Jin-Quan Wang

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.

Catalytic fixation of CO2 to cyclic carbonates by phosphonium chlorides immobilized on fluorous polymer

Phosphonium chloride covalently bound to the fluorous polymer is proved to be an efficient and recyclable homogeneous CO2-soluble catalyst for organic solvent-free synthesis of cyclic carbonates from epoxides and CO2 under supercritical CO2 conditions. The catalyst can be easily recovered by simple filtration after reaction and reused with retention of high activity and selectivity. In addition, the effects of various reaction variables on the catalytic performance are also discussed in detail. The process represents a simpler access to preparing cyclic carbonates with the ease of homogeneous catalyst recycling.

Carbon dioxide chemistry: Examples and challenges in chemical utilization of carbon dioxide

The development of catalytic methods for chemical transformation of CO2 into useful compounds is of paramount importance from a standpoint of C1 chemistry and so-called green chemistry. The kinetic and thermodynamic stability of CO2 molecule presents significant challenges in designing efficient chemical transformations based on this potential feedstock. In this context, efforts to convert CO2 to useful chemicals will inevitably rely on its activation through molecular catalysts, particularly transition-metal catalysts. Two preparative processes employing solid catalyst or CO2-philic homogeneous catalyst were devised for environmentally benign synthesis of organic carbonates and oxazolidinones under solvent-free conditions. Those processes represent pathways for greener chemical fixations of CO2 to afford industrial useful materials such as organic carbonates and oxazolidinones with great potential applications.

Supercritical carbon dioxide and poly(ethylene glycol): an environmentally benign biphasic solvent system for aerobic oxidation of styrene

Aerobic oxidation of styrene catalyzed by PdCl2/CuCl can be smoothly performed in the supercritical carbon dioxide and poly(ethylene glycol) biphasic system. A high conversion of styrene and yield of acetophenone were obtained in the presence of a relatively low catalyst loading. This environmentally benign biphasic catalytic system can be applied to the Wacker oxidation of various alkenes. Furthermore, the PdCl2-mediated oxidation of styrene was preferentially converted into benzaldehyde using a biphasic scCO2/PEG system. The PEG could effectively immobilize and stabilize the catalysts. The present biphasic system could facilitate product separation and catalyst recycling. The effects of the reaction parameters such as solvent and CO2 pressure were also investigated in detail.

A CO2/H2O2-tunable reaction: direct conversion of styrene into styrene carbonate catalyzed by sodium phosphotungstate/n-Bu4NBr

Inspired by biomimetic oxybromination, a binary catalyst system composed of sodium phosphotungstate and n-Bu4NBr (TBAB) was developed for facile synthesis of styrene carbonate in a single operation from styrene and CO2 using 30% H2O2 as an oxidant. Notably, the presence of a base like NaHCO3 markedly improved the formation of styrene carbonate. Interestingly, an oxidized product, i.e.phenacyl benzoate, could be obtained exclusively in good yield directly from styrene in the absence of CO2 under the appropriate reaction conditions.

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.

Polyethylene Glycol–Enhanced Chemoselective Synthesis of Organic Carbamates from Amines, CO2, and Alkyl Halides

Abstract An efficient and environmentally benign method for the synthesis of organic carbamates was developed. Amines, CO2, and alkyl halides underwent a three-component reaction with the aid of K2CO3 and polyethylene glycol (PEG, MW = 400), affording the organic carbamates under ambient conditions. PEG could presumably act as a solvent and phase-transfer catalyst (PTC). Notably, the presence of PEG could also depress the alkylation of both the amine and the carbamate, thus resulting in enhanced selectivity toward the target carbamate. Supplemental materials are available for this article. Go to the publishers online edition of Synthetic Communications® to view the free supplemental file.