A Palladium−Ferrocenyl Tetraphosphine System as Catalyst for Suzuki Cross-Coupling and Heck Vinylation of Aryl Halides:  Dynamic Behavior of the Palladium/Phosphine Species

Abstract

The system combining the new ferrocenyl tetraphosphine 1,1‘,2,2‘-tetrakis(diphenylphosphino)-4,4‘-di-tert-butylferrocene (called Fc(P)4tBu, 1) and [PdCl(η3-C3H5)]2 has been found to be an active catalyst for the cross-coupling of aryl halides with aryl boronic acids (i.e., Suzuki reaction) and for the vinylation of aryl halides with alkenes (Heck reaction). A variety of chlorides reacts in good yield with aryl boronic acids in the presence of 1−0.01% catalyst. The more reactive aryl bromides were reacted with aryl boronic acids or alkenes in the presence of 0.01−0.0001% catalyst. This system compares well with other catalytic systems that have been described for Suzuki or Heck reactions. 1H, 13C, and 31P NMR studies in solution were conducted with the view to obtain a better understanding of the interaction involving the palladium dimeric precursor and the tetraphosphine. The initial formation of kinetic and then different thermodynamic species was evidenced. A dynamic evolution from labile Pd(π-allyl)/tetraphosphine species toward the well-defined, stable, and nonfluxional complexes [PdCl2{Fc(P)4tBu}] (3) and [Pd2Cl4{Fc(P)4tBu}] (4) is observed. This behavior is different from the other known active tetraphosphine Tedicyp [cis,cis,cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)cyclopentane]. The palladium mononuclear 3 and dinuclear 4 complexes were isolated and fully characterized in the solid state by X-ray diffraction analysis and in solution by multinuclear NMR. The blocked conformation in solution of compounds 1, 3, and 4 respectively leads to original AA‘BB‘, ABMX, and A2B2 31P NMR spin-systems for the four phosphorus atoms.