Some enantioselective borane reductions of prochiral ketones catalysed by polymer-supported oxazaborolidines bound via the boron atom

Abstract

Polystyrenes containing arylboronic acid residues were prepared and treated with (1R,2S)-(–)-norephedrine to give polymer-supported (PS) chiral oxazaborolidines. The latter were investigated as catalysts for the reductions of acetophenone and/or propiophenone by the borane–dimethyl sulfide complex. A variety of reaction conditions was used. The results were compared with those obtained using the non-polymeric analogue of the catalysts prepared by reaction of (1R,2S)-(–)-norephedrine with phenylboronic acid. When used under the best reaction conditions of those investigated (30 mol% of catalyst in tetrahydrofuran at 20 °C) the percentage ees obtained with the better PS catalyst were almost the same as those obtained using the non-polymeric analogue under similar reaction conditions. It was shown that the PS reaction with propiophenone was > 95% complete in 20 min. The PS catalyst could be recycled successfully at least three times.