Palladium-Catalyzed Sonogashira Reaction for the Synthesis of Arylalkynecarboxylic Acids from Aryl Bromides at Low Temperature

A variety of aryl bromides were coupled with propiolic acid in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and catalyst Pd(PPh3)(4) to afford the corresponding arylalkynecarboxylic acids in good yields at low temperature. This method showed good tolerance toward functional groups such as alkoxy, ketone, ester, aldehyde, cyano, nitro, and hydroxy. Under these conditions, propiolic acid showed higher reactivity than any other compound containing terminal alkyne groups. According to the mechanistic studies, the key reaction step for the high reactivity of propiolic acid might be ligand exchange between the acetylide and bromide at palladium, and/or reductive elimination, but not the oxidative addition step.