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

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-P-31 NMR; both revealed a net triligated coordination preference of the ferrocenyl tetraphosphine Fc(P)(4)Bu-t 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.