Stereoconvergent Access to <i>Z</i>‐Allylborons from <i>E</i>/<i>Z</i>‐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.