Theoretical Study On C-O/C-C Bond Construction By Ligand-to-Ligand Hydrogen Transfer Catalyzed By Nickel Complex

In the field of organic chemistry,the construction of C-O bonds and C-C bonds is one of the most basic ways to complicate small molecules.The coupling reactions of constructing of C-O and C-C bonds have attracted considerable attentions in recent years.The coupling reactions take the simple organic small molecules as raw materials to synthesize organic compounds composed of carbon skeleton structures with the requirements of atomic economy and high selectivity.In the past decades,transition metals have been chosen to catalyze various coupling reactions with strong catalytic activities.The in-depth study of the catalytic reaction mechanism has become the prerequisite and basis for promoting the development of catalytic reactions at this stage.In present thesis,on the basis of density functional theory(DFT)calculation,a systematic study is performed to explore the reaction mechanism for constructing C-O bond in a dehydrogenative cross-coupling reaction and C-C bond in a hydroalkylation reaction with the aim to clarify the microscopic reaction mechanism and explore the structural changes in macroscopic organic reaction from the molecular level.The present work could not only accelerate the development of transition metals in the field of organic catalysis,but also provide some useful information for the design of efficient catalysts and optimization of experimental reaction conditions.The main research contents of this thesis are summarized as follows:(1)Based on the density functional theory,the mechanism of dehydrogenative cross-coupling reaction of benzaldehyde with benzyl alcohol catalyzed by Ni-NHC was studied.The entire Ni(0)/Ni(II)catalytic cycle consists of four basic steps:ligand exchange,oxidative addition,hydrogen transfer,and reductive elimination.The calculation results show that the hydrogen transfer process in the oxidative addition step is the rate-determining step,which is achieved by C-H~aactivation without generating a nickel hydride intermediate with higher energy,which is called the ligand-to-ligand hydrogen transfer(LLHT).The LLHT mechanism is verified by analyzing the bond lengths,electron populations and orbital interactions.In addition,in order to clarify the important role of external oxidants and hydrogen acceptors,the effects of electronic effects of different substrates on the reaction energy barrier were also explored,and it was proved that?,?,?-trifluoroacetophenone could effectively accelerate the LLHT process with lower activation energy barrier.(2)By using density functional theory calculations,the mechanism of hydroalkylation of phenylbutadiene and acetophenone catalyzed by nickel(0)was studied to elucidate the detailed reaction mechanism of the whole catalytic cycle.The calculation results show that the Ni(0)/Ni(II)catalytic cycle consists mainly of three steps:oxidative addition,proton transfer and reductive elimination of C-C bonds.Wherein,the oxidative addition step involves two reaction pathways,and the most preferable pathway is completed by the ligand-to-ligand hydrogen transfer process,which is the rate-determining step of the entire reaction,that is,the activated hydrogen atom migrates directly from the substrate ethanol to another substrate phenylbutadiene,without forming a less stable nickel hydride intermediate throughout the process.