CHIRAL OXAZABOROLIDINES BOUND VIA THE BORON ATOM TO POLYMERS PREPARED USING 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 oxazaborolidines were used to catalyse the reduction of prochiral ketones using the borane-dimethyl sulfide complex, a reduction which also occurs in the absence of catalyst to give racemic products. Using the catalysts in this competitive situation together with measurement of the enantiomeric excesses (ees) obtained provides a sensitive way of comparing the performances of the various polymer supports. Best results were obtained using lightly crosslinked polymers prepared using approximately equimolar amounts of 2-vinylthiophene and styrene. However, the ees obtained using even these catalysts were significantly smaller than those obtained using the non-polymeric analogue of the catalysts. This is most probably due to diffusion barriers limiting the access of the ketone and/or the borane to the catalytic sites on the polymers.