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Asymmetric Lewis acid organocatalysis of the Diels–Alder reaction by a silylated C–H acid

Science  26 Feb 2016:
Vol. 351, Issue 6276, pp. 949-952
DOI: 10.1126/science.aae0010

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Silicon marries a chiral counterion

Acid is among the oldest and most versatile of chemical catalysts, but its symmetrical protons can't guide reactions to favor a product over its mirror image. Chemists have resolved this shortcoming through the use of chiral conjugate bases. While the proton activates the substrate, the nearby counterion asymmetrically biases the space around it. Gatzenmeier et al. extend this approach to Lewis acid catalysis by silyl cations, which can activate a variety of substrates in complementary fashion to protons (see the Perspective by Dumoulin and Masson). By pairing these silyl groups with chiral carbon-based anions, they achieve highly enantioselective catalysis of Diels-Alder reactions.

Science, this issue p. 949; see also p. 918

Abstract

Silylium ion equivalents have shown promise as Lewis acid catalysts for a range of important C–C bond-forming reactions. Here we describe chiral C–H acids that upon in situ silylation, generate silylium-carbanion pairs, which are extremely active Lewis acid catalysts for enantioselective Diels–Alder reactions of cinnamates with cyclopentadiene. Enantiomeric ratios of up to 97:3 and diastereomeric ratios of more than 20:1 are observed across a diverse set of substitution patterns with 1 mole percent (mol %) of C–H acid catalyst and 10 mol % of a silylating reagent. The results show promise for broad applications of such C–H acid–derived silylium ion equivalents in asymmetric Lewis acid catalysis.

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