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| Author(s): |
Fábio da Silva Vinhado
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Ribeirão Preto. |
| Institution: | Universidade de São Paulo (USP). Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (PCARP/BC) |
| Defense date: | 2005-03-18 |
| Examining board members: |
Yassuko Iamamoto;
Gregoire Jean Francois Demets;
Ana Maria da Costa Ferreira;
Ynara Marina Idemori;
Ulf Friedrich Schuchardt
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| Advisor: | Yassuko Iamamoto |
| Abstract | |
The oxyfunctionalisation of alkanes under mild conditions is a hard reaction due to the high energy of the C-H bond in these molecules. Some enzymes, from which it should be highlighted cytochromes P450 monooxygenases, are able to promote these oxidations with high efficiency and selectivity. Synthetic metalloporphyrins are often studied as chemical models of the cytochrome P450, since these enzymes contains a Fe(III)porphyrin in their prosthetic groups. This work reports studies on the oxyfunctionalisation of two alkanes, namely cyclohexane and n-pentane, with iodosylbenzene (PhIO) catalysed by the MnIIIporphyrins [Mn{T(4-N-MePy)P}]5+, [Mn(TPFP)]+, [Mn(TDCSPP)]3- and [Mn(TF4TMAPP)]5+ in homogeneous solutions as well as supported on chemically modified silica surfaces, via ionic and/or covalent binding, using acetone, dichloromethane and benzene as reaction solvents. It has been observed high yields of oxygenated products (alcohols and ketones), rather in reactions catalysed by [Mn(TF4TMAPP)]5+, in both conditions, homogeneous and heterogeneous, making this catalyst one of the most promising catalyst for oxidations of organic compounds, mainly when it is immobilised on the supports SiSO3 - e SiSO3 - (IPG), since these supported catalysts are easily obtained and, in addition, the strong electrostatic interactions between the cationic MnIIIporphyrin and the anionic supports avoid the leaching of the Mn-complex from the solid support during the reaction. On the reactivity, it has been noticed difference on the values of alcohol/ketone ratios, mainly for oxidations in acetone. Experiments under inert atmosphere clearly yielded higher values of alcohol/ketone ratio, indicationg the participation of O2 from the air in the reaction mechanism, apart of the solvent used. Studies of intermolecular kinetic isotope effect in the competitive reactions among n-pentane ´ perdeuterated n-pentane indicated, from the values of kH/kD between 6 and 7 obtained for the reactions in the three solvents, that the rate-limiting step is the hydrogen atom abstraction of the alkane from a metal-oxo species, much probably MnV=O, common to all reactions investigated, which should be operating by the classical oxygen rebound mechanism. A more detailed study of the catalytic oxidations in acetone, that conducted to lower values of alcohol/ketone ratio, allowed the detection of 1-(acetyloxi)-2-propanone, which is formed only in reactions with catalyst, under air atmosphere and in presence of substrate. Furthermore, cyclohexane oxidations catalysed by [Mn(TF4TMAPP)]-SiSO3 in presence of bromotrichloromethane, an alkyl radical trapping, in different solvents (dichloromethane, 1,2-dichloroethane, acetone and benzene) allowed us to determine a viscosity effect in the formation of the products alcohols and ketones, that indicated as less viscous is the solvent as high is the escape of alkyl radicals from the solvent cage, being it responsible by the lower values of alcohol/ketone ratios observed in acetone oxidations. From these studies, it has been determined the oxygen rebound mechanism is operating in all catalytic oxidations investigated in this work, but there is a significant escape of alkyl radicals from the solvent cage, mainly for reactions performed in less viscous solvents. The material [Mn(TPFP)]-APDES, obtained by the sol gel method via covalent binding between pentafluoro groups of the metalloporphyrin and ?NH2 groups of the silane, showed a satisfactory catalytic activity in oxidations of n-pentane and cyclohexane. (AU) | |