Zn(OTf)2-Promoted Chemoselective Esterification of Hydroxyl Group Bearing Carboxylic Acids

Selective esterification of aliphatic and aromatic carboxylic acids with various alcohols is studied using triphenylphosphine, I-2, and a catalytic amount of Zn(OTf)(2). Use of this catalyst allows the formation of esters at a faster rate with good to excellent yield by activating the in situ generated acyloxyphosphonium ion intermediate. During the esterification process, both their aromatic and aliphatic hydroxyl groups are fully preserved from trans-esterification. The results show that the bulkiness and the reactivity of this doubly activated intermediate III control the selectivity and the rate of the reaction, respectively. The method is also useful for direct amidation reactions.