Synthesis and characterization of some cationic η3-propargylpalladium complexes

Some cationic eta(3)-propargylpalladium complexes were prepared upon treatment of the corresponding eta(1)-propargyl- or eta(1)-allenylbis(triphenylphosphine)palladium(II) chloride with Ag[BF4] or Na[BPh4]. The effectiveness of the latter reagent suggests that a eta(1)-propargyl- or eta(1)-allenyl(chloro)palladium complex equilibrates with a cationic eta(3)propargylpalladium complex with the liberation of a C1(-) ligand. A qualitative comparison of trends in a series of analogous equilibrium systems suggests that the eta(3)-coordination models favored to a greater extent when (i) propargyl ligands have an alkyl substituent at the propargylic position, (ii) phosphine ligands are bidentate, such as dppe, (iii) polar solvents are used, and (iv) the liberating ligand is a Cl- one. A possible implication of eta(3)-coordination of propargyl ligands in a catalytic cycle of Pd-catalyzed transformations of propargylic or allenylic substrates is presented.