Applications of α-Cationic Phosphines as Ancillary Ligands in Homogeneous Catalysis

The unique architecture of alpha-cationic phosphanes, in which one or more positively charged groups are directly attached to the phosphorus center dramatically affects both the physicochemical properties of the corresponding ligands and their donor/ acceptor properties. Specifically, the introduction of charges increase the solubility of the resulting ligand in polar media allowing the derived metal complexes to be utilized in highly polar solvents such as ionic liquids, or to be easily separated from typical organic media. On the other hand, the presence of a cationic group in close proximity to the donor position weakens the sigma-donor ability of the P-atom when compared with neutral analogues, and concomitantly enhances the pi-acceptor character of the resulting ligand. In this Highlight Review these aspects are overviewed, putting special emphasis on the implications for transition metal catalyzed processes. Recent developments are critically evaluated and future perspectives for this exciting area of ligand design are discussed.