ABSTRACT
Enantioselective syntheses using chiral catalysts have been the object of extensive research for a number of years (1). Most of this work has used soluble chiral organometallic complexes to promote a variety of synthetically useful reactions. Since these catalysts are molecular species they have a single 'active site' surrounded by an appropriate chiral pocket defmed by the ligands present in the complex. With the appropriate chiral ligand such species are usually very selective, frequently giving products having enantiomeric excesses (ee's) in the 95%-99% range. These catalysts, being in solution with the reaction products, can be difficult to separate from the reaction products and once separated are not easily reconstituted for potential re-use. From an economic standpoint, this loss of the catalyst can be appreciable, not so much because of the loss of the metal but, more importantly, the chiral ligand which can be many times more expensive than the metal. One method for minimizing this factor has been to use these soluble catalysts with a very high substrate/catalyst ratio with the catalyst frequently being present at only a few hundred ppm This low catalyst load, however, can lead to prolonged reaction times.
