Iridium‐Catalyzed Asymmetric Allylic Alkylation of Boron Enolates to Construct Acyclic All‐Carbon Quaternary Stereocenters

Abstract

Enolates are ubiquitous intermediates in organic synthesis. Among them, boron enolates exhibit distinctive reactivity patterns and selectivities due to the presence of a boron atom, making their synthesis highly attractive. Although methods for accessing ketone‐ or ester‐derived boron enolates are well‐developed, much less progress has been made in the development of aldehyde‐derived boron enolates due to aldehydes' high tendency toward self‐condensation. Therefore, the practical applications of aldehyde‐derived boron enolates are significantly hindered. We present herein an efficient method for the preparation of aldehyde‐derived boron enolates via the 1,2‐hydroboration of ketenes with boranes, avoiding the use of acidic R2BCl/R2BOTf and bases and leading to improved functional group tolerance. Utilizing this convenient protocol, we developed an Ir‐catalyzed asymmetric allylic alkylation of boron enolates, yielding a wide array of chiral aldehydes bearing acyclic all‐carbon quaternary centers with high chemo‐, regio‐, and enantioselectivity, which are prevalent in various natural products and bioactive molecules. The synthetic utility and practicality of this method are demonstrated through gram‐scale reactions and asymmetric syntheses of the ent‐5HT1A antagonist as well as biological activity studies in inhibiting the growth of plant pathogens.