Solvent-enhanced coupling of sterically hindered reagents and aryl chlorides using functionalized ionic liquids
A highly efficient and poison-resistant system for the Suzuki coupling reaction based on hydroxyl-functionalized ionic liq. solvents has been established. Coupling of RC6H4X (X = Br, I, Cl) and 2-bromothiophene with R1C6H4B(OH)2 in 2-hydroxyethyl-substituted imidazolium ionic liqs. [1-(HOCH2CH2)-3-Me-2-RC3H2N2]A (1-7; C3H4N2 = imidazole; R, A: 1, H, Cl; 2, H, Br; 3, H, BF4; 4, H, PF6; 5, H, Tf2N; 6, Me, Cl; 7, Me, Tf2N), catalyzed by PdCl2 in the presence of Na2CO3 base gave biphenyls RC6H4C6H4R1 (R = 4-NO2, 4-CN, 4-OMe, H, 2-Me, 4-MeCO; R1 = H, 2-Me) and 2-phenylthiophene, resp., with 51-99% yields. Whereas the yields for aryl bromides and iodides are generally high, aryl chlorides gave moderate to good yields. The ionic liq. plays a crit. role in catalyst/substrate activation directly facilitating "ligand-free" coupling reactions.