Selective Acylation of Phenols in Boron Trifluoride Diethyl Etherate Solution and the Mechanistic Implication

In the presence of boron trifluoride diethyl etherate (BF3•OEt2), direct acylation of phenols with free carboxylic acid is chemoselective and regioselective and no demethylation, if any, was observed.The para-directing effect of BF3•OEt2 is attributed to the large steric hindrance of the boron trifluoride-phenolic hydroxyl group complex, which blocks the ortho-acylation from occurrence.Microwave irradiation could not change the regioselectivity of BF3•OEt2 except the reaction time being greatly shortened.