Preparation of monodentate phosphinite ligands: their applications in palladium catalyzed Suzuki reactions

Two phosphinites 2,6-OMe, 4-Me, 1-OPR₂C₆H₂ (5a: R=Ph; 5b: R=^tBu) were prepared in good yields. Two methoxy groups are placed on the 2,6 positions of the phosphinites deliberately thereby to avoid a plausible orthometallation process while coordinating toward palladium metal. Further reaction of 5a with PdCl₂ in the ratio of 2:1 and 1:1 gave 5a ligated palladium complexes {(5a)PdCl(μ-Cl)}₂7a and {(5a)₂PdCl₂} 8a, respectively. As revealed from their crystal structures, the former is a dimeric complex with anticipated molecular arrangement; nevertheless, the latter is a monomeric complex with unexpected, energetically unfavorable cis form. By contrast, only dimeric form was observed from the reaction of 5b with PdCl₂. It is believed that the bulky substituents, ^tBu, on 5b prevent the formation of a monomeric complex in cis form. Fair to good efficiencies were observed for the Suzuki-Miyaura cross-coupling reactions employed in situ-prepared 5/Pd(OAc)₂ as the catalytic precursor.