Formation Mechanism of Achiral Amphiphile-Templated Helical Mesoporous Silicas

Efficient preparation of stereopure amphiphilic 1,2-amino alcohols by using preparative enantioselective HPLC

Chiral amphiphiles are useful for controlling the structures and properties of supramolecular assemblies, but their stereocontrolled synthesis is generally difficult, because their long alkyl chains tend to bring unfavorable effects on the solubility, reactivity, and crystallinity of molecules. Typical examples are amphiphilic 1,2-amino alcohols (S)-1 and (1S,2S)-2 developed by our group, which were known to serve as chiral reaction media for controlling the stereochemistry of asymmetric photoreactions. We previously developed synthetic schemes for these 1,2-amino alcohols, but their synthetic efficiencies were unsatisfactory (13 steps with 2% overall yield for (S)-1; eight steps with 8% yield for (1S,2S)-2). As the main reason of such low efficiencies, the stereocontrolling methods we previously employed (diastereomer-salt crystallization for (S)-1; stereoselective reactions for (1S,2S)-2) were not ideal. Here, we report highly improved synthetic schemes for (S)-1 and (1S,2S)-2 based on the enantioselective high performance liquid chromatography (HPLC) separation of intermediates in preparative scales. Compared with the previous schemes, the new schemes are advantageous in fewer number of steps, higher overall yield, and lower risk of racemization (seven steps with 15% overall yield for (S)-1; seven steps with 26% overall yield for (1S,2S)-2).

Chiral Nanoceramics

The study of different chiral inorganic nanomaterials has been experiencing rapid growth during the past decade, with its primary focus on metals and semiconductors. Ceramic materials can substantially expand the range of mechanical, optical, chemical, electrical, magnetic, and biological properties of chiral nanostructures, further stimulating theoretical, synthetic, and applied research in this area. An ever-expanding toolbox of nanoscale engineering and self-organization provides a chirality-based methodology for engineering of hierarchically organized ceramic materials. However, fundamental discoveries and technological translations of chiral nanoceramics have received substantially smaller attention than counterparts from metals and semiconductors. Findings in this research area are scattered over a variety of sources and subfields. Here, the diversity of chemistries, geometries, and properties found in chiral ceramic nanostructures are summarized. They represent a compelling materials platform for realization of chirality transfer through multiple scales that can result in new forms of ceramic materials. Multiscale chiral geometries and the structural versatility of nanoceramics are complemented by their high chiroptical activity, enantioselectivity, catalytic activity, and biocompatibility. Future development in this field is likely to encompass chiral synthesis, biomedical applications, and optical/electronic devices. The implementation of computationally designed chiral nanoceramics for biomimetic catalysts and quantum information devices may also be expected.

Chiral mesoporous silica synthesized by a facile strategy for loading and releasing poorly water-soluble drug

Most of the mesoporous chiral mesoporous silica (CMS) was synthesized by the chiral surfactant-directing method. In this study, a facile method was designed to synthesize CMS. In this method, achiral amphiphile was used as templating agents, and dilute ammonia solution was applied to induce the chirality of the CMS. Meanwhile, its morphology can be controlled by changing the concentration of the aqueous ammonia solution. The obtained CMS was characterized by dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results showed that all of the CMS possessed highly ordered mesostructures, and as the concentration of ammonia decreases, the chirality of the CMS becomes more obvious. Water-insoluble drug curcumin (Cur) was used as a model drug. The characteristics of CMS before and after drug loading were further detected by Fourier transform infrared spectrometer (FT-IR), N₂ adsorption-desorption and differential scanning calorimetry (DSC). The result showed that Cur was successfully loaded inside the pores of the CMS and remained an amorphous state due to steric inhibition. Additionally, CMS could significantly increase the release rate of Cur under different pH conditions.

Chiral Resolution using Supramolecular-Templated Mesostructured Materials

Chiral resolution using non-functionalized mesoporous particles is demonstrated for a variety of enantiomeric pairs. This is achieved through the use of supramolecular templated silica materials prepared with guanosine monophosphate (NGM-1) and folic acid (NFM-1) which enable direct chiral transcription onto the surface of the mesopores after solvent extraction and post calcination of the template. The chiral selectivity and kinetics of the mesoporous materials are measured by circular dichroism (CD) spectroscopy on adsorbed molecules with different affinities for the pore surface. NGM-1 and NFM-1 have opposite enantiomeric selectivity for enantiomeric pairs. These results significantly increase the potential of mesoporous materials for chiral separation and enantiomeric catalysis.

Synthesis of Helical Phenolic Resin Bundles through a Sol-Gel Transcription Method

Chiral and helical polymers possess special helical structures and optical property, and may find applications in chiral catalysis and optical devices. This work presents the preparation and formation process of helical phenolic resins through a sol-gel transcription method. A pair of bola-type chiral low-molecular-weight gelators (LMWGs) derived from valine are used as templates, while 2,4-dihydroxybenzoic acid and formaldehyde are used as precursors. The electron microscopy images show that the phenolic resins are single-handed helical bundles comprised of helical ultrafine nanofibers. The diffused reflection circular dichroism spectra indicate that the helical phenolic resins exhibit optical activity. A possible formation mechanism is proposed, which shows the co-assembly of the LMWGs and the precursors.

Emerging chirality in nanoscience

Chirality in nanoscience may offer new opportunities for applications beyond the traditional fields of chirality, such as the asymmetric catalysts in the molecular world and the chiral propellers in the macroscopic world. In the last two decades, there has been an amazing array of chiral nanostructures reported in the literature. This review aims to explore and categorize the common mechanisms underlying these systems. We start by analyzing the origin of chirality in simple systems such as the helical spring and hair vortex. Then, the chiral nanostructures in the literature were categorized according to their material composition and underlying mechanism. Special attention is paid to highlight systems with original discoveries, exceptional structural characteristics, or unique mechanisms.

Enantioselectively controlled release of chiral drug (metoprolol) using chiral mesoporous silica materials

Chiral porous materials have attracted burgeoning attention on account of their potential applications in many areas, such as enantioseparation, chiral catalysis, chemical sensors and drug delivery. In this report, chiral mesoporous silica (CMS) materials with various pore sizes and structures were prepared using conventional achiral templates (other than chiral surfactant) and a chiral cobalt complex as co-template. The synthesized CMS materials were characterized by x-ray diffraction, nitrogen physisorption, scanning electron microscope and transmission electron microscope. These CMS materials, as carriers, were demonstrated to be able to control the enantioselective release of a representative chiral drug (metoprolol). The release kinetics, as modeled by the power law equation, suggested that the release profiles of metoprolol were remarkably dependent on the pore diameter and pore structure of CMS materials. More importantly, R- and S-enantiomers of metoprolol exhibited different release kinetics on CMS compared to the corresponding achiral mesoporous silica (ACMS), attributable to the existence of local chirality on the pore wall surface of CMS materials. The chirality of CMS materials on a molecular level was further substantiated by vibrational circular dichroism measurements.

Seeding-growth of helical mesoporous silica nanofibers templated by achiral cationic surfactant

Helical mesoporous silica nanofibers with parallel nanochannels were synthesized in high yield via a novel seeding-growth method by using the achiral cationic surfactant cetyltrimethylammonium bromide (CTAB) as template without auxiliary additives. A general entropy-driven model taking into account the icelike structure water due to the hydrophobic effect was proposed to explain the formation of helical mesoporous silica nanofibers. It was indicated that helical silica mesostructures could result from a thick layer of highly ordered icelike water around thin silicate seed rods with a proper concentration, which was verified by the effect of various anions and organic additives on the formation of helical mesoporous silica.