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Chiral Organic Ion Pair Catalysts Assembled Through a Hydrogen-Bonding Network

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Science  02 Oct 2009:
Vol. 326, Issue 5949, pp. 120-123
DOI: 10.1126/science.1176758

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Catalytic Assembly

Most asymmetric catalysts employed in organic chemistry are assembled through binding of chiral ligands to one or more metal ions. Uraguchi et al. (p. 120, published online 27 August) show that a highly selective catalyst can assemble through hydrogen bonding from a small collection of small molecules in the absence of metals. In the solid state, the catalyst comprises a central chiral phosphonium cation bound to two phenol molecules, which in turn bind a charge-compensating phenoxide ion. In solution, an oxazolone derivative appears to be able to bind in place of the phenoxide, spurring its stereoselective addition to a broad range of esters.

Abstract

Research to develop structurally discrete, chiral supramolecular catalysts for asymmetric organic transformations has met with limited success. Here, we report that a chiral tetraaminophosphonium cation, two phenols, and a phenoxide anion appear to self-assemble into a catalytically active supramolecular architecture through intermolecular hydrogen bonding. The structure of the resulting molecular assembly was determined in the solid state by means of x-ray diffraction analysis. Furthermore, in solution the complex promotes a highly stereoselective conjugate addition of acyl anion equivalents to α,β-unsaturated ester surrogates with a broad substrate scope. All structural components of the catalyst cooperatively participate in the stereocontrolling event.

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