Mechanism for the Carboxylative Coupling Reaction
of a Terminal Alkyne, CO<sub>2</sub>, and an Allylic Chloride Catalyzed
by the Cu(I) Complex: A DFT Study
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Abstract
DFT calculations have been carried
out to study the detailed mechanisms for carboxylative-coupling reactions
among terminal alkynes, allylic chlorides, and CO<sub>2</sub> 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 PhCCH occurs as the first step
to give the acetylide (IPr)Cu–CCPh, from which CO<sub>2</sub> 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–CCPh
and (IPr)CuOCOCCPh (a species derived from CO<sub>2</sub> insertion)
with an allylic chloride molecule occur through an S<sub>N</sub>2
substitution pathway. The two S<sub>N</sub>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