Direct vs indirect route in the activation of aroylpalladium(II) complexes by electron transfer

Based on mechanistic investigations, it is established that it is not possible to invert the reactivity of aroyl halides, ArCO-X, and make them react with electrophiles, in the presence of a palladium catalyst and a source of electron. Indeed, the key intermediate ArCO-Pd-XL₂ (L = PPh₃) whose activation by electron transfer in the presence of an electrophile would produce the coupling product ArCO-E, is less easily reduced than the corresponding aroyl halide. ArCO-X + E⁺ + 2e - ×^Pd(0) → ArCO-E + X^-. However, an indirect palladium catalyzed process for the synthesis of ArCO-E derivatives is proposed, from aryl halides, carbon monoxide and an electron source. Ar-X + CO + E⁺ + 2e →^Pd(0) ArCO-E + X^-. The efficiency of the second process comes from the fact that the intermediate ArCO-Pd-XL₂, formed by reaction of CO with Ar-Pd-XL₂, is more easily reduced than the aryl halide and the transient Ar-Pd-XL₂ complex.