Lewis Base Activation of Lewis Acids:  Catalytic, Enantioselective Addition of Silyl Ketene Acetals to Aldehydes

The concept of Lewis base activation of Lewis acids has been reduced to practice for catalysis of the aldol reaction of silyl ketene acetals and silyl dienol ethers with aldehydes. The weakly acidic species, silicon tetrachloride (SiCl4), can be activated by binding of a strongly Lewis basic chiral...

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Bibliographic Details
Published in:Journal of the American Chemical Society 2005-03, Vol.127 (11), p.3774-3789
Main Authors: Denmark, Scott E, Beutner, Gregory L, Wynn, Thomas, Eastgate, Martin D
Format: Article
Language:English
Subjects:
Chemistry
Exact sciences and technology
Organic chemistry
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Summary:The concept of Lewis base activation of Lewis acids has been reduced to practice for catalysis of the aldol reaction of silyl ketene acetals and silyl dienol ethers with aldehydes. The weakly acidic species, silicon tetrachloride (SiCl4), can be activated by binding of a strongly Lewis basic chiral phosphoramide, leading to in situ formation of a chiral Lewis acid. This species has proven to be a competent catalyst for the aldol addition of acetate-, propanoate-, and isobutyrate-derived silyl ketene acetals to conjugated and nonconjugated aldehydes. Furthermore, vinylogous aldol reactions of silyl dienol ethers are also demonstrated. The high levels of regio-, anti diastereo-, and enantioselectivity observed in these reactions can be rationalized through consideration of an open transition structure where steric interactions between the silyl cation complex and the approaching nucleophile are dominant.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja047339w