Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO₂

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H₂CCHAr using CO₂ 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 CO₂ 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

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

Mechanistic Insight into the Gold-Catalyzed Carboxylative Cyclization of Propargylamines

DFT calculations have been carried out to study the detailed mechanisms for the carboxylative cyclization of propargylamine using CO₂ catalyzed by NHC-gold­(I) complexes. The calculation results indicate that the reaction starts with an N-coordinated species, [(NHC)­Au­(propargylamine)]­Cl, which undergoes isomerization to an alkyne-coordinated species. An amine–carbon dioxide interaction gives a carbamate ion species, from which a nucleophilic attack of the in-plane lone pair of electrons in the carbamate anion moiety on one of two coordinated alkyne carbons leads to formation of a five-membered-ring intermediate. The final product is generated through deprotonation and protonation processes. Through a detailed mechanistic study, we found that the substrate propargylamine assists (catalyzes) the deprotonation and protonation processes. Careful study of the solvent effect indicates that solvents, which are polar and capable of hydrogen bonding, promote the catalytic reactions through stabilizing the carbamate ion intermediate species

Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO₂

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H₂CCHAr using CO₂ 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 CO₂ 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

How the Coordinated Structures of Ag(I) Catalysts Affect the Outcomes of Carbon Dioxide Incorporation into Propargylic Amine: A DFT Study

Density functional theory calculations have been carried out to explore the detailed mechanisms for carbon dioxide incorporation of N-unsubstituted propargylic amine catalyzed by Ag­(I) catalysts. We show that the reaction undergoes substrate adsorption or displacement, isomerization from amine-coordinated species to the alkyne-coordinated species, CO₂ attack, and proton transfer, giving the carbamate intermediate. Subsequently, the reaction would bifurcate at the intermolecular ring-closing step, which produces five-membered ring (5MR) and six-membered ring (6MR) products at the same time, thus raising a regioselectivity issue. Our calculations reveal that the outcomes of the reaction critically depend on the coordination number and the basicity of the ligands. Higher coordinate number and stronger basicity of the ligands would stabilize the 5MR transition state over the 6MR counterpart. Such a preference can be rationalized by using transition state energy decomposition. All of these results could promote the rational design of noble metal/organic base combined catalysts with higher selectivity

A Synergistic Catalytic Mechanism for Oxygen Evolution Reaction in Aprotic Li–O₂ Battery

The large polarization of a Li–O₂ battery is derived from oxygen evolution reaction (OER) processe. To achieve a long-life Li–O₂ battery with high round-trip efficiency, various catalysts have been extensively investigated for oxygen cathodes, especially for OER processes. Here, we designed an in situ growth of α-MnO₂/RuO₂ composite on a graphene nanosheet with a carbon-embedded structure as the cathode electrode for a Li–O₂ battery. The synergistic catalytic effect between the α-MnO₂ and RuO₂ has significantly improved the OER kinetics. The fabricated Li–O₂ battery can deliver a high reversible capacity of 2895 mAh/g_composite with a low charge overpotential of 0.25 V (0.34 V lower than bare RuO₂ cathode). The results revealed that more LiO₂ intermediates formed when α-MnO₂ was introduced into the RuO₂ electrode during the oxidation of Li₂O₂. The facilitation of the initial Li extraction was confirmed by density functional theory (DFT) calculations, which shows that the α-MnO₂ and RuO₂ interfaces can stabilize the primary Li ions and Li_2–<i>x</i>O₂ intermediates, respectively. Subsequently, Li_2–<i>x</i>O₂ would be easily oxidized to O₂ by RuO₂ catalyst. With the synergy between α-MnO₂ and RuO₂, the initial delithiation process and O₂ evolution are promoted simultaneously. By combining theoretic and experimental results, we proposed a synergistic catalytic mechanism for the OER processes