Theoretical Study On The Mechanism Of Catalytic Hydrogenation Of Ketones And Imines

Hydrogenation of ketones and imines is an important process in fine chemical industry and pharmaceutical synthesis. Recently, hydrogenation of ketones and imines catalyzed by transition-metal complexes has attracted considerable attention and remarkable progresses have been made. Investigating the catalytic mechanism and the structure-activity relationship of the catalyst can provide theoretical guide for the development of new high-efficiency catalysts. In chapter 3-5 of this dissertation, the mechanisms of hydrogenation of ketones or imines catalyzed by three typical transition–metal complexes which were experimentally described were investigated in detail by using density functional theory.Organophosphorus compounds are widely used as pesticides and herbicides at present. On the other hand, these compounds pose significant toxicity toward mammalian organisms and the environment. The need for improved methods of environmental decontamination has provided the impetus for numerous studies aiming at enhancing the decomposition rate of these compounds. Aminolysis is an important method for the decomposition of organophosphorus compounds. In chapter 6 of this dissertation,the aminolysis of phosphinate is investigated theoretically.Density functional theory calculations have been performed to explore the detailed mechanism of the carbonyl hydrogenation catalyzed by the first well-defined bifunctional iron catalyst, [2,5-(SiMe₃)₂-3,4-(CH₂)₄(η⁵-C₄COH)]Fe(CO)₂H]. The catalytic cycle consists of two steps, namely, hydrogen transfer and dihydrogen activation. The hydrogen transfer reaction proceeds via a concerted addition mechanism involving the Fe–H and cyclopentadienyl–OH hydrogen atoms outside the coordination sphere of Fe. The dihydrogen activation processes with and without the assistance of alcohols have been studied. The two pathways both start with the coordination of H₂ to an unsaturated iron intermediate to form a stableη2-H₂ complex. The calculation indicates that the activation barrier is significantly lowered with the assist of alcohols. For the whole catalytic cycle, the rate-determining step is the hydrogen transfer process. The calculated free energy barriers are in close to the estimated experimental barrier height.The mechanism of imine hydrogenation catalyzed by thiolate complexes of Rh(III) bearing a hydrotris(3,5-dimethylpyrazolyl)borato ligand has been investigated via the DFT calculations. The overall catalytic cycle for heterolytic cleavage of H₂ and hydrogenation of N-benzylidenemethylamine by the model catalyst [TpRh(SPh)2(MeCN)] is presented in detail. According to our calculations, the reaction is likely to proceed through the following steps: (1) the addition of H₂ to the rhodium center to generate aη2-H₂ adduct; (2) the formation of iminium cation: deprotonation of the H₂ ligand to form the iminium and hydride; (3) the hydride transfer process. Two possible pathways (Path A and Path B) have been considered for the step of formation of the iminium. For Path A, the dihydrogen is heterolytically cleaved firstly at the Rh–S bond via a four-center transition state to yield the hydride thiol product and then the proton on S transfers to the N atom of the imine. In Path B, the proton transfer from theη2-H₂ ligand directly to the N of the imine. The calculated results indicate that Path B is more favorable. For the whole catalytic cycle, the hydride transfer is the rate-determining step.In next work, the mechanism of H₂-hydrogention of acetophenone catalyzed by Cp*Ir(TsDPEN-H), which is a typical catalyst for transfer hydrogenation, is evaluated using DFT method. The whole catalytic cycle includes two parts: the hydrogenation of catalyst and hydrogen transfer. The hydrogenation of catalyst both in neutral and acidic conditions were considered for comparison. In neutral condition, the hydrogenation of catalyst with H₂ proceeds through the following steps: (1) the oxidative addition of H₂ to the iridium center to generate a dihydride intermediate; (2) the reductive elimination of one Ir-bound hydrogen to produce the amino-hydride complex. The oxidative addition step is the rate-determining step with a high barrier. In acidic condition (TfOH), the catalyst can easily be protonated by TfOH. The protonated moiety can be stabilized by TfO– in solution. The energy barrier of the hydrogenation of catalyst is significantly lowered when the reaction is acid catalyzed. The hydrogen transfer proceeds via bifunctional mechanism.In chapter 6, density functional computations are applied to study the mechanism and the energy profile for the aminolysis of dimethyl phenylphosphinate. Two competing reaction pathways, the concerted process and the two-step stepwise pathway, were mainly discussed. The results obtained reveal that both pathways involve hydrogen transfers. The stepwise pathway is slightly more favorable. The general base catalysis of the process was also examined. The catalytic role of a second ammonia molecule is executed by facilitating the hydrogen transfer processes and by decreasing all energy barriers. The energy barriers calculated for the first step of the stepwise process for catalyzed and uncatalyzed aminolysis are usually much higher than those for the second step. The data show that the most favorable pathway of the reaction is through the general-base-catalyzed neutral stepwise mechanism.