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

The rate and mechanism of the oxidative addition of aryl halides (PhI, PhBr) to Pd(O) complexes generated in situ upon addition of phosphines (PPh 3, PnBu3, dppf) to the macrocyclic triolefinic complex Pd0(1a) have been investigated in THF or DMF. The macrocyclic ligand la is known to allow a good recycling of the catalyst in catalytic reactions. It is established that the ligand la 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(PPh 3)4 > {Pd0(1a) + 4 PPh3), as a consequence of an equilibrium between Pd0(1a) and Pd 0(PPh3)3 which decreases the concentration of Pd0(PPh3)3 and consequently that of the reactive Pd0(PPh3)2. As expected, (Pd 0(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 phosphine/Pd loading, the Pd(0) complex ligated by PnBu3 is more reactive than that ligated by PPh3 and allows activation of PhBr at 25°C in DMF. When n = 2, Pd 0(PnBu3)2 is the unique species, which reacts with PhBr, but its concentration is controlled by the concentration of la, which favors the formation of the unreactive Pd0(η2-1a) (PnBu3)2 in a reversible reaction. The rate of the oxidative addition is limited by the dissociation of Pd0(η 2-1a)(PnBu3)2 to the reactive Pd 0(PnBu3)2 at high PhBr concentrations (>0.04 M). The reaction with PhI involves both Pd0(PnBu3) 2 and Pd0(η2-1a)(PnBu3) 2 as reactive species. The addition of 1 equiv of dppf to Pd 0(1a) leads to a complex mixture of Pd(0) complexes in THF. A reactivity order with PhI has been established, {Pd0(1a) + 2 PPh 3) > {Pd0(1a) + 1 dppf), in THF at 25°C. © 2008 American Chemical Society.