Theoretical Mechanistic Study Of The Selective C-C/C-O Bond Activation And Formation Reaction By Metal-metal Synergistic Effect

The carbon-carbon?C-C?activation and C-C/carbon-heteroatom?C-X,X=N,O,S?bond formation reactions can generate compounds with special fine skeleton structure using relatively simple raw materials,which is in line with the pursuit of high atomic efficiency.It has been a hot topic in the field of chemistry,because it has powerful functional and wide application value in chemical industry,environmental protection,petroleum,materials,medicine and other important fields.At present,the selective C-C bond activation and C-C/C-X bond formation reactions by metal-metal synergistic effect have been paid more and more attention.However,limited by the means of detection,it is usually difficult to capture intermediates in experiment,which leads to a large controversy over such reaction mechanism.At present,computations have become essential to elucidate structures and properties of molecules,and mechanisms of reactions.Therefore,the catalytic reaction mechanism and essence of metal-metal synergistic effect have been revealed by modeling the catalysis using computational programs in this thesis.These works will provide theoretical guidance on designing new synergistic catalysts with lower cost and higher performance and promote the development of C-C bond activation and C-C/C-X bond formation reactions in chemistry.In this thesis,using density functional theory?DFT?,mechanisms of the selective C-C bond activation and C-C/C-O bond formation reactions by metal-metal synergistic effect were discussed.Thesis consists of six chapters;the first chapter is an introduction including catalysis,synergetic catalysis and the application of synergistic effect in the reactions of C-C bond activation and C-C/C-X bond formation.The second chapter is calculated theory.The third to the sixth chapters are the main content of the thesis:1.The cyanoesterification reaction of ethyl cyanoformate and but-2-yne catalyzed by nickel?0?/Lewis acid was theoretically investigated using DFT.The results show that an unusual biphosphine cycle including rate-determining oxidative addition of a C-CN bond,alkyne migratory insertion,and reductive elimination steps is more favorable than the generally accepted monophosphine cycle.The strong electron-withdrawing property of LA strengthens the coordination ability of the Ni center with alkyne to form a thermodynamically favorable five-coordinate nickel?II?cyanide carboxylate species,which then avoids the large deformation of the transition state of alkyne migration insertion to reduce activation energy and accelerate the reaction.Furthermore,the LA has a non-negligible impact on the mechanistic origin of selective C-CN bond activation.Thus,the proposed biphosphine cycle successfully rationalizes the experimental observation and provides a theoretical basis for the rational design of such synergetic catalysis system with higher performance.2.The mechanism of carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride was investigated using DFT.A novel?two-state reactivity?mechanism and an electronic state transformation?EST?were proposed for the first time.The important elementary steps consist of hydride transfer,benzene coordination,dehydrogenation,oxidative addition,hydride-proton exchange,and reductive elimination.The ground-state potential energy surface switches from nearly degenerate triplet and antiferromagnetic singlet states to a closed-shell singlet state through the minimum energy cross point?MECP?and EST in the"two-state reactivity"mechanism,which effectively decreases the activation barrier.Furthermore,the synergetic effect between trinuclear titanium centers and hydrides has been rationalized:the hydride transfers are responsible for reducing partial?bonds to?bonds of benzene,whereas the trinuclear titanium moiety plays a key role in the C-C?bond activation and reformation.3.The mechanism of nickel?II?/palladium?II?dual catalyzed cross-Ullman reaction of aryl bromide and 4-methoxyphenyltriflate was systematically investigated using DFT.The theoretical calculations clearly disclose a unique double electron transmetalation mechanism involving Ni^I-Ni^IIIII catalytic cycle.The most favorable catalytic cycle consists of four key elementary steps:the highly orthogonal selective oxidative addition of the C-OTf and C-Br bonds to Pd⁰?dppp?and Ni^I?bpy?Br,transmetalation and reductive elimination.The rate-determining step is the oxidative addition of the C-Br bond to Ni^I?bpy?Br with the?G^°?value of 20.3 kcal/mol.The orthogonal substrate reactivity exhibited by Pd⁰?dppp?and Ni^I?bpy?Br palys a decisive role in the cross-selectivity of such reaction.A stronger reductant which would reduce Ni^II?bpy?Br₂ to Ni⁰?bpy?could imped this reaction.4.Photoredox-mediated iridium?III?/nickel?II?dual catalyzed C-O cross-coupling of aryl bromide and alcohol was chosen to illuminate the ambiguous mechanism of such dual catalysis using DFT.The reaction occurs via a novel oxidation state modulation mechanism,merging unusual oxidative quenching(Ir^III-*Ir^III-Ir^IV-Ir^III)and nickel catalytic(Ni^II-Ni^I-Ni^III-Ni^I-Ni^II)cycles,rather than radical mechanism.Interestingly,the quinuclidine is engaged in a proton-coupled electron transfer process to accelerate the reaction and regulate chemoselectivity of C-O versus C-C coupling.Such knowledge will provide theoretical foundation to exploit novel and inexpensive photoredox-mediated dual catalytic system.