Cheng-Xia Miao

Bifunctional metal-salen complexes as efficient catalysts for the fixation of CO2 with epoxides under solvent-free conditions.

A bifunctional cobalt-salen complex containing a Lewis acidic metal center and a quaternary phosphonium salt unit anchored on the ligand effectively catalyzes the synthesis of cyclic carbonates from CO2 and epoxides under mild conditions without the utilization of additional organic solvents or co-catalysts. The effects of various reaction variables on the catalytic performance were studied in detail and indicate an optimized reaction temperature of about I00 degrees C and CO2 pressure of around 4 MPa, although the reaction proceeds smoothly even at pressures as low as 2 MPa. The catalyst is applicable to a variety of epoxides, producing the corresponding cyclic carbonates in good yields in most cases. Furthermore, the catalyst can be easily recovered and reused several times without significant loss of its catalytic activity. This process thus represents a greener pathway for the environmentally benign chemical fixation of CO2 to produce cyclic carbonates.

Efficient synthesis of dimethyl carbonate from methanol, propylene oxide and CO2 catalyzed by recyclable inorganic base/phosphonium halide-functionalized polyethylene glycol

The cycloaddition of propylene oxide and CO2 to form propylene carbonate promoted by a phosphonium salt covalently bound to polyethylene glycol (PEG), and the transesterification of propylene carbonate with methanol to dimethyl carbonate (DMC) mediated by PEG-supported K2CO3, were separately investigated. Inorganic base/phosphonium halide-functionalized PEG (K2CO3/BrBu3PPEG6000PBu3Br) was shown to be active for DMC synthesis from propylene oxide, CO2 and methanol under mild reaction conditions, even under low CO2 pressure (2 bar). The effects of various reaction variables on the activity and selectivity performance are discussed in detail. The catalyst was readily separated and reused, without catalyst leaching being detected by 31P NMR. Notably, excellent yields of DMC and complete conversion of propylene carbonate were reached under the optimized reaction conditions. This procedure was successfully applied to the synthesis of other dialkyl carbonates.

Self-neutralizing in situ acidic CO2/H2O system for aerobic oxidation of alcohols catalyzed by TEMPO functionalized imidazolium salt/NaNO2.

A reversible in situ acidic catalytic system comprising recyclable TEMPO functionalized imidazolium salt ([Imim-TEMPO][Cl])/NaNO(2)/CO(2)/H(2)O was developed for selective transformation of a series of aliphatic, allylic, heterocyclic, and benzylic alcohols to the respective carbonyl compounds. Notably, the system avoids any conventional acid and can eliminate unwanted byproducts, facilitate reaction, ease separation of the catalyst and product, and also provide a safe environment for oxidation involving oxygen gas.

Ethylene carbonate as a unique solvent for palladium-catalyzed Wacker oxidation using oxygen as the sole oxidant

Ethylene carbonate (EC) as a unique solvent for the Wacker oxidation of higher alkenes and aryl alkenes has been successfully developed using molecular oxygen as the sole oxidant, in which colloidal Pd nanoparticles stabilized in EC are considered to facilitate its reoxidation under cocatalyst-free conditions.

Tert-butyl nitrite: a metal-free radical initiator for aerobic cleavage of benzylic CC bonds in compressed carbon dioxide

Tert-butyl nitrite easily releasing alkoxyl radical and NO in combination with compressed CO2 under metal free condition was applied to efficiently and selectively initiate aerobic cleavage of benzylic CC bonds. Notably, compressed CO2 in this study not only provides a safe reaction environment but also tunes the selectivity.

The Free-Radical Chemistry of Polyethylene Glycol: Organic Reactions in Compressed Carbon Dioxide

The thermal oxidative degradation of polyethylene glycol (PEG) is known to occur in an oxygen atmosphere at elevated temperatures. In this study, PEG radicals assumed to result from thermal oxidative degradation are successfully applied, in combination with compressed CO(2), to initiate a range of free-radical reactions, such as selective formylation of primary and secondary aliphatic alcohols, oxidation of benzylic alcohols, benzylic C==C bond cleavage, and benzylic sp(3) C--H oxidation, demonstrating enormous synthetic potential in a cost-efficient, practically useful, and environmentally friendly manner; not requiring any catalyst or additional free-radical initiator. We find that both PEG and molecular oxygen are prerequisites in order to perform these reactions smoothly. Given that dense CO(2) is immune to free-radical chemistry; it is an ideal solvent for such reactions. As a result, compressed CO(2) allows reactions initiated by PEG radicals to be tuned by subtly adjusting reaction parameters such as the CO(2) pressure, thereby enhancing the product selectivity. By attaining a high selectivity towards the desired products this methodology is practical for organic syntheses.

Catalytic Processes for Chemical Conversion of Carbon Dioxide into Cyclic Carbonates and Polycarbonates

Chemical fixation of CO2 has received much attention because CO2 is the most inexpensive and renewable carbon resource from the viewpoint of green chemistry and atom economy. The kinetic and thermodynamic stability of CO2 molecule presents significant challenges in designing efficient chemical transformations based on this potential feedstock. Currently, cyclic carbonates and polycarbonates are both valuable products, and the coupling of CO2 and epoxides is one of the most promising and eco-friendly methods for chemical conversion of CO2 into cyclic carbonates and/or polycarbonates. In this review, we will mainly discuss the synthesis of polycarbonates or cyclic carbonates catalyzed by metalsalen complexes through adjusting the architecture of the ligands and optimizing reaction conditions, such as temperature, pressure, co-catalysts, epoxide concentration. Furthermore, the recent progress for the synthesis of cyclic carbonates via the coupling reactions of epoxides and CO2 mediated by both homogeneous and heterogeneous catalysts is particularly reviewed.

Catalytic Processes for Chemical Conversion of Carbon Dioxide into Cyclic Carbonates and Polycarbonates~!2008-03-31~!2008-04-14~!2008-06-10~!

Chemical fixation of CO2 has received much attention because CO2 is the most inexpensive and renewable carbon resource from the viewpoint of green chemistry and atom economy. The kinetic and thermodynamic stability of CO2 molecule presents significant challenges in designing efficient chemical transformations based on this potential feedstock. Currently, cyclic carbonates and polycarbonates are both valuable products, and the coupling of CO2 and epoxides is one of the most promising and eco-friendly methods for chemical conversion of CO2 into cyclic carbonates and/or polycarbonates. In this review, we will mainly discuss the synthesis of polycarbonates or cyclic carbonates catalyzed by metalsalen complexes through adjusting the architecture of the ligands and optimizing reaction conditions, such as temperature, pressure, co-catalysts, epoxide concentration. Furthermore, the recent progress for the synthesis of cyclic carbonates via the coupling reactions of epoxides and CO2 mediated by both homogeneous and heterogeneous catalysts is particularly reviewed.