| Transition metals typically exhibit a variety of oxidation states due to their unfilled d orbitals,and can form stable complexes with a variety of organic compounds by?-bond or?-bond interaction.In addition,the feature of electronic configuration outside the transition metal core can affect its coordination mode and coordination number,and thus change the electronic distribution of reactants and improve their reactivity.Because of this property,transition metals are often used as catalysts for the synthesis of complex molecules,among which Au and Cu are widely used in various organic synthesis reactions.In this thesis,Au-catalyzed[4+2]cycloaddition and Michael addition reaction,as well as Cu-catalyzed[4+2]cycloaddition were studied theoretically by employing density functional theory?DFT?method.For the[4+2]cycloaddition and Michael addition reactions catalyzed by Au catalyst,IDSCRF model,B3LYP method and mixed basis sets were used to optimize the geometrical structures of all reactants,intermediates,transition states and products in experimental solvent at 353 K,with frequency analysis conducted at the same computational level based on geometry optimization.LANL2DZ pseudo basis set was used for Au atom,while 6-31G*basis set was used for C,H,O and N atoms.All energies reported for the Au-catalyzed system are relative Gibbs free energies obtained in 1,2-dichloroethane solvent at 353K,by employing IDSCRF model at IDSCRF-B3LYP/Gen level.For the reaction catalyzed by Cu catalyst,all geometrical optimization and frequency analysis were carried out at the IDSCRF-UB3LYP/DGDZVP level.All energies reported for the Cu-catalyzed system are relative Gibbs free energies obtained in experimental solvent at 253 K.For the Au-catalyzed[4+2]cycloaddition and Michael addition reactions of propargyl ester with 1,2-benzoisoxazole,we have studied the possible reaction mechanism of ethyl propargyl ester and tert-butyl propargyl ester with1,2-benzoisoxazole in 1,2-dichloroethane solvent at 353 K.Initially,both the[4+2]cycloaddition and Michael addition reactions go through the same pathway.The Au-L catalytic center interacts with propargyl ester or tert-butyl propargyl ester to form a gold complex firstly,and activates the triple-bonded carbon atoms at the same time.Then,the nitrogen atom in 1,2-benzoisoxazole attacks the triple-bonded carbon atom of the gold complex,and forms a six-membered heterocyclic intermediate through cyclization.Next,the N-O bond of the five-membered ring in aforementioned heterocyclic intermediate breaks and leads to two different intermediates.This final step is the rate-determining step?RDS?of both Michael addition and[4+2]cycloaddition reactions,which needs to get across a Gibbs free energy barrier of 31.6 and 31.9 kcal·mol-1,respectively.When taking ethyl propargyl ester as substrate,the C-O bond on the six-memeberd ring,other than the C-O bond in the ether group in previously formed intermediate breaks to form a ring-opening intermediate.This ring-opening intermediate dissociates one molecule of 2-cyanophenol to form the initially-formed gold complex.Subsequently,The O atom in 2-cyanophenol attacks the triple-bonded C atom,followed by the migration of hydroxyl H atom to Au atom.Finally,this H atom migrates to another triple-bonded C atom from Au atom,and the gold catalyst was regenerated accompanied by the formation of the Michael addition product.When tert-butyl propargyl ester is taken as the substrate,the C-O bond in the tert-butyl ether group,other than the C-O bond on the six-memeberd ring in previously formed intermediate breaks,accompanying by H migration from methyl in tert-butyl group to one of the triple-bonded C atom to form one molecule of isobutylene.Finally,the[4+2]cycloadiiton product is formed through getting rid of the gold catalyst.We have also calculated other possible reaction paths for the[4+2]cycloaddition reaction and compared with the optimal path.Due to the difference of stabil ity of the six-membered ring and four-memebered ring transition states involved in the reaction process,the corresponding energy barrier results are different.In addition,we have also studies and these two reactions in the gas phase and compared with the results obtained in solution.It's found that the Gibbs free energy barriers obtained in 1,2-dichloroethane solvent under experimental temperature are lower than these obtained in gas phase,and coincides better with corresponding experimental data.Our computational results have clarified the microscopic reaction mechanism of the aforementioned two reactions,which is of significance for further study of similar gold-catalyzed reactions.For the Cu-catalyzed inverse electron-demand aza-Diels-Alder reaction of enol ethers with azoalkenes,we have optimized the geometrical structures of possible stationary points on the reaction path,and verified its possible reaction mechanism.Computational results show that the reaction may proceed in a synergistic way.Under the help of copper catalyst,a molecule of hydrogen chloride is firstly removed from the reactant to form azoene,and the formed azoene coordinates with copper center to form a new complex.Then,another reactant interacts with this complex and forms a six-membered intermediate through synergistic[4+2]cyclization.Finally,coordination interaction between the six-membered intermediate and the catalyst is released,and product is formed accompanied by regeneration of the catalyst.We have also investigated and discussed different coordination modes of the ligand with copper center.It's found that the reaction needs to get over 17.5 and 14.8kcal·mol-1 of Gibbs free energy barriers respectively under two different coordination modes of ligand.These results indicate the coordination mode of ligand has influence on the reactivity,which is attributed to different steric hindrance arisen from different coordination mods of ligand.The aforementioned computational results can shed some light on better understanding of the microscopic mechanism of this and other similar reactions. |
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