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(η3-1-PhC3H4)(η5-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), Pd2(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, PBut3). As anticipated, catalysts generated fr...