The aim of this research program is the design and study of soluble transition metal complexes that enantioselectively catalyze synthetically valuable transformations of organic compounds. Asymmetric catalysts can provide the most efficient and practical routes to biologically active optically pure materials, and the need for enantiomerically pure materials calls for highly stereoselective catalysts for a broad range of substrates. This proposal focuses on the development and study of chiral salicylidenaminato (salen)-based epoxidation catalysts. Initial studies have revealed that salen-based systems provide practical access to chiral epoxides from a unfunctionalized olefins with very good to moderate selectivity. Strategies to increase enantioselectivity to very high levels are outlined based on a proposed highly predictive stereochemical model for oxygen atom transfer from metals to olefins. The model will be thoroughly tested using crystallographic and solution phase characterization of catalyst precursors and reactive intermediates. Correlation of substrate and catalyst structure with observed enantioselectivities can then permit valuable insight into the transition structure geometries of oxygen-atom-transfer from metal to olefin. The chiral salen complexes are very accessible through simple high-yield synthetic procedures, and this allows screening of a variety of potential catalysts. Strategies are described for systematic tuning of the steric properties of the salen ligands, including the preparation of sterically hindered salicylaldehyde derivatives and the synthesis of a variety of new C2-symmetric chiral diamines. Catalyst lifetimes will be maximized through variation of ligand and metal, and identification of the decomposition pathways under carefully controlled reaction conditions. The synthetic utility of the existing and newly developed catalysts will be illustrated through reactions on a series of unfunctionalized and highly complex substrates, and through the execution of large scale asymmetric epoxidations that afford valuable chiral products from simple olefins. With proper development, chiral salen based systems will provide previously inaccessible chiral building blocks and end-products of great value to pharmaceutical synthesis. The possible utility of existing and newly developed salen based complexes as asymmetric catalysts in other reactions will also be examined. Targeted applications include chlorohydrin cyclization, sulfide oxidation, and Lewis acid-promoted alkylations and cycloadditions of carbonyl compounds.
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