Study On Carbonylation Of Monocyclic Aromatic Hydrocarbons With CO₂

Aromatic carboxylic acids and their derivatives are important organic synthesis intermediates.Direct carboxylation of simple aromatics and CO₂to obtain aromatic carboxylic acids is an acylation technology with 100%atom utilization.By adding chlorosilane（R₃Si Cl）with alkyl or aryl groups,the direct carboxylation of monocyclic aromatic hydrocarbons and CO₂catalyzed by Lewis acid was effectively promoted.Under mild reaction conditions,the conversion of aromatics is achieved with high selectivity.Under the pressure of 3MPa CO₂,the catalyst Al Cl₃and the co-catalyst Ph₃Si Cl realized the carboxylation of toluene under mild conditions,and the yield of toluic acid was 74%.The addition of alkyl halosilanes is essential and the stronger the alkyl power supply,the better the promotion of the reaction.In addition,different aromatic hydrocarbons are converted into corresponding carboxylic acids under the reaction system,and aromatic hydrocarbons containing electron-donating substituents are favorable for the reaction.The mixture samples were prepared by stirring Al Cl₃,Ph₃Si Cl and cyclohexane under 3 MPa CO₂pressure for 3 h.The measured sample has an obvious C=O characteristic absorption peak at 1602 cm^-1,speculated that Ph₃Si Cl and Al Cl₃coordinate with CO₂to form a complex.This complex may be composed of silyl haloformic acid.The ester is formed by coordination with AlCl₃,and the reaction mechanism of Al Cl₃and Ph₃Si Cl co-activating CO₂is proposed.AlCl₃may activate toluene or CO₂first when no additive is used.In order to explore the activation mechanism,conducted a density functional theory（DFT）study on the possible activation process.Using the B3LYP method,quantum chemistry calculations were carried out at the level of 3-21G,and the possible mechanism of the direct carboxylation of toluene initiated by Al Cl₃was discussed.Through energy analysis,it was found that the activation energy required for the Al₂Cl₆to participate in the reaction was lower than that of the Al Cl₃,indicating that Al Cl₃might participate in the reaction in the form of dimer.Through comparison,it is concluded that the reaction mechanism of Al₂Cl₆activating toluene first and then carboxylating with CO₂is most likely to occur,which provides theoretical guidance for the Lewis acid-catalyzed carboxylation of aromatic hydrocarbons with CO₂ in the future.