Many transition metal-catalyzed reactions use a second metal as reagent. For instance, the Negishi Cross-Coupling involves Pd or Ni as catalysts and Zn reagents as organometallic nucleophiles. However, over the past decade, new methods to form C–C bonds using two metals in catalytic amounts have emerged. The most popular example is the Sonogashira Cross-Coupling, which uses Pd and Cu as co-catalysts.
Very often a transmetalation step connect two catalytic cycles. In these reactions one metal acts as “group transfer” from one reagent to the final catalyst, taking the transfered group from another reagent or creating it in its own coordination sphere. There are other numerous examples that proposed two different catalytic cycles connected by a transmetalation step. You can find a critical review of these processes:
Related with our study on the transmetalation between gold and palladium complexes (Angew. Chem. Int. Ed. 2012, 20, 4917–4920), we have developed a bimetallic system that makes possible the cross coupling of very bulky groups using the Stille reaction.
The low sterically demanding linear gold (I) complexes act as intermediate in the transmetalation of bulky aryl groups between organostannanes and palladium complexes. Our study shows that the Sn/Au transmetalation step is critical for the success of the reaction, and that it is favored by the release of the steric constraint in the organotin compound after the aryl exchange to the gold compound.