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(μ-O2CR)­(PPh3)]n complexes (1: Ar = Ph, 2-MeC6H4, 2,6-Me2C6H3; R = Me, tBu) 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 = tBu) reduces the reactivity as compared with the acetate ligand (R = Me). Complex 1 is in equilibrium with the monomeric species [PdAr­(O2CR-κ2O)­(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.