Benzene and heterocyclic rings formation in cycloaddition reactions catalyzed by RuCp derivatives: DFT studies

The mechanism of the RuCp(COD) Cl catalyzed cyclotrimerization of acetylene, as well as the cyclocotrimerization of two alkynes with one molecule of ethylene, R-C=N (R = H, Me, Cl, COOMe), CX2 (X = O, S, Se), and HN=C=X (X = O, S) investigated by means of high level DFT/B3LYP calculations, has been reviewed. The key reaction step is in all cases the oxidative coupling of two alkyne ligands to give a metallacyclopentatriene intermediate (or metallacyclopentadiene in other systems). This metallacycle adds unsaturated molecules, containing C equivalent to C, C=C and C=X bonds, or RC equivalent to N, CX2, and HN=C equivalent to X, in a concerted fashion, directly to the Ru=C bond, forming bicyclic carbenes. The cycle is completed by a rearrangement followed by an exothermic displacement of the arene or pyridine, by two acetylene molecules regenerating the catalytically active species. Small differences are found depending on the molecules and bonds involved. These reactions are reviewed and the proposed mechanisms compared with other available studies. (C) 2011 Elsevier B.V. All rights reserved.