The mechanism of directed remote asymmetric reduction of carbonyl groups via homochiral boronate esters

Abstract

In order to determine whether the remote asymmetric induction in the reduction of compounds such as 1 and 2 using borane is really controlled by intramolecular chelates of type 3, rather than dioxaborolane oxygen-borane chelates of type 12, a study was undertaken to examine related reductions involving the corresponding homochiral acetal 30 and comparative reductions of the dioxaborolane 14 and the acetal 23b. While this study showed that reductions of the dioxaborolane 14 and the acetal 23b with borane and L-Selectride were virtually identical, this result did not necessarily indicate that dioxaborolane oxygens or acetal oxygens were directing borane reduction. However, that the more likely explanation for the remote asymmetric induction observed for 1 and 2 being mediated by complex 3 was confirmed by the fact that the acetal 30 gave no asymmetric induction with borane. A crystal structure of the phenylboronate ester 10 has been carried out.