B(C6F5)3‐Catalyzed Reduction of Ketones and Imines Using Silicon‐Stereogenic Silanes: Stereoinduction by Single‐Point Binding

We recently employed a silicon‐stereogenic silane as astereochemical probe to clarify the mechanism of the B(C6F5)3‐catalyzed hydrosilylation of ketones. When using a prochiral ketone, reasonable stereoinduction was seen, originating from the stereogenicity at the silicon atom, a chirality transfer from silicon to carbon through single‐point binding of the chiral silane to the carbonyl oxygen atom. In the present investigation, we further elaborated on this remarkable observation by systematic variation of the ketone substitution pattern. We then included prochiral imines as well to test for diastereocontrol. Unexpectedly, these substance classes, ketones and imines, yielded diametrically opposed results in the reduction with a silicon‐stereogenic silane. While the level of diastereoselection was decent in the C=O reduction (dr ≈ 80:20), no asymmetric induction was detected in the C=NR reduction. On the basis of these experimental data and our previous mechanistic insight, we propose different reaction pathways for the reduction step of these related B(C6F5)3 catalyses. Aside from these mechanistic implications, we also report an unusual 1,6‐ rather than conventional 1,2‐reduction of a sterically encumbered diaryl‐substituted ketone.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) Probed with silicon: Chirality at Si induces decent diastereoselectivity in the B(C6F5)3‐catalyzed carbonyl reduction whereas no stereoinduction is observed in the related imine reduction. Mechanisms of action are suggested for the irreversible, stereochemistry‐determining hydride transfer from a borohydride. Moreover, an unsual 1,6‐reduction with a borohydride is disclosed for a sterically congested ketone.