Density functional study of the complete pathway for the Heck reaction with palladium diphosphines

The reaction mechanism for the complete catalytic cycle of the Heck reaction (between phenyl bromide, C₆H₅Br, and ethylene, C₂H₄, in the presence of the base, NEt₃ to form the product styrene, C₆H₅–C₂H₃), catalyzed by diphosphinopalladium complexes, Pd(PR₃)₂ {R = H, Me, Ph}, was investigated by using density functional theory (DFT). The relative free energies of the fully-optimized species in gas phase at 298.15 K and 1 atm were corrected for solvation in DMSO at 1 mol/L by using conductor-like polarizable continuum model (CPCM). The calculations indicate a four-step mechanism for the catalysis, including oxidative addition of C₆H₅Br, migratory insertion of C₆H₅ to C₂H₄, β-hydride transfer/olefin elimination of product, and catalyst regeneration by removal of HBr. Our calculations demonstrate that Pd π-complexes can be formed with phenyl bromide and ethylene before the oxidative addition occurs. Subsequently, various reaction paths were studied for the oxidative addition of phenyl bromide to palladium complexes, coordinated by phosphine(s) and/or ethylene. Interestingly, all pathways lead to palladium monophosphine as the active catalyst. Careful exploration was made on two possible pathways for the migratory insertion and β-hydride-transfer/olefin elimination: (1) the neutral path with bromide bound to Pd and (2) the cationic path with prior bromide ion dissociation. The neutral path is preferred to the cationic path, especially when the more bulky phosphines such as triphenylphosphine are involved.