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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(η3-C3H5)(tmiy)(PR3)]BF4 (tmiy=tetramethylimidazolin-2-ylidene, PR3=phosphane), precursors to monoligated Pd0 species, with aryl electrophiles yielding 2-arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over PdIV and σ-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 PdII complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin-2-ylidene).