Rates and mechanisms of oxidative addition to zerovalent palladium complexes generated in situ from mixtures of Pd0(dba)2 and triphenylphosphine

Article abstract of DOI:10.1021/om00032a045

The composition of mixtures of Pd⁰(dba)₂ (dba = dibenzylideneacetone) and triphenylphosphine was examined in THF and DMF, as well as their reactivity vis à vis oxidative addition of PhI. It is concluded that, at equilibrium, these catalytic systems contain lesser available amounts of the species active in oxidative addition, viz. the low-ligated zerovalent palladium intermediate Pd⁰(PPh₃)₄ , than Pd⁰(PPh₃)₄ solutions do for an identical concentration of zerovalent palladium. This arises because, in contradiction with usual assumptions, dba is a better ligand than triphenylphosphine, for the low-ligated active Pd⁰(PPh₃)₂ , as evidenced by the small values (0.14) of the equilibrium constants of Pd⁰(dba)(PPh₃)₂ + PPh₃ + solvent ? solvent-Pd⁰(PPh₃)₃ + dba in THF or DMF. As a result, oxidative addition of PhI to mixtures of Pd⁰(dba)₂ and 2 equiv of triphenylphosphine proceeds at an overall rate that is ca. 10 times less than that to Pd⁰(PPh₃)₄. However, it is shown that oxidative addition to the two systems proceeds via the same transient intermediate, the solvated low-ligated Pd⁰(PPh₃)₂ ' moiety, evidencing that coordination by dba is not involved in the transition state of oxidative addition. This validates a posteriori previous assumptions on such transition states made in the literature, particularly for rationalization of enantiomeric selectivity when chiral phosphines are used.