Polymer-bound ethylenediamine-borane reagent: A new class of polymeric reducing agent

A polymer-supported ethylenediamine (EDA)-borane reagent was prepared under mild reaction conditions. The reagent was found to be efficient and selective in the reduction of aldehydes to the corresponding alcohols in high yields at room temperature. The polymeric reagent was prepared on a Merrifield resin and on a 1,6-hexanediol diacrylate-crosslinked polystyrene resin (HDODA-PS). HDODA-PS was prepared by suspension polymerization and chloromethylated by a Friedel-Crafts reaction. The preparation of the polymer-bound EDA-borane reagent involves a series of polymer analogous reactions, such as amination of the chloromethyl resin, conversion to amine hydrochloride, and, finally, complexation with sodium borohydride. The reagent exhibited good chemoselectivity in the reduction of aldehydes. The reduction reactions were carried out with a 2 molar excess of the polymeric reagent. In the competitive reduction of a 1:1 molar mixture of benzaldehyde and acetophenone, benzaldehyde was selectively reduced to benzyl alcohol. The spent resin could be recycled by treating with sodium barohydride. The recycled resin was used several times without much loss in chemical reactivity and mechanical stability. The HDODA-PS resin was found to be more efficient than was the Merrifield resin in the preparation of the reagent as well as in the reduction reactions. (C) 2001 John Wiley & Sons, Inc.