Mechanism of the ruthenium-catalyzed formation of pyrane-2-one and their sulfur and selen analogs from acetylene and CX2 (X = O, S, Se): a theoretical study

Complete catalytic cycles for the (actual or potential) cycloaddition of acetylene with CX2 (X=O, S, Se), mediated by CpRu(COD)Cl as the precatalyst, are proposed on the basis of DFT/B3LYP calculations. The common initial step is the replacement of labile COD by two molecules of acetylene to afford a bisacetylene complex. Oxidative coupling of the acetylene ligands results in the formation a coordinatively saturated metallacyclopentatriene complex. This metallacycle adds the C=X bond of CX2 directly to the Ru=C bond in a concerted fashion, forming an unusual bicyclic carbene intermediate. The activation energies for this addition are 14.2, 15.3 and 30.0 kcal mol(-1) for X=O, S and Se, respectively, thus rendering the CSe2 addition the most difficult. In a subsequent reductive elimination a coordinatively unsaturated metallaheteronorbornene intermediate is formed, which eventually rearranges into RuCpCl complexes bearing pyrane-2-one, thiopyrane-2-thione and selenopyrane-2-selenone ligands coordinated in an eta(4) fashion. The energy barrier for the reductive elimination step decreases in the order O>S>Se, being 28.1, 17.7 and 14.6 kcal mol(-1), respectively. Overall, the reaction with CS2 is thus the most favorable. Completion of the cycles is achieved by an exothermic displacement of the respective heterocyclic product by two acetylene molecules, regenerating the bisacetylene complex.