Theoretical Study On The Mechanism Of Acid-Catalyzed Schiff Base Reaction Of Dihydroxy Aromatic Dialdehydes

Imine(Schiff base)materials have the advantages of easy synthesis,low cost,reversible and only water by-products.They are widely used in biochemistry,catalysis,anticorrosion,covalent organic framework(COF)materials,dynamic covalent chemistry and so on.The reaction mechanism of 2,5-dihydroxy p-phenylenedialdehyde(DHTA)and p-phenylenediamine(PDA)catalyzed by acid was firstly studied at B3LYP/6-31G(d,p)theoretical level.Then we investigated the Schiff base assembling reaction of 4,4'-dihydroxy-[1,1-benzene]-3,3'-diformaldehyde(DHBDA)with PDAs.The reaction mechanisms,effects of the OH groups,selectivity of different conformation products in the gas phase,in benzene,water and acetic acid solution with both explicit and implicit solvent effects were focused on.The main conclusions of this thesis are as follows:The acid-catalyzed Schiff base reaction is carried out in two steps: the first step is nucleophilic addition,forming a C-N single bond,the second step is dehydration,and the C=N double bond formation is accompanied by proton transfer.The second step is the decisive step of the reaction.The auxiliary molecules of water and acetic acid can reduce the energy barrier of cis and trans Schiff bases by about 10 kcal/mol.The auxiliary effect of water is stronger than that of acetic acid;the benzene solvent molecules hardly promote the reaction.For the cis reaction path with the water molecule assisted,due to the H2 O molecule forming hydrogen bond with the hydroxyl group of DHTA other than the oxygen of C=O group,the proton transferring process of the dehydration step changes from synchronization to stepwise,a proton first transfers to the aided water,then another proton transfers from water to the oxygen of C=O.Comparing the cis and trans paths,it is found that in most cases,the trans path has a dynamic advantage within 4 kcal/mol.The energy of the trans product is much lower than that of the cis product over 10 kcal/mol(6 kcal/mol without the aid of solvent molecules and benzene molecules),so the trans path has a greater thermodynamic advantage,the reaction It is more inclined to form relatively stable trans products.Due to the two conformations of DHBDA,the same-side and the opposite-side with respect to the main axis of biphenyl,and the energy barrier for the conversion between conformations is about 1 kcal/mol,the reaction between DHBDA and PDAs with different conformations is considered.The gas phase Schiff base reaction,regardless of kinetic or thermodynamic,opposite-side products are dominant,and it is easier to form one-dimensional linear ordered COFs.In liquid water or acetic acid solution,the same-side products are more stable,especially for the reaction of DHBDA with the second PDA,the advantage of the same-side products are more obvious(7-10 kcal/mol),so the macrocyclic Schiff base products are easier to form.The results show that the structure and function of COFs can be adjusted by changing the reactants and the reaction environment.