DFT perspective on the selectivity and mechanism of ligand-free Heck reaction involving allylic esters and arenediazonium salts

Abstract A density functional theory (DFT) study has been carried out with respect to the cationic and neutral mechanism of ligand-free Heck-Matsuda cross-coupling reaction involving allylic esters and arenediazonium salts. Full Heck-Matsuda (HM) catalytic cycles were explored in the presence of different possible combinations of ligands based on previous mass-spectrometry studies. DFT calculations were performed to examine the olefinic substrate influences on the regioselectivity of this ligand-free Heck reaction. A multi-step mechanism was revealed for the oxidative addition of palladium (0) center to carbon-nitrogen bond of arenediazonium cation. The cationic migratory insertion pathway is favored by a chelated coordination involving the palladium center and the allylic ester. In contrast, for neutral complexes, a bidentate coordination of acetate ligand is the favored pathway. Regarding the hydride elimination step, the resulting free energy profile is quite flat, revealing that the Heck product and the hydride complex are easily formed for both cationic and neutral cycles. The reductive elimination step assisted by acetate is a very exothermic process, going through small relative reaction barriers. These results constitute new insights to a better understanding of the selectivity and the mechanism of ligand-free Heck-Matsuda reactions.