Stereoselective Chiral Recognition of Amino Alcohols with 2,2′-Dihydroxybenzil

Optically Pure Co(III) Complex Absorbed by Electrochemiluminescence-Active Covalent Organic Framework as an Enantioselective Recognition Platform to Give Opposite Responses Toward Amino Alcohol Enantiomers

Although covalent organic frameworks (COFs) accompanied by electrochemiluminescence (ECL) behavior have been introduced in recent years, they are still rarely applied for ECL-based enantioselective sensing, especially giving high recognition efficiency. In the current study, an achiral ionic COF comprised of the pyridinium unit is synthesized in the linkage of the carbon-nitrogen cation bond through the Zincke reaction. Interestingly, the synthesized ionic COF can generate clear ECL owing to the presence of electroactive species. Then, the ECL-active achiral COF is employed to absorb the chiral Co(III) complex for enantioselective sensing. As a result, the developed ECL sensor displays discriminative responses toward amino alcohol enantiomers. When the chiral Co(III) complex with (R)-configuration is used, the examined (S)-amino alcohols result in ECL enhancement, whereas (R)-amino alcohols lead to ECL quenching. The maximum ECL intensity ratio between (S)- and (R)-amino alcohols is up to 47.7. In addition, the recognition mechanism is investigated in detail. Finally, a good linear relation between enantiomeric composition and ECL intensity is developed and appropriate for the accurate analysis of the enantiomeric purities of unknown samples. In short, we believe that this study constructs an effective strategy to combine the respective advantages of COFs and ECL for high-efficiency enantioselective sensing.

Circular Dichroism Sensing: Strategies and Applications

The analysis of the absolute configuration, enantiomeric composition, and concentration of chiral compounds are frequently encountered tasks across the chemical and health sciences. Chiroptical sensing methods can streamline this work and allow high-throughput screening with remarkable reduction of operational time and cost. During the last few years, significant methodological advances with innovative chirality sensing systems, the use of computer-generated calibration curves, machine learning assistance, and chemometric data processing, to name a few, have emerged and are now matched with commercially available multi-well plate CD readers. These developments have reframed the chirality sensing space and provide new opportunities that are of interest to a large group of chemists. This review will discuss chirality sensing strategies and applications with representative small-molecule CD sensors. Emphasis will be given to important milestones and recent advances that accelerate chiral compound analysis by outperforming traditional methods, conquer new directions, and pioneering efforts that lie at the forefront of chiroptical high-throughput screening developments. The goal is to provide the reader with a thorough understanding of the current state and a perspective of future directions of this rapidly emerging field.

Straightforward synthesis of chiral sulfonate-based ionic liquids from amino alcohols for chiral recognition

New sulfonate-based chiral salts were prepared from amino alcohols and sodium hydroxymethanesulfonate, vinyl sulfonate, or sultone. The synthesis started with different amino acids from the chiral pool and gave the desired products in just four steps. After cation metathesis, the salts were explored as chiral solvating agents (CSAs) in NMR studies. The new chiral ionic liquids (CILs) were successfully able to interact with different chiral guest molecules and formed diastereomeric aggregates. In some cases, baseline separation was observed. The influence of the structural differences in the CIL as well as the structural requirements of the guest molecule is discussed.

A Pair of Multifunctional Cu(II)-Dy(III) Enantiomers with Zero-Field Single-Molecule Magnet Behaviors, Proton Conduction Properties and Magneto-Optical Faraday Effects

Multifunctional materials with a coexistence of proton conduction properties, single-molecule magnet (SMM) behaviors and magneto-optical Faraday effects have rarely been reported. Herein, a new pair of Cu(II)-Dy(III) enantiomers, [DyCu2(RR/SS-H2L)2(H2O)4(NO3)2]·(NO3)·(H2O) (R-1 and S-1) (H4L = [RR/SS] -N,N'-bis [3-hydroxysalicylidene] -1,2-cyclohexanediamine), has been designed and prepared using homochiral Schiff-base ligands. R-1 and S-1 contain linear Cu(II)-Dy(III)-Cu(II) trinuclear units and possess 1D stacking channels within their supramolecular networks. R-1 and S-1 display chiral optical activity and strong magneto-optical Faraday effects. Moreover, R-1 shows a zero-field SMM behavior. In addition, R-1 demonstrates humidity- and temperature-dependent proton conductivity with optimal values of 1.34 × 10-4 S·cm-1 under 50 °C and 98% relative humidity (RH), which is related to a 1D extended H-bonded chain constructed by water molecules, nitrate and phenol groups of the RR-H2L ligand.

A Real-Time Strategy for Chiroptical Sensing and Enantiomeric Excess Determination of Primary Amines via an Acid-Base Reaction

Two achiral aromatic carboxylic acids that included the 1,8-naphthalimide group and an imidazolium cation were synthesized and exploited as chiroptical sensors. These compounds showed the real-time discrimination and enantiomeric excess determination of chiral amines and amino alcohols via an acid-base interaction, especially for UV-silent chiral compounds.

Application of aromatic amide-derived atropisomers as chiral solvating agents for discrimination of optically active mandelic acid derivatives in 1 H nuclear magnetic resonance spectroscopy

Non-biaryl atropisomers and their stereochemistry have attracted much attentions in the past years. However, application of the non-biaryl atropisomers as chiral solvating agents is yet to be explored. In this work, four aromatic amide-derived atropisomeric phosphine ligands (hosts) were used as chiral solvating agents to recognize various mandelic acid derivatives (guests) in ¹ H nuclear magnetic resonance (NMR) spectroscopy. It is found that chiral center configurations of the four hosts have different effects on the enantiorecognition to the used guests. In addition, the host and guest interaction was further investigated by determination of the host-guest complex stoichiometry using the Job's method and density functional theory calculation, respectively. Moreover, chiral analysis accuracy of these hosts was evaluated through relationship between enantiomeric excess values of 4-chloromandelic acid provided by NMR and gravimetry, respectively.

On the non-innocence and reactive versus non-reactive nature of α-diketones in a set of diruthenium frameworks

Slightly modified ligand designs in diruthenium setups have major impacts on the reactivity/stability of coordination complexes. The 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione bridge is also potentially redox non-innocent.

Point-to-Axial Chirality Transmission: A Highly Sensitive Triaryl Chirality Probe for Stereochemical Assignments of Amines

A readily available stereodynamic and the electronic circular dichroism (ECD)-silent 2,5-di(1-naphthyl)-terephthalaldehyde-based probe has been applied for chirality sensing of primary amines. The chiral amine (the inductor) forces a change in the structure of the chromophore system through the point-to-axial chirality transmission mechanism. As a result, efficient induction of optical activity in the chromophoric system is observed. The butterflylike structure of the probe, with the terminal aryl groups acting as changeable "wings", allowed for the generation of exciton Cotton effects in the region of 1Bb electronic transition in the naphthalene chromophores. The sign of the exciton couplets observed for inductor-reporter systems might be correlated with an absolute configuration of the inductor, whereas the linear relationship between amplitudes of the specific Cotton effect and enantiomeric excess of the parent amine gives potentiality for quantitative chirality sensing. Despite the structural simplicity, the probe turned out to be unprecedentedly highly sensitive to even subtle differences in the inductor structure (i.e., O vs CH2).

An ionic-based carbon dot for enantioselective discrimination of nonaromatic amino alcohols

Here, ionized chiral carbon dots, (S,S)-C-dots-1 (λex = 430 nm, λem = 480 nm), were synthesized via a facile route with relatively high quantum yield (∼24.4%) and used as a fluorescent chiral sensor. One of the advantages of the synthetic process is that it avoids the loss of the chiral center. That is, the chiral bromo compound can directly form an ionic pair with the pyridyl group, which is derived from the amine precursor in the first step. Furthermore, (S,S)-C-dots-1 shows clear discrimination toward different configurations of nonaromatic amino alcohols in the presence of Cu(ii). When the (R)-isomer is added to a solution of (S,S)-C-dots-1 + Cu(ii), it shows much higher fluorescent intensity than the (S)-isomer. The values of IR/IS are 2.9 and 2.3 for 2-aminobutan-1-ol and 2-aminopropan-1-ol, respectively. In summary, we believe that this work can expand the synthetic routes and potential applications of functional carbon dots in the field of enantioselective sensing.

Stereochemical analysis of chiral amines, diamines, and amino alcohols: Practical chiroptical sensing based on dynamic covalent chemistry

Practical chiroptical sensing with a small group of commercially available aromatic aldehydes is demonstrated. Schiff base formation between the electron-deficient 2,4-dinitrobenzaldehyde probe and either primary amines, diamines, or amino alcohols proceeds smoothly in chloroform at room temperature and is completed in the presence of molecular sieves within 2.5 hours. The substrate binding coincides with a distinct circular dichroism signal induction at approximately 330 nm, which can be correlated to the absolute configuration and enantiomeric composition of the analyte. The usefulness of this sensing method is highlighted with the successful sensing of 18 aliphatic and aromatic amines and amino alcohols and five examples showing quantitative %ee determination with good accuracy.

Macroscopic helical chirality and self-motion of hierarchical self-assemblies induced by enantiomeric small molecules

Transfer of molecular chirality to supramolecular chirality at nanoscale and microscale by chemical self-assembly has been studied intensively for years. However, how such molecular chirality further transfers to the macroscale along the same path remains elusive. Here we reveal how the chirality from molecular level transfers to macroscopic level via self-assembly. We assemble a macrostripe using enantiomeric camphorsulfonic acid (CSA)-doped polyaniline with hierarchical order. The stripe can twist into a single-handed helical ribbon via helical self-motion. A multi-scale chemo-mechanical model is used to elucidate the mechanism underlying its chirality transfer and induction. The molecular origin of this macroscopic helical chirality is verified. Results provide a comprehensive understanding of hierarchical chirality transfer and helical motion in self-assembled materials and even their natural analogues. The stripe exhibits disparate actuation behaviour under stimuli of enantiomeric amines and integrating such chiral perception with helical self-motion may motivate chiral biomimetic studies of smart materials.

Chirality transfer by chemical self-assembly has been studied intensively for years but chirality transfers along the same path remains elusive. Here the authors use a multiscale chemo-mechanical model to elucidate the mechanism underlying the chirality transfer via self-assembly in hierarchical camphorsulfonic acid doped polyaniline.

Concentration-Independent Stereodynamic g-Probe for Chiroptical Enantiomeric Excess Determination

Enantiomeric excess (ee) determination is crucial in many aspects of science, from synthesis to materials. Within this subject, coupling molecular sensors with chiroptical techniques is a straightforward approach to the stereochemical analysis of chiral molecules, especially in terms of process immediacy and labor. Stereodynamic probes typically consist of racemic mixtures of rapidly interconverting enantiomeric conformers able to recognize a chiral analyte and greatly amplify its chiroptical readout. A great number of sensors have been developed, but their activity is generally restricted to one or a few classes of chemicals, and the analysis outcome relies on precise knowledge of the probe and analyte concentrations. This aspect in particular limits the potential practical applications. Here we report an oxo-vanadium(V) aminotriphenolate complex that was found to act as a concentration-independent stereodynamic sensor for a wide range of compounds. The bare complex is CD-silent, but coordination of an enantioenriched substrate immediately gives rise to intense Cotton effects in the visible region. Furthermore, a geometry change during the substrate-complex interaction leads to a marked optical response, as witnessed by a strong red-shift of the probe absorption bands, thus allowing the generation of dichroic signals in an "interference-free" area of the spectrum. This peculiarity allows for a linear correlation at high wavelengths between the ee of the analyte and anisotropy g-factor. This parameter derives from the differential circularly polarized light absorption of the sample but is independent of concentration. The newly developed sensor based on a simple coordination process has an unprecedented general character in terms of substrate scope and employment.