Theory Study On The Mechanisms Of Two Organic Reactions

Organic synthesis, which is a reaction process of synthesizing organisms from elementary substances or compounds, is an important part in the filed of chemistry. As we all know, carbon atoms are connected in the form of covalent bond in organic compounds and carbon-carbon bonds are not easy to break. So organic synthetic reactions proceed usually in suitable experimental conditions, such as heating, catalyst, suitable solvent, light, pressure and so on. Understanding the reaction mechanisms of organic synthetic reactions is very important for us to devise new synthetic route, choose appropriate catalyst and suitable solvent, and so on. However, it is very difficult to explore the reaction mechanisms of organic reactions, unless we get more information at molecular level. So exploring the reaction mechanisms of organic reactions through theoretical calculation methods is particularly important.In this thesis, we have investigated the reaction mechanisms for two organic reactions theoretically using density functional theory (DFT), which is helpful for us to understand thoroughly the reaction processes as well as the role of catalyst and solvent in reaction. The first one is the catalytic mechanism on tail-to-tail dimerization of methyl methacrylatethe catalyzed by metal-free N-heterocyclic carbine. And the second one is a study on the multicomponent reaction mechanism of primary amine and dialkyl acetylenedicarboxylate with1,3-dimethylalloxan.1. A density functional theory study at the M05-2X/6-311++G(2d,2p)//M05-2X/6-31G(d, p) level has been conducted to gain insight into the catalytic mechanism of the metal-free N-heterocyclic carbene (NHC) catalyzed tail-to-tail dimerization of methyl methacrylate. We have suggested four possible reaction channels (including channels A, B, C, and D), which have been investigated in detail in this paper. The channel A is similar with the proposed mechanism in the experiment, including the processes of nucleophilic attacks, proton transfers and dissociation. While the novel channel B does not involve direct intramolecular1,2-H shift process, which is different from channel A. In addition, channels C and D are proposed on the basis of the channel B. Among of them, the E-isomer product can be obtained via the channels A and B, while the Z-isomer product was acquired through the channels C and D. The calculated results indicate that the channels B and D are much more energetically favorable under the experimental conditions. Moreover, We have explained the stereoselective phenomenon in two aspects associated with kinetics and thermodynamics, which is agreement with the experimental results (E:Z=95:5). The further calculations and analysis of NBO charge reveal the role of the catalyst N-heterocyclic carbene in the dimerization. The obtained novel mechanistic insights should be valuable for not only understanding the detailed mechanisms of the NHC-catalyzed tail-tail dimerization but also rational design of more efficient and high stereoselective NHC catalyst.2. In an attempt to explore the mechanism for the multicomponent reaction of primary amine and dialkyl acetylenedicarboxylate with1,3-dimethylalloxan, two reaction mechanisms were investigated using density function theory (DFT) at the B3LYP/6-311++G(d, p) level of theory, including the solvent effect of dichloromethane simulated by PCM. Firstly, we suggested fussy proton transfer mechanism according to the proposed mechanism in the experiment. Then, we suggested and studied the novel "non-proton transfer mechanism", which avoids all the complicated proton transfer processes. The calculation results indicated that the new mechanism is the most energetically favorable pathway. Specifically, it contains only two tandem processes:Firstly, the propylamine reacts with dimethyl acetylenedicarboxylate to form a zwitterionic intermediate. Secondly, it is a stepwise [2+3] cycloaddition of the zwitterionic intermediate with1,3-dimethylalloxan to give the final product via elimination of a methanol molecule. The reaction can occur easily at room temperature, which is in good agreement with available experiment. We believe this mechanistic study should be of great value in introducing choices of solvent and rational design of new synthesis reactions of more therapeutic potential barbiturate derivatives.