- Decatungstate catalyst supported on silica and γ-alumina: Efficient photocatalytic oxidation of benzyl alcohols
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Four supported catalysts with the same tungsten loading were prepared by depositing decatungstate species W10O4-32, through wet impregnation, on the surface of γ-alumina and silica at different pH values. The prepared samples were characterized using BET measurements as well as XRD, UV-vis DR, and XP spectroscopies. Higher dispersion of W(VI) oxo-species was obtained in the silica-supported catalysts compared with the corresponding alumina-supported ones. Within the same support, the dispersion was higher when the impregnation pH is lower than the point of zero charge (pzc) of the support. The decatungstate anions were present mainly on the silica surface without any modification, whereas these underwent a partial depolymerization on their deposition on the γ-alumina surface. The extent of depolymerization was less in the sample prepared at pH above pzc. These findings were explained in terms of the mode of deposition of the W(VI) species from the solution onto the support surface. The photocatalytic activity of the aforementioned catalysts, concerning the photooxidation of 1-phenylethanol, depends on the fraction of the W10O4-32 supported species rather than on the W(VI) dispersion. Thus, extremely high conversions have been obtained over the silica-based catalysts and also over the γ-alumina-based catalyst prepared at relatively high pH. These catalysts also are very effective in the photooxidation of a series of secondary and primary benzyl alcohols, in which benzyl ketones and benzoic acids were formed as the only or major products, respectively. The easy separation of the solid catalyst from the reaction mixture, the high activity, selectivity, and stability as well as the retained activity in subsequent catalytic cycles, make these supported catalysts suitable for a small-scale synthesis. Based on product analysis and kinetic data on the heterogeneous oxidation of benzyl alcohols, we suggest that a hydrogen abstraction transfer (HAT) mechanism predominates with respect to an electron transfer (ET) one in these reactions.
- Tzirakis, Manolis D.,Lykakis, Ioannis N.,Panagiotou, George D.,Bourikas, Kyriakos,Lycourghiotis, Alexis,Kordulis, Christos,Orfanopoulos, Michael
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p. 178 - 189
(2008/09/18)
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- Biomimetic catalysis with immobilised organometallic ruthenium complexes: Substrate- and regioselective transfer hydrogenation of ketones
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Chloro-(η6-arene) complexes of ruthenium(II) with N-sulfonyl-1,2-ethylenediamine ligands that have one or two styrene side chains have been synthesised and characterised. The chloro ligand was substituted with a diphenylphosphinato ligand and the resulting organometallic complexes are transition state analogues for the ruthenium-catalysed transfer hydrogenation of benzophenone. Following the protocol of molecular imprinting, these complexes were copolymerised with ethylene glycol dimethacrylate (EGDMA) in the presence of a porogen. The polymers were ground and sieved, and the phosphinato ligand was substituted with a chloro ligand, thus generating a shape-selective cavity in close proximity to the catalytically active metal centre. When tested for their ability to catalyse the reduction of benzophenone, the imprinted polymers showed a significantly higher activity (up to a factor of seven) than control polymers without cavities. Out of a mixture of seven different aromatic and aliphatic ketones, benzophenone was preferentially reduced when the imprinted polymer was used. Furthermore, the specificity of the catalyst for diaryl ketones has been confirmed in a reaction with a bifunctional substrate, 4-acetyl-benzophenone; the diaryl ketone was reduced faster with the imprinted catalyst than the acetyl group. The opposite regioselectivity was observed with the control polymer. Both the activity and the selectivity of the imprinted catalysts are dependent on how the ruthenium complexes are attached to the polymer backbone. A double connection proved to give superior results.
- Polborn, Kurt,Severin, Kay
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p. 4604 - 4611
(2007/10/03)
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- Multiple-photon chemistry in the benzophenone photoreduction during laser-jet photolysis: Effect of alcohol solvent on cross-coupling versus hydrogen abstraction of the electronically excited hydroxydiphenylmethyl radical
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The ground state of the hydroxydiphenylmethyl radical (1) leads to benzpinacol (2) through head-to-head coupling and the diols 3 as cross-coupling product. In contrast, under the high-intensity conditions of the laser-jet photolysis, the excited radical 1* couples in the para position to afford the benzophenone derivatives 4 (head-to-tail coupling) (higher spin density at the para position/AM1 calculations). The major two-photon product is benzhydrol (5). The pronounced increase in hydrogen atom abstraction from MeOH to iPrOH by 1* is explained in terms of the greater electrophilic character of the electronically excited radical 1* versus its ground state.
- Adam, Waldemar,Walther, Barbara
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p. 10399 - 10404
(2007/10/03)
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- Two-Photon Chemistry in the Laser Jet: Photochemistry of the Diphenylhydroxymethyl Radical
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In the laser-jet mode (argon ion laser) the excited hydroxydiphenylmethyl radical (3*) underwent H expulsion in inert solvents (benzene, acetonitrile) to produce benzophenone (two-photon product), besides dimerization of its ground state 3 to benzpinacol (one-photon product).In ethanol, the laser-jet photolysis of benzophenone afforded as new products (two-photon chemistry) benzhydrol, 4-(1-hydroxyethyl)benzophenone (1), together with benzpinacol and 1,1-diphenyl-1,2-propandiol (4), which are also observed in low-intensity photolyses (one-photon chemistry).Benzhydrol is formed through H abstraction from the excited radical 3* and ketone 1 possibly by coupling of an excited radical 3* at its para position (highest spin density) with a transient 1-hydroxyethyl radical.Photolysis of α-phenylbenzoin (2) produced benzpinacol, benzophenone, pinacol 4, benzhydrol, ketone 1, and the benzoyl-derived products benzil and benzaldehyde.Also benzhydrol was observed as a two-photon product.Key Words: Two-photon chemistry / Laser jet / Hydroxydiphenylmethyl radical / Hydrogen abstraction / Hydrogen atom expulsion
- Adam, Waldemar,Oestrich, Rolf Schulte
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p. 2463 - 2466
(2007/10/02)
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- Synthesis of Hydroxy Ketones by Chemoselective Alkylation of Keto Aldehydes with Dialkylzincs Catalyzed by Amino Alcohol, Diamine, or Dilithium Salt of Piperazine
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Hydroxy ketones were obtained in high yields by the chemoselective alkylation of formyl group of keto aldehydes with dialkylzincs in the presence of such catalysts as 2-dimethylaminoethanol, N,N,N',N'-tetramethylethylenediamine, ordilithium salt of pipera
- Soai, Kenso,Watanabe, Masami,Koyano, Masashi
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p. 2124 - 2125
(2007/10/02)
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- SELECTIVE PROTECTION OF CARBONYL GROUPS BY TAKING ADVANTAGE OF A COMBINATION OF SOLID SUPPORT AND GIRARD'S REAGENT.
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Some Girard's derivatives of 4-phenylbenzophenone were prepared in ethanol solution, in the presence of Amberlite IRC-50 as a catalyst. Supported samples of the derivatives were then prepared by addition of silica gel to the solution, followed by evaporat
- Chihara,Waniguchi,Wakabayashi,Taya
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p. 2479 - 2483
(2007/10/02)
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- NEW OBSERVATIONS ON THE PHOTOPINACOLIZATION OF BENZOPHENONE IN ALIPHATIC ALCOHOLS
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Irradiation of benzophenone in alcohol solutions produces unsymmetrical pinacols and para-coupled products as well as benzpinacol; the product composition depends on light intensity.
- Rubin, Mordecai B.
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p. 4615 - 4618
(2007/10/02)
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