Rate and Mechanism of the Oxidative Addition of Aryl Halides to Palladium(0) Complexes Generated in Situ from a Pd(0)−Triolefinic Macrocyclic Complex and Phosphines

Abstract

The rate and mechanism of the oxidative addition of aryl halides (PhI, PhBr) to Pd(0) complexes generated in situ upon addition of phosphines (PPh3, PnBu3, dppf) to the macrocyclic triolefinic complex Pd0(1a) have been investigated in THF or DMF. The macrocyclic ligand 1a is known to allow a good recycling of the catalyst in catalytic reactions. It is established that the ligand 1a affects the kinetics of oxidative addition, as monitored by electrochemical techniques. As far as PPh3 is concerned, a reactivity order with PhI has been established in THF, Pd0(PPh3)4 > {Pd0(1a) + 4 PPh3}, as a consequence of an equilibrium between Pd0(1a) and Pd0(PPh3)3 which decreases the concentration of Pd0(PPh3)3 and consequently that of the reactive Pd0(PPh3)2. As expected, {Pd0(1a) + 2 PPh3} is more reactive than {Pd0(1a) + 4 PPh3}. In contrast to PPh3, the addition of n equivalents of PnBu3 to Pd0(1a) in DMF leads to the formation of Pd0(η2-1a)(PnBu3)2 and Pd0(PnBu3)3 (n > 2), characterized by 31P NMR. At equal phosphi...