Negishi cross-coupling reactions, palladium mechanisms

Negishi, E., Takahashi, T., Baba, S., Van Horn, D. E., Okukado, N. Nickel- or palladium-catalyzed cross coupling. 31. Palladium- or nickel-catalyzed reactions of alkenylmetals with unsaturated organic halides as a selective route to arylated alkenes and conjugated dienes scope, limitations, and mechanism. J. Am. Chem. Soc. 1987,109, 2393-2401. 

As in all C-C cross-coupling reactions, the Negishi reaction mechanism consists in three steps (Fig.4.1) oxidative addition, transmetalation, and reductive elimination. The former and the latter are common to all the other cross-coupling reactions, whereas the transmetalation step is particular of this reaction. Unfortunately, this transmetalation has been less studied compared to the ones in the Stille [12-17] or Suzuki reactions, [18-21] in spite of the fact that the transmetalation between organozinc and palladium complexes is also involved in other relevant processes, such as the hydroalkylation of styrenes, [22] the asymmetric allylation of aryl aldehydes, [23] the coupling propargylic benzoates and aldehydes, [24] or the double-transmetalation oxidative cross-coupling reaction [25, 26]. 

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