Structure-Reactivity Relationships in Negishi Cross-Coupling Reactions

Competition experiments have been performed to determine the relative reactivities of substituted bromobenzenes and of different arylzinc reagents in the [Pd(PPh3)(4)]-catalyzed Negishi cross-coupling reaction in THF at 25 degrees C. The cross-coupling reactions are accelerated by electron acceptors in the bromobenzenes, the effect of which increases in the order ortho < meta < para. On the other hand, electron acceptors in the arylzinc halides diminish the reaction rates. Hammett correlations show that substituent variations in the bromobenzenes (rho = +2.5) have a larger effect than substituent variations in the arylzinc halides (rho = -0.98).