Stereoselective reduction of ketones using extracoordinate silicon: C-2-symmetric ligands

Several C-2-symmetric diimidazole ligands were prepared by forming amides o r imines from suitably functionalized imidazole derivatives. Extracoordinat e silanes were formed by the addition of imidazolide anions to trialkoxysil anes; neutral imidazoles did not lead to observable extracoordination as ju dged by Si-29 NMR. The extracoordinate hydrosilanes enantioselectively redu ce prochiral ketones with good yield and moderate to good stereoselectivity (up to 70% e.e.). However, the induction observed with these C-2-symmetric ligands was comparable to that observed with catalysis by simple amino aci ds. The mechanistic implications of these observations are discussed. (C) 1 999 Elsevier Science S.A. All rights reserved.