Study On The Carboxylation Of C_sp2-H Bond Of Polycyclic Aromatic Hydrocarbons And CO₂

Aromatic carboxylic acid,as a kind of very important fine chemical raw materials,can be widely used in dyes,medicine,coatings,plasticizers and spices and other related fields.Oxidation reaction and multi-step reaction exist in the traditional production process of aromatic carboxylic acids,which make the preparation of aromatic carboxylic acids have such defects as low atom utilization rate,high cost,many by-products,harsh reaction conditions and serious environmental pollution.As one of the main greenhouse gases,CO₂ has strong thermodynamic stability and kinetic inertia,which make CO₂ not easy to be activated.The fixation of CO₂ can not only eliminate CO₂ to a certain extent,but also obtain high value-added chemicals.Three molecules of Lewis acid Al X₃（X=Cl,Br）can form a ring complex with one molecule of CO₂,which increases the electrophilic activity of CO₂ to activate CO₂,thus allowing CO₂ to participate in S_E reaction.R₃SiCl can react with Al X₃ and CO₂ to form an intermediate complex similar to chloroformate,in which oxygen on the carbonyl group interacts with Al X₃.The complex can directly electrophilic attack the C_sp2-H bond of polycyclic aromatic hydrocarbons to form corresponding aromatic carboxylic acids.The reaction has good atomic economy and no environmental pollution.Using AlBr₃ as a catalyst and Ph₃SiCl as an additive,the influence of AlBr₃ and Ph₃SiCl on direct carboxylation of naphthalene and CO₂ was studied by controlled variable method,and the optimized reaction conditions were obtained.Under the conditions of 1.0 mmol naphthalene,1.0 mmol AlBr₃,1.0 mmol Ph₃SiCl,3.0 MPa CO₂,chlorobenzene as solvent,40℃and 10 h,the conversion rate of direct carboxylation of naphthalene with CO₂ was 84.6%,and the yield was 78.8%.Under the same reaction conditions,AlBr₃ and Ph₃SiCl could also promote the direct carboxylation of other polycyclic aromatic hydrocarbons with CO₂,resulting in high yields of corresponding aromatic carboxylic acids.Meanwhile,Ph₃SiCl could be recycled through recrystallization.The proposed reaction mechanism of AlBr₃ and Ph₃SiCl promoting the direct carboxylation of naphthalene with CO₂ was S_EAr reaction mechanism.Through a series of characterization methods,the intermediate complexes was verified during the reaction.The DFT calculation method with B3LYP/6-31+G（d,p）basis sets was used to obtain the optimized geometric configuration of reactants,intermediates and products in the direct carboxylation of naphthalene with CO₂ promoted by AlBr₃ and Ph₃SiCl.The energy corresponding to the optimized geometric configuration was calculated to obtain the heat absorption and exothermic conditions in each step of the reaction process,which laid a theoretical foundation for the direct carboxylation of naphthalene with CO₂.