Catalytic enantioselective transformations of borylated substrates: Preparation and synthetic applications of chiral alkylboronates

Abstract

Organoboronic acid derivatives are well-established intermediates for the preparation of alcohols and amines, and in the formation of C–C bonds via different reactions, including homologations, carbonyl allylboration, or transition-metal-catalyzed cross-coupling chemistry. In the past decade, there has been great interest in the development of catalytic enantioselective methods for the preparation of chiral, optically enriched organoboronates as precursors of enantioenriched compounds. While the mainstream strategy remains the late-stage borylation of organic functional groups, our group has focused on an alternate strategy focused on modification of boron-containing substrates. In this way, acyclic and cyclic secondary alkyl- and allyl-boronates were prepared through catalytic enantioselective processes such as [4 + 2] cycloadditions, isomerizations, allylic substitutions, and conjugate additions. The resulting optically enriched boronates have been successfully utilized in the syntheses of complex natural products and drugs. One remaining challenge in the chemistry of secondary alkylboronate derivatives is their cross-coupling, especially with control of stereoselectivity. In this regard, our recent approach featured the conjugate asymmetric borylation of β-boronyl acrylates, providing the first enantioselective preparation of highly optically enriched 1,1-diboronyl derivatives. The chirality of these geminal diboron compounds is conferred through the use of two distinct boronate adducts, which can be coupled chemo- and stereoselectively with a variety of aryl and alkenyl halides under palladium catalysis.