Structure-Activity Relationships of Oligocationic, Ammonium-Functionalized Triarylphosphines as Ligands in the Pd-Catalyzed Suzuki-Miyaura Reaction

A series of oligocationic triarylphosphine ligands, containing a varying number (from 2 to 6) of meta-ammoniomethyl substituents, have been synthesized, characterized, and tested as ligands in the Pd-catalyzed Suzuki-Miyaura cross-coupling reaction. By systematic catalytic investigations and spectroscopic (NMR and UV/Vis) studies of these ligands in the palladium(0) complexes as well as of their neutral amine analogues and of triphenylphosphine, structure-activity relations have been established. The substitution of a triarylphosphine core with cationic meta-ammoniomethyl substituents increases the steric demand (cone angle) of the phosphine ligand and decreases its sigma-donation strength. Furthermore, Coulombic inter-ligand repulsion forces are introduced. The catalytic activity in the Suzuki-Miyaura reaction within this class of ligands is governed by the number of cationic charges in the ligand structure, rather than by their steric properties or their sigma-donating strength. With an increasing number of charges, the preference for the formation of coordinatively unsaturated phosphine-palladium species increases, which leads to a higher catalytic activity through faster catalyst activation.