Lewis Base-Catalyzed, Lewis Acid-Mediated Reactions (n → σ*)

The tremendous growth of interest in silicon chemistry was initially related to the ability of silicon to break the 'Lewis-Langmuir octet rule'. The chemistry of chiral, hypercoordinate silicon species has attracted increasing attention because of the capability of these compounds to catalyze a variety of stereoselective reactions. This chapter focuses on the interaction between the nonbonding electron pairs of the Lewis base (LB) donor and the o-antibonding orbitals of the silicon-based Lewis acid. When a silicon-based Lewis acid is involved in the formation of an acid-base adduct, a redistribution of the density over the peripheral ligands takes place. Such 'redistribution' results in bond lengthening as the coordination number of the Lewis acid increases with the expansion of the coordination sphere. The scope of Lewis base-catalyzed, Lewis acid-mediated reactions was expanded in 2003 to include stereoselective aldol additions of silyl enol ethers of achiral methyl ketones to aldehydes promoted by SiCl4 and catalyzed by phosphoramides.