Elucidating the Origin of Regioselectivity in Palladium-Catalyzed Aromatic Fluorination: Mechanistic Investigation and Microkinetic Analysis

Nucleophilic fluorination of aromatic rings can be done via a cross-coupling reaction catalyzed by palladium and biaryl monophosphine ligands such as AlPhos. Nevertheless, aromatic fluorination via palladium catalysis can lead to regioisomers, a serious problem for this kind of reaction. An ortho-deprotonation mechanism was proposed for this reaction in previous studies. However, this mechanism does not explain why the ortho product is not formed from para and meta substrates, and the details of the mechanism are not fully understood yet. This theoretical work discloses the mechanism behind regioisomers formation, which takes place via unimolecular ortho-elimination of HF. The computed free energy profile of the reaction was followed by a detailed microkinetic analysis, which explain the experiments quantitatively. The present study shows that the rotation of the aryne coordinated to the palladium to form a complex leading to the ortho product is critical for regioselectivity. This rotation has a high free energy barrier due to steric repulsion with the adamantane group of the ligand. Such a feature impedes the formation of ortho product from para and meta substrates and allows the formation of the ortho product selectively from the ortho substrate.