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Heterochiral Dimers (heterochiral + dimer)
Selected AbstractsEnantiorecognition on solid chiral selectors using microbatch technology: an example of limitation in case of strong association in the racemateBIOMEDICAL CHROMATOGRAPHY, Issue 6 2005Christian Roussel Abstract When they were independently tested, the enantiomers of N,N,-bis(salicylidene)- trans -1,2-cyclohexanediamine showed a large difference in adsorption on new chiral selectors using microbatch technology. Surprisingly, when these enantiomers were applied on the same supports as a racemic mixture, no discrimination was observed even though suitable adsorption existed. When a mixture enriched in one enantiomer (scalemic mixture) was applied, the resulting supernatant contained the racemic form and the enantiomer in excess was adsorbed on the support together with a part of racemate. This behavior, which militates in favor of a strong heterochiral dimer formation in the racemate, was revealed using microbatch technology but remained hidden on classical column chromatography on chiral support. Molecular dynamics calculations corroborate this hypothesis, showing a favorite binding mode of the heterochiral dimer, which is stabilized by various inter- and intramolecular interactions. Our findings may be considered as a new limitation of microbatch technology, but they may have some inference in case of chiral amplification using the N,N,-bis(salicylidene)- trans -1,2-cyclohexanediamine enantiomers as chiral ligands. Copyright İ 2005 John Wiley & Sons, Ltd. [source] Synthesis, Characterization, and Electrochemical Properties of Dinuclear Complexes Assembled from Asymmetric CoIII Bis(dioximates) and Boronic AcidsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2005Renata Dreos Abstract Bis(methylphenylglyoximate)cobalt(III) complexes exist both as cis and trans isomers due to the asymmetry of the equatorial ligand, and, when the axial ligands are different, the trans isomer is chiral. The reaction of racemic trans -[CH3Co(mpgH)2py] (1) with either 3- or 4-pyridylboronic acid affords dimeric units arranged on a crystallographic symmetry center such that the pyridyl nitrogen of one moiety coordinates to the Co atom of the symmetry-related unit. In principle, three structurally different dimeric species (two homodimers and one heterodimer) can be obtained. Time-resolved 1H NMR spectra of a 1:1 mixture of racemic 1 and either 3- or 4-pyridylboronic acid in CDCl3/CD3OD show that the reaction does not converge toward a unique species in solution. Nevertheless, X-ray structures show that the heterochiral dimers are the only products that crystallize from the reaction mixture. The nature of the dioximate side groups does not affect the geometry of the dimeric arrangements assembled by 4-pyridylboronic acid ("molecular box"). On the contrary, the geometry of the species assembled by 3-pyridylboronic acid varies from the "molecular parallelogram" obtained from the bis(dimethylglyoximates) to the highly squeezed "molecular box" obtained from bis(methylphenylglyoximates). Cyclic voltammetry studies show that the metal centers in the dimeric species do not interact with each other and undergo a simultaneous redox process. However, depending on the geometry of the systems, the redox process involves a single four-electron reduction for 3 and 5 or two consecutive two-electron reduction steps for 4 and 6. (İ Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Predictable close-packing similarities between cis - and trans -2-hydroxy-1-cyclooctanecarboxylic acids and trans -2-hydroxy-1-cyclooctanecarboxamideACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2002Alajos Kálmán In order to extend the experimental data already available on the close packing of cyclopentanes substituted with vicinal COX (X,=,OH, NH2) and OH groups to the analogous cyclohexanes, cycloheptanes and cyclooctanes, (1R*,2S*) -cis- 2-hydroxy-1-cyclooctanecarboxylic acid (8C), (1R*,2R*)- trans -2-hydroxy-1-cyclooctanecarboxylic acid (8T) and (1R*,2R*)- trans -2-hydroxy-1-cyclooctanecarboxamide (8T*) were subjected to X-ray crystal structure analysis. In 8T and 8T*, the hydrogen bonds form infinite ribbons of dimers joined by (12) rings with Ci symmetry. Two types of dimer alternate along each ribbon. The dimers differ in the donor and acceptor roles of the functional groups. This pattern was previously deduced topologically among the possible forms of association for heterochiral dimers [Kálmán et al. (2002). Acta Cryst. B58, 494,501]. As they have the same pattern of hydrogen bonds, 8T and 8T* are isostructural. The additional donor (i.e. the second hydrogen of the NH2 group) present in 8T* links the adjacent ribbons so as to form smaller (8) rings between them. The crystals of the cis stereoisomer 8C are built up from antiparallel hydrogen-bonded helices. The topology and symmetry of this structure are the same as for the close packing of (1R*,2R*,4S*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxamide [Kálmán et al. (2001). Acta Cryst. B57, 539,550]; only the hydrogen-bond donors and acceptors are interchanged, in the same way as in the two dimer types of 8T and 8T* ribbons. This analogy suggests that helices may originate as homochiral dimers with C2 symmetry and polymerize into helices during crystal formation. The conformational characteristics of the heterochiral dimers observed in the title compounds and in closely related structures are discussed. [source] Basic forms of supramolecular self-assembly organized by parallel and antiparallel hydrogen bonds in the racemic crystal structures of six disubstituted and trisubstituted cyclopentane derivativesACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2001Alajos Kálmán A selection of stereoisomeric 2-hydroxy-1-cyclopentanecarboxamides, a 4- tert -butyl derivative and three tert -butyl derivatives of the respective carboxylic acid were subjected to X-ray crystallography. The optically active molecules (I),(VI) form racemic crystals. Each racemic structure is basically determined by two intermolecular hydrogen bonds of O,H,O=C,XH and O=C,X,H,OH types (X = O, NH). The partially similar patterns of close packing observed reflect five basic forms of supramolecular self-assembly. In the racemic crystals of chiral molecules, there are homo- and heterochiral chains of molecules formed by the principal (O,H,O=C) hydrogen bonds. These chains assemble either in a parallel or antiparallel mode. The parallel homochiral chains (hop) observed in structure (II), (1R*,2R*)-2-hydroxy-1-cyclopentanecarboxamide, demand the polar space group Pca21, while the parallel heterochiral chains (hep) are organized in antiparallel layers with space group P21/n in structure (VI), (1R*,2S*,5R*-5- tert -butyl-2-hydroxy-1-cyclopentanecarboxylic acid). Heterochiral chains in an antiparallel array (hea) are found in (I), (1R*,2S*)-2-hydroxy-1-cyclopentanecarboxamide, and (V) [(1R*,2S*4S*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxylic acid, space group P21/c]. Structures (IV), (1R*,2S*,4R*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxylic acid, and (III), (1R*,2R*,4S*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxamide, reveal that homochiral chains in an antiparallel array (hoa; cross-linked by heterochiral dimers held together by the second hydrogen bonds) can be formed by either translation (space group P) or a screw axis (space group P21/c). These alternatives are denoted hoa1 and hoa2. Similarly, within each pattern (hea, hep and hop) two slightly different alternatives can be expected. The partial similarities in the identified five patterns of hydrogen bonding are described by graph-set notations. Structures (I), (IV) and (V) can be characterized by a common supramolecular synthon, while the highest degree of similarity is shown by the isostructurality of (I) and (V). [source] | ||||||||||