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