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 s-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 s-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.