How Does an Earth-Abundant Copper-Based Catalyst Achieve Anti-Markovnikov Hydrobromination of Alkynes? A DFT Mechanistic Study

Abstract

The first catalytic hydrohalogenation of alkynes was recently achieved using a copper(I) N-heterocyclic carbene (NHC) complex, and the reaction was found to be syn and anti-Markovnikov selective. The present work is a density functional theory (DFT) computational study (B3LYP and M06) on the detailed mechanism of this remarkable catalytic reaction. The reaction begins with a phenoxide additive turning over the precatalyst (NHC)CuCl into (NHC)Cu(OAr), which subsequently transmetalates with the hydride source Ph2SiH2 to deliver the copper(I) hydride complex (NHC)CuH. (NHC)CuH undertakes hydrocupration of the substrate RC≡CH via alkyne coordination and subsequent migratory insertion into the Cu–H bond, forming (E)-(NHC)Cu(CH═CHR). The migratory insertion step determines the syn selectivity because it occurs by a concerted pathway, and it also determines the anti-Markovnikov regioselectivity that arises from the charge distributions across the Cu–H and C≡C bonds. The brominating agent (BrCl2C)2 uses the bromo...