Direct Condensation of Carboxylic Acids with Alcohols Catalyzed by Hafnium(IV) Salts

Science 10 November 2000:
Vol. 290. no. 5494, pp. 1140 - 1142

The replacement of current chemical processing with more environmentally benign alternatives is an increasingly attractive subject (1). Esterification is one of the most fundamental and important reactions in organic synthesis [(2), and references cited therein]. Although several methods have been explored and developed [(2), and references cited therein], the use of large amounts of condensing reagents and activators should be avoided in order to promote green chemistry and atom efficiency. The direct condensation of carboxylic acids with alcohols using small amounts of catalysts is the most ideal method, but in most cases, large excess amounts of either carboxylic acids or alcohols are used in this condensation to give esters in high yield (3-18). We describe highly efficient and direct ester condensation with equimolar amounts of carboxylic acids and alcohols catalyzed by hafnium(IV) chloride and alkoxides (19-21).