Lewis Base Activation of Lewis Acids in Asymmetric Aldol Reactions of Silyl Ketene Acetals

323 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.The concept of Lewis base activation of Lewis acids has been effectively reduced to practice for catalysis in the chiral phosphoramide catalyzed/SiCl 4 promoted aldol reaction of silyl ketene acetals and silyl dienol ethers with aldehydes. The weakly acidic species, silicon tetrachloride (SiCl 4), is activated by binding of a strongly Lewis basic chiral phosphoramide, leading to in situ formation of a chiral silyl cation. This species has proven a competent catalyst for the aldol addition of acetate-, propanoate- and isobutyrate-derived silyl ketene acetals to conjugated and non-conjugated 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. Kinetic studies undertaken using ReactIR spectroscopy show that the reaction displays saturation kinetics in SiCl4. This suggests that the phosphoramide bound silyl cation complex is the catalyst resting state. Spectroscopic studies have garnered support for this conclusion through the observation of the phosphoramide bound silyl cation complex by 29Si NMR.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD