Abstract An efficient insertion reaction of α-diazoketone to various carboxylic acids was achieved by using Cu(acac) 2 as a catalyst. Treatment of the diazo compound with a carboxylic acid (1.2 equiv) in the presence of Cu(acac) 2 (0.1 equiv) at room temperature afforded the corresponding ketoester in good yield. Various kinds of functional groups were tolerated under the reaction conditions.