Oxidative coupling of activated alkynes with palladium(0) olefin complexes: Side production of the highly symmetric hexamethyl mellitate species under mild conditions at low alkyne/complex molar ratios

The reactions of oxidative coupling of dimethylacetylene dicarboxylate (DMA) with palladium (0) olefin complexes bearing a pyridylthioether as ancillary ligand give rise to the corresponding palladacyclopentadienyl complexes as the main product and in some cases to the highly symmetric hexamethyl mellitate molecule, especially when a low alkyne(DMA)/complex ratio is employed. A plausible mechanism based on several pieces of experimental evidence is proposed, and the most effective complexes promoting the hexamethyl mellitate synthesis were identified.