Stereoconvergent Access to Z‐Allylborons from E/Z‐Mixed 1,3‐Dienes via Cu‐Guanidine Catalysis

Abstract

Merging the stereoisomeric mixture of substrate into a single product through stereospecific transformation is a challenging but higher‐order synthetic strategy, which perfectly meets the demand of cost control in the precise chemical production. In this work, the stereoconvergent synthesis of Z‐allylic boronates was realized with protoborylation of E/Z‐mixed 1,3‐dienes catalyzed by a novel copper‐guanidine complex. The reaction could proceed smoothly under very mild conditions with good functional group tolerance, and convert diverse aryl‐substituted 1,3‐dienes into the desired Z‐1,4‐addition products with excellent chemo‐, regio‐, and stereoselectivities in minutes. Detailed mechanistic studies also helped to disclose the origin of stereoconvergency. Both E‐ and Z‐diene were found directly undergoing a rapid borylation without E/Z isomerization of C=C bond, followed by a convergent formation of the same thermodynamically stable allylcopper intermediate before the slow protonation step occurred.