| Abstract
| - A complete catalytic cycle for the cyclotrimerization of acetylene with the CpRuCl fragment hasbeen proposed and discussed based on DFT/B3LYP calculations, which revealed a couple of uncommonintermediates. The first is a metallacyclopentatriene complex RuCp(Cl)(C4H4) (B), generated throughoxidative coupling of two alkyne ligands. It adds another alkyne in η2 fashion to give an alkyne complex(C). No less than three successive intermediates could be located for the subsequent arene formation.The first, an unusual five- and four-membered bicyclic ring system (D), rearranges to a very unsymmetricalmetallaheptatetraene complex (E), which in turn provides CpRuCl(η2-C6H6) (F) via a reductive eliminationstep. The asymmetry of E, including Cp ring slippage, removes the symmetry-forbidden character fromthis final step. Completion of the cycle is achieved by an exothermic displacement (21.4 kcal mol-1) of thearene by two acetylene molecules regenerating A. In addition to acetylene, the reaction of B with ethyleneand carbon disulfide, the latter taken as a model for a molecule lacking hydrogen atoms, has also beeninvestigated, and several parallels noted. In the case of the coordinated alkene, facile C−C coupling to theα carbon of the metallacycle is feasible due to an agostic assistance, which tends to counterbalance thereduced degree of unsaturation. Carbon disulfide, on the other hand, does not coordinate to ruthenium,but a CS bond adds instead directly to the RuC bond. The final products of the reactions of B withacetylene, ethylene, and carbon disulfide are, respectively, benzene, cyclohexadiene, and thiopyrane-2-thione, the activation energies being lower for acetylene.
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