First Copper(I) Ferrocenyltetraphosphine Complexes: Possible Involvement in Sonogashira Cross-Coupling Reaction?

Abstract

Preparation and characterization of the first examples of copper(I) ferrocenylpolyphosphine complexes are reported. The molecular structure of complex {P,P′,P′′-[1,1′,2,2′-tetrakis(diphenylphosphino)-4,4′di-tert-butylferrocene]iodocopper(I)} (1) was solved by X-ray diffraction studies, and its fluxional behavior in solution was investigated by VT-31 P NMR; both revealed a net triligated coordination preference of the ferrocenyl tetraphosphine Fc(P) 4t Bu with copper. The tetradentate ligand is an active auxiliary in Sonogashira alkynylation; therefore the general question of copper as a competitive coordination partner in the Pd/Cu-catalyzed Sonogashira reaction was raised and discussed. Electronically neutral, activated, and deactivated aryl bromides were employed for coupling with phenylacetylene with various [(Pd)/ (Cu)/(tetraphosphine)] systems. The catalytic investigations shown that 1 mol % of complex 1 in combination with palladium is far more effective and selective for Sonogashira coupling than 5 mol % of CuI and palladium in the coupling to phenylacetylene of the deactivated aryl bromide 4-bromoanisole. This system efficiently avoids the concurrent and deleterious consumption of phenylacetylene by formation of diyne or enynes. To our knowledge, this is the first time that this kind of high selectivity is induced in Sonogashira alkynylation by initial ligand complexation to copper instead of palladium. These results demonstrate that coordination of Cu halide cocatalyst is a factor that should no longer be neglected in mechanistic and applied studies of the Sonogashira reaction.

Silica-supported palladium: Sustainable catalysts for cross-coupling reactions

“…The exact mechanism of the homogeneous copper co-catalyzed Sonogashira reaction is unknown and is generally supposed to take place through two independent catalytic cycles, as shown in Scheme 21. While the first cycle involving Pd is classical from C C cross-coupling formation, the second cycle generally involving copper (I) iodide is still poorly understood [54].…”

Cross‐Coupling Reactions to sp Carbon Atoms

“…It is widely accepted that the initial generation of a π-alkyne-Cu complex 13 increases the acidity of the terminal alkyne and thus facilitates the base-assisted formation of the copper acetylide 10. However, recent reports suggested further processes involving copper, such as prestabilization of CuI with multidentate polyphosphine ligands and ligand exchange between the participating catalytically active metals [23].…”

Sonogashira Reactions Using Nanocatalysts

“…However, it has been shown that CuI-polyphosphane adducts can be formed and ligands can be transferred from one metal to another, showing that copper-ligand interactions might also be likely in tandem palladium/copper cross-coupling reactions. 146,147 The processes involving the copper-free Sonogashira reaction have been investigated more deeply, even theoretically, 148,149 and the suggested mechanism is depicted in Figure 4.4a. This proposed catalytic cycle is initiated, as usual, by the oxidative addition of the aryl or vinyl halide to the catalytic species [Pd(0)L 2 ].…”