Use of Conglomerate Mixed Crystals to Deracemize a Stable Racemic-Compound-Forming System.

Deracemization extended to racemic-compound-forming systems is demonstrated. We present here the first results of an alternative for the resolution of systems that exhibit a stable racemic compound but also a closely related conglomerate-forming system. If the couples of enantiomers forming the racemic compound and the enantiomers of the stable conglomerate can syncrystallize in mirror-related partial solid solutions, it is possible to deracemize the racemic mixture of mixed crystals to access to a single handedness. The evidence for this possibility is given in three examples by using temperature-cycling-induced deracemization.

Synthesis and Chiroptical Properties of a Chiral Isotopologue of syn-Cryptophane-B.

We report the synthesis and absolute configuration (AC) of a chiral isotopologue of syn-cryptophane-B. Low chiral signatures were measured by polarimetry and electronic circular dichroism, whereas most significant chiroptical effects were observed by vibrational circular dichroism (VCD) and Raman optical activity (ROA). The comparison of experimental VCD and ROA spectra with those predicted by DFT calculations allows the determination of the AC of the two enantiomers as (-)589-MP-syn-2 and (+)589-PM-syn-2.

Chiral analysis of E-ε-viniferin enantiomers, towards a new chemotaxonomic marker of the vine.

BACKGROUND: The accurate characterization of grapevine cultivars (Vitis vinifera) is crucial for grape growers, winemakers, wine sellers, consumers and authorities, considering that mistakes could involve significant damage to the wine economic system. To avoid any misunderstanding, morphological, molecular and chemical tools are developed to positively identify grape varieties. RESULTS: E-ε-viniferin is a stilbene dimer mainly present in the woody part of grapevine and present as a mixture of two enantiomers: (7aR, 8aR)-(-)-E-ε-viniferin (1) and (7aS, 8aS)-(+)-E-ε-viniferin (2). In addition to phenotypic and genotypic approaches, a chemotaxonomic method using E-ε-viniferin enantiomers as chemical markers of grapevine cultivars was investigated. The isolation and purification of E-ε-viniferin enantiomers by preparative high-performance liquid chromatography (HPLC) and chiral HPLC from 14 red and eight white grapevine cane cultivars enabled us to determine the proportion of each enantiomer and therefore to calculate the enantiomeric excess for each variety. The relative abundance of each E-ε-viniferin enantiomer permitted us to distinguish grape varieties, as well as to establish cultivar relationships and patterns through statistical analysis. CONCLUSION: This pioneering work highlighting the enantiomeric excess of E-ε-viniferin as a chemical marker of grapevine paves the way for further studies to understand what mechanisms are involved in the production of these enantiomers in grapevine. © 2023 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Asymmetric Allenylation of Alkynes mediated by Chiral Organoselenated Reagents under Oxidative Conditions.

The reactivity of novel chiral lactamide-substituted diselenide-based reagents under oxidative conditions was exploited to develop a metal-free method for the preparation of enantioenriched allenylamides from simple alkynes in good yields, and with enantiomeric excesses up to 99 %. The key of the success in this method is attributed to the hydrogen-bonded lactamide appendages that ensure configurational stability of chiral vinyl selenoxide intermediates for an optimal enantiotopic ß-syn-elimination step.

Silane-Based SAMs Deposited by Spin Coating as a Versatile Alternative Process to Solution Immersion.

Functionalization of silica surfaces with silane-based self-assembled monolayers (SAMs) is widely used in material sciences to tune surface properties and introduce terminal functional groups enabling subsequent chemical surface reactions and immobilization of (bio)molecules. Here, we report on the synthesis of four organotrimethoxysilanes with various molecular structures and we compare their grafting by spin coating with the one performed by the conventional solution immersion method. Strikingly, this study clearly demonstrates that the spin coating technique is a versatile, fast, and more convenient alternative process to prepare robust, smooth, and homogeneous SAMs with similar properties and quality as those deposited via immersion. SAMs were characterized by PM-IRRAS, AFM, and wettability measurements. SAMs can undergo several chemical surface modifications, and the reactivity of amine-terminated SAM was confirmed by PM-IRRAS and fluorescence measurements.

Improving Orientation, Packing Density, and Molecular Arrangement in Self-Assembled Monolayers of Bianchoring Ferrocene-Triazole Derivatives by "Click" Chemistry.

This work describes the self-assembled monolayers (SAMs) of two ferrocene derivatives with two anchoring groups (at the bottom and at the top of the SAM) deposited on ultraflat template-stripped gold substrates by cyclic voltammetry and analyzed by complementary surface characterization techniques. The SAM of each molecule is deposited by three different protocols: direct deposition (one step), click reaction on the surface (two steps), and reverse click reaction on the surface (two steps). The SAM structure is well studied to determine the SAM orientation, SAM arrangement, and ferrocene position within the SAM. Electron transfer kinetics have also been studied, which agree with the quality of each SAM. With the help of two anchoring groups and click-chemistry active functional groups, we have shown that the two molecules can be deposited by controlling the position of ferrocene at either end. We further investigated the involvement of the triazole five-membered ring in the electron transfer mechanism. We have found that a carbon spacer between ferrocene and triazole improves the SAM packing. This study enhances the understanding of tethering thiol and thiol acetate anchoring groups on gold by a controlled orientation, which may help in the development of functional molecular devices requiring two anchoring groups.

Are the Physical Properties of Xe@Cryptophane Complexes Easily Predictable? The Case of syn- and anti-Tris-aza-Cryptophanes.

We report the synthesis and optical resolution of C3-symmetrical tris-aza-cryptophanes anti-3 and syn-4, as well as the study of their interaction with xenon via hyperpolarized 129Xe NMR. These molecular cages are close structural analogues of the two well-known cryptophane-A (1; chiral) and cryptophane-B (2; achiral) diastereomers since these new compounds differ only by the presence of three nitrogen atoms grafted onto the same cyclotribenzylene unit. The assignment of their relative (syn vs anti) and absolute configurations was made possible, thanks to the combined use of quantum calculations at the density functional theory level and vibrational circular dichroism spectroscopy. More importantly, our results show that despite the large structural similarities with cryptophane-A (1) and -B (2), these two new compounds show a very different behavior in the presence of xenon in organic solutions. These results demonstrate that prediction of the physical properties of the xenon@cryptophane complexes, only based on structural parameters, remains extremely difficult.

Enantiopure cryptophane derivatives: Synthesis and chiroptical properties.

This review addresses the synthesis of enantiopure cryptophane and the study of their chiroptical properties. Cryptophane derivatives represent an important class of macrocyclic compounds that can bind a large range of species in solution under different conditions. The overwhelming majority of these host molecules is chiral, and their chiroptical properties have been thoroughly investigated. The first part of this review is dedicated to the optical resolution and the synthesis of enantiopure cryptophane derivatives. In a second part, the study of the chiroptical properties of these molecular hosts by different techniques such as electronic and vibrational circular dichroism and Raman optical activity is detailed. These techniques allow the determination of the absolute configuration of cryptophane derivatives and provide useful information about their conformation in different conditions.

Chirality induction to achiral molecules by silica-coated chiral molecular assemblies.

Hybrid silica-organic nanohelices are used to organize a large variety of nonchiral small organic molecules or inorganic anions to nanometer-sized assemblies. Such chiral organization of achiral molecules induces chiroptical properties as detected by vibrational or electronic circular dichroism (CD), as well as from circularly polarized luminescence (CPL).

Loading Linear Arrays of CuII Inside Aromatic Amide Helices.

The very stable helices of 8-amino-2-quinolinecarboxylic acid oligoamides are shown to uptake CuII ions in their cavity through deprotonation of their amide functions with minimal alteration of their shape, unlike most metallo-organic structures which generally differ from their organic precursors. The outcome is the formation of intramolecular linear arrays of a defined number of CuII centers (up to sixteen in this study) at a 3 Šdistance, forming a molecular mimic of a metal wire completely surrounded by an organic sheath. The helices pack in the solid state so that the arrays of CuII extend intermolecularly. Conductive-AFM and cyclic voltammetry suggest that electrons are transported throughout the metal-loaded helices in contrast with hole transport observed for analogous foldamers devoid of metal ions.

Observing the Migration of Hydrogen Species in Hybrid Perovskite Materials through D/H Isotope Exchange.

Unlike heavier elements, the migration of hydrogen species in perovskite materials cannot be directly tracked using imaging or mass spectrocopy techniques. Our results show that quantitative analysis of D/H exchange in PbCH3ND3I3 allows indirect monitoring of H migration by following the N-D vibration using polarization-modulated infrared reflection-absorption spectroscopy. Kinetic analysis shows that the isotope exchange process is pseudo-first order and particularly sensitive to the intensity of light and relative humidity, and, to a lesser degree, sample thickness. In the presence of light (450 nm), the D/H exchange is accelerated up to 10-fold with respect to samples in the dark but slows again for higher light intensities.. The technique also allows the direct assessment of the efficiency of protective layers toward deterioration of hybrid organic-inorganic perovskite devices by moisture. Comparison of different monolayer-forming fluorinated molecules reveals important differences in rates attesting to variations between their efficiency in blocking access to the active layer by water molecules.

Hierarchical chirality expression of gemini surfactant aggregates via equilibrium between chiral nucleotide and nonchiral mono-anions.

The assembling behaviors of nonchiral dicationic amphiphilic molecules (gemini) in the presence of the mixture of chiral anionic nucleotides and nonchiral anions are investigated. We demonstrate that subtle balance of various physico-chemical parameters and the competition between chiral and nonchiral anions at the interface of gemini assemblies influences the expression of molecular chirality at the micrometer scale through the hierarchical molecular assembly.

Resolution, structures, and vibrational circular dichroism of helicoidal trinickel and tricobalt paddlewheel complexes.

It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co3 (dpa)4 (CH3 CN)2 ]2+ (dpa = the anion of 2,2'-dipyridylamine) can be resolved using the chiral [As2 (tartrate)2 ]2- anion (AsT) and that these complexes demonstrate a strong chiroptical response in the ultraviolet-visible and X-ray energy regions. Here we report that the nickel congener, [Ni3 (dpa)4 (CH3 CN)2 ]2+ , can likewise be resolved using AsT. Depending on the stereochemistry of the enantiopure AsT anion, one or the other of the trinickel enantiomers crystallize from CH3 CN and diethyl ether in space group P421 2 as the (NBu4 )2 [Ni3 (dpa)4 (CH3 CN)2 ](AsT)2 ·[solvent] salt. After resolution, the AsT salts were converted into the PF6 - salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure [Ni3 (dpa)4 (CH3 CN)2 ](PF6 )2 ·2CH3 CN and [Co3 (dpa)4 (CH3 CN)2 ](PF6 )2 ·CH3 CN·C4 H10 O compounds crystallize in space groups C2 and P21 , respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH3 CN. Vibrational circular dichroism (VCD) data obtained in CD3 CN demonstrate the expected mirror image spectra for the enantiomers, the observed peaks arising from the dpa ligand. The VCD response is significant, with Δε values up to 6 Lmol-1 cm-1 and vibrational dissymmetry factors on the order of 10-3 . Density functional theory calculations well reproduce the experimental spectra, showing little difference between the peak position, sign, and intensity in the VCD for the cobalt and nickel complexes. These results suggest that VCD enhancement of these peaks is unlikely, and their remarkable intensity may be due to their rigid helicoidal structure.

Nanoscale Archimedean Tilings Formed by 3-Miktoarm Star Terpolymer Thin Films.

3-Miktoarm star terpolymer architecture (3µ-ABC), consisting of three dissimilar polymer chains, A, B, and C connected at a junction point, provides a unique opportunity in the design of complex nanoscale patterns such as Archimedean tilings that are not accessible from linear ABC terpolymers. In this work, the synthesis and the self-assembly of 3-miktoarm star terpolymers, namely, polystyrene-arm-poly(2-vinylpyridine)-arm-polyisoprene (3µ-SPI) into Archimedean tiling patterns is described. Several 3µ-SPI terpolymers are produced via a mid-functional PS-b-P2VP, synthesized by sequential anionic polymerization, using a 1,1-diphenylethylene bearing a tert-butyldimethylsilyl-protected hydroxyl functionality as a core molecule. PI arms with different sizes are then linked to the deprotected hydroxyl function of the core molecule via a Steglich esterification. Solvent-annealed 3µ-SPI thin films exhibit nanoscale prisms arranged into a (4.6.12) Archimedean tiling pattern. Depending on the size of the low etch-resistant PI arm and the solvent selected to promote the self-assembly of 3µ-SPI thin films, the voided columns occupy the square or decagonal sites of the (4.6.12) pattern. The use of this (4.6.12) tiling produced for the first time from self-assembled 3µ-ABC thin films can be a promising route to build 2D photonic crystals having complete photonic band gaps, where the light propagation is completely prohibited.

Chiroptical study of cryptophanes subjected to self-encapsulation.

In 1,1,2,2-tetrachloroethane-d2 , the 129 Xe NMR spectrum of the Xe@cryptophane-223 complex bearing seven acetate groups (Xe@1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem. 2015;80:6143-6151). To interpret this unexpected behaviour, a 1 H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The 1 H NMR spectra of 1 reveal that a self-encapsulation phenomenon takes place in DMSO-d6 and 1,1,2,2-tetrachloroethane-d2 solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2-tetrachloroethane, in which the self-encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(-)254 ]-1 and [CD(+)254 ]-1 is changed. These results have then been compared with those obtained with cryptophane-223 bearing only one acetate group on the propylenedioxy linker (compound 2) and with cryptophane-223 bearing six acetate groups (compound 3). A self-encapsulation phenomenon is also observed with compound 2. Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1.

Optically Active Polyoxometalate-Based Silica Nanohelices: Induced Chirality from Inorganic Nanohelices to Achiral POM Clusters.

In order to investigate the principle of chiral induction from nanometric silica helices to polyoxometalate (POM) clusters, a series of optically active silica POM-based nanohelices (NANOPOMs) have been prepared by electrostatic grafting and direct adsorption of α-Keggin polyoxometalate [α-PW12 O40 ]3- to well-defined left- and right-handed silica nanohelices. UV/Vis, Raman, DRIFT, TEM, HR-TEM, EDS and circular dichroism (CD) spectroscopy were used to characterize these NANOPOMs, and confirm the presence of POM clusters as well as their interactions with the helical support. The optical activity of the left-handed and right-handed NANOPOMs has been proven by CD spectroscopy. Their CD spectra are mirror images of one another, showing cotton effects at around 214 and 276 nm, this last contribution corresponding to the oxygen-to-tungsten charge-transfer bands of Keggin polyoxoanions. The CD signal of POM clusters is strongly enhanced for NANOPOMs built by adsorption of POM onto silica nanohelices, indicating a better induced optical activity to POM clusters. These nanohelices are stable, recoverable and active catalysts in the oxidation of sulfides. To the best of our knowledge, the present research represents the first examples of optically active POM-containing silica nanohelices in which achiral POM clusters have been grafted onto silica nanohelices, and display chiroptical effects.

Chiroptical properties of cryptophane-111.

The two enantiomers of cryptophane-111 (1), which possesses the most simplified chemical structure of cryptophane derivatives and exhibits the highest binding constant for xenon encapsulation in organic solution, were separated by HPLC using chiral stationary phases. The chiroptical properties of [CD(+)254]-1 and [CD(-)254]-1 were determined in CH2Cl2 and CHCl3 solutions by polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) experiments and were compared to those of cryptophane-222 (2) derivative. Synchroton Radiation Circular Dichroism (SRCD) spectra were also recorded for the two enantiomers of 1 to investigate low-lying excited states in the 1Bb region. Time-dependent density functional theory (TDDFT) calculations of the ECD and SRCD as well as DFT calculations of the VCD and ROA allowed the [CD(-)254]-PP-1 and [CD(+)254]-MM-1 absolute configurations for 1 in CH2Cl2 and CHCl3 solutions. Similar configurations were found in the solid state from X-ray crystals of the two enantiomers but the chemical structures are significantly different from the one calculated in solution. In addition, the chiroptical properties of the two enantiomers of 1 were independent of the nature of the solvent, which is significantly different to that observed for cryptophane-222 compound. The lack of solvent molecule (CH2Cl2 or CHCl3) within the cavity of 1 can explain this different behaviour between 1 and 2. Finally, we show in this article that the encapsulation of xenon by 1 can be evidenced by ROA following the symmetric breathing mode of the cryptophane-111 skeleton at 150 cm-1.

Direct Observation of Siloxane Chirality on Twisted and Helical Nanometric Amorphous Silica.

Synthesis of chiral inorganic or hybrid nanomaterials through sol-gel transcription of chiral organic templates has attracted a great deal of interest for more than a decade. However, the chiral nature of these inorganic matrices has never been directly observed. For the first time, we report a direct evaluation of chirality on noncrystalline silica chiral nanoribbons by vibrational circular dichroism (VCD) measurements. Strong Cotton effect around 1150-1000 cm-1 from Si-O-Si asymmetric stretching vibration was observed. Surprisingly, calcination of these hybrid nanoribbons doubled the intensity of Cotton effects. On the basis of transmission electron microscopy observations, IR, VCD, NMR, and Raman spectroscopies, we demonstrate that the silica chirality originates from twisted siloxane network composed of chiral arrangement of the Si-O-Si bonds. Our findings clearly prove the presence of chiral organization of amorphous silica network, making them very promising chiral platforms for chiral recognition, optical applications, or asymmetric catalysis.

Epoxy-terminated self-assembled monolayers containing internal urea or amide groups.

We report the synthesis of new coupling agents with internal amide or urea groups possessing an epoxy-terminal group and trimethoxysilyl-anchoring group. The structural characterizations of the corresponding self-assembled monolayers (SAMs) were performed by polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS). The molecular assembly is mainly based on the intermolecular hydrogen-bonding between adjacent amide or urea groups in the monolayers. Because of the steric hindrance of amide or urea groups, the distance between the alkyl chains is too large to establish van der Waals interactions, inducing their disorder. The reactivity of the epoxy-terminal groups was successfully investigated through reaction with a fluorescent probe. We show that SAMs containing internal urea or amide groups exhibited a higher density of accessible epoxide groups than the corresponding long-chain (C22) glycidyl-terminated SAM.

Immobilization of cryptophane derivatives onto SiO2/Au and Au substrates.

The synthesis of a cryptophane molecule bearing five methoxy substituents and an alkanethiol chain, 4, as well as its subsequent grafting onto a gold surface, is reported. Immobilization of cryptophane derivatives onto silica (SiO2/Au) surfaces was also performed by reacting a cryptophane molecule bearing one or six acid functions, 5 or 6, respectively, with an amino-terminated self-assembled monolayer (SAM). Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) was used to characterize the two types of cryptophane monolayers. Surface coverage of cryptophane monolayers was estimated by comparing the PM-IRRAS intensity of cryptophane bands with that calculated from the optical constants of pentamethoxy-cryptophane for a compact monolayer. A very efficient grafting of 4 onto a gold surface was found, with a surface coverage close to 100%. On the other hand, the reaction of mono-acid, 5, or hexa-acid, 6, cryptophanes with amino-terminated SAM was less efficient, since the surface coverage did not exceed 15%. Finally, a good surface coverage (75%) was also obtained by using a cysteamine coupling agent to modify 5 before its grafting onto a gold surface.