Strong Enantiomeric Preference on the Macroion-Counterion Interaction Induced by Weakly Associated Chiral Counterions

ASYMMETRIC SYNTHESIS: A GLANCE AT VARIOUS METHODOLOGIES FOR DIFFERENT FRAMEWORKS

Abstract:

Asymmetric reactions have made a significant advancement over the past few decades and involved the production of enantiomerically pure molecules using enantioselective organocatalysis, chiral auxiliaries/substrates, and reagents via controlling the absolute stereochemistry. The laboratory synthesis from an enantiomerically impure starting material gives a combination of enantiomers which are difficult to separate for chemists in the fields of medicine, chromatography, pharmacology, asymmetric synthesis, studies of structure-function relationships of proteins, life sciences and mechanistic studies. This challenging step of separation can be avoided by the use of asymmetric synthesis. Using pharmacologically relevant scaffolds/pharmacophores, the drug designing can also be achieved using asymmetric synthesis to synthesize receptor specific pharmacologically active chiral molecules. This approach can be used to synthesize asymmetric molecules from wide variety of reactants using specific asymmetric conditions which is also beneficial for environment due to less usage and discharge of chemicals into the environment. So, in this review, we have focused on the inclusive collation of diverse mechanisms in this area, to encourage auxiliary studies of asymmetric reactions to develop selective, efficient, environment-friendly and high yielding advanced processes in asymmetric reactions.

Side Group of Hydrophobic Amino Acids Controls Chiral Discrimination among Chiral Counterions and Metal-Organic Cages

The self-assembly of chiral Pd₁₂L₂₄ metal-organic cages (MOCs) based on hydrophobic amino acids, including alanine (Ala), valine (Val), and leucine (Leu), into single-layered hollow spherical blackberry-type structures is triggered by nitrates through counterion-mediated attraction. In addition to nitrates, anionic N-(tert-butoxycarbonyl) (Boc)-protected Ala, Val, and Leu were used as chiral counterions during the self-assembly of d-MOCs. Previously, we showed that l-Ala suppresses the self-assembly process of d-Pd₁₂Ala₂₄ but has no effect on l-Pd₁₂Ala₂₄, i.e., chiral discrimination. Here, we indicate when the amino acid used as the chiral counterion has a bulkier side group than the amino acid in the MOC structure, no chiral discrimination exists; otherwise, chiral discrimination exists. For example, Ala can induce chiral discrimination in all chiral MOCs, whereas Leu can induce chiral discrimination only in Pd₁₂Leu₂₄. Moreover, chiral anionic d- and l-alanine-based surfactants have no chiral discrimination, indicating that bulkier chiral counterions with more hydropohobic side groups can erase chiral discrimination.

Nanosheets and Hydrogels Formed by 2 nm Metal-Organic Cages with Electrostatic Interaction

We report the mechanism of hydrogel formation in dilute aqueous solutions (>15 mg/mL) by 2 nm metal-organic cages (MOCs). Experiments and all-atom simulations confirm that with the addition of small electrolytes, the MOCs self-assemble into 2D nanosheets via counterion-mediated attraction because of their unique molecular structure and charge distribution as well as σ-π interactions. The stiff nanosheets are difficult to bend into 3-D hollow, spherical blackberry type structures, as observed in many other macroion systems. Instead, they stay in solution and their very large excluded volumes lead to gelation at low (∼1.5 wt %) MOC concentrations, with additional help from hydrophobic and partial π-π interactions similar to the gelation of graphene oxides.