New bis(fluoro-ponytailed) bipyridine ligands for Pd-catalyzed Heck reactions under fluorous biphasic catalysis condition

Three highly fluorinated bipyridine derivatives (4,4′-bis(RfCH2OCH2)-2,2′-bpy) {Rf=HCF2(CF2)7 (1a), n-C8F17 (1b), n-C10F21 (1c)} have been synthesized using 4,4′-bis(BrCH2)-2,2′-bpy and the corresponding fluorinated alkoxides. The fluorine contents of ligands 1a–c are 58.3, 59.8, and 62.3%, respectively. Despite its high fluorine content, the ligand 1a with a –CF2H polar terminal group is more soluble in organic solvents. The ligand 1b is a white solid and is still moderately soluble in CH2Cl2. The ligand 1c has a high fluorophilicity, the partition ratio being 42:1 for the n-C8F18/CH2Cl2 system. The reaction of ligands 1a–c with [PdCl2(CH3CN)2] results in the novel Pd complexes [PdCl2(4,4′-bis-(RfCH2OCH2)-2,2′-bpy)] where Rf=HCF2(CF2)7 (2a), n-C8F17 (2b), n-C10F21 (2c), respectively. The Pd complex 2b is a pale yellow solid, and has been tested unsatisfactorily for FBC. Insoluble in organic solvents, the Pd complex 2c dissolves only in fluorinated solvents, for instance FC77, which is mainly n-C8F18. The novel Pd complex 2c has been tested as a catalyst in Heck reactions under a fluorous biphasic catalysis condition. It was found that the Pd complex 2c, after an easy separation, keeps its catalytic activity (>90% yield), even after seven runs. The TGA studies indicate that the Pd complexes 2a–c are stable up to 330 °C.