The compound Pd(η3-1-PhC3H4)(η5-C5H5) reacts essentially quantitatively with a variety of phosphines L to form cross-coupling catalysts of the type PdL2 and has recently been shown to be a much more effective catalyst precursor for Suzuki–Miyaura cross-coupling reactions in comparison to more commonly utilized precursors such as Pd(PPh3)4, Pd2(dba)3, and Pd(OAc)2, which do not effectively generate two-coordinate species PdL2. This advantage is expected to apply also to e.g. Heck-Mizoroki and Sonogashira cross-coupling reactions, both of which are generally believed to be catalyzed by species of the type PdL2. Therefore, comparisons of the efficacies of catalyst systems based on Pd(η3-1-PhC3H4)(η5-C5H5), Pd(PPh3)4, Pd2(dba)3, and Pd(OAc)2 are made utilizing the conventional coupling reactions of aryl halides with methyl acrylate and styrene for Heck–Mizoroki coupling and with phenylacetylene for Sonogashira coupling. As anticipated, catalyst systems based on Pd(η3-1-PhC3H4)(η5-C5H5) are found to be significantly more active.

It has been demonstrated previously that Pd(η3-1-PhC3H4)(η5-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, Pd2(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/2PPh3 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, Pd2(dba)3/PPh3, and Pd(OAc)2/PPh3, the latter two activated with appropriate amounts of PPh3. Complementary studies involving the use of PBu t 3 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.