Remarkable Ligand Effect of P(2-MeOC6H4)3 on Palladium-Catalyzed Direct Arylation

Palladium-catalyzed direct arylation polymerization (DArP) has recently attracted much attention as a simple and easy means of synthesizing p-conjugated polymers. We have reported a novel catalytic system for DArP. While the already known catalytic systems for direct arylation commonly require the use of a highly polar solvent such as DMA, our system is sufficiently reactive in less polar THF and toluene. A key to this unique catalytic property is the use of P(2-MeOC6H4)(3) (L1), having o-MeO substituents as the supporting ligand. In this paper, we examine the role of L1 through the isolation of presumed catalytic intermediates. The arylpalladium acetate complexes [PdAr(OAc)(L1)]n (Ar = Ph (1a), 2,6-Me2C6H3 (5b)) are prepared by oxidative addition of ArBr to [Pd{P(o-tolyl)3}2] in the presence of L1, followed by anionic ligand exchange with AgOAc. X-ray diffraction analysis reveals that 1a is a dimeric compound (n = 2) bridged by two types of mu-OAc ligands, whereas 5b is a monomeric species (n = 1) coordinated with PO-chelating L1 (L1-kappa(PO)-P-2). Although the solid-state structures are different, both complexes form an equilibrium mixture of two kinds of monomeric compounds in solution, [PdAr(OAc-kappa(O))(L1-kappa(PO)-P-2)] (5) and [PdAr(OAc-kappa O-2)(L1-kappa(P))] (2). The equilibrium ratio of 5 to 2 is nearly independent of Ar ligands, and the reaction rate with 2-methylthiophene (3) is little affected by Ar ligands and solvents (toluene, THF, and DMA). These tendencies make a sharp contrast to those observed for the PPh3 analogues [PdAr(OAc)(PPh3)]n (Ar = Ph, 2,6-Me2C6H3), whose reactivity changes with Ar ligands and solvents remarkably. A mechanistic understanding of the unique ligand effect of L1 is presented.