Imines metal enolate equivalents

Since then, efficient catalytic asymmetric methods have been developed for the addition of silyl enol ethers or silyl ketene acetals to imines with chiral metal catalysts [29-34], Recently, direct catalytic asymmetric Mannich reactions which do not require preformation of enolate equivalents have appeared. 

In 1973, Mukaiyama and his co-workers reported the use of silyl enol ethers as ketone enolate equivalents. Silyl enol ethers react with aldehydes in the presence of a stoichiometric amount of TiCU as a Lewis acid (Scheme 3-78). The Lewis acid is considered to electrophilically activate aldehydes. Since this landmark discovery, many efforts have been made to improve the original protocol, especially focusing on the use of a catalytic amount of Lewis acid catalysts.A wide variety of metal complexes and nonmetallic cationic compounds is now applicable to this reaction as a catalyst. Not only aldehydes but also acetals, ketones, and imines have been extensively employed as electrophiles for the aldol reactions. The reaction generally proceeds via an acyclic transition state, in which the electron-rich double bond of enol silyl ethers approach carbonyls activated by a Lewis acid (Scheme 3-79). In most cases, acyclic transition state with an antiperiplanar orientation of reactants well accounts for observed diastereoselectivities. ... 

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