Synthesis of a Water‐Stable 2, 4‐Bis(trifluoromethyl)phenyl‐Substituted Phosphonous Acid Palladium Complex and its Catalytic Activity in Cross‐Coupling Reactions

This manuscript describes the selective synthesis of2, 4‐bis(trifluoromethyl)phenylphosphinic acid, ( R f )P(O)(H)OH.Reaction of chlorobis(diethylamino)phosphane with a mixture of 2, 4‐(CF 3 ) 2 C 6 H 3 Li and 2, 6‐(CF 3 ) 2 C 6 H 3 Li results in the formation of 2, 4‐bis(trifluoromethyl)phenylbis(diethylamino)phosphane, ( R f )P(NEt 2 ) 2 . The following in situ reaction with gaseous HBr leads to the cleavage of the phosphorus nitrogen bonds resulting in the selective formation of the corresponding dibromophosphane ( R f )PBr 2 .2, 4‐Bis(trifluoromethyl)phenylphosphinic acid is available via two different synthetic strategies starting from the dibromophosphane ( R f )PBr 2 . Reduction of the dibromophosphane with tributyltin hydride affords selectively the phosphane ( R f )PH 2 . The following oxidation to the phosphinic acid ( R f )P(O)(H)OH, however, only proceeds in moderate yields. A more efficient synthesis is achieved via the hydrolysis of the dibromophosphane ( R f )PBr 2 in dichloromethane solution. 2, 4‐bis(trifluoromethyl)phenylphosphinic acid, ( R f )P(O)(H)OH, is surprisingly stable towards air and water. The water soluble phosphinic acid reacts sluggishly with a 30 % H 2 O 2 solution, yielding only 23 % of the corresponding phosphonic acid ( R f )P(O)(OH) 2 after seven days at ambient temperature.In principle a tautomerization of the phosphinic acid ( R f )P(O)(H)OH to the corresponding phosphonous acid ( R f )P(OH) 2 – analogously to the phosphane oxide ( R f ) 2 P(O)H and the phosphinous acid ( R f ) 2 POH – is conceivable. However, the phosphonous acid could not be detected with spectroscopic methods. The energy difference between the phosphinic acid and the phosphonous acid is calculated by quantum chemical calculations at the B3LYP/6‐311+G(2df, p) level of theory to 30.1 kJ · mol –1 . This high value explains why the phosphonous acid tautomer could not be detected by spectroscopic methods so far.As transition metal complexes featuring phosphonous acid ligands are of significant interest for the application as homogeneous catalysts we studied the coordination chemistry of the related phosphinic acid in more detail. The preligand 2, 4‐bis(trifluoromethyl)phenylphosphinic acid reacts smoothly with palladium dichloride in THF to the dinuclear complex [Pd 2 (μ‐Cl) 2 {[2, 4‐(CF 3 ) 2 C 6 H 3 P(OH)O] 2 H} 2 ], featuring a phosphonous acid phosphonato unit, with a strong hydrogen bridge substantiated by an O ··· O distance of 247.7(3) pm. The dinuclear phosphonous acid palladium complex exhibits a high catalytic activity in the Suzuki cross‐coupling. Alternatively, the phosphinic acid can be used as preligand for the in situ formation of a phosphonous acid palladium complex. These compounds combine catalytic activity with a very high stability towards air and water.