Suzuki reaction
The mechanism of the Suzuki reaction is best viewed from the perspective of the palladium catalyst 1. The first step is the oxidative addition of palladium to the halide 2 to form the organopalladium species 3. Reaction (metathesis) with base gives intermediate 4, which via transmetalation with the boron-ate complex 6 (produced by reaction of the boronic acid 5 with base) forms the organopalladium species 8. Reductive elimination of the desired product 9 restores the original palladium catalyst 1 which completes the catalytic cycle. The Suzuki coupling takes place in the presence of a base and for a long time the role of the base was not fully understood. The base was first believed to form a trialkyl borate (R3B-OR), in the case of a reaction of an trialkylborane (BR3) and alkoxide (−OR); this species could be considered as being more nucleophilic and then more reactive towards the palladium complex present in the transmetalation step. Duc and coworkers investigated the role of the base in the reaction mechanism for the Suzuki coupling and they found that the base has three roles: Formation of the palladium complex [ArPd(OR)L2], formation of the trialkyl borate and the acceleration of the reductive elimination step by reaction of the alkoxide with the palladium complex.