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 r eactivity vis a 'vis oxidative addition of PhI. It is concluded that, at equilibrium, these catalytic systems contain lesser available amoun ts of the species active in oxidative addition, viz. the low-ligated z erovalent palladium intermediate ''Pd0(PPh3)2'', than Pd0(PPh3)4 solut ions do for an identical concentration of zerovalent palladium. This a rises because, in contradiction with usual assumptions, dba is a bette r ligand than triphenylphosphine, for the low-ligated active ''Pd0(PPh 3)2'', as evidenced by the small values (0.14) of the equilibrium cons tants 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 mixtu res of Pd0(dba)2 and 2 equiv of triphenylphosphine proceeds at an over all rate that is ca. 10 times less than that to Pd0(PPh3)4. However, i t 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 tra nsition state of oxidative addition. This validates a posteriori previ ous assumptions on such transition states made in the literature, part icularly for rationalization of enantiomeric selectivity when chiral p hosphines are used.