The Role of Bidentate Fluorenylphosphines in Palladium-Catalyzed Cross-Coupling Reactions

Seven new bidentate phosphine ligands, in which two 9-(dicyclohexylphosphino)fluoren-9′-yl units are bridged with n-alkanediyl- (C1−C5) or o/m-xylenediyl linkers were synthesized and characterized as the respective air-stable phosphonium salts. Pd complexes of the new bidentate ligands proved to be highly active catalysts for Buchwald−Hartwig amination and Suzuki and Sonogashira coupling using aryl bromides and chlorides as substrates. A study comparing the catalytic activity of the Pd complexes of the bidentate phosphine ligands with those of closely related monodentate fluorenyl phosphines gave insights into the influence of the various bridging units on the catalytic transformations. In Suzuki and amination reactions the diphosphine 2b with the shortest linker, a −CH2− unit, turned out to be by far the best ligand. In the Sonogashira coupling monodentate phosphines rendered the most active Pd catalysts, while 2b fails.