Chiral phosphoramide-catalyzed aldol additions of ketone enolates. Preparative aspects

Trichlorosilyl enolates of ketones (enoxytrichlorosilanes) were demonstrated to be highly reactive aldol addition reagents. Trichlorosilyl enolates of cyclohexanone (E-enolate) and propiophenone (Z-enolate) reacted readily at room temperature with a wide variety of aldehydes to afford aldol addition products in high yield and diastereoselectivity (E --> syn, Z --> anti). These reactions were shown to be highly susceptible to acceleration by catalytic quantities of chiral phosphoramides. In particular, a phosphoramide derived from (S,S)-stilbenediamine was remarkably effective not only in accelerating the reaction but also in modulating the diastereoselectivity and in providing the aldol addition products in good to excellent enantioselectivity. The diastereoselectivity of the unpromoted process has been interpreted as a consequence of reaction via a pentacoordinate, trigonal bipyramidal (tbp) silicon complex through a boatlike transition structure. The phosphoramide-catalyzed reactions are more complicated and are believed to proceed via hexacoordinate, octahedral complexes through chairlike transition structures. A systematic examination of the influence of solvent, concentration, addition rate, and catalyst loading on rate and stereoselectivity is also described.