η~5-cyclopentadienyl-η~3-1- 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

摘要

It has been demonstrated previously that Pd(η~3-1-PhC _3H_4)(η~5-C_5H_5) (I) reacts with a number of phosphines L to form two-coordinate, zerovalent species of the type PdL_2, known to be effective cross-coupling catalysts for Suzuki-Miyaura and Heck-Mizoroki cross-coupling reactions. Formation of PdL _2 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(PPh_3)_4, Pd_2(dba) _3, and Pd(OAc)_2, which do not efficiently generate PdL_2. 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 PdL_2, 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(PPh_3)_4, trans-PdCl_2(PPh_3)_2, Pd_2(dba) _3/PPh_3, and Pd(OAc)_2/PPh_3, the latter two activated with appropriate amounts of PPh_3. Complementary studies involving the use of PBu _3~t 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(C _2Ph)_2(PPh_3)_2, long considered to be an intermediate in the activation of trans-PdCl_2(PPh _3)_2 but not heretofore, to our knowledge, shown to be viable.