Unexpected bell-shaped effect of the ligand on the rate of the oxidative addition to palladium(0) complexes generated in situ from mixtures of Pd(dba)2 and para-substituted triarylphosphines

As with PPh3, mixtures of Pd(dba)(2) and nL (L=para-Z-substituted triphenylphosphines, n greater than or equal to 2) in DMF lead to the formation of Pd(dba)L-2 and PdL3 in equilibrium with PdL2. The equilibrium between Pd(dba)L-2 and PdL3 is more in favor of PdL3 when the phosphine is less electron rich. In other words, the exchange of the dba ligand by a phosphine from Pd( dba)L, to form PdL, is more favored when the phosphine is less electron rich, The less ligated complex PdL2 is the reactive species in the oxidative addition with phenyl iodide. It was therefore expected that the rate of the oxidative addition would increase when the phosphine is more electron rich. However, surprisingly, when the palladium(0) complex is generated from mixtures of Pd(dba)(2) and n L (n greater than or equal to 2), the oxidative addition does not follow a linear Hammett correlation and the reactivity of the palladium(0) complex exhibits a maximum value. This is due to two antagonist effects. Indeed, the overall reactivity in the oxidative addition is governed by two factors: the intrinsic reactivity of PdL2 and its concentration. When the phosphine becomes more electron rich, the complex PdL, becomes more nucleophilic and its intrinsic reactivity in the oxidative addition increases. However, when the phosphine becomes more electron rich, the concentration of PdL2 decreases because the equilibrium between the palladium(0) complexes becomes more in favor of Pd(dba)L-2. These results emphasize the crucial role of the dba ligand on the reactivity of palladium(0) complexes generated in situ in mixtures of Pd(dba)(2) and phosphines. (C) 1998 Elsevier Science S.A. All rights reserved.