Mechanism for the Carboxylative Coupling Reaction of a Terminal Alkyne, CO2, and an Allylic Chloride Catalyzed by the Cu(I) Complex: A DFT Study

DFT calculations have been carried out to study the detailed mechanisms for carboxylative-coupling reactions among terminal alkynes, allylic chlorides, and CO2 catalyzed by N-heterocyclic carbene copper(I) complex (IPr)CuCl. The competing cross-coupling reactions between terminal alkynes and allylic chlorides have also been investigated. The calculation results show that a base-assisted metathesis of (IPr)CuCl with PhC CH occurs as the first step to give the acetylide (IPr)Cu-C CPh, from which CO2 insertion and reaction with an allylic chloride molecule, respectively, lead to carboxylative-coupling and cross-coupling reactions. It was found that both the reactions of (IPr)Cu-C CPh and (IPr)CuOCOC CPh (a species derived from CO2 insertion) with an allylic chloride molecule occur through an S(N)2 substitution pathway. The two S(N)2 transition states (calculated for the carboxylative coupling and cross coupling) are the rate-determining transition states and show comparable stability. How the reaction conditions affect the preference of one pathway over the other (carboxylative coupling versus cross coupling) has been discussed in detail.