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Oxidation of Alcohols Catalyzed by TEMPO

Author: MiaoChengXia
Tutor: HeLiangNian
School: Nankai University
Course: Organic Chemistry
Keywords: Carbon dioxide ionic liquid water 2,2,6,6-tetramethyl-piperidine-N-oxy oxidation of alcohol tert-butyl nitrite benzylic olefins Co-salen epoxide
CLC: O643.32
Type: PhD thesis
Year: 2010
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Abstract

With the attention of environmental problems and the development of Green Chemistry, chemists pay more and more attention to developing friendly catalytic systems and synthetic methods. This dissertation concentrates on founding green system instead of the traditional chemical process by reasonably taking advantage of green solvent or designing green catalyst and so on. Then the green system can keep the reactivity, reduce the reaction steps, and avoid using toxic materials. The oxidation of alcohols into the corresponding aldehydes or ketones is one of the most important functional group transformations in organic synthesis, which plays an important role in fundamental research and production of fine chemicals. In this content, selective oxidation of alcohols catalyzed by 2,2,6,6-tetramethyl-piperidine-N-oxy (TEMPO) with high activity is a significant method under mild conditions. This dissertation mainly focuses on the oxidation of alcohols catalyzed by TEMPO and its derivatives using oxygen as the oxidant under solvent-free or green solvent conditions.1) An effective catalytic system comprising a 2,2,6, 6-tetramethyl-piperidine-N-oxy (TEMPO) functionalized imidazolium salt ([Imim-TEMPO]+X-), a carboxylic acid substituted imidazolium salt ([Imim-COOH]+X-), and sodium nitrite (NaN02) was developed for the aerobic oxidation of aliphatic, allylic, heterocyclic and benzylic alcohols to the respective carbonyl compounds with excellent selectivity up to>99%, even at ambient conditions. Notably, the catalyst system could preferentially oxidize a primary alcohol to the aldehyde rather than a secondary alcohol to the ketone. Moreover, the reaction rate was greatly enhanced when a proper amount of water is present. And a high turnover number (TON 5000) was achieved in the present transition metal-free aerobic catalytic system. Additionally, the functionalized imidazolium salts were successfully reused at least four times. In addition, a tentative mechanism of the aerobic oxidation was also proposed that the designed transition metal-free aerobic system was completed by a sequential bicycle or tricycle involving a two-electron transfer step.2) A reversible in situ acidic catalytic system consisting of recyclable TEMPO functionalized imidazolium salt ([Imim-TEMPO][Cl])/NaNO2/CO2/H2O was developed for selective transformation of a series of aliphatic, allylic, heterocyclic, and benzylic alcohols to the respective carbonyl compounds. Notably, the system avoided any conventional acid and can eliminate unwanted byproducts, facilitate reaction, ease separation of catalyst and product, and also provide a safe environment for oxidation involving oxygen gas. In the cases of primary aliphatic alcohols, the longer the chain, the better the activity. And the reason was that the formation of alkylcarbonic acid in the system could compete with the oxidation of alcohols.3) The first bimagnetic ionic liquid based on imidazole was synthesized and the paramagnetic properties were investigated by a superconducting quantum interference device (SQUID) at 25℃. In addition, the magnetic ionic liquid was demonstrated to be an effective catalyst for oxidation of benzylic, heterocyclic alcohols at low temperature under solvent-free conditions or using water as the solvent. Moreover, the magnetic ionic liquid is also convenient to be recovered by simple extraction. And the yield just had a slight decrease while the selectivity retained after five runs. Besides, the reaction should not undergo an’oxoammonium’mechanism judged by ESI-MS (electrospray ionization mass spectrometry) and the substrate scope.4) Recyclable TEMPO-functionalized polyethylene glycol [PEG6000-(TEMPO)2] in combination with cuprous chloride were developed for biomimetic oxidation of a series of benzylic, allylic, heterocyclic alcohols and 2-phenylethanol into the corresponding aldehydes or ketones in high selectivity and in moderate to high conversion in compressed CO2, which enhanced the catalytic activity as well as improved the selectivity.The oxidation of alcohols into the corresponding aldehydes or ketones is the same important as the oxidative cleavage of olefin C=C bond to obtain the carbonyl compounds. CO2 with many unique properties appears to be an ideal solvent for use in oxidation and free-radical reaction. Tert-butyl nitrite releasing alkoxyl radical and NO under high temperature was applied to efficiently and selectively initiate the cleavage of benzylic C=C bond under compressed carbon dioxide. And compressed carbon dioxide played an important role in adjusting the selectivity of the desired product. Besides, a free-radical mechanism was proposed.CO2 can be used as both friendly solvent and green material. Chemical fixation of CO2 is of great interest from the viewpoint of better utilization of carbon resources and the increasing concern on our environment. One of the most promising methodologies in this area is the atom efficient synthesis of five-membered cyclic carbonates through the cycloaddition of CO2 to epoxides. A bifunctional cobalt-salen complex containing a Lewis acidic metal center and a quaternary phosphonium salt unit anchored on the ligand effectively catalyzed the synthesis of cyclic carbonates from CO2 and epoxides under mild conditions without the utilization of additional organic solvents or co-catalysts. Furthermore, the catalyst could be easily recovered and reused several times without significant loss of its catalytic activity. In addition, a mechanism based on cooperative effect of Lewis acid/base was also proposed.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalytic reaction
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