Computational Insight into the Mechanism of Nickel-Catalyzed
Reductive Carboxylation of Styrenes using CO<sub>2</sub>
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Abstract
DFT
calculations have been carried out to study the detailed mechanisms
for the nickel-catalyzed reductive carboxylation of ester-substituted
styrenes H<sub>2</sub>CCHAr using CO<sub>2</sub> to form α-carboxylated
products. Two possible mechanisms, the oxidative coupling mechanism
and the nickel hydride mechanism, were calculated and compared. Our
calculations show that, for the oxidative coupling mechanism, a metallacycle
thermodynamic sink is generated from oxidative coupling between CO<sub>2</sub> and a styrene substrate molecule on the nickel(0) metal center,
which should be avoided in order for smooth reductive carboxylation
of styrenes. For the nickel hydride mechanism, a nickel hydride species
is the active species, from which styrene insertion into the Ni–H
bond followed by reductive elimination produces the α-carboxylated
product. Calculations show that either of these two steps (insertion
and reductive elimination) can be the rate-determining step, and both
transition states are only slightly more stable than the oxidative
coupling transition state leading to the thermodynamic sink. Because
of the competitive nature between the two mechanisms, the reaction
conditions and other factors (substituent, pressure, and ligand) significantly
affect the reaction outcome, all of which have been discussed in detail