Theoretical Study On Coupling Reaction Of Carbon Dioxide With Benzylidenecyclopropane Mediated By Nickel(0) Complexes

At present, the problem of energy and environment is the globalfocus. Using carbon dioxide as a starting material in syntheses ofnew chemical products has received much attention. One of thepromising methodologies in chemical CO₂fixation is couplingbetween carbon dioxide with unsaturated compounds to affordcarboxylic acids. A remarkable variety of catalysts have beencontinuously explored for the coupling reactions between carbondioxide and unsaturated compounds over the past decades.However, due to transition states can’t be observed and reactionintermediates are difficultly detected just by experimental methods,there generally appears to be a lack of unabridged and certainmechanistic studies till now. Alternatively, the quantum chemistrymeans can combine with experimental methods to providecomprehensive and thorough understanding for reactionmechanisms.The purpose of this work is to clarify the detailed mechanism ofthe CO₂/benzylidenecyclopropane coupling reaction catalyzed byNi（0） complexes at B3LYP level of density functional theory, withthe hopes of shedding light on factors that affect the energy barrier,and hence offer theoretical guidance for experimental studies. Themain contents and results are as follows. （1） The coupling reaction of CO2and benzylidenecyclopropaneoccurs through dissociative mechanism （monoligand mechanism）.Viewing the geometric structure of the intermediates and transitionstates involved in bisligand routes, one can find out the stericrepulsive interactions between ligands （MTBD or DBU） and thereaction substrate benzylidenecyclopropane are all larger.Contrarily, the steric repulsive interaction in monoligandcompounds is smaller and the space for the coupling of reactants isnot congested. Therefore, monoligand routes have the lower energybarriers, and the reaction will proceed along monoligand routes.（2） Self-Consistent Reaction Field （SCRF） calculations indicatethat when DBU is used as the amine ligand, in toluene, the titlereaction proceeds via routeⅠto provide product A. The elementarysteps involving in the reaction are as following: （ⅰ） the coordinationof benzylidenecyclopropane to Ni to produce theπ-complex 1A, （ⅱ）carbon dioxide insertion into Ni–C bond to form five-memberedcomplex, which is the rate-determining step.In contrast, the use of more polar solvent （acetonitrile） favorsrouteⅡ. Our calculations predicted that acetonitrile molecule canplay the role of not only the solvent but also the ligand in thecatalytic reaction. Under such condition, the reaction needs rathermild conditions, so thermodynamic factors control the distributionof products in acetonitrile. The reaction proceeds via the followingelementary steps: （ⅰ） the formation ofπ-complex 1A, （ⅱ） the ligandexchange of acetonitrile and DBU to form 1Aac, （ⅲ） thetautomerization of 1Aac to form four-membered complex 2Bac,which is the rate-determining step of this process, （ⅳ） carbon dioxide insertion to form six-membered cyclic metalla-carboxylate4Bac.（3） The SCRF calculations for MTBD ligand show that the energybarrier for the rate-determining step ofⅢgradually decreases alongwith the increase of the solvents’polarity. This point consists withthe experiment results that the yield of C improves along with theincrease of the solvents’polarity. But the result of calculationsindicates that the energy barrier for routeⅢis not the lowest nomatter in what kind of solvent, which is not consistent withexperimental observations. This may be because the B3LYP methodoverestimates the energies of transition states. Difference fromrouteⅡ, in routeⅢCO₂ inserts into Ni-Csp3bond to form a newsix-membered cyclic metalla-carboxylate.（4） We find the energy barriers for each route with MTBD ligandare lower than that with DBU ligand. In addition, theseven-membered-ring of DBU is symmetrical conformation in thewhole reaction.