Pd(η3-1-PhC3H4)(η5-C5H5), an Unusually Effective Catalyst Precursor for Suzuki-Miyaura Cross-Coupling Reactions Catalyzed by Bis-Phosphine Palladium(0) Compounds

It has previously been shown that the easily handled, heat- and air-stable compound Pd(eta(3)-1-PhC3H4)(eta(5)-C5H5) (Vb) reacts rapidly with a wide variety of tertiary phosphines L to produce near-quantitative yields of the corresponding Pd(0) compounds PdL2, which are widely believed to be the active species in many often-used cross-coupling catalyst systems based on Pd(PPh3)(4) (I), Pd-2(dba)(3) (II), PdCl2 (III), and Pd(OAc)(2) (IV). However, catalyst precursors I-IV are in fact known to preferentially generate sterically hindered, three-coordinate Pd(0) species rather than two-coordinate PdL2, and thus Vb is hypothetically expected to be a better catalyst precursor for e.g. Suzuki-Miyaura cross-coupling reactions. Utilizing the conventional Suzuki-Miyaura cross-coupling reaction of phenylboronic acid with bromoanisole, comparisons are made of the efficacies of catalyst systems based on Vb with those based on compounds I-IV (L = the representative phosphines PPh3, PCy3, PBu3t). As anticipated, catalysts generated from Vb are significantly more competent and, as a bonus, Vb makes palladium(0) complexes PdL2 available under rigorously anhydrous conditions.