Direct Arylation of Heterocycles: The Performances of Ferrocene‐Based Polyphosphane Ligands in Palladium‐Catalyzed CH Bond Activation

Abstract

Abstract The palladium‐catalyzed direct arylation of alkylated‐ furan, thiophene, and thiazole and benzoxazole heterocycles with electronically and sterically deactivated bromoarenes was selectively and efficiently promoted by ferrocenyl polyphosphanes. In this CH bond activation reaction of heteroaromatics, the performances of polydentate di‐, tri‐, and tetraphosphane ligands were compared, showing that the triphosphane 1,1′,2‐tris(diphenylphosphino)‐4‐ tert ‐butylferrocene 3 was the most effective for the coupling. The introduction of more electron‐donating ( i Pr) or electron‐withdrawing (furyl) groups on the phosphorus atoms did not improve the ligand performances. The coordination behavior of 3 towards palladium(II) and other group 10 metals, Ni II and Pt II , was studied and the corresponding 1,2‐P chelating phosphorus complexes were isolated in high yields and fully characterized by multinuclear 1 H, 13 C, 31 P NMR ( 3 ⋅PdCl 2 , 3 ⋅PtCl 2 ) and X‐ray structures ( 3 ⋅NiBr 2 ). It was found that the triphosphane ligand 3 was able to accommodate bidentate chelating cis coordination to group 10 metals, and additionally a much rarely described bis monodentate trans configuration. The combination of these two modes of coordination in a multimetallic species was clearly evidenced for the first time for a ferrocenyl polyphosphane. This versatility in bonding is a clear difference of coordination potential of this catalytically more efficient triphosphane compared to analogous ferrocenyl di‐ or tetraphosphanes.