Theoretical Study On The Mechanism Of Metal Carbene Participation In O-H Bond Insertion And CO₂ Reduction

Carbene has a very important position in the field of chemistry.The insertion reaction of X-H bonds(X=N,O,S,C,P,etc.)catalyzed by transition metal carbene,is an important method for constructing C-X bonds in organic synthesis.This method has the characteristics of good atom economy and high reactivity,and is widely used in the synthesis of natural products and pharmaceutical molecules.Experimental studies have found that the reaction of metal carbene with cyclopropanol can simultaneously produce O-H insertion product and ring-opening product.However,the specific reaction mechanism is not clear.In addition,the mechanism by which bimetallic carbene reduces CO₂ is unclear.Understanding the reaction mechanism has important guiding significance for further optimizing the reaction and designing a catalyst with excellent performance.In this thesis,the reaction mechanism of the following two reactions are studied by DFT method:(1)CuI catalyzes the reaction of cyclopropanol and diazonate in toluene solvent;(2)azacarbene dinuclear Ni(0)reduces CO₂.Through research,the following main conclusions are obtained:(1)We have studied in detail the mechanism of CuI catalyzing the O-H insertion reaction and C-C cracking reaction of diazonate and cyclopropanol in toluene solvent.Calculations show that in the O-H insertion reaction path,the hydroxyl oxygen of cyclopropanol nucleophilically attacks the carbene carbon to form oxonium ylide.The activation energy at this step is 15.3 kcal/mol.The proton on the hydroxyl group is then transferred to the adjacent carbonyl oxygen to form a metal-associated enol intermediate with an activation energy of only 3.1 kcal/mol.The activation energy of O-H insertion product is up to 29.8 kcal/mol by oxonium ylide[1,2]-H migration.The high activation energy barrier indicates that it is impossible to obtain O-H insertion through oxonium ylide[1,2]-H migration product.In addition,the total energy barrier of the path of the metal associative enol intermediate via ligand transfer and[1,3]-H migration of 42.1 kcal/mol is also not conducive to the reaction.This indicates that free enol is the species of[1,3]-H migration.By introducing a diazonate to adsorb on Cu,the metal associated enol dissociates the metal ligand to form a free enol and only needs to absorb 0.6 kcal/mol of heat.In the end,the free enol assisted the[1,3]-H transfer to form a product through two cyclopropanols.The activation energy barrier for the enol[1,3]-H migration was 16.4 kcal/mol,which is also determined by the rate of the O-H insertion pathway step.In the cyclopropanol ring-opening path,the reaction starts when cyclopropanol rings on Cu.The energy barrier at this step is 0.4 kcal/mol,and the energy of the intermediate produced is 7.3 kcal/mol lower than that of the reactant.This step is still thermodynamically advantageous.After overcoming the energy barrier C-Cu bond cleavage of 22.5 kcal/mol,the activation energy of carbene formed by the decomposition of the diazoate on Cu is 7.3 kcal/mol.After carbene formation,the activation energy barrier for C-C bond formation is 4.4 kcal/mol,and the energy barrier for the[1,5]-H migration is 0.7 kcal/mol.The activation energy barrier of the rate determining step of the O-H insertion pathway is 16.4 kcal/mol,and the activation energy barrier of the rate determining step of the cyclopropanol ring-opening pathway is 22.5 kcal/mol.The ring-opening pathway is 6.1 kcal/mol higher than the O-H insertion pathway,indicating that the formation of O-H insertion product is more kinetically favorable.(2)Calculation of CO₂ reduction mechanism of azacarbene dinuclear Ni(0)shows that the adsorption of CO₂ contributes to the dissociation of the benzene ring coordinated with Ni,and the activation energy for dissociation of both benzene rings is 2.4 kcal/mol.During the formation of the CO₂reduction precursor,the activation energy barrier to be overcome by the molecular inversion step in the rate-determining step is 15.3 kcal/mol.Different from the previously reported coordination mode of CO₂and metal,we found in the study that the oxygen atom of CO₂ can coordinate with two metal Ni at the same time.In addition,the previously reported coordination modes only appeared separately,but now two different coordination modes can appear simultaneously.More importantly,calculations show that the reduction reaction of the bimetallic double-adsorbed CO₂ complex occurs in the triplet state.First,a transient CO₂ dimer is formed,followed by a disproportionation reaction to generate CO₃^2-and CO containing fragments,and at the same time,the intersystem crossing and returning to the singlet state.Double nickel can reduce two molecules of CO₂,and the activation energy for reducing the first and second CO₂ is 12.1 kcal/mol and 16.2 kcal/mol,respectively.During the reaction,two molecules of free CO are dissociated,and the dissociation energies are 14.7 kcal/mol and 16.4 kcal/mol,respectively.The dissociated CO reacts with the catalyst molecule 1 or A3,and it is easy to remove the benzene ring or CO₂ to form intermediate B3 at room temperature.B3 is then converted to B4,an intermediate of double nickel on the same side,by BTS3 with an activation energy barrier of 11.4 kcal/mol.Intermediate B4 finally exothermicly converted to 12.3kcal/mol into CO₂ reduction end product P.