Mechanisms in the Reaction of Palladium(II)-π-Allyl Complexes with Aryl Halides: Evidence for NHC Exchange Between Two Palladium Complexes

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(eta(3)-C3H5)(tmiy)(PR3)]BF4 (tmiy = tetramethylimidazolin-2-ylidene, PR3=phosphane), precursors to monoligated Pd-0 species, with aryl electrophiles yielding 2-arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd-IV and sigma-bond metathesis pathways, and that transmetalation is the rate-determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd-II complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin-2-ylidene).