Synthesis and Characterization of Some Cationic η3-Propargylpalladium Complexes

Some cationic η3-propargylpalladium complexes were prepared upon treatment of the corresponding η1-propargyl- or η1-allenylbis(triphenylphosphine)palladium(II) chloride with Ag[BF4] or Na[BPh4]. The effectiveness of the latter reagent suggests that a η1-propargyl- or η1-allenyl(chloro)palladium complex equilibrates with a cationic η3-propargylpalladium complex with the liberation of a Cl− ligand. A qualitative comparison of trends in a series of analogous equilibrium systems suggests that the η3-coordination mode is 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 η3-coordination of propargyl ligands in a catalytic cycle of Pd-catalyzed transformations of propargylic or allenylic substrates is presented.