ASYMMETRIC REDUCTION OF ACETOPHENONE WITH CHIRAL POLYMERIC REAGENTS - SUPPORTED SODIUM-BOROHYDRIDE ON CHIRAL AMINO-ALCOHOL BOUND TO A POLYMER

We have shown the possibility of enantiomeric induction in the reduction of prochiral ketones by NaBH4 supported on a chiral support. The supports involved polymers (porous or gel type) containing chiral amino-alcohol groups in the side chain. Three chemical ways were tested to obtain these types of support: (i) chemical modification of styrene divinylbenzene copolymers (type 1); (ii) Merrifield synthesis (type 2); or (iii) polymerization of a functional chiral monomer (type 3). The reduction with type 1 copolymers gives good yield (60%) and enantiomeric excess (ee) (56%). The ee increases with introduction of a spacer between the amino and alcohol functions. The ee obtained for the reduction of acetophenone with type 2 copolymers depend on the method used for coupling the amino-alcohol. With triethyl amine the ee is low (38%) but if the cesium salt method is used ee reaches 60%. The cesium salt method in comparison of triethylamine method has the advantage of avoiding quaternary salt formation, this salt acting with NaBH4 as an ion exchange resin. The best ee (75%) are obtained with supports synthesized by polymerization of a chiral functional monomer. For these supports the yield depends on the texture: 20% for porous support and 50% for gel type support. In comparison model compounds in homogeneous condition lead to very low ee (10%).