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_N2 substitution pathway. The two S_N2 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