Theoretical Study On The Hydrogenation Of C=O Bond Catalyzed By Manganese Complexes And The Isomerization Of Allylic Alcohol Catalyzed By Bases

In this thesis,the density functional theory(DFT)method was employed to study the hydrogenation of C≡O double bonds in ketones and carbon dioxide catalyzed by cheap metal manganese complexes,and the reaction mechanism of the asymmetric hydrogenation of ketones and the hydrogenation of carbon dioxide catalyzed by transition metal complexes was explored.Besides,the effects of ligand,solvent,substituent group on the catalyst activity,as well as the source of the stereoselectivity of the catalytic reaction were discussed.In addition,the reaction mechanism of the isomerization of allylic alcohol catalyzed by bases,and the effect of substrate substituents on chemical reactivity was discussed.This thesis further deepens the understanding of the molecular mechanism of homogeneous catalysis through theoretical study on the catalytic hydrogenation reaction of transition metal Mn complexes and the basecatalyzed isomerization of allylic alcohol.This thesis mainly includes the following four parts:1.The DFT method was employed to investigate the reaction mechanism of hydrogen activation and asymmetric transfer hydrogenation(ATH)of acetophenone catalyzed by well-defined phosphine-free Mn(I)NNS complex.The calculated results indicated that the Mn-NNS complex has potential high catalytic hydrogenation activity.Direct hydrogen activation was the rate-determining step of the whole catalytic cycle and alcohol could effectively reduce the energy barrier of hydrogen activation.In addition,the hydrogen transfer(HT)step was a concerted but asynchronous process,and hydride transfer taken precedes the proton transfer.RDG analysis shown that the stereoselectivity of ATH mainly origins from the non-covalent interaction between the substrate and the catalyst.This thesis compared the effects of different anti-ligands(X=CO,Cl,H,OMe,NCMe,CCMe,CHCHMe)on the catalytic hydrogenation activity of the Mn complex.This work will provide a certain theoretical reference for the design of new phosphine-free metal catalysts for the hydrogenation of polar C=O double bonds.2.The DFT method was employed to study the mechanism of carbon dioxide hydrogenation to formic acid catalyzed by manganese complex.The whole catalytic cycle mainly included two stages:hydrogen activation and carbon dioxide hydrogenation.The calculated results shown that the participation of formic acid significantly reduces the reaction energy barrier of hydrogen activation.The hydrogenation process of carbon dioxide followed the outer-sphere mechanism and hydrogen transfer was a stepwise process,in which the hydride transfer was the rate-determining step,with an energy barrier of 5.0 kcal/mol.In addition,the modulation effect of R group on sulfur atom in Mn complex was explored.The results shown that when R was an electron-withdrawing group,it could reduce the activation barriers of hydrogen cracking and proton transfer during carbon dioxide hydrogenation.The transfer of hydride was facilitated when the R group was an electron-donating group,and the whole reaction with a smallest energy span of 19.2 kcal/mol when R=CF3.3.The DFT method was carried out exploring the reaction mechanism of three cascade cycles for the hydrogenation of carbon dioxide to methanol(CO2+3H2→CH3OH+H2O)catalyzed by a manganese pincer complex,[Mn(Ph2PCH2SiMe2)2N(CO)2].The three cascade cycles were involved:the hydrogenation of CO2 to formic acid,the hydrogenation of formic acid to methanediol and the decomposition of methanediol to formaldehyde and water,and the hydrogenation of formaldehyde to methanol,respectively.The calculated results demonstrated that the hydrogen activation was the rate-determining step of every catalytic cycle without solvent,with an energy barrier of 15.0 kcal/mol.However,the energy span of the whole reaction progress is 27.1 kcal/mol,indicating that the reaction is difficult to occur at room temperature.Furthermore,the solvent was found to be of importance in this reaction.For the three different solvents,the formic acid hydrogenation stage was verified as the rate-determining steps of this reaction in water,toluene and THF solvents,the corresponding energy spans were 21.3,20.8 and 20.4 kcal/mol,respectively.These imply that this manganese complex could be a promising catalyst for efficient conversion of CO2 and H2 to methanol at temperatures below 100-150℃.4.Allylic alcohols,as common and readily available building blocks,could be converted into other widely used carbonyl compounds through isomerization reactions.However,these generally involve expensive transition metal complexes as the catalyst,with few exceptions.The development and design of cheap metal catalysts or non-metal catalysts to catalyze such reactions is one of the research hotspots in the isomerization of allylic alcohol.In this thesis,the DFT method was employed to explore mechanistic patterns of allylic alcohol catalyzed by bases,such as KOH,NaOH,LiOH,tBuOK,tBuONa,tBuOLi,1,5,7-triazabicyclo[4.4.0]dec-5ene(TBD),1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2a]pyrimidine(MTBD)and 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU).The calculated results indicated that the main role of the base was like an initiator,which assisted the transfer of H1 atom in allylic alcohol from C1 to C3.Additionally,studies about the modulation of substrate substituents suggest that electron-withdrawing groups are favorable for the C1-H1 cleavage,while electron-donating groups are favorable for the hydrogen transfer.This thesis applied the Hirshfeld charge,molecular electrostatic potential(MEP),NAO energy,and ITA quantities to examine the influence of bases and substrate substituents on the chemical reactivity of allylic alcohol isomerization and revealed the impact of electronic effects on the reaction.This work should provide a much-needed theoretical guidance to design better non-metal catalysts for the isomerization of allylic alcohol compounds and their derivatives.