Combinatorial and Evolution-Based Methods in the Creation of Enantioselective Catalysts.

Combinatorial methods in the development of enantioselective homogeneous catalysts constitute a new branch of catalysis research. The goal is to prepare libraries of potential asymmetric catalysts, rather than choosing the traditional one-catalyst-at-a-time approach. Several conceptional advancements have been reported in the parallel preparation of chiral ligands. Currently the most meaningful systems constitute modularly constructed ligands on solid supports, which allow high degrees of structural diversity and thus the maximum probability of finding enantioselective catalysts or even new types of ligands for asymmetric catalysis. Search strategies have been developed which amongst other things, lead to catalysts not likely to have been discovered by traditional methods. Genuine application of such strategies involve thousands of catalysts and require high-throughput screening systems capable of assaying enantioselectivity. The first high-throughput ee-screening systems were in fact developed for use in the directed evolution of enantioselective enzymes, a process based on "evolution in the test tube" in which the appropriate methods of random mutagenesis, gene expression, and ee assays are combined. Since no screening system is likely to be universal, different approaches are necessary. Thus far these include assays based on UV/Vis, fluorescence, circular dichroism, mass spectrometry, and even modified gas chromatography as well as special forms of capillary electrophoresis. One of the most efficient systems involves the concept of the mass-spectrometric detection of deuterium-labeled pseudo-enantiomers and pseudo-prochiral compounds with which about 1000 exact ee determinations can be achieved per day, although the assay is restricted to kinetic resolution and/or reactions of prochiral compounds bearing enantiotopic groups. Super-high-throughput screening for enantioselectivity is possible in many cases by making use of chirally modified capillary array electrophoresis in a parallel step. Accordingly, 7000 to 30 000 ee determinations can be carried out per day. These and other analytical developments are expected to stimulate further research in the combinatorial search for asymmetric homogeneous catalysts and in the directed evolution of enantioselective enzymes for use in organic chemistry.