MECHANISM OF OXIDATIVE ADDITION OF PALLADIUM(0) WITH AROMATIC IODIDES IN TOLUENE, MONITORED AT ULTRAMICROELECTRODES

The mechanism and rates of oxidative addition of substituted iodobenzenes to tetrakis(triphenylphosphine) palladium(0) is investigated in toluene by means of electrochemistry at ultramicroelectrodes. The mechanism is shown to be identical with that previously reported in THF and to correspond to a similar slope for Hammett's correlation (ρ = +2 in THF vs +2.3 ± 0.2 in toluene). Moreover, the important change in polarity (ϵs≈ 7.58 in THF and 2.38 in toluene) does not affect the enthalpy of activation and leads to similar activation entropies in both solvents. This suggests that the transition state of addition of aryl halides to the coordinatively unsaturated zerovalent palladium complex Pd°(PPh3)2has no significant ionic character. © 1990, American Chemical Society. All rights reserved.