Microgel-supported Oxazaborolidines: Novel Catalysts for Enantioselective Reductions

Microgel-bound oxazaborolidines have been prepared and used as catalysts in the enantioselective reduction of prochiral ketones. The preparation of these soluble, crosslinked polymer molecules was accomplished by solution polymerisation. The approach involved the preparation of microgels bearing free boronic acid functionalities and their subsequent conversion to oxazaborolidines. The selectivities of these supported catalysts are in most cases comparable to those achieved with low molecular weight analogues. The advantage of the application of microgel-bound catalysts is their good solubility together with low viscosity of the solution. Reagents of this type can easily be removed by ultrafiltration and the process can be performed in a continuous mode in a membrane reactor.