Trimerization of acetylene

We are omitting reactions such as the trimerization of acetylene on transition metal catalysts, even though they could be considered cycloaromatization reactions... 

It is noteworthy that the starting tf -benzene complex was prepared by cyclo-trimerization of acetylene by IrCl(C2H4)(triphos) [16]. All of the attempts to react the fragment [Ir(triphos)]+ with benzene were unsuccessful, which reflects the difficulty met by a transition-metal fragment to overcome the energy barrier to /4-benzene coordination. 

The free y hydroxy-acetylenic acids in the thiophene series are very unstable and trimerize very readily. The ease of dimerization and trimerization of acetylenic compounds in the presence of a hydroxyl group, conjugated double bond or a second triple bond is very characteristic and was observed with certain acetylenic alcohols and diacetylenic glycols in this series. 

A second point that needs to be clarified is why allowed reactions actually have a barrier at all. In fact, some so-called allowed reactions possess particularly high barriers so that they hardly proceed. One such case, discussed by Houk et al. (1979), concerns the allowed trimerization of acetylene to yield benzene. Despite being extremely exothermic (AH° = — 143 kcal mol ) this reaction appears to have an activation barrier in excess of 36 kcal mol"1. Since the orbitals of the reactants correlate smoothly with those of the products, in accord with the Woodward-Hoffmann rules, the origin of barrier formation in allowed reactions generally, needs to be clarified. 

Benzene Petroleum, coal tar, trimerization of acetylene, natural products, etc. Colourless liquid, b.p.80°C m.p.5.5°C. stable. Has relatively pleasant odour, (carcinogenic). Industrially a very important compound). 

An example which we have studied only cursorily, but which nevertheless reveals much about the importance of molecular distortions in chemical reactions, is the trimerization of acetylene to form benzene (Fig. 9). This reaction was brought to our attention by the work of Vollhardt, who found that a cyclic triacetylene, 1,5,9-cyclo-dodecatriyne, undergoes reaction only at elevated temperature to give products suggestive of the intermediacy of tris-cyclobutenanenzene25. ... 

Because the trimerization of acetylene is so enormously exothermic (—143.6 kcal/mol) and is allowed by the Woodward-Hoffmann rules, one might expect a very low activation energy for this transformation. The decrease in entropy might be costly, but should not be prohibitive in intramolecular cases. Experimentally, acetylene undergoes reaction at 400 °C in the gas phase to give a wide variety of products, of which benzene constitutes only a small fraction26. ... 

The general mechanistic picture presented above applies to a number of synthetically useful systems including the first to be discovered, in which trimerization of acetylene to benzene, and propargyl alcohol to a 1 1 mixture of 1,2,4- and 1,3,5-trisubstituted benzene derivatives was catalyzed by (Ph3P)2Ni(CO)2. Phosphine nickel carbonyls are usually much more highly regioselective for 1,2,4-... 

Another example concerns the trimerization of acetylene over [(CoH5)3P]4Ni. This could be represented by the following scheme ... 

The reaction mechanism and the development of the aromaticity along the trimerization of acetylene to yield benzene (Scheme 8, Figure 12) have been analysed by the ELF in the same framework of structural stability domains described before.88... 

Figure 14 Evolution of the ELF,r, ELF and average ELF -ELF along the IRC path of trimerization of acetylene...

Bistriphenylphosphinedicarbonylnickel, [(C6H5>3Pl2Ni(CO)2. This complex catalyzes trimerization of acetylenes to aromatic compounds, and in the presence of acetylene promotes dimerization of butadiene to cyclooctadiene-1,5. ... 

Figure 2. The equilibrium structure of (HCCH)i3 calculated [167] with a polarizable model potential. The central molecule is seen essentially end on. The 12 surrounding molecules are in three layers. The upper and lower layers have three molecules and resemble the structure of the cyclic trimer of acetylene. The middle layer of six acetylene molecules has a pinwheel like arrangement. Puckering of the rings in the layers yields T shaped orientations between acetylenes in different layers, along with the essentially T shaped arrangements for adjacent molecules within each layer. The number of favorable (T shaped) quadrupole quadrupole interactions among acetylenes is thereby enhanced.

Ni or Ti catalyzed tetramerization or trimerization of acetylene and reactions with alcohols, amines, carboxylic acids, thiols (see 1st edition). 

Woodward-Hoffmann allowed reactions (4 + 2 or 2 + 2 + 2), where G is given by Here, there is a high barrier of 62kcal/mol for the trimerization of acetylene, where 2 A sx(Tnr ) is very large (ca. 297 kcal/mol). The barrier gets lowered to 22 kcal/mol for the Diels-Alder reaction where 2 A sx(tttt ) is comparatively much smaller (ca. 173 kcaFmol). Thus, with differences of 120kcal/ mol in G, an/factor of 0.3 (as quantified recently for radical reaction [22-24]) lowers the barrier by 40 kcaVmol. 

Cycloaddition reactions of alkynes aided by transition metals were reviewed Various trimerization processes of acetylenic compounds have been reported. Titanium chloride catalyses the trimerization of acetylenic compounds, by way of intermediate complexes that can be isolated and characterized. This is shown in Table 2 for TiCU and 2-butyne. Acetylenes activated by ether groups in the propargyl position undergo trimerization catalysed by NiBri/Mg. Acetylenes without activation also undergo the same reaction, but with lower yields. Iron 7i-complexes can catalyse stepwise polymerization of alkynes ... 

S. Ikeda and his coworkers studied a mechanism of acetylene polymerization in connection with olefin polymerization by various Ziegler-Natta catalysts. They found that polymerization yields not only highly polymerized polyacetylene but also benzene, which is a cyclic trimer of acetylene, and that the ratio of these two products depends... 

Trimerization of acetylene into benzene is known to proceed on a single crystal of palladium and on fine particles of palladium dispersed on a substrate. Among them, Pd (111) surface is the most active for the trimerization because the surface has a site with three fold symmetry at which three acetylene molecules are adequately adsorbed for the trimerization into benzene geometry-controlling reaction. In the trimerization involving a palladium cluster, it is expected that the catalytic activity of the trimerization begins to appear at a critical size as the cluster size increases because a small cluster does not have such an active site with three-fold symmetry but a larger cluster should have. 

Fig. 23. Catalytic formation of benzene molecules on different palladiiun cluster sizes by the temperature-programmed-reaction (TPR) experiment. The benzene was produced through the cyclo trimerization of acetylene catalyzed by size-selected palladium clusters, Pdni supported on a MgO film. (Adapted from Ref. 35.)...

Oligomerization. Maitlis has reviewed the trimerization of acetylenes in protic solvents to benzenoids catalyzed by this complex. 

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