Anion Induction

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Science  09 Jun 2006:
Vol. 312, Issue 5779, pp. 1443
DOI: 10.1126/science.312.5779.1443b

In traditional asymmetric catalysis, a chiral catalyst binds directly to the reagent and thereby facilitates the reaction path to one product isomer while hindering the path to its enantiomer, or mirror image. Mayer and List show that asymmetric induction can also arise from pairing of an achiral cationic catalyst with a chiral counterion. Chiral amine derivatives can promote transfer hydrogenation of α,β-unsaturated aldehydes. The authors have now probed the same reaction class using protonated morpholine, an achiral amine, in combination with binaphthol-based chiral phosphate anions. This catalyst system is particularly effective for aromatic substrates, yielding product distributions that favor one enantiomer by 98:1 or higher ratios. Sterically unhindered aliphatic substrates, such as citral and farnesal, are also reduced in high enantiomeric excess. Because the reaction proceeds in aprotic solvent and requires a secondary, rather than tertiary, amine salt catalyst, the authors propose that induction occurs via an ion pair between the phosphate and an iminium intermediate, formed by amine displacement of the aldehyde oxygen. — JSY

Angew. Chem. Int. Ed. 45, 10.1002/anie.200600512 (2006).

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