Application of Tautomerism of Ferrocenyl Secondary Phosphine Oxides in Suzuki−Miyaura Cross-Coupling Reactions

Several new ferrocenyl secondary phosphine oxides, (η⁵-C₅H₄-P(O)(Ph)(H))₂Fe (7aB) and (η⁵-C₅H₄-P(O)(R)(H))(η⁵-C₅H₅)Fe (7M, a: R = Ph; c: R = Cy), were prepared and characterized by spectroscopic means. Further reaction of Pd(COD)Cl₂ with 2 molar equiv of (η⁵-C₅H₄-P(O)(R)(H))₂Fe (7bM, b: R = ^tBu) led to the formation of the palladium dimer [(7b′M)(7b′M-H⁺)Pd(μ-Cl)]₂, 8. In addition, the reaction of Pd(OAc)₂ with 1 molar equiv of 7bM yielded a bis-7bM-coordinated palladium acetate, [(7b′M)(7b′M-H⁺)Pd(OAc)], 9. The molecular structures of 7aB, 7aM, 7bM, 8, and 9 were determined by single-crystal X-ray diffraction methods. A tautomeric equilibrium between 7aB and its isomeric form (η⁵-C₅H₄-P(OH)(Ph))₂Fe (7a′B) indeed took place. This was also true for the case of 7aM and its isomeric form (η⁵-C₅H₄-P(OH)(Ph))(η⁵-C₅H₅)Fe (7a′M). Palladium-catalyzed Suzuki−Miyaura reactions employing 7aB and 7bM as ligands gave satisfactory results.