CHIRAL OXAZABOROLIDINES BOUND VIA THE BORON ATOM TO POLYMERS PREPAREDUSING 2-VINYLTHIOPHENE - POLYMER-SUPPORTED CATALYSTS FOR THE ENANTIOSELECTIVE REDUCTION OF PROCHIRAL KETONES BY BORANE

Polymer-supported chiral oxazaborolidines bound to the polymer via the boron atom, have been prepared from a series of crosslinked polymers synthesized using 2-vinylthiophene. The polymer-supported oxazaborolid ines were used to catalyse the reduction of prochiral ketones using th e borane-dimethyl sulfide complex, a reduction which also occurs in th e absence of catalyst to give racemic products. Using the catalysts in this competitive situation together with measurement of the enantiome ric excesses (ees) obtained provides a sensitive way of comparing the performances of the various polymer supports. Best results were obtain ed using lightly crosslinked polymers prepared using approximately equ imolar amounts of 2-vinylthiophene and styrene. However, the ees obtai ned using even these catalysts were significantly smaller than those o btained using the non-polymeric analogue of the catalysts. This is mos t probably due to diffusion barriers limiting the access of the ketone and/or the borane to the catalytic sites on the polymers.