Use of achiral and meso ligands to convey asymmetry in enantioselective catalysis.

The field of asymmetric catalysis has progressed rapidly since the early 1980s, as judged by the increasing number of reports each year and the greater frequency with which enantioselective catalysts are employed in industry.1-3 This intense research effort has greatly expanded the variety of reactions that can be performed with good substrate generality and in a highly enantioselective fashion. Despite this remarkable success, most asymmetric catalysts do not meet the criteria required to be attractive to industry, such as high turnover frequency, long catalyst lifetimes, tolerance of air and moisture, and high enantioselectivity. Therefore, new catalysts will need to be developed. The approach to developing new catalysts has not changed greatly since the early days of asymmetric catalysis. In general, chemists rely on their intuition to design and optimize new catalysts. Research in asymmetric catalysis has been driven by the need to develop and optimize enantioselective processes. As a result, investigations into the reaction mechanisms have received less emphasis, and our understanding is not well developed. Further contributing to this deficiency is the complexity exhibited by many catalytic systems. Thus, the era of rationally designed catalysts may be in the distant future. Emerging strategies to develop asymmetric catalysts based on combinatorial methods hold great promise.4-29 Several research groups have successfully synthesized families of ligands and catalysts for use in asymmetric catalysis, while others have used † University of Pennsylvania. ‡ Merck Research Laboratories. 3297 Chem. Rev. 2003, 103, 3297−3344