Palladium Monophosphine Intermediates in Catalytic Cross-Coupling Reactions: A DFT Study

Abstract

The mechanism of the cross-coupling of phenylboronic acid with acetic anhydride, a viable model of the widely used Suzuki reaction, has been studied by DFT calculations at the BP86/6-31G* level of theory. Multiple interconnected reaction pathways are considered that start from the neutral Pd(PMe3)2 molecule, the two-coordinate anionic [Pd(PMe3)OAc]- complex, and the three-coordinate anionic [Pd(PMe3)2OAc]- complex. The calculated catalytic cycles seem mechanistically and energetically plausible. According to the calculations, oxidative addition of acetic anhydride to either of the two anionic species leads to the formation of anionic palladium(II) monophosphine complexes with two acetate ligands located either cis or trans to each other. As a consequence, there are two competing anionic pathways for the transmetalation reaction with phenylboronic acid which involve only monosphosphine complexes. Both pathways are energetically feasible, with a slight preference for the cis variant. Reductive elimination of the product acetone in the presence of a second phosphine molecule leads to the regeneration of the anionic bisphosphine complex [Pd(PMe3)2OAc]- while the monophosphine complex [Pd(PMe3)OAc]- is formed in the absence of excess phosphine. Our calculations suggest that in Suzuki-type coupling reactions several catalytic pathways involving neutral and anionic palladium species may generally contribute to the catalytic turnover. In the present model system, the anionic pathways are most favorable.