Radical Chemistry Of High-density Carbon Dioxide Polyethylene Glycol

During the past decade, chemists and engineers have been developing industrial or commercial processes to do just that-use liquid CO₂ in place of or in combination with organic solvents to create more "green" operations. Great strides have been made in understanding CO₂'s solvent properties. A reaction in almost any organic solvent using air or O₂ as the oxidant- the least expensive and most atom-efficient route- will form waste byproducts arising from oxidation of the solvent. Consequently, oxidation reactions in CO₂ have been investigated extensively over the past decade.Aerobic oxidation of styrene catalyzed by PdCl₂/CuCl can be smoothly performed in the supercritical carbon dioxide and poly (ethylene glycol) biphasic system. 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 products separation and catalyst recycling.The thermal/oxidative degradation of polyethylene glycol (PEG) is known to occur under oxygen atmosphere at elevated temperature. The utility of this concept of practically utilizable free-radical chemistry of PEG induced by molecule oxygen in dense carbon dioxide is demonstrated to be successfully applied to important and fundamental organic reactions with enormous synthetic potentials. Current applications include selective formylation of primary and secondary aliphatic alcohols, and oxidation of benzylic alcohols, and benzylic C=C cleavage reactions, and benzylic sp3 C-H oxidation. We find that both PEG and molecule oxygen are prerequisite to performing those reactions smoothly. Given that dense CO₂ is immune to free radical chemistry; it is an ideal solvent for such free radical reactions. As a result, dense CO₂ in this elegant study allows such reactions initiated by PEG radical able to be tuned by subtly adjusting reaction parameter like CO₂ pressure, thus leading to enhancing the product selectivity. Attaining high selectivity towards the desired product makes this methodology more practical in organic synthesis. These scrutinous findings inspired with a serendipity in the course of continuing effort devoted to developing efficient sustainable process for the oxidation of organic substrates like alcohols and olefins in PEG/dense CO₂ biphasic system lead to creating a novel concept of utilizable free-radical chemistry of PEG, and would whereby offer an environmentally friendly, metal-free, cost-efficient and viable access to a diverse set of synthetic useful transformations without any additional free radical initiator nor a catalyst.Silica-supported quaternary ammonium salt and ionic liquid proved to be an efficient heterogeneous catalyst for solventless synthesis of cyclic carbonates from epoxides and carbon dioxide under supercritical conditions, which requires no additional organic solvents either for the reaction or for the separation of product. Silica in these studies could promote the reaction. The effects of reaction time, temperature and other reaction parameters on the reaction are investigated. High yields with excellent selectivity were obtained. The purity of product separated directly by filtration from the reaction mixture, reached 99% without further purification process. Moreover, the catalyst can be easily recovered by filtration and reused over 4 times with slightly loss of its catalytic activity. The process represents a simple, ecologically safer, cost-effective route to cyclic carbonates with high product quality, as well as easy product recovery and catalyst recycling.