Preparation Of Immobilized TEMPO Catalyst And Its Catalytic Property In Oxidation Of Alcohols

The oxidation of alcohols into the corresponding aldehydes or ketones is one of themost important functional group transformations in organic synthesis，which plays animportant role in fundamental research and production of fine chemicals.2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPO) has attracted much attention for whichit has expressed efficient performance in the oxidation of alcohol under mild condition.With the attention of environmental problems and the development of Green Chemistry，chemists pay more and more attention to developing friendly catalytic systems. Theimmobilization of TEMPO is one of the most important subjects in the study of catalyticoxidation. In our present investigation, a new preparation method of crosslinked polymer(glycidyl methacrylate) microspheres (CPGMA) as the carrier to manufacturerimmobilized TEMPO catalyst has been designed. On this basis, a co-catalyst system wasconstituted with the polymer-supported catalyst as major catalyst was used to realize thecatalytic oxidation of benzyl alcohol and cyclohexanol with molecular oxygen asoxidant, and their catalytic characters were examined and investigated.First of all, crosslinked polymer (glycidyl methacrylate) microspheres (CPGMA)were prepared with glycidyl methacrylate as monomer and ethylene dimethacrylate ascrosslinker by suspension polymerization. The ring opening reaction between the epoxygroups on the CPGMA microspheres and4-hydroxy-2,2,6,6-tetramethyl-piperidinyl-1-oxy (4-OH-TEMPO) was carried out toimmobilize TEMPO on the polymer microspheres. TEMPO-immobilized microspheres(TEMPO/CPGMA) were obtained and were characterized by several methods. Theeffects of the main factors in the immobilization were examined so that the reaction conditions were optimized. The experimental results showed that the ring openingreaction of the epoxy group allowed the immobilization of TEMPO to be smoothlyrealized on the CPGMA polymer microspheres, on which there were many epoxy groups.The ring opening reaction of the epoxy group is a nucleophilic substitution reaction withthe SN2reaction mechanism, and N, N′-dimethyl formamide with a strong polarity is anappropriate solvent and a suitable temperature is85°C.After obtaining the the microspheres TEMPO/CPGMA, a co-catalyst system wasconstituted with TEMPO/CPGMA and CuCl and was used in the oxidation reaction ofbenzyl alcohol by molecular oxygen. The catalytic activity of the co-catalyst system andthe effects of the main factors were examined. The results show that the co-catalystsystem consisting of the heterogeneous catalyst TEMPO/CPGMA and CuCl efficientlycatalyzed the oxidation reaction of benzyl alcohol by molecular oxygen. Under mildconditions, benzyl alcohol was transformed to benzaldehyde with100%selectivity and ayield of90%. The suitable molar ratio of the main catalyst TEMPO/CPGMA to theco-catalyst CuCl is1:1.2and the suitable amount of TEMPO/CPGMA is0.90g. TheTEMPO/CPGMA catalyst has excellent recyclability.Bidentate Schiff base copper complex formed by benzaldehyde (BA) andortho-aminophenol (AP) was bonded chemically on crosslinked poly(glycidylmethacrylate) microspheres CPGMA via macromolecular reactions, resulting in thesupporter microspheres BAAP-CPGMA, on which Schiff base ligands wereimmobilized. And then through the coordionation reaction with copper salt, theimmobilized-Schiff base copper complex microspheres [Cu(BAAP)2]-CPGMA wereobtained. A co-catalyst was constituted with the microspheres [Cu(BAAP)2]-CPGMAand2,2,6,6-four methyl piperidine nitroxide (TEMPO) free radical, and it was used inthe oxidation reaction of benzyl alcohol by molecular oxygen. The catalytic activity ofthis co-catalyst system was examined and the catalytic mechanism was researched. Theexperimental results show that the co-catalyst system consisting of the homogeneousmain catalyst TEMPO and the heterogeneous sub-catalyst [Cu(BAAP)2]-CPGMA can efficiently catalyzed the oxidation reaction of benzyl alcohol by molecular oxygen undermild conditions including at room temperature and at normal pressure for oxygen, andbenzyl alcohol is transformed to benzaldehyde with100%selectivity and with a yield of93%. Furthermore, the co-catalyst has excellent cycle and reuse property.At last，a combination of the TEMPO/CPGMA microspheres as heterogenouscatalyst and Fe(NO3)3as homogenous co-catalyst was constituted, and thiscombinatorial catalyst was used in the oxidation reaction of cyclohexanol by molecularoxygen. The catalytic property of this combinatorial catalyst and the catalyticmechanism were investigated, and the effects of the main factors on this oxidationreaction were examined. The experimental results show that this combinatorial catalystcan effectively catalyze the oxidation reaction of cyclohexanol by molecular oxygen andmakes cyclohexanol to transform to cyclohexanone as the sole product, showingexcellent catalytic selectivity. Both two species in Fe(NO3)3, Fe3+ion and NO3-ion, actwell, and they together play a co-catalyst role. For this combinatorial catalyst,TEMPO/CPGMA microspheres and Fe(NO3)3, the suitable molar ratio of theimmobilized TEMPO to Fe(NO3)3is equal to1:1.0, and under the optimized conditions,including at the temperature of55°C, at ordinary pressure of oxygen and with a suitableadded amount of this combinatorial catalyst, the cyclohexanone yield can get up to44.1%, realizing the transforming of cyclohexanol to cyclohexanone under the mildconditions.