The Trimerization of Acetylene

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.  

Our computations indicate that there is a large activation barrier for this reaction27. Both MINDO/3 and ab initio STO-3G calculations were used in preliminary investigations, and the results of a surface scan by the former in which D3h sym- 

Although these surface scans are approximate, and probably suffer somewhat from the requirement of D3h symmetry, they do reveal that this remarkably exothermic, and thermally allowed, reaction has an unusually high activation barrier. Acetylene is neither a good donor nor a good acceptor, and the approach of three acetylenes, even in a geometry which produces both in-plane and out-of-plane aromatic sextets, results in no strong HOMO-LUMO interactions. Repulsive interactions due to the overlap of filled orbitals of the three molecules occur, but the filled and vacant orbitals of the acetylenes are too far apart in energy for any appreciable stabi- 

This example clearly shows the importance of the energy of geometrical distortions upon activation energies, and makes the origin of the high reactivities of strained molecules somewhat more obvious. In two other studies of the influence of geometrical distortions upon reaction rates, we have found that the electronic characteristics of molecules can be profoundly altered by relatively minor geometrical alterations  

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

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... 

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 ... 

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

Bimetallic catalysts (lanthanide halides or 3 diketonates and alkyl-aluminium or lithium derivatives) have been reported. SmCp is also a catalyst for the trimerization of acetylenes to substituted benzenes and... 

The trimerization of acetylene is a prototypical multimolecular process where orbital symmetry allowedness is insufficient to render the reaction kinetically facile. In general, because the gap is a simple sum of the individual excitations for each bond that is broken during the reaction (STA TEMENTS1 and 2), multimolecular processes are expected to possess large excitation gaps. This means that such processes can become facile only if the reactants are linked in proximity to each other, at a distance... 

Santos JC, Polo V, Andres J (2005) An electron localization function study of the trimerization of acetylene reaction mechanism and development of aromaticity. Chem Phys Lett 406 393-397... 

Promotion energy gap patterns were observed also for allowed cycloadditions and their bond exchange analogs. An interesting example is the trimerization of acetylene to benzene, equation (13), which possesses a very large barrier (rj62 kcal mol at the MP3/6-31G //HF/6-31G level ), despite the fact that the reaction is symmetry allowed, and can potentially proceed with an enormous thermodynamic advantage of A7f Rs —140 kcal mol . 

Acetylenes may frequently be trimerized by treatment with triphenyl- or trialkyl-chromium compounds to give substituted benzenes and bis-w-arene chromium complexes. The course of the reaction depends on the stoicheiometry of the reaction mixture. Figure 50 illustrates the possible mode of formation of the benzene derivatives. The complex Me2Co(PPh3)7r-CjH5 can catalyze the trimerization of acetylene itself to give benzene. 

The trimerization of acetylenes by transition metal complexes may give n -arene complexes and these reactions are discussed in Chapter S. 

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