Mechanism Of C-N Coupling And C-N Cleavage Catalyzed By Transition Metal Complexes

Both imines and amides are nitrogen-containing compounds,which are important carriers of biochemical processes and widely exist in natural products and organic compounds in nature.Imines have been widely used in the production such as agriculture,dyes,daily necessities,medicine,etc.The formation of imines through C-N bond coupling to form imine has attracted the interest of chemists and has become a research hotspot in the field of coupling reactions.The C-N bond of amides is not easily activated and required high energy to break.The cleavage of the C-N bond can provide the product amine and alcohol,which are of great significance to life activities.Thus the cleavage of the C-N bond of the amide also has become a research hotspot.Although great experimental progress has been made in the research on C-N bond cleavage of amides,there are still many problems such as the poor atom efficiency,harsh reaction conditions,and the high cost.This makes the simple and efficient cleavage of the C-N bond of amides to achieve directional transformation is a challenging topic in the field of chemical research.Based on the relevant experimental studies,this thesis investigates the reaction mechanism of two catalytic reaction systems,the C-N coupling of amines and alcohols to form imines and the C-N bond breaking of amides,by using density functional theory（DFT）.The important intermediates and the optimal transition states involved in the reaction paths were optimized,and the detailed mechanism of the interest reaction was clarified.The thermodynamic and kinetic properties of the reaction were explored,and the performance and the effects of ligands and additives on the reaction were analyzed.The main research contents of this paper are as follows:1.The reaction mechanism of the phosphine-free transition metal NN-Mn（I）complex-catalyzed acceptorless dehydrogenative coupling（ADC）of benzyl alcohol with aniline to imines was investigated computationally by using the B3LYP method.The synthesis of imines via the acceptor-free dehydrogenative coupling catalytic reaction of alcohols with amines is a potential green and sustainable synthetic route,in which the by-products are H₂O and clean energy H₂.Based on the calculation results,the detailed mechanism of the reaction is summarized.The first step is the addition of a base,which allows the HBr to be removed and the catalyst to be activated.The second step is the dehydrogenation of the alcohol to form an aldehyde,followed by the release of hydrogen gas.In the dehydrogenation process,two types of dehydrogenation were examined,the bifunctional hydrogen transfer（BDHT）mechanism and the conventionalβ-H elimination,and the calculations show that the alcohol dehydrogenation prefers to the former BDHT mechanism.The third step is that the activated catalytic species promotes the coupling of amine and aldehyde,and then the formed species dehydrates to obtain imine.The calculations show that the base added at the beginning of the reaction is effective in stimulating the activation of the precatalyst.Experimentally,the catalytic efficiency varies depending on the base added.The best catalytic performance of NN-Mn（I）complex being achieved with the additive of potassium tert-butoxide（^tBu OK）but a significant decrease when sodium tert-butoxide（^tBu ONa）is added.Our calculations show that there is little difference in the energy barriers to activate the catalysts by two different bases.The initial catalytic performance may be affected by the solubility of the corresponding produced salt in the solvent.2.The reaction mechanism of the preparation of amines and alcohols from benzoic anilide catalyzed by the Ru-PNNH pincer complex was studied by B3LYP method.From the calculations it is concluded that there is a synergistic interaction between the metal centre and the ligand during the catalytic reaction and that the reaction mechanism involves several basic steps.At the initial stage,the basic substance potassium tert-butoxide（^tBu OK）is added to activate the catalyst Ru-PNNH to obtain the active catalytic species.Then the first step of the catalytic reaction cycle is the activation of hydrogen molecular,which is coordinated with the metal center Ru.The second step is the broken of the carbonyl C-N bond of the amide to generate amine.The third step is the activation of another hydrogen molecular,and the process is similar to the first step.The fourth step is to generate product alcohol.In both the second and fourth steps,both the inner and outer sphere mechanisms were considered and calculated.The results indicate that the outer sphere mechanism is superior to the inner mechanism.The title reaction follows a metal-ligand synergistic hydrogen transfer mechanism.The second step is the rate-determining step in the overall reaction.