Combinatorial Catalysis in Asymmetric Synthesis

The importance and practicality of asymmetric synthesis as a tool to obtain optically pure or enantiomerically enriched compounds have been fully acknowledged by chemists in the areas of synthetic organic, medicinal, agricultural, and natural products chemistry and their related industries [64]. Among the asymmetric transformations, the most desirable and also the most challenging is catalytic asymmetric synthesis, where one chiral catalyst molecule can create millions of targeted chiral product molecules in a process that has been termed ''chirality multiplication.'' Thus, asymmetric catalysis represents a significant economic advantage over stoichiometric asymmetric synthesis, and, consequently, the development of new methods for discovering asymmetric catalytic transformations stands as an important and emerging objective in chemistry. Among others, divergent ligand synthesis strategies, where the transformation of an advanced ligand intermediate into a series of different chiral ligands is performed, appear to be especially fruitful in this area. In most cases, the ligand bears the chiral information and, therefore, creates a discriminating chiral environment in close proximity to the active metal site of the catalyst. Asymmetric synthesis is an especially suitable area for combinatorial catalyst discovery since asymmetric catalysis is often mechanistically very complex and critical variables in the parameter space to be evaluated for catalyst development and optimization are mutually dependent. Thus, by rapid parallel synthesis and metal complexation of chiral ligand libraries, coupled to automated or high-throughput screening, multidimensional problems in the discovery and in the optimization of catalysts may be efficiently addressed.

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