Theoretical study on monoligated Pd-catalyzed cross-coupling reactions of aryl chlorides and bromides

The mechanism of oxidative addition of aryl halides to Pd(PR3)(2) (R = Me, Et, iPr, tBu, Ph) was investigated by using density functional theory methods enhanced with a polarized continuum solvation model. Different reaction pathways were discussed on the basis of Gibbs free-energy profiles in a tetrahydrofuran solution. The calculations indicated that monophosphine PdPR3 was catalytically more active than bisphosphine Pd(PR3)(2) for oxidative addition. However, among different PR3 ligands (R = Me, Et, iPr, tBu, Ph), the free-energy barriers for oxidative addition to PdPR3 did not change significantly (i.e., less than 2 kcal/mol). This gave rise to an important question: why was P(t-Bu)(3) the only catalytically active ligand toward aryl chlorides among the above five ligands? It was proposed on the basis of the calculated data that the difference of the dissociation energies from PdL2 to PdL and L (L = ligand) between the various PR3 ligands dictated their dissimilar reactivity in oxidative addition.