Redox reactions in palladium catalysis: on the accelerating and/or inhibiting effects of copper and silver salt additives in cross-coupling chemistry involving electron-rich phosphine ligands.

Challenging a catalytic cycle: Pd(0) catalysts are readily oxidized by Cu and Ag salts to give dinuclear Pd(I) complexes and Cu(I) or Ag(I) cubanes (see scheme). The reactivities of the resulting Pd(I) dimers are consistent with several observations of additive effects in cross-coupling chemistry. The results indicate the possibility for alternative catalytic cycles involving dinuclear Pd(I) complexes over the currently accepted synergistic cycles involving Pd(0)/Pd(II) intermediates and Cu or Ag.