高密度二氧化碳介质中的聚乙二醇自由基化学

【作者】 王金泉；

【导师】 何良年；

【作者基本信息】 南开大学 ， 有机化学， 2009， 博士

【摘要】 随着近年来对环境问题的日益重视,发展环境友好的合成方法已越来越引起有机化学家的关注,超临界二氧化碳作为有机溶剂的代替品已经进行了广泛的研究,在氢化反应中已实现工业化。氧化反应是有机化学中最基本的也是最重要的反应之一,工业过程多以氧气作为氧化剂,但反应过程会受传质的限制。超临界二氧化碳在氧化反应中展现了诸多优点,如:本身不会被氧化、可提高氧气的操作压力、改善体系的传质等。本论文主要着眼于利用超临界二氧化碳的独特性质,发展环境友好的超临界二氧化碳中的氧化反应,超临界二氧化碳在反应中不仅可以代替有机溶剂,而且可以加速反应,控制反应的选择性等。此外,二氧化碳既是最主要的温室气体,同时也是地球上分布最广、储量最丰富的碳资源之一。无论从资源利用还是环境保护的角度考虑,二氧化碳的固定和化学转化的研究都具有重要意义。论文的另一部分主要着眼于发展新的催化方法实现二氧化碳转化利用。Wacker氧化是氧化反应中最重要的反应之一,无论在工业生产还是有机合成中都占有重要的地位。目前,Wacker氧化的发展主要受限于贵重钯金属催化剂的失活,催化剂钯难以回收再用。我们研究发现,Wacker反应可以在超临界二氧化碳和聚乙二醇两相体系中顺利进行,超临界二氧化碳不仅可以提供安全的氧气操作环境,而且可以促进氧化反应的进行,反应结束后还可以用来萃取产物。聚乙二醇不仅可以固定催化剂,而且还可以稳定钯催化剂,使催化剂有更长的使用寿命。另外,对苯乙烯类底物,通过调节助催化剂,可选择性的生成Wacker反应产物和C=C键断裂产物。聚乙二醇作为一种环境友好、生物可降解的高分子化合物,被广泛用作热能储存材料、反应溶剂、相转移催化剂等。聚乙二醇氧化降解此前已经有很多报导,主要经过了自由基过程,而聚乙二醇氧化降解过程一直没有在有机合成中得到应用。通过一系列实验发现,聚乙二醇降解产生的自由基可以引发自由基反应,包括苄位C=C双键的断裂反应、苄位sp~3 C-H键的氧化反应、苄醇的氧化反应和甲酰化反应。另外,加入少量的金属钴盐或者锰盐可以促进苄位sp~3C-H键的氧化反应。结合机理的研究,我们提出了聚乙二醇自由基引发的反应机理。另外,高密度的二氧化碳在聚乙二醇自由基引发的反应中发挥了重要作用,如加速反应,调节产物的选择性等。二氧化碳既是最主要的温室气体,同时也是地球上分布最广、储量最丰富的碳资源之一。无论从资源利用还是环境保护的角度考虑,二氧化碳的固定和化学转化的研究都具有重要意义。其中环氧化物和二氧化碳原子经济性地合成具有重要应用价值的如环状碳酸酯是化学固定二氧化碳有效方法之一。离子液体在催化二氧化碳与环氧化物的环加成反应方面有很多报导。离子液体和环状碳酸酯都是高沸点化合物,分离环状碳酸酯往往需要减压蒸馏,使得过程复杂而且需要消耗大量的能量。为更好地解决产物的分离问题,我们分别研究了二氧化硅负载季铵盐和二氧化硅负载的咪唑类离子液体催化二氧化碳和环氧化物的环加成反应,用二氧化硅负载离子液体不但有利于催化剂的回收,而且可以提高催化剂的催化活性。采用超临界二氧化碳作为反应介质,反应结束后,释放出二氧化碳,只需要过滤就可得到98%以上纯度的产物,不需要任何有机溶剂。此外,该工艺操作简单,可用于流动式(固定床)反应。更多还原

【Abstract】 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.更多还原