Theoretical Study of Alternative Pathways for the Heck Reaction through Dipalladium and Ligand-Free Palladium Intermediates

The pathways for the Heck reaction, in particular, carbon-carbon bond formation between ethylene and phenyl bromide, catalyzed by dipalladium intemiediates and substrate-bound palladium complexes, were investigated by density functional calculation. For the sterically hindered phosphine ligand, (PBu3)-Bu-t, the free energy barrier for phenyl bromide oxidative addition to Pd-2((PBu3)-Bu-t)(2) is significantly higher than that to Pd((PBu3)-Bu-t) due to the higher steric interaction. However, for the PMe3 ligand, phenyl bromide oxidative addition by Pd,(PMe3)(2) has a lower free energy barrier than that by Pd(PMe3). Although the dipalladium is less likely to be active when coordinated by two sterically hindered phosphines, such as (PBu3)-Bu-t, it could serve as the active catalyst when less sterically encumbered. For Pd-2(PMe3)(2). the free energy profile for the complete Heck reaction cycle shows that, like the monopalladium pathway, oxidative addition of phenyl bromide is the rate-limiting step. Substrate-bound palladium complexes were also investigated as models for ligand-free conditions. Of the substrate-bound palladium complexes examined-free Pd, PdBr-, and Pd(eta(2)-C2H4)-the olefin-coordinated intermediate has the lowest free energy barrier for the phenyl bromide oxidative addition. Examination of the complete Heck reaction cycle for PdBr-- and Pd(eta(2)-C2H4)-based intermediates shows that ethylene stabilizes atomic palladium, which then proceeds to oxidative addition and migratory insertion. After the C-C bond coupling, a second bromide or other ligand binds to stabilize the low-coordinated palladium complex before it proceeds to beta-H transfer/olefin elimination and catalyst recovery.