The complexity of the transmetalation step in a Pd-catalyzed Negishi reaction has been investigated by combining experiment and theoretical calculations. The reaction between trans-[PdMeCl(PMePh2)2] and ZnMe2 in THF reveal some unexpected and relevant mechanistic aspects not observed for ZnMeCl as nucleophile. The operative reaction mechanism is not the same when the reaction is carried out in the presence or in the absence of an excess of phosphine in the medium. In the absence of added phosphine an ionic intermediate with THF as ligand ([PdMe(PMePh2)2(THF)]+) opens ionic transmetalation pathways.
During the sutdy of the Negishi reaction we have also found that Pd takes a roundtrip before coupling: The large rate difference of the slow reductive elimination and the fast transmetalation-retrotransmetalation steps brings Pd to visit many times its complexes 3, 2, and 1, before coupling from 2 takes place. This fast recurrence of transformations offers many opportunities for the formation of undesired homocoupling products in Negishi reactions, via statistically accessible “mistaken” retrotransmetalation events in the multitude of roundtrip processes preceding the slower irreversible reductive elimination of the coupling product.
We recently found a system that allows us to study what happens when during the Stille reaction the transmetallation is faster than the isomerization reactions of the palladium complexes involved.