Methoxycarbonylation of olefins catalyzed by palladium complexes bearing P,N-donor ligands

Abstract

The methoxycarbonylation of alkenes catalyzed by palladium(II) complexes with P,N-donor ligands, 2-(diphenylphosphinoamino)pyridine (Ph₂PNHpy), 2-[(diphenylphosphino)methyl]pyridine (Ph₂PCH₂py), and 2-(diphenylphosphino)quinoline (Ph₂Pqn) has been investigated. The results show that the complex [PdCl(PPh₃)(Ph₂PNHpy)]Cl or an equimolar mixture of [PdCl₂(Ph₂PNHpy)] and PPh₃, in the presence of p-toluensulfonic acid (TsOH), is an efficient catalyst for this reaction. This catalytic system promotes the conversion of styrene into methyl 2-phenylpropanoate and methyl 3-phenylpropanoate with nearly complete chemoselectivity, 98% regioselectivity in the branched isomer, and high turnover frequency, even at alkene/Pd molar ratios of 1000. Best results were obtained in toluene–MeOH (3 : 1) solvent. The Pd/Ph₂PNHpy catalyst is also efficient in the methoxycarbonylation of cyclohexene and 1-hexene, although with lower rates than with styrene. Related palladium complexes [PdCl(PPh₃)L]Cl (L = Ph₂PCH₂py and Ph₂Pqn) show lower activity in the methoxycarbonylation of styrene than that of the 2-(diphenylphosphinoamino)pyridine ligand. Replacement of the last ligand by (diphenylphosphino)phenylamine (Ph₂PNHPh) or 2-(diphenylphosphinoaminomethyl)pyridine (Ph₂PNMepy) also reduces significantly the activity of the catalyst, indicating that both the presence of the pyridine fragment as well as the NH group, are required to achieve a high performing catalyst. Isotopic labeling experiments using MeOD are consistent with a hydride mechanism for the [PdCl(PPh₃)(Ph₂PNHpy)]Cl catalyst.