G. Giffels et al. / Tetrahedron: Asymmetry 9 (1998) 691–696
695
stable conditions until the concentration of active oxazaborolidine drops below a critical value. This
subsequently leads to an observable decrease of conversion as well as enantiomeric excess. Nevertheless
the total turnover number of the oxazaborolidines could be enhanced during the operation time up to
a value of 560 (!) which would be equivalent to a catalyst concentration of only 0.18 mol% in a batch
experiment (entry 4, average ee 91%).
In addition to the higher ttn, the workup of the product is simplified because the catalyst is retained by
the membrane. As the conversion is almost quantitative, evaporation of the solvent followed by simple
distillation afforded the chiral products in high purities (>99%).
In summary it is shown that the use of polymer enlarged oxazaborolidines in a membrane reactor is a
very efficient method for producing chiral alcohols with excellent enantioselectivity, space–time yields
and much higher total turnover numbers for the catalyst than in batch experiments. These advantages
make the use of membrane reactors a very promising technique for homogeneous catalysis in the future.
Investigations on other kinds of catalysts are currently in progress.14 Besides polymer bound catalysts
a dendritic enlargement is also gaining more and more attention. The possible application of membrane
separation for this kind of catalyst has been announced repeatedly.15 First experiments revealed that
retention rates higher than 98% are possible.
Acknowledgements
Part of this work was financially supported by the BMBF. Helpful discussions with our partners within
this project are gratefully acknowledged.
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1
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1
11. The detailed results of our kinetic investigations comprising a kinetic model which allows predictions of conversions and
ees will be published elsewhere.