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

The system combining the new ferrocenyl tetraphosphine 1,1‘,2,2‘-tetrakis(diphenylphosphino)-4,4‘-di- tert -butylferrocene (called Fc(P) 4 t Bu, 1 ) and [PdCl(η 3 -C 3 H 5 )] 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. 1 H, 13 C, and 31 P 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 [PdCl 2 {Fc(P) 4 t Bu}] ( 3 ) and [Pd 2 Cl 4 {Fc(P) 4 t Bu}] ( 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 A 2 B 2 31 P NMR spin-systems for the four phosphorus atoms.