η5-Cyclopentadienyl-η3-1-phenylallylpalladium, an Unusually Effective Catalyst Precursor for Copper-Free and Copper-Assisted Sonogashira Cross-Coupling Reactions Catalyzed by Bis-phosphine Palladium(0) Compounds: The Role of Bis-acetylide Intermediates in the Reduction of a Palladium(II) Catalyst Precursor to a Palladium(0) Catalyst

It has been demonstrated previously that Pd(eta(3)-1-PhC3H4)(eta(5)-C5H5) (I) reacts with a number of phosphines L to form two-coordinate, zerovalent species of the type PdL2, known to be effective cross-coupling catalysts for Suzuki-Miyaura and Heck-Mizoroki cross-coupling reactions. Formation of PdL2 from I is generally rapid and near quantitative, and I has therefore been found to be much more effective than more commonly utilized precursors such as Pd(PPh3)(4), Pd-2(dba)(3), and Pd(OAc)(2), which do not efficiently generate PdL2. This advantage is expected to apply as well to Sonogashira cross-coupling reactions, which are also believed to be catalyzed by species of the type PdL2, and it is shown that the I/2PPh(3) catalyst system is much more active for the representative, conventional coupling reaction of bromobenzene with phenyl acetylene to form diphenylacetylene than are conventional catalyst systems based on Pd(PPh3)(4), trans-PdCl2(PPh3)(2), Pd-2(dba)(3)/PPh3, and Pd(OAc)(2)/PPh3, the latter two activated with appropriate amounts of PPh3. Complementary studies involving the use of PBu3t with I, which generates an extremely active catalyst system, and the involvement of the cocatalyst CuI are also described, as is the catalytic competence of trans-Pd(C2Ph)(2)(PPh3)(2), long considered to be an intermediate in the activation of trans-PdCl2(PPh3)(2) but not heretofore, to our knowledge, shown to be viable.