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

2008-12-08T00:00:00Z (GMT) by Panida Surawatanawong Michael B. Hall
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, P<sup><i>t</i></sup>Bu<sub>3</sub>, the free energy barrier for phenyl bromide oxidative addition to Pd<sub>2</sub>(P<sup><i>t</i></sup>Bu<sub>3</sub>)<sub>2</sub> is significantly higher than that to Pd(P<sup><i>t</i></sup>Bu<sub>3</sub>) due to the higher steric interaction. However, for the PMe<sub>3</sub> ligand, phenyl bromide oxidative addition by Pd<sub>2</sub>(PMe<sub>3</sub>)<sub>2</sub> has a lower free energy barrier than that by Pd(PMe<sub>3</sub>). Although the dipalladium is less likely to be active when coordinated by two sterically hindered phosphines, such as P<sup><i>t</i></sup>Bu<sub>3</sub>, it could serve as the active catalyst when less sterically encumbered. For Pd<sub>2</sub>(PMe<sub>3</sub>)<sub>2</sub>, 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<sup>−</sup>, and Pd(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)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<sup>−</sup>- and Pd(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)-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 β-H transfer/olefin elimination and catalyst recovery.