Synthesis Of Pyridine-bridged Bifunctional Organocatalysts And Application For Cycloaddition Of Carbon Dioxide

CO₂ is one of the greenhouse gases that cause global warming.Therefore,the fixation and transformation of CO₂ has become a research hotspot in the field of chemical research.Among the many chemical conversion pathways of CO₂,the cycloaddition reaction of CO₂ with epoxides to prepare cyclic carbonates has become the most widely studied type in chemical reaction,because it has environmental friendly,high atomic economy and wide application features.However,the thermodynamic stability of CO₂is very high,wherein the ring opening step of the epoxide is the rate controlling step in cycloaddition reaction of CO₂.Cyclic carbonates are widely used in industrial production,but the current industrial production of cyclic carbonates still use highly toxic phosgene as raw material.In order to develop a highly efficient catalyst for the cycloaddition of CO₂,we designed a series of catalysts base on bifunctional pyridine bridged imidazolium salt and applied them to the cycloaddition of CO₂.The cycloaddition reaction of CO₂ with epoxide can be achieved by the action of bifunctional synergistic catalysis under mild conditions.Our group developed a series of synthesis methods for the asymmetric pyridine-bridged pincer-type ligand,we synthesized 14 kinds of imidazolium or benzimidazole salts by introducing carboxyl or hydroxyl functionalized onto 2,6-dibromopyridine.It is used as catalyst for CO₂ conversion,we also studied the the catalytic mechanism and catalytic performance of the catalyst by changing the type of halide ion and the position of the hydroxyl group on the benzene ring.Firstly,the catalyst was used to catalyze the cycloaddition reaction of CO₂ with epoxide.Based on the experimental results,we came to the following conclusions:?1?The catalysts of benzimidazole substituent has higher catalytic activity than the imidazole substituent;?2?The catalysts base on hydroxyl functionalized has higher catalytic activity than the carboxyl functionalized,due to poor solubility of the carboxyl catalyst in the reaction system;?3?The catalysts with iodide ionization had higher catalytic activity than the catalysts with chloride ion and bromide ion,because the iodide ionization has stronger leaving ability;?4?The position of the hydroxyl group on the benzene ring had a great influence to the reaction proformance,and the catalytic activity of the para-hydroxy substituent catalysts had higher catalytic activity than meta-and ortho-hydroxyl substituent catalysts;?5?The hydroxyl group on the catalyst is the key role to improve catalystic activity,and its catalytic activity is drastically reduced after esterification of the hydroxyl group.Oxazolidinones are important precursors or raw materials of chemical pharmaceuticals.One-pot synthesis of oxazolidinone by CO₂,epoxide and aromatic amines is one of the most efficient methods to synthesise oxazolidinone.However,the reported synthesis methods usually requires high temperature and high CO₂ pressure.We also apply the pyridine bridged bifunctional imidazolium salt catalyst to the one-pot synthesis of oxazolidinone by CO₂,epoxide and aromatic amines,the catalyst t can be catalytically prepared oxazolidinone under relatively mild conditions?90°C,0.3 MPa CO₂?,and the substrate universality are very well.The catalyst base on carboxyl functionalized show the highest catalytic efficiency.