Cross-coupling reactions phosphorus-palladium complexes

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

More recently, it has been demonstrated that octaethyl-l,l -diphosphafer-rocene 41 is able to give a stable bis-complex [Pd(41)2] whose structure has been established by X-ray analysis [68]. In this complex, the two parallel lone pairs of one diphosphaferrocene are side-on coordinated to the tetrahedral Pd(0). The bis-chelate 18-electron [PdP4] structure can probably equilibrate with the 14-electron [PdP2] species in which one phosphorus atom of each diphosphaferrocene is axially coordinated to palladium and the other is free. Whatever, [Pd(41)2] is a very robust catalyst for the Suzuki cross-coupling reaction. For example, PhB(OH) and 4-bromoacetophenone are coupled at 110 °C by [Pd(41)2] with a TON of 980,000. 

Mixed phosphorus compounds, bearing at least two different tervalent phosphorus moieties have been developed and have found several applications in catalysis. Iri li described the synthesis of aminophosphine-phosphinite ligands 149 and 150, from the commercially available 3-piperidinemethanol and 3-(methylamino) pro-pan-l-ol. Their palladium and platinum complexes 151 and 152, containing metallocyclic rings, were obtained as air-stable compounds with high yields, and characterized by P, and NMR spectroscopy, FTIR spectroscopy and X-ray crystallography. The Pd complexes were effective catalysts on Suzuki-Miyaura cross-coupling reactions of various aryl chlorides and atyl bromides with phe-nylboronic acid. 

Both the Michaelis-Arbuzov and the Michaelis-Becker reactions are often stereospecific, when compounds containing a chiral phosphorus center are used (retention of configuration), but this property has not been exploited in the synthesis of natural products or their analogs. This is probably a result of often harsh reaction conditions that make them incompatible with the synthesis of more complex, P-chiral molecules. Recently, however, a palladium-catalyzed version of the Michaelis-Becker reaction (a cross-coupling... 

Since the phosphorus atoms of these macrocycles are all included in phosphole rings, they readily invert close to room temperature. The macrocycles 195-197 can therefore adopt their conformations to the stereochemical requirements of the complexed metals. Macrocycles 195 and 196 can chelate either one (198) (M = Mo(CO)4 [166], PdCb [167]) or two (199-200) (M = Mo(CO)4 [166]) metal-containing units via their diagonal phosphorus atoms. The structure of the cage complex 200 shows a Mo-Mo distance of 5.883 A [166]. Life-times of the palladium catalyst based on the cyclophane ligand 195 in Stille cross-coupling and Heck reactions demonstrate extraordinary resistance of the catalyst towards degradation [167]. 

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