Publications
2001
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(2001) Journal of Organic Chemistry. 66, 25, p. 8650-8653 Abstract
The discovery of new environmentally friendly methods for selective catalytic oxidation of alcohol substrates to aldehyde and ketones is an important goal in the development of modern methods for chemical synthesis. 1 In this context, of special significance is the use of intrinsically non-waste-producing oxidants such as molecular oxygen from air and hydrogen peroxide. It has been shown in the past that stable nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) is able to mediate the oxidation of primary alcohols to aldehydes with a variety of terminal oxidants. 2 Especially common is the use of hypochlorite, 3 but the use of electrocatalytic conditions, 4 peracetic acid together with a catalytic amount of bromide, 5m-chloroperbenzoic acid, 6 bromite, 7 persulfate, 8 and hydrogen peroxide together with hydrogen bromide and methylrhenium trioxide as catalyst have also been reported. 9 The use of these nitroxyl reaction systems has one or more disadvantages including a high price of oxidant, formation of considerable amounts of organic waste, and the use of halide (chloride and/or bromide)-containing oxidants which in turn form nondesirable halide-containing wastes. To obviate these disadvantages, molecular oxygen can also as be used as terminal oxidant in the presence of cupric ion catalysts, 10 a ruthenium catalyst, RuCl2(PPh3)3, 11 or enzymes with or without metal complexes.12 These methods have the advantage of being intrinsically waste-free synthetic systems as concerns the oxidant. However, they have other disadvantages. The copper-based method is largely ineffective for simple (nonallylic or nonbenzylic) aliphatic especially secondary alcohols. In addition, relatively high catalytic loads, usually at least 10 mol % each of TEMPO and cupric ion are needed. The ruthenium-based system overcomes the disadvantage of the lack of reactivity for simple aliphatic alcohols, but it requires an expensive noble metal and preferably an intrinsically oxidatively unstable triphenylphosphine ligand, which has to be present in order to obtain appreciable catalyst activity. The enzyme-based method affords only low to modest conversions after long reaction times, and the rate of these enzymatically catalyzed oxidation reactions cannot be increased by significantly increasing the temperature because of the thermal lability of the enzymes.
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(2001) Synthetic Metals. 124, 1, p. 59-61 Abstract
The film preparation properties and amplified spontaneous emission (ASE) features of a conjugated polymer blended with a zirconium-organosilicon xerogel glassy matrix were studied. Dip and spin coating methods were used to perform polarization and washing steps for the conjugated polymer. Silicon wafer covered with a 7μm thick silica layer was used as a waveguide substrate. The characterization of the absorption and fluorescence spectra of the films signified the evidence of an enhanced gap between the two bands.
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(2001) Journal of the American Chemical Society. 123, 35, p. 8531-8542 Abstract
The mechanism of aerobic oxidation of aromatic and alkyl aromatic compounds using anthracene and xanthene, respectively, as a model compound was investigated using a phosphovanadomolybdate polyoxometalate, H5PV2Mo10O40, as catalyst under mild, liquid-phase conditions: The polyoxometalate is a soluble analogue of insoluble mixed-metal oxides often used for high-temperature gas-phase heterogeneous oxidation which proceed by a Mars-van Krevelen mechanism. The general purpose of the present investigation was to prove that a Mars-van Krevelen mechanism is possible also in liquid-phase, homogeneous oxidation reactions. First, the oxygen transfer from H5PV2Mo10O40 to the hydrocarbons was studied using various techniques to show that commonly observed liquid-phase oxidation mechanisms, autoxidation, and oxidative nucleophilic substitution were not occurring in this case. Techniques used included (a) use of 18O-labeled molecular oxygen, polyoxometalate, and water; (b) carrying out reactions under anaerobic conditions; (c) performing the reaction with an alternative nucleophile (acetate) or under anhydrous conditions; and (d) determination of the reaction stoichiometry. All of the experiments pointed against autoxidation and oxidative nucleophilic substitution and toward a Mars-van Krevelen mechanism. Second, the mode of activation of the hydrocarbon was determined to be by electron transfer, as opposed to hydrogen atom transfer from the hydrocarbon to the polyoxometalate. Kinetic studies showed that an outer-sphere electron transfer was probable with formation of a donor-acceptor complex. Further studies enabled the isolation and observation of intermediates by ESR and NMR spectroscopy. For anthracene, the immediate result of electron transfer, that is formation of an anthracene radical cation and reduced polyoxometalate, was observed by ESR spectroscopy. The ESR spectrum, together with kinetics experiments, including kinetic isotope experiments and 1H NMR, support a Mars-van Krevelen mechanism in which the rate-determining step is the oxygen-transfer reaction between the polyoxometalate and the intermediate radical cation. Anthraquinone is the only observable reaction product. For xanthene, the radical cation could not be observed. Instead, the initial radical cation undergoes fast additional proton and electron transfer (or hydrogen atom transfer) to yield a stable benzylic cation observable by 1H NMR. Again, kinetics experiments support the notion of an oxygen-transfer rate-determining step between the xanthenyl cation and the polyoxometalate, with formation of xanthen-9-one as the only product. Schemes summarizing the proposed reaction mechanisms are presented.
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(2001) Catalysis Letters. 72, 3-4, p. 241-243 Abstract
New metallosilicate catalysts were prepared by reacting a silanol capped dendrimer, Si[CH2CH2Si(CH3)2OH]4 with MCp2Cl2 (M = TiIV, MoVI, WVI and VV). The resulting Si[CH2CH2Si(CH3)2OMCp 2Cl]4 compounds were incorporated in a silica matrix by the sol-gel method. The catalytic activity of the metallosilicates after calcination revealed excellent activity and selectivity towards epoxidation of alkenes with tert-butylhydroperoxide. Maximum activity was observed with molybdenum-containing materials. Analysis of the catalytic activity revealed that the catalysts were truly heterogeneous.
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(2001) Chemical Communications. 5, p. 487-488 Abstract
A silane functionalized by octafluoroacetophenone was polymerized by the sol-gel method to form an insoluble silicate with perfluoroketone pendants; the silicate was used as a heterogeneous catalyst for the activation of aqueous hydrogen peroxide and the oxidation of aromatic amines and alkenes.
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(2001) Synthetic Metals. 119, 1-3, p. 143-144 Abstract
Composites of PPV in PVA, PVK and other polymeric matrixes were prepared by an in-situ polymerization of 1,4 phenylene dimethylene-bis-(tetramethylene sulfonium chloride) or □□□-dibromo-p-xylene. The monomer was converted directly to PPV in the polymeric matrix. This polymerization technique was successfully carried out by the use of a base, at room temperature, without requiring removal of oligomers and was completed in several minutes. EL and PL of the composites were measured and compared with PPV. A blue shift was found in the PL spectrum of the PPV-PVA composite compared to PPV. Other composites showed similar PL to that of PPV.
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(2001) European Journal of Inorganic Chemistry. 3, p. 789-794 Abstract
An antimony-substituted polyoxomolybdate, [PSbIII(H2O) -Mo11O39]4-, of the α-Keggin structure, has been synthesized. Although the molecular structure is typical, we have found from the analysis of the X-ray diffraction data that the crystal is anisotropic with alignment of the molecular units so that the substituted antimony atoms are all directed along the polar c axis and on the same side of the molecular unit. UV/Vis, ESR and magnetic susceptibility data support a formulation of [PSbIII(H2O)Mo11O39]4- whereby there is partial electron delocalization with some formation (10-20%) of MoV. Oxidation of [PSbIII(H2O)Mo11O39]4- with ozone or bromine yields the oxidized species [PSbV(O)Mo11O39]4- and [PSbV(Br)Mo11O39]3-. In the anaerobic oxydehydrogenation of alcohols, [PSbV(O)Mo11O39]4- is more active than [PSbV(Br)Mo11O39]3-. Catalytic aerobic oxidation of benzylic alcohols to aldehydes was also possible with no over-oxidation and formation of carboxylic̈ acids.
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