The reactivity of Pd(0) complexes generated by addition of PPh3 (PPh3/Pd = 2, 4) to either Pd02(dba-n,n‘-Z)3 (n,n‘-Z = 4,4‘-F, 4,4‘-H, 4,4‘-MeO, 3,3‘,4,4‘,5,5‘-OMe) or Pd0(dba-n,n‘-Z)2 (n,n‘-Z = 4,4‘-Br, 4,4‘-Cl, 4,4‘-H, 4,4‘-CH3, 3,3‘,5,5‘-OMe) in DMF is affected by the electron-donating or -accepting properties of the groups Z substituted on the aromatic rings of dba. Whatever the nature of Z, the unreactive major complexes Pd0(η2-dba-n,n‘-Z)(PPh3)2 are formed, which are in equilibrium with the common reactive complex Pd0(PPh3)2 and dba-n,n‘-Z. The latter controls the concentration of the reactive Pd0(PPh3)2 and, consequently, also controls the rate of the overall oxidative addition with phenyl iodide. The more electron donating the Z group, the lower the affinity of dba-4,4‘-Z for Pd0(PPh3)2. As a result, the overall rate of the oxidative addition with PhI is faster when Z is an electron-donating group. For a given Z, the overall oxidative addition is faster when using Pd02(dba-n,n‘-Z)3 instead of Pd0(dba-n,n‘-Z)2. Therefore, the rate of the oxidative addition can be modulated by changing the electronic properties of the dba ligands determined by substituents on its phenyl groups and by changing the structure of the precursors: Pd02(dba-n,n‘-Z)3 versus Pd0(dba-n,n‘-Z)2.
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Macé et al. (2006) studied this question.
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