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Chiral Modifiers (chiral + modifier)

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Chemistry100%

Selected Abstracts

Enantiopure (9-Anthryl)(2-piperidyl)- and (9-Anthryl)(2-pyridyl)methanols , Their Use as Chiral Modifiers for Heterogeneous Hydrogenation of Keto Esters over Pt/Al2O3

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2007
Arlette Solladié-Cavallo
Abstract A route toward the synthesis of the erythro isomer of (9-anthryl)(2-piperidyl)methanol is presented as well as resolution and assignment of the structure (through NMR). The use of both the erythro and threo enantiopure isomers of this new amino alcohol, and its precursor [(9-anthryl)(2-pyridyl)methanol], as chiral modifiers for the Pt/Al2O3 hydrogenation of ethyl lactate showed that the erythro isomer is not necessarily the most efficient chiral modifier. This is probably because of the 9-anthryl group. The enantioselectivities that this compound provides are not, as one would expect, higher than those observed with the naphthyl group.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

Molecular modeling of chiral-modified zeolite HY employed in enantioselective separation

CHIRALITY, Issue 6 2007
Siricharn S. Jirapongphan
Abstract Insight into enantioselective separation utilizing chiral-modified zeolite HY could be useful in designing a chiral stationary phase for resolving pharmaceutical compounds. A model was employed to better understand the enantioseparation of valinol in zeolite HY that contains (+)-(1R;2R)-hydrobenzoin as a chiral modifier. This model incorporates the zeolite support and accounts for the flexible change. Results from grand canonical Monte Carlo and molecular dynamics simulations indicate that the associated diastereomeric complex consists of a single (+)-(1R;2R)-hydrobenzoin and a single valinol molecules located in the zeolite HY supercage. Supercage-based docking simulation predicted an enantioselectivity of 2.6 compared with that of 1.4 measured experimentally. Also, the supercage-based docking simulation demonstrated a single binding motif in the S complex, and two binding motifs in the R complex. The multiple binding modes in the R complex resulted in its lower stability. This is hypothesized to be the origin of the weaker binding between (,)-(R)-valinol and the chiral modifier, and explains why (+)-(R)-valinol is retained more in the chiral-modified zeolite system studied. Chirality, 2007. © 2007 Wiley-Liss, Inc. [source]

Enantiopure (9-Anthryl)(2-piperidyl)- and (9-Anthryl)(2-pyridyl)methanols , Their Use as Chiral Modifiers for Heterogeneous Hydrogenation of Keto Esters over Pt/Al2O3

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2007
Arlette Solladié-Cavallo
Abstract A route toward the synthesis of the erythro isomer of (9-anthryl)(2-piperidyl)methanol is presented as well as resolution and assignment of the structure (through NMR). The use of both the erythro and threo enantiopure isomers of this new amino alcohol, and its precursor [(9-anthryl)(2-pyridyl)methanol], as chiral modifiers for the Pt/Al2O3 hydrogenation of ethyl lactate showed that the erythro isomer is not necessarily the most efficient chiral modifier. This is probably because of the 9-anthryl group. The enantioselectivities that this compound provides are not, as one would expect, higher than those observed with the naphthyl group.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

Non-Linear Effect of Modifier Composition on Enantioselectivity in Asymmetric Hydrogenation over Platinum Metals

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2003
Wolf-Rüdiger Huck
Abstract Prominent non-linear behavior was observed when mixtures of cinchona alkaloids were applied as chiral modifiers in enantioselective hydrogenations over Pt/Al2O3 and Pd/TiO2. The phenomenon is traced to differences in the strength and geometry of alkaloid adsorption on the metal surface. In ethyl pyruvate hydrogenation under close to ambient conditions the weaker adsorbing quinidine (QD) outperformed the generally preferred modifier cinchonidine (CD) and afforded the highest ee (96,98%) at 1,5,bar. In the partial hydrogenation of 4-methoxy-6-methyl-2-pyrone to the dihydro derivative 4 CD gave 73% ee to (S)- 4 and QD provided 72% ee to (R)- 4, and still the alkaloid mixture containing less than 5,mol,% CD afforded 15% ee to (S)- 4. This non-linear behavior may be advantageous in asymmetric synthesis as low purity chiral compounds can be applied as efficient modifiers for Pt or Pd. [source]

Chirally Modified Platinum Generated by Adsorption of Cinchonidine Ether Derivatives: Towards Uncovering the Chiral Sites

CHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2007
Norberto Bonalumi
Abstract The adsorption behavior of O -methyl and O -trimethylsilyl derivatives of cinchonidine (CD), employed as chiral modifiers for heterogeneous enantioselective hydrogenations on supported Pt catalysts, has been investigated by using attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory (DFT) electronic structure calculations. The ATR-IR spectroscopic investigation provided detailed insight of the adsorbed modifiers under conditions close to those employed during catalytic processes, and electronic structure calculations were used as a complement to the experiments to uncover the implications of conformational changes in generating the topology of the surface chiral site. The structural investigation of the adsorbed modifiers revealed a relationship between the spatial positions of the ether substituents and the enantiodifferentiation induced by the modified catalyst observed in the hydrogenation of ,-activated ketones. Experiments and calculations corroborate a model, according to which the addition of a bulky ether group to CD reshapes the chiral sites, thus generating catalytic chiral surfaces with different and, in some cases (e.g. hydrogenation of ketopantolactone), even opposite enantioselective properties to those obtained with CD without altering the absolute configuration of the modifier. The study also confirms that active surface conformations of cinchona modifiers are markedly different from those existing in vacuum and in solution, thus underlying the necessity of investigating the surface-modifier interaction in order to understand enantioselectivity. [source]