This proposal describes the design and development of small-molecule, chiral catalysts that generate highly reactive intermediates and engage them in stereoselective reactions through networks of non-covalent interactions. The underlying principle in this effort is a new mechanistic framework wherein hydrogen-bond donors induce ionization of neutral substrates by abstracting and binding weakly basic, anionic leaving groups. The resulting cationic intermediate can be induced to undergo enantioselective reactions through a combination of electrostatic, H-bonding, and other weak interactions within the chiral ion pair. The anion binding catalysis concept provides a general approach to enantioselective catalytic reactions of cationic intermediates such as oxocarbenium ions, iminium ions, unstabilized carbocations, halonium ions, aromatic heterocyclic cations, and radical cations. One crucially important aim will be to answer fundamental mechanistic questions relating to anion-binding catalysis. Primary among these are a clear understanding of the mechanism of anion abstraction, the relationship between the kinetics of abstraction and the thermodynamic stability of the resulting catalyst-anion complexes, and the manner in which these two characteristics influence outcomes in enantioselective catalytic reactions. The mechanistic insights drawn from this work will be applied to catalyst-controlled glycosylation reactions, wherein diastereoselectivity in additions to sugar-derived oxocarbenium ions will be sought through the agency of chiral H-bond-donor catalysts. In a second aim, the stereocontrolled synthesis of chlorinated compounds is addressed through the identification of methods for enantioselective, catalytic mono- and di-chlorination reactions. Chiral H-bond donor-chloride complexes will be generated in association with highly reactive carbocationic or chloronium ion intermediates. Collapse of these ion pairs will result in the formation of the desired chlorinated products in enantioenriched form. In a third aim, the goal will be to generate and engage delocalized cations in stereocontrolled reactions, through the development of a catalytic method for enantioselective cycloadditions to oxidopyrylium ions.
The fourth aim i s directed toward the discovery of methods for controlling enantioselectivity in electron hole-catalyzed reactions. This effort will involve the design and application of H-bond donor catalysts that bind quinone derivatives and thereby increase their oxidation potential. The resulting complex can promote one-electron oxidation of organic substrates, and induce enantioselective reactions of the resulting radical cation intermediates.
The chemical methods discovered in the proposed research will facilitate the discovery and lower the cost of production of new medicines. These efforts will uncover catalysts that will be used in both industry and academia for the synthesis of biologically important compounds, and will advance the field of mechanistic chemistry by developing a more complete and predictive understanding of the interactions that are responsible for selectivity.
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