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

Abstract

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 CX bond of CX₂ directly to the RuC 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 η⁴ 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.