Stereoselective reduction of ketones using extracoordinate silicon: C2-symmetric ligands

Several C2-symmetric diimidazole ligands were prepared by forming amides or imines from suitably functionalized imidazole derivatives. Extracoordinate silanes were formed by the addition of imidazolide anions to trialkoxysilanes; neutral imidazoles did not lead to observable extracoordination as judged by 29Si NMR. The extracoordinate hydrosilanes enantioselectively reduce prochiral ketones with good yield and moderate to good stereoselectivity (up to 70% e.e.). However, the induction observed with these C2-symmetric ligands was comparable to that observed with catalysis by simple amino acids. The mechanistic implications of these observations are discussed.