Theoretical Study of Alternative Pathways for the Heck Reaction through Dipalladium and "Ligand-Free" Palladium Intermediates
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abstract
The pathways for the Heck reaction, in particular, carbon-carbon bond formation between ethylene and phenyl bromide, catalyzed by dipalladium intermediates and substrate-bound palladium complexes, were investigated by density functional calculation. For the sterically hindered phosphine ligand, PtBu3, the free energy barrier for phenyl bromide oxidative addition to Pd2(PtBu3)2 is significantly higher than that to Pd(PtBu3) due to the higher steric interaction. However, for the PMe3 ligand, phenyl bromide oxidative addition by Pd2(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 PtBu3, it could serve as the active catalyst when less sterically encumbered. For Pd2(PMe 3)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(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(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 -H transfer/olefin elimination and catalyst recovery. 2008 American Chemical Society.
