Ligandless, Anionic, Arylpalladium Halide Intermediates in the Heck Reaction

We report the isolation and reactivity of a series of "ligandless," anionic arylpalladium complexes of the general structure [Pd(Ar)Br(2)](2)(2-) by the reaction of ((t)Bu(3)P)Pd(Ar)(Br) and bromide. These anionic complexes insert olefins at room temperature, and these fast insertions indicate that the anionic complexes are kinetically competent to be intermediates in Heck-Mizoroki reactions conducted under "ligandless" conditions (lacking added dative ligand). Kinetic studies showed that the anionic complexes insert olefins much faster than the corresponding neutral, P(t-Bu)(3)-ligated complexes. Addition of halide to the reaction of the neutral complex ((t)Bu(3)P)Pd(Ar)(Br) and styrene led to a significant rate acceleration, suggesting that the anionic complex forms rapidly and reversibly in situ from the neutral species prior to migratory insertion. These data, along with studies on the regioselectivity for reaction of aryl halides with butyl vinyl ether in the presence of the different starting catalysts, are consistent with the intermediacy of the same anionic, arylpalladium intermediates in Heck reactions catalyzed by palladium complexes containing bulky trialkylphosphine ligands as in reactions conducted under ligandless conditions.