Palladium–catalysed alkyne alkoxycarbonylation with P,N chelating ligands revisited: a density functional theory study

A revised in situ base mechanism of alkyne alkoxycarbonylation via a Pd catalyst with hemilabile P,N-ligands (PyPPh₂, Py = 2-pyridyl) has been fully characterised at the B3PW91-D3/PCM level of density functional theory. Key intermediates on this route are acryloyl (η³-propen-1-oyl) complexes that readily undergo methanolysis. With two hemilabile P,N-ligands and one of them protonated, the overall computed barrier is 24.5 kcal mol^-1, which decreases to 20.3 kcal mol^-1 upon protonation of the second P,N-ligand. This new mechanism is consistent with all of the experimental data relating to substituent effects on relative reaction rates and branched/linear selectivities, including new results on the methoxycarbonylation of phenylacetylene using (4-NMe₂Py)PPh₂ and (6-Cl-Py)PPh₂ ligand. This ligand is found to decrease catalytic activity over PyPPh₂, thus invalidating a formerly characterised in situ base mechanism.