Kinetic Data on the Synergetic Role of Amines and Water in the Reduction of Phosphine-Ligated Palladium(II) to Palladium(0)
Résumé
Tertiary amines such as EtN(iPr)2 do not reduce [PdCl2(PPh3)2] into a Pd0 complex. The latter is formed only after the addition of water (generation of HO-), as was reported by Grushin and Alper for NEt3. The mechanism of the PdII/Pd0 reduction performed in the presence of an excess amount of PPh3 is established quantitatively by means of electrochemical techniques that provide kinetic data (determination of the equilibrium and rate constants) for the disappearance of [PdCl2(PPh3)2] and the formation of [Pd0(PPh3)3]. [PdCl(OH)(PPh3)2] is formed, which allows reductive elimination between OH and the ligated PPh3 (zero-order reaction for PPh3), and this leads to a Pd0 complex. The reducing agent is ligated PPh3, which ultimately yields (O)PPh3. The rate of the overall reduction process is controlled by the amount of water that imposes the concentration of HO-.