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.