RATES AND MECHANISMS OF OXIDATIVE ADDITION TO ZEROVALENT PALLADIUM COMPLEXES GENERATED IN-SITU FROM MIXTURES OF PD0(DBA)2 AND TRIPHENYLPHOSPHINE

The composition of mixtures of Pd0(dba)2 (dba = dibenzylideneacetone) and triphenylphosphine was examined in THF and DMF, as well as their reactivity vis a '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)2'', 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 reversible 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.