Application of nanofiltration to re-use the sharpless asymmetric dihydroxylation catalytic system

In this paper, successive batch steps of an osmium-catalyzed asymmetric dihydroxylation reaction using Sharpless conditions and nanofiltration of post-reaction mixture were coupled, allowing us to enhance the cumulative catalyst turn over number to about 3.7 times over six cycles. The nanofiltration step provides for isolation of the chiral product, whereas the catalytic system (osmium and chiral ligand) is re-used in the following batch cycle. In this work the osmium average rejection through the selected nanofiltration membrane, Starmem™120, was 83%; this result may indicate the existence of free osmium in solution and implies residual product osmium contamination at an average value of 1.5 mg Os/g-product. Effective application of this methodology to the model reaction requires improvement of catalyst rejection, which calls for an effective complexation of osmium by the ligand. Nevertheless, the enantioselectivity of the reaction was maintained constant over the six cycles at a value of 69%.