Michael reactions catalysed by polymer-supported quaternary ammonium salts derived from cinchona and ephedra alkaloids

Abstract

Reaction of chloromethylated crosslinked polystyrenes with cinchonidine, cinchonine, or (–)-N-methylephedrine gave polymer-supported chiral quaternary ammonium chlorides. The hydroxide, hydrogen carbonate, fluoride, and chloride forms of these polymers, and also of a commercial polymer containing achiral quaternary ammonium groups, catalyse one or more of the following Michael reactions: (1) The addition of methyl 1-oxoindan-2-carboxylate to methyl vinyl ketone, (2) the addition of ethyl 2-oxo-cyclohexanecarboxylate to methyl vinyl ketone, and (3) the addition of thio-p-cresol to cyclohex-2-enone. Catalysts prepared from cinchonidine achieve optical yields of ⩽27% of the S-adduct from reaction (1), whilst those catalysts prepared from cinchonine achieve optical yields of ⩽13% of the R-adduct. This is one of the few instances where the use of polymer-supported chiral quaternary ammonium salts has given substantial optical yields. With reactions (2) and (3) the optical yields were all <2.5%. The catalysts are chemically fragile but, at least with reaction (1), can be re-used several times.