In 1979, Houk et al. performed a detailed theoretical analysis and offered an exlanation for the apparent high reaction barrier. The high activation energy was attributed to the unfavorable repulsions involving closed-shell electrons upon the approach of acetylene [89]. This study was reaffirmed years later by the calculations of Bach et al. (Scheme 1-3) [90]. 

The Co(I)-catalysed co-oligomerization of nitriles with alkynes (Bonnemann 1978 [67]) is important in preparative chemistry. Reaction of 2 mol acetylene with nitriles results in a virtually quantitative yield of 2-substituted pyridines. The competing cyclotrimerization of acetylene to benzene can be suppressed by using an excess of nitrile ... 

---