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 t hem react with electrophiles, in the presence of a palladium catalyst and a source of electron. Indeed, the key intermediate ArCO-Pd-XL2 (L = PPh,) whose activation by electron transfer in the presence of an el ectrophile would produce the coupling product ArCO-E, is less easily r educed than the corresponding aroyl halide. [GRAPHICS] However, an ind irect palladium catalyzed process for the synthesis of ArCO-E derivati ves is proposed, from aryl halides, carbon monoxide and an electron so urce. [GRAPHICS] The efficiency of the second process comes from the f act that the intermediate ArCO-Pd-XL2, formed by reaction of CO with A r-Pd-XL2, is more easily reduced than the aryl halide and the transien t Ar-Pd-XL2 complex. (C) 1997 Elsevier Science Ltd.