Theoretical Study On The Mechanism Of Transition Metal-Catalyzed C-H Functionalization And Alkyne Carbonylation

C-H functionalization catalyzed by transition metal complexes is an important step in organic synthesis,which is a great significance in the synthesis of biology,pesticides,medicine and fine chemicals.At the same time,the process of carbonylation of alkynes catalyzed by transition metal complexes is also a hot topic in recent years.It is a challenging task to design coordination groups of transition metal complexes with specific electronic distribution and spatial properties around metal ions.So that the transition metal complexes can have high chemical selectivity only for a specific organic synthesis reaction.In recent years,the rapid development of theoretical and computational chemistry had provided an effective tool for studying organic catalytic reactions,explaining experimental phenomena and predicting reaction products.In particular,the catalytic reaction mechanism and intermediate structure which were difficult to obtain in the experimental process could obtain accurate theoretical data through computational chemistry.The reaction mechanism of C-H functionalization and alkyne carbonylation catalyzed by transition metal complexes were studied by density functional theory(DFT),which can reveal the reaction path of transition metal complexes,find excellent catalysts and reaction substrates with high activity and selectivity,design new reactions.Then we can use the calculated data to guide the experimental research.In this thesis,density functional theory(DFT)was used to calculate the C-H functionalization and alkyne carbonylation catalyzed by Pd(Ⅱ)and Ni(0)transition metal complexes at the molecular level.The effects of central ions,ligands,substrates and stereo conformations of transition metal complexes on the catalytic process were revealed.The reasons for the different yields of reaction products in the experimental results were explained from the thermodynamic and kinetic perspectives,which provided theoretical guidance for the design of experimental processes and the selection of high-performance catalysts in the future.The main research results of this thesis were as follows:(1)The Pd(Ⅱ)complexes catalyzedγ-C(sp~3)-H olefination of organic amines cyclization reaction was investigated by density functional theory(DFT).The results showed that the reaction included two catalytic cycles.The first catalytic cycle included the ligand exchange between the ligand of the transition metal catalyst and the substrate,bicarbonate-assisted C-H activation,olefin insertion and reduction ofβ-hydrogen elimination.In the second catalytic cycle,the generated olefinic intermediate was added.It reduced to obtain the Aza-Wacker oxidation product and completed the cyclization process.The reductionβ-hydrogen elimination step was the key step to determine the reaction rate.Hydrogen carbonate and acetic acid promoted the cleavage of C-H bond through hydrogen bond interaction,and promoted the deprotonation process of olefination intermediates through proton coupled electron transfer(PCET)mechanism.The stereoselectivity of E-conformation products was attributed to the strong interaction between the parallel bridging structures formed by metals and olefins in Pd-πintermediate.We successfully explained the reasons for the promotion effect of pyridyl ligands on the reaction found in the experiment,which not only deepened the understanding of the mechanism of Pd(Ⅱ)complex catalyzed the functionalization of organic aminesγ-C(sp~3)-H,but also promoted the development of highly selective Pd catalytic system in the C-H functionalization reaction.(2)The mechanism of methylsilane and vinylsilane arylation catalyzed by transition metal Ni(0)complex was studied.The results showed that the reaction path was ligand exchange between Ni(0)complex catalyst and substrate,C-H bond cleavage on aromatics,complete oxidation addition and insertion,β-Si elimination process and C-Si reduction elimination process,finally the competition steps of methylsilane and arylation.The selectivity was determined by the competition between C-Si and C-C reduction elimination.N-heterocyclic carbene(NHC)ligands played a key role in the two competitive reactions.In order to understand this ligand effect,we gradually reduced the size of the carbene ring structure by changing the N-substituent on the two competitive paths.It was found that the smaller N-heterocyclic carbene(NHC)was more conducive to the formation of C-Si due to the repulsion between the aromatic group and the ligand.On the contrary,the size of the N-heterocyclic carbene(NHC)had no regular effect on the activation energy of the transition state in C-C reduction.Considered the electronic effect of the ligand,the larger carbene ring(NHC)exhibited better electrostatic matching with the substrate,which can promote the formation of C-C bonds.The results of this study had an important guiding significance for the design of heterocyclic carbene ligands which could control the reaction selectivity.(3)In this thesis,the mechanism and origin of ligand-controlled Pd(Ⅱ)-catalyzed regioselective carbonylation of alkynes were discussed by density functional theory(DFT)calculation.The results showed that the reaction mechanism of alkyne carbonylation process included O-(N-)cyclization,CO insertion,N-H(O-H)fracture,C-N(C-O)reduction elimination and catalyst regeneration.Reduction elimination was the rate determining step of the whole reaction.The chemical selectivity of the reaction was determined by the cyclization reaction.A new cooperative deprotonation/cyclization model was proposed to rationalize the ligand-regulated chemical selectivity.The C-N reduction elimination was more advantageous in kinetics and thermodynamics than the C-O reduction elimination,which was related to the different bond formation modes of C-N and C--O bonds.Our calculation results well reproduced the ligand regulatory selectivity of the experimental results.Compared the ligand effects of nitrogen-containing ligand L1 and phosphorus-containing ligand L2,it was found that the electron-deficient ligand promoted the flow of electrons in the cyclization process,so the electron-deficient nitrogen-containing ligand L1 was beneficial to the O-cyclization reaction path.In contrast,the large steric hindrance and electron-rich phosphorus-containing ligand L2was not conducive to this path due to the electronic effect and the steric hindrance between the substrate and the ligand.These speculations on the mechanism and the process of molecular simulation would help to design new ligands to control the chemical and regioselective reactions of substrates with competitive groups.The research on the above systems provided an important reference value for the design of organic synthesis experiments and the selection of high-performance catalytic systems in the future.