Synthesis and reactivity of a cationic palladium complex as possible intermediate in a Suzuki-Miyaura cross-coupling reaction

Abstract

A stoichiometric reaction of a palladium fluorido complex with an aromatic boronic ester yielded a fluorinated 4-aryl phenylalanine derivative (aryl=4-C6H4SF5) as Suzuki-Miyaura cross-coupling product. Due to the high reactivity of the metal complex, the reaction proceeded smoothly at ambient temperature. Low-temperature NMR investigations revealed a possible role of trans‐[Pd{BF(4-C6H4SF5)(pin)}(PheEt)(PiPr3)2] (5, PheEt=bound phenylalanine derivative=4-C6H4CH2C{NHC(O)CH3}(CO2Et)2, pin=pinacolato=O2C2Me4) as a potential intermediate of the transmetallation step. Compound 5 resulted from fluoride transfer from palladium to boron. The similar complex trans-[Pd(BF4)(PheEt)(PiPr3)2] (7) was generated on treatment of the fluorido complex with NaBF4. Complex 7 is not stable at room temperature. Degradation gave the phosphonium salt [PiPr3PheEt][BF4] (4). Interestingly, the same compound was also found in the initially mentioned cross-coupling reaction as minor product.