Chiral resolution of l- and d-alanine and a racemic macrocyclic nickel(II) complex: synthesis and crystal structures

Abstract: We report new 3D fragment descriptors to model parameters and properties of stereoisomeric molecules and conformers. New 3D fragment descriptors have been applied to discriminate between stereoisomers in predictive QSPR modeling of the standard free energy (∆G°) for the 1:1 inclusion complexation of 76 chiral guests with β-cyclodextrin (β-CD) and 40 chiral guests with 6-amino-6-deoxy-β-cyclodextrin (am-β-CD) in water at 298 K. The in-house software, mfSpace (Molecular Fragments Space), was used for QSPR modeling, generation and coding of the 3D fragment descriptors. The program implements the Singular Value Decomposition for Multiple Linear Regression analysis as machine learning method. We used ensemble modeling techniques which include the generation of many individual models, the selection of the most relevant ones and followed by their joint application to test compounds, i.e., applying a consensus model for average predictions. The models based on 2D and 3D fragment descriptors provide the best predictions in external fivefold cross-validation: root mean squared error RMSE = 1.1 kJ/mol and determination coefficient $$R_{{det}}^{2}$$Rdet2 = 0.918 (β-CD), RMSE = 0.89 kJ/mol and $$R_{{det}}^{2}$$Rdet2 = 0.910 (am-β-CD).

Abstract: We report new 3D fragment descriptors to model parameters and properties of stereoisomeric molecules and conformers. New 3D fragment descriptors have been applied to discriminate between stereoisomers in predictive QSPR modeling of the standard free energy (∆G°) for the 1:1 inclusion complexation of 76 chiral guests with β-cyclodextrin (β-CD) and 40 chiral guests with 6-amino-6-deoxy-β-cyclodextrin (am-β-CD) in water at 298 K. The in-house software, mfSpace (Molecular Fragments Space), was used for QSPR modeling, generation and coding of the 3D fragment descriptors. The program implements the Singular Value Decomposition for Multiple Linear Regression analysis as machine learning method. We used ensemble modeling techniques which include the generation of many individual models, the selection of the most relevant ones and followed by their joint application to test compounds, i.e., applying a consensus model for average predictions. The models based on 2D and 3D fragment descriptors provide the best predictions in external fivefold cross-validation: root mean squared error RMSE = 1.1 kJ/mol and determination coefficient $$R_{{det}}^{2}$$ = 0.918 (β-CD), RMSE = 0.89 kJ/mol and $$R_{{det}}^{2}$$ = 0.910 (am-β-CD).

Abstract: Three enantiomeric pairs of novel homochiral coordination compounds (HCCs) with the formula {[Zn(D-hpg)(4,4′-bipy)(H2O)] · (NO3)}n 1-D, {[Zn(L-hpg)(4,4′-bipy)(H2O)] · (NO3)}n 1-L, {[Zn(D-hpg)(4,4′-bipy)(H2O)] · (ClO4)}n 2-D, {[Zn(L-hpg)(4,4′-bipy)(H2O)] · (ClO4)}n 2-L, [Zn(D-hpg)2(4,4′-bipy)] · (4,4′-bipy) · 2H2O 3-D, [Zn(L-hpg)2(4,4′-bipy)] · (4,4′-bipy) · 2H2O 3-L (D-Hhpg = D-(−)-4-Hydroxyphenylglycine, L-Hhpg = L-(+)-4-Hydroxyphenylglycine, 4,4′-bipy = 4,4′-bipyridine) have been synthesized and characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, thermal analysis and powder X-ray diffraction. Compounds 1 and 2 are isostructural 2D layer structures with NO3– or ClO4– anions between the layers. While compound 3 features a 0D structure without counter anions. Compounds 1-D and 2-D both contain 1D right-hand helical chains via D-Hhpg ligand, while 1-L and 2-L both contain 1D left-hand helical chains via L-Hhpg ligand. It's worth noting that 3-D forms left-hand supramolecular double helical chains, while 3-L forms right-hand supramolecular double helical chains. Compounds 1–3 are further extended into 3D supramolecular architectures through hydrogen-bonding interactions. Circular dichroism and luminescent properties of compounds 1–3 were investigated at room temperature. Our results highlight that chiral ligands have an important effect on regulating the chirality/helicity of complexes and the frameworks can be controlled via applying different zinc salts and adjusting the pH values.

Abstract: Two dinuclear carbonato-bridged nickel(II) complexes formulated as [Ni(rac-L)]2(µ-CO3)(H2O)(im)4(ClO4)2 (1) and [Ni(SS-L)]2(µ-CO3)(H2O)(im)4(ClO4)2 (2) (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, im = imidazole) were isolated from the reactions of [Ni(rac-L)](ClO4)2 and [Ni(SS-L)](ClO4)2 with imidazole in the air, respectively, and a hydrocarbonato-coordinated nickel(II) complex formulated as [Ni(rac-L)](HCO3)(ClO4) (3) was reacted with obtained when [Ni(rac-L)](ClO4)2 reacted with l-cysteine under weakly basic conditions in the air. We found that the macrocyclic nickel(II) complexes can easily take up and fix atmospheric CO2 at room temperature. Single-crystal X-ray diffraction analyses revealed of all three complexes that the central Ni(II) atoms all have a six-coordinated distorted octahedral coordination geometry, and the carbonate anion bridges two Ni(II) atoms in a tridentate fashion to form dimers in complexes 1 and 2, and the hydrocarbonate coordinates with Ni(II) in a didentate fashion in complex 3. The monomers of {[Ni(RR-L)]2(µ-CO3)(H2O)}2+/{[Ni(SS-L)]2(µ-CO3)(H2O)}2+ are connected through hydrogen bonds to generate one-dimensional right- and left-handed helical chains in complex 1. The chiral nature of complex 2 has been confirmed by CD spectroscopy.

Abstract: Ever since Pasteur noticed that tartrate crystals exist in two non-superimposable forms that are mirror images of one another--as are left and right hands--the phenomenon of chirality has intrigued scientists. On the molecular level, chirality often has a profound impact on recognition and interaction events and is thus important to biochemistry and pharmacology. In chemical synthesis, much effort has been directed towards developing asymmetric synthesis strategies that yield product molecules with a significant excess of either the left-handed or right-handed enantiomer. This is usually achieved by making use of chiral auxiliaries or catalysts that influence the course of a reaction, with the enantiomeric excess (ee) of the product linearly related to the ee of the auxiliary or catalyst used. In recent years, however, an increasing number of asymmetric reactions have been documented where this relationship is nonlinear, an effect that can lead to asymmetric amplification. Theoretical models have long suggested that autocatalytic processes can result in kinetically controlled asymmetric amplification, a prediction that has now been verified experimentally and rationalized mechanistically for an autocatalytic alkylation reaction. Here we show an alternative mechanism that gives rise to asymmetric amplification based on the equilibrium solid-liquid phase behaviour of amino acids in solution. This amplification mechanism is robust and can operate in aqueous systems, making it an appealing proposition for explaining one of the most tantalizing examples of asymmetric amplification-the development of high enantiomeric excess in biomolecules from a presumably racemic prebiotic world.

Abstract: The exogenous delivery of cometary and asteroidal material is observed today and undoubtedly has showered the Earth through its prior history ([ 1 ][1]). Because carbonaceous meteorites contain amino acids displaying asymmetry that, if not as extensive, has the same sign (L) as terrestrial amino

Abstract: This critical review is devoted to an active field of research on chiral separation, membrane-based enantioseparation technique, which has potential for large-scale production of single-enantiomer compounds. Adsorption-type enantioselective membranes and membrane-assisted resolution systems with non-enantioselective solid membranes have attracted much attention recently. The principles and recent developments of both enantioselective liquid and solid membranes and membrane-assisted processes for chiral resolution will be summarized comprehensively in this review, in which the contents are of interest to a wide range of readers in a variety of fields from analytical, organic and medicinal chemistry, to pharmaceutics and materials, to process engineering for fabricating pharmaceuticals, agrochemicals, fragrances and foods, and so on (148 references).