ASYMMETRIC REDUCTION OF ACETOPHENONE WITH CHIRAL POLYMERIC REAGENTS - SUPPORTED SODIUM-BOROHYDRIDE ON CHIRAL QUATERNARY AMMONIUM BINDED 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 involve polymers (porous or gel type) containing chiral quaternary ammonium groups in the side chain. Two chemical ways were tested to obtain these types of support, chemical modification of styrene divinylbenzene copolymers [polymers supported chiral alcohol quaternary ammonium salts (type 1)] or 4-vinylpyridine divinylbenzene copolymers [polymers supported 1,2,3,4-tetrahydro-N,N-dialkyl pyridinium salts (type 2)]. The reduction with type 1 copolymers gives good yield and enantiomeric excess (ee) (50%), the kinetic of the reduction is governed by the accessibility of the reactant then by the texture of the support and the presence of a spacer ar-m. The reduction with type 2 copolymers gives also good results (yield 70%, ee 56% in the best case). The kinetic is also governed by site accessibility, on the contrary ee is function only of the chemical aspect of the chiral centre.