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The mechanism of oxidative addition of aryl halides to Pd(PR 3 ) 2 (R = Me, Et, i Pr, t Bu, 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 PdPR 3 was catalytically more active than bisphosphine Pd(PR 3 ) 2 for oxidative addition. However, among different PR 3 ligands (R = Me, Et, i Pr, t Bu, Ph), the free-energy barriers for oxidative addition to PdPR 3 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 PdL 2 to PdL and L (L = ligand) between the various PR 3 ligands dictated their dissimilar reactivity in oxidative addition.
Li et al. (2008) studied this question.