ON THE MECHANISM OF FORMATION OF HOMOCOUPLED PRODUCTS IN THE CARBON-CARBON CROSS-COUPLING REACTION CATALYZED BY PALLADIUM COMPLEXES CONTAINING RIGID BIDENTATE NITROGEN LIGANDS - EVIDENCE FOR THE EXCHANGE OF ORGANIC GROUPS BETWEEN PALLADIUM AND THE TRANSMETALATING REAGENT

The results of some carbon-carbon coupling reactions of organic halides with organomagnesium, -zinc, and -tin reagents catalyzed by Pd(Ar-BIAN) complexes, i.e., complexes containing the rigid bidentate nitrogen ligands bis(arylimino)acenaphthene, are described. From a detailed mechanistic study on the formation of homocoupled products, especially in the reaction of benzyl bromide with p-tolylmagnesium, p-tolylzinc, and organotin reagents, it has been shown that the formation of homocoupled products in catalytic cross-coupling reactions proceeds via a mechanism involving the exchange of organic groups between the organopalladium(II) complex, formed after oxidative addition of the organic halide, and the transmetalating reagent. This leads to the formation of a mixture of two organopalladium(II) complexes and two transmetalating reagents, from which cross-coupled and homocoupled products can be formed. The exchange of organic groups is observed in the reaction of organopalladium(Ar-BIAN) complexes with transmetalating reagents, under conditions such that coupling has not yet occurred (by the appropriate choice of temperature, reaction time, and solvent). Other mechanisms which could in principle lead to the formation of homocoupled products, such as halogen-metal exchange, reductive elimination from a Pd(IV) or a dinuclear Pd(II) complex, reactions via free radical pathways, or disproportionation reactions, are excluded on the basis of our experimental data for the Pd(Ar-BIAN)-catalyzed reaction of benzyl bromide with p-tolylmagnesium bromide and p-tolylzinc chloride.