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)}2 7a 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.