MTA SZTE Stereochemistry Research Group

Szeged, Hungary

MTA SZTE Stereochemistry Research Group

Szeged, Hungary
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Szollosi G.,MTA SZTE Stereochemistry Research Group | Kovacs L.,University of Szeged | Kozma V.,University of Szeged | Kolcsar V.J.,University of Szeged
Reaction Kinetics, Mechanisms and Catalysis | Year: 2017

A study on the asymmetric Michael addition of a fluorine containing carbon nucleophile to β-nitrostyrene was carried out to find an easily obtainable cinchona alkaloid derivative, which provides high stereoselectivities, and may be conveniently immobilized over inorganic materials to obtain efficient chiral heterogeneous catalysts. It was shown that high enantioselectivities are reached in the addition of ethyl 2-fluoroacetoacetate catalyzed by β-isocupreidine accompanied by good diastereomeric ratios. This cinchona derivative prepared in one step from quinidine was immobilized by cation-exchange between the layers of an aluminum phyllosilicate, as evidenced by XRD measurements. However, due to the protonation of the tertiary amino group, the material lost its catalytic activity in the Michael addition. The immobilization of the deprotonated alkaloid over the particle surface of an anion exchanger layered double hydroxide resulted in an inorganic–organic hybrid material, with catalytic performance approaching that of the soluble organocatalyst. Upon reuse, gradual deactivation of this heterogeneous catalyst was observed due to the leaching of the organic material. However, using less polar media could increase the lifetime of the hybrid catalyst. The results of catalytic measurements indicated that the Michael addition might occur on the solid surface. Based on the catalytic behavior of the heterogeneous catalyst, bonding possibilities of the cinchona derivative to the surface of the layered hydroxide by electrostatic interactions and hydrogen bonding are suggested. © 2017 Akadémiai Kiadó, Budapest, Hungary

Chen K.-Y.,University of Groningen | Wezenberg S.J.,University of Groningen | Carroll G.T.,University of Groningen | London G.,University of Groningen | And 4 more authors.
Journal of Organic Chemistry | Year: 2014

The design, synthesis, and dynamic behavior of a series of novel tetrapodal molecular switches and motors containing common functional groups for attachment to various inorganic and organic surfaces are presented. Using a Diels-Alder reaction, an anthracene unit with four functionalized alkyl substituents ("legs") was coupled to maleimide-functionalized molecular switches or motors under ambient conditions. Terminal functional groups at the "legs" include thioacetates and azides, making these switches and motors ideal candidates for attachment to metallic or alkyne-functionalized surfaces. UV/vis absorption spectroscopy shows that the molecular switches and motors retain their ability to undergo reversible photoinduced and/or thermally induced structural changes after attachment to the tetrapodal anthracene. © 2014 American Chemical Society.

Szollosi G.,MTA SZTE Stereochemistry Research Group | Csampai A.,Eötvös Loránd University | Somlai C.,University of Szeged | Fekete M.,University of Szeged | And 2 more authors.
Journal of Molecular Catalysis A: Chemical | Year: 2014

The heterogeneous asymmetric direct aldol reactions between aldehydes (2-nitrobenzaldehyde, 2-methylpropanal) and ketones (acetone, cyclohexanone) in the presence of polystyrene (PS) resin supported di- and tripeptides were studied under otherwise identical experimental conditions at room temperature in batch reactor. For all three asymmetric aldol reactions reversal of enantiomeric excess was observed on H-Pro-Pro-Glu(OH)-PS catalyst (ee 39-50%) as compared to the H-Pro-Glu(OH)-PS catalyst (ee 52-95%). The determinant role of the conformation of the intermediate adducts held responsible for chiral induction is interpreted by quantum chemical calculations. © 2013 Elsevier B.V. All rights reserved.

Szollosi G.,MTA SZTE Stereochemistry Research Group | Fekete M.,University of Szeged | Gurka A.A.,University of Szeged | Bartok M.,MTA SZTE Stereochemistry Research Group | Bartok M.,University of Szeged
Catalysis Letters | Year: 2014

We report new results on the aldol reactions between aldehydes of three different types (aromatic, aliphatic and cycloaliphatic) and acetone/cycloalkanones as reaction partners, driven by organic-inorganic hybrid catalyst Pro/λ-Al2O3. In contrast to the homogeneous liquidphase reaction, over Pro/λ-Al2O 3reversal of the enantioselection in up to 20-40 % ee depending on the structure of the aldehyde was observed in reactions of acetone. Reversal of the ee in the presence of c-Al2O3cannot be generalized, as it has only been observed for acetone among the ketones studied by us. It was proven using methods of a great variety such as ultrasonic irradiation, reuse measurements on used catalyst and the filtrate of the first reaction, measurements on the L-Pro-L-Pro(OH) dipeptide, studies using mixtures of L-Pro and D-Pro that the organic-inorganic hybrid catalyst Pro/λ-Al 2O3formed in situ is responsible for reversal of the ee. In the reactions of cycloalkanones there is presumably competition between the liquid-phase and the surface reaction over Pro/ c-Al2O 3with preference for the former. Based on these results a surface reaction pathway was proposed. Although, the ees obtained under heterogeneous catalytic conditions are low, further studies may lead to application of this unusual phenomenon for obtaining chiral heterogeneous catalysts suitable for the preparation of the desired enantiomer of a chiral compound using the same chiral source. © Springer Science+Business Media New York 2013.

Kunfi A.,MTA SZTE Stereochemistry Research Group | Mastalir A.,University of Szeged | Bucsi I.,University of Szeged | London G.,MTA SZTE Stereochemistry Research Group
Reaction Kinetics, Mechanisms and Catalysis | Year: 2016

Supported Pd catalysts were investigated in the Heck coupling reactions: Pd/C; Pd/BaSO4; Pd EnCat. We optimized the experimental conditions of the Heck reaction of bromobenzene and styrene by using various bases in the presence of quaternary ammonium salts. It was found that the examined catalysts work as a reservoir of the catalytically active Pd species during the reaction. However, they have a different activity and recyclability under the investigated reaction conditions. Pd EnCat displayed the highest activity and selectivity in NMP with Na2CO3 as a base and Bu4NCl as an additive. By using this catalytic system, the reactions of para-substituted aryl halides with styrene and methyl methacrylate afforded excellent conversions and selectivities. © 2016 Akadémiai Kiadó, Budapest, Hungary

Gurka A.A.,University of Szeged | Szori K.,MTA SZTE Stereochemistry Research Group | Bartok M.,University of Szeged | Bartok M.,MTA SZTE Stereochemistry Research Group | London G.,MTA SZTE Stereochemistry Research Group
Tetrahedron Asymmetry | Year: 2016

Parameters influencing dual stereocontrol in aldol reactions of water miscible acetone with aromatic aldehydes in the presence of a large amount of water using hydroxyproline based catalysts were studied. Stereocontrol was achieved by changing the acidity and basicity of the reaction media by the addition of achiral salts in the presence of a single chiral catalyst. Under acidic conditions (NH4Cl salt) the (R)-aldol product was formed in excess while basic aqueous media (carboxylate salts) led to the enrichment of the (S)-enantiomer. Reaction conditions under which the reaction is feasible were optimised and the effect of the structure of the hydroxyproline-based catalysts was investigated. The results show that the formation of a biphasic micellar system and the presence of an appropriate catalyst are both crucial for the reaction to occur. Although the catalyst structure influenced the formation and stabilisation of the micellar system to a large extent, its effect on the enantioselectivities were found to be less pronounced. © 2016 Elsevier Ltd

Makra Z.,University of Szeged | Szollosi G.,MTA SZTE Stereochemistry Research Group
Catalysis Communications | Year: 2014

The effect of the solvent, modifier structure and concentration on the enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid over Pd/Al 2O3 modified by cinchona alkaloids was influenced by the addition of achiral amines to the reaction slurry. The solvent dependence in the presence of achiral amines showed that additives are involved in the rate determinant step of the reaction, whereas the effect of the dilution pointed to the presence of the acid dimers in the intermediate responsible for enantioselection. The dependence of the enantioselectivity on the cinchonidine concentration in the presence of amines and results obtained using cinchona derivatives and mixtures thereof were in line with our earlier assumptions related to the participation of the amine additive in the formation of the surface intermediate. Based on these and previously published results possible structures of this intermediate are suggested. © 2013 Elsevier B.V.

Szollosi G.,MTA SZTE Stereochemistry Research Group | Makra Z.,University of Szeged | Kovacs L.,University of Szeged | Fulop F.,MTA SZTE Stereochemistry Research Group | And 3 more authors.
Advanced Synthesis and Catalysis | Year: 2013

The development of a novel heterogeneous catalytic asymmetric cascade reaction for the synthesis of tetrahydroquinolines from 2-nitrophenylpyruvates is reported. Optically enriched 3-hydroxy-3,4-dihydroquinolin-2(1H)-ones are prepared by enantioselective hydrogenation of the activated keto group over a Cinchona alkaloid-modified Pt catalyst, reduction of the nitro group and spontaneous cyclization cascade. Acceleration of the enantioselective hydrogenation of the activated keto group over the catalyst modified by Cinchona alkaloids ensured high tetrahydroquinolinone selectivities. The scope of the reaction was checked using twelve substrates. Both yields and enantioselectivities were significantly influenced by the nature and position of the substituents on the phenyl ring. Substituents adjacent to the nitro group considerably increased the product yield, due to their effect on the nitro group′s reduction rate; however, had only a limited effect on enantioselectivities. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Cao J.,ETH Zurich | London G.,ETH Zurich | London G.,MTA SZTE Stereochemistry Research Group | Dumele O.,ETH Zurich | And 6 more authors.
Journal of the American Chemical Society | Year: 2015

Three series of stable, neutral, π-extended bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these chromophores feature skeletons with [4n+2] π-electron perimeters, the two 8 π-electron pentalene subunits strongly influence bonding and spectral properties. 1H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable to antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICS-XY-scans suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene moieties. X-ray diffraction analyses revealed planar bispentalene cores with large double- and single-bond alternation in the pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TD-DFT calculations, both optical and electrochemical data showed much smaller HOMO-LUMO energy gaps compared to other neutral, acene-like hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the molecular properties of the presented bispentalenes are dominated by the antiaromatic pentalene-subunits despite the [4n+2] π-electron perimeter of the skeletons. (Chemical Equation Presented). © 2015 American Chemical Society.

Kovacs L.,University of Szeged | Szollosi G.,MTA SZTE Stereochemistry Research Group | Fulop F.,University of Szeged | Fulop F.,MTA SZTE Stereochemistry Research Group
Journal of Flow Chemistry | Year: 2015

The asymmetric heterogeneous catalytic cascade reaction of ethyl 2-nitro-3-methylphenylpyruvate has been investigated over platinum modified by cinchonidine in continuous-flow system using a fixed-bed reactor. The high selectivities and enantioselectivities of the (R)-3-hydroxy-3,4-dihydro-8-methylquinolin-2(1H)-one obtained in previous studies in batch reactor were not reached. The catalyst was in situ prehydrogenated and premodified with cinchonidine, and the reaction conditions optimized for batch reactor were changed in order to increase the yield and enantioselectivity of the desired product under flow conditions. Results obtained in the flow apparatus contributed to the understanding of the reaction pathway through which the quinolone is formed. It was shown that, at low conversions, the intermediate aminohydroxyester desorbs preferentially and is further transformed by readsorption and cyclization to the quinolone derivative after complete disappearance of the 2-nitrophenylpyruvate. However, at high conversion, the formation of the quinolone may also occur instantaneously on the Pt surface following the two competitive reduction steps. The ratio of the product formed through these two pathways is determined by the reaction conditions and the system used. © 2015 Akadémiai Kiadó.

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