MECHANISTIC STUDIES ON THE ARYL-ARYL INTERCHANGE REACTION OF ARPDL2I (L = TRIARYLPHOSPHINE) COMPLEXES

The aryl-aryl interchange reaction of ArPdL2I complex Im was found to follow pseudo-first-order kinetics. A marked inhibition in the presenc e of excess phosphine and/or excess iodide was observed, suggesting th at a dissociative pathway was involved, contrary to the analogous alky l-aryl interchange reaction studied previously. Phosphine flooding exp eriments could not be performed due to a competing phosphonium salt fo rmation reaction that occurred in the presence of excess phosphine. A deuterium labeling experiment indicated that the interchange reaction proceeded via the reductive elimination to form the phosphonium salt, suggesting that excess phosphine was acting as a trap for intermediate palladium(0) species preventing the generation of the interchanged pa lladium(II) complex. Substituent effect studies of the interchange rea ction indicated that it was inhibited by electron-withdrawing groups o n both the phosphine and palladium-bound aryl groups and by increased steric bulk on both the phosphine and palladium-bound aryl groups. Und er catalytic conditions, the distribution of phosphines formed from th e aryl-aryl interchange during palladium-mediated cross-coupling react ions could be modeled by statistics. Various strategies for eliminatin g the formation of byproducts caused by the interchange during cross-c oupling reactions were screened and optimized.