Direct Arylation of 2-Methylthiophene with Isolated [PdAr(μ-O2CR)(PPh3)]n Complexes: Kinetics and Mechanism

The palladium-catalyzed direct arylation of aromatic compounds With aryl halides has been proposed to involve an arylpalladium carboxylate intermediate. However, isolated arylpalladium complexes, which undergo C H bond cleavage of aromatic substrates without the aid of additional activators or promoters, have been scarcely documented. This paper reports that [PdAr(mu-O2CR)(PPh3)](n) complexes (1: Ar = Ph, 2-MeC6H4, 2,6-Me2C6H3; R = Me, Bu-t) successfully react with 2-methylthiophene (2) in the absence of additives to afford 5-aryl-2-methylthiophenes (3) in high yields. The reactivity increases with increasing bulkiness of the Ar group, whereas the bulky pivalate ligand (R = Bu-t) reduces the reactivity as compared with the acetate ligand (R = Me). Complex 1 is in equilibrium with the monomeric species [PdAr(O2CR-kappa O-2)(PPh3)] (5) in solution, as confirmed by IR spectroscopy. Kinetic examinations have suggested that the direct arylation proceeds via 5, which undergoes C-H bond cleavage of 2. Complex 1 serves as a good catalyst for direct arylation of 2 with aryl bromides.