Transmetalation in the Suzuki-Miyaura coupling: the fork in the trail.

The Suzuki-Miyaura coupling is one of the few transition-metal-catalyzed C-C bond-forming reactions that have been used in applications ranging from discovery chemistry to manufacturing processes. Although coupling proceeds through the generic three-stage 'oxidative addition, transmetalation, reductive elimination' sequence, there are a number of features that differentiate the Suzuki-Miyaura process from other transition-metal-catalyzed cross-couplings. Most of these features are centered around, or are a consequence of, activation of the boron reagent for transmetalation through one or both of two distinct pathways. This review focuses on the evidence that has been presented for this 'fork in the trail', and the potential to apply such mechanistic insight to the design of reaction conditions.