Theoretical design of new ligands to boost reaction rate and selectivity in palladium-catalyzed aromatic fluorination.

The development of palladium-catalyzed fluorination with biaryl monophosphine ligands has faced two important problems that limit its application for bromoarenes: the formation of regioisomers and insufficient catalysis for heteroaryl substrates as bromothiophene derivatives. Overcoming these problems requires more ligand design. In this work, reliable theoretical calculations were used to elucidate important ligand features necessary for achieving more rate acceleration and selectivity. These features include increasing the ligand-substrate repulsion and creating a negative charge in the space around the fluoride ion bonded to the palladium. The investigated L5 ligand presents these features, and the calculations predict that this ligand completely suppresses the regioisomer formation in the difficult case of 4-bromoanisole. In addition, the free energy barriers are decreased by 2-3 kcal mol^-1 in comparison with the catalysis involving the AlPhos ligand. Thus, the present study points out a direction for new developments in palladium-catalyzed fluorination.