Enantioselective sensing of carboxylic acids with a bis(urea)oligo(phenylene)ethynylene foldamer

Organometallic Chirality Sensing via "Click"-Like η6-Arene Coordination with an Achiral Cp*Ru(II) Piano Stool Complex

Piano stool complexes have been studied over many years and found widespread applications in organic synthesis, catalysis, materials and drug development. We now report the first examples of quantitative chiroptical molecular recognition of chiral compounds through click-like η6-arene coordination with readily available half sandwich complexes. This conceptually new approach to chirality sensing is based on irreversible acetonitrile displacement of [Cp*Ru(CH3CN)3]PF6 by an aromatic target molecule, a process that is fast and complete within a few minutes at room temperature. The metal coordination coincides with characteristic circular dichroism inductions that can be easily correlated to the absolute configuration and enantiomeric ratio of the bound molecule. A relay assay that decouples the determination of the enantiomeric composition and of the total sample amount by a practical CD/UV measurement protocol was developed and successfully tested. The introduction of piano stool complexes to the chiroptical sensing realm is mechanistically unique and extends the scope of currently known methods with small-molecule probes that require the presence of amino, alcohol, carboxylate or other privileged functional groups for binding of the target compound. A broad application range including pharmaceutically relevant multifunctional molecules and the use in chromatography-free asymmetric reaction analysis are also demonstrated.

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.

Chiroptical Recognition of Carboxylates with Charge-Neutral Double-Stranded Zinc(II) Helicates

Chirality analysis of small molecules for the determination of their enantiopurity is nowadays ruled by streamlined chromatographic methods which utilize chiral stationary phases. Chiroptical probes which rely on host-guest interactions are so far overshadowed by the latter but have the benefit of depending only on common spectroscopic techniques such as CD spectroscopy to distinguish enantiomers and to quantify their ratio. Interest into this receptor-based approach is constantly rising because non-invasive high-throughput screenings with a minimal waste production can be performed. In this study we investigate the possibility to utilize metal-based containers in form of charge-neutral helicates able to recognize anions for this purpose. Key building block of the helicates are triazole units which show rotational freedom and give rise to either a meso-structure or a racemic mixture of the right- and left-handed complex. A chiroptical response of the probe is observed upon recognition of chiral mono- or dicarboxylates and chirality analysis of tartrate is conducted by CD spectroscopy.

Effective enantioselective recognition by steady-state fluorescence spectroscopy: Towards a paradigm shift to optical sensors with unusual chemical architecture

A series of amino acid-derived 1,2,3-triazoles presenting the amino acid residue and the benzazole fluorophore connected by a triazole-4-carboxylate spacer was studied for enantioselective recognition using only steady-state fluorescence spectroscopy in solution. In this investigation, the optical sensing was performed with D-(-) and L-(+)-Arabinose and (R)-(-) and (S)-(+)-Mandelic acid as chiral analytes. The optical sensors showed specific interactions with each pair of enantiomers, allowing photophysical responses, which were used for their enantioselective recognition. DFT calculations confirm the specific interaction between the fluorophores and the analytes corroborating the observed high enantioselectivity of these compounds with the studied enantiomers. Finally, this study investigated nontrivial sensors for chiral molecules by a mechanism different than turn-on fluorescence and has the potential to broad chiral compounds with fluorophoric units as optical sensors for enantioselective sensing.

Hydrogen Bonded Foldamers with Axial Chirality: Chiroptical Properties and Applications

Folding phenomenon refers to the formation of a specific conformation widely featured by the intramolecular interactions, which broadly exist in biomacromolecules, and are closely related to their structures and functions. A variety of oligomeric folded molecules have been designed and synthesized, namely "foldamer", exhibiting potentials in pharmaceutical and catalysis. Molecular folding is a promising strategy to transfer chirality from substituents to the whole skeleton, when chirality transfer, amplification, evolution, and other behaviors could be achieved. Investigating chirality using foldamer model deepens the understanding of the structure-function correlation in biomacromolecules and expands the molecular toolbox towards chiroptical and asymmetrical chemistry. Substitutes with abundant hydrogen bonding sites conjugated to a rotatable aryl group afford a parallel β-sheet-like conformation, which enables the emergence and manipulation of axial chirality. This concept aims to give a brief introduction and summary of the hydrogen bonded foldamers with anchored axial chirality, by taking some recent cases as examples. Design principles, control over axial chirality and applications are also reviewed.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Chiroptical sensing of amino acids, amines, amino alcohols, alcohols and terpenes with π-extended acyclic cucurbiturils

The efficiency and scope of two acyclic π-wall extended cucurbiturils, M2 and M3, exhibiting rapidly interconverting helical conformers for chiroptical sensing of amines, amino acids, alcohols, and terpenes at micromolar concentrations in water is evaluated. The formation of 1 : 1 host-guest complexes results in spontaneous induction of circular dichroism signals that can be used for accurate determination of the absolute configuration and enantiomeric composition of the analyte based on a simple mix-and-measure protocol.

Distance-Dependent Chiral Communication between Two Quinoline Oligoamide Foldamers Connected by Alkyl Chains

A series of macrocycles that contain two quinoline oligoamide foldamers (QOFs) using various length of alkyl chains as linkers (2, 3, 6, 8 or 12 hydrocarbons) were synthesized. The two QOFs interact with each other through the linkers and the intramolecular helix chiral communications between the two QOFs were studied by ¹ H NMR spectroscopy and crystal structures. Investigations show that the intensity of the intramolecular helix chiral communications between the two QOFs is dependent on the length of the linkers, and the interaction between the two QOFs increases with decreasing length of the linkers. When the length of the linkers decreased to C2 linkers, only one conformer is present in solution. Moreover, increasing the length of the foldamers would enhance the intramolecular helix chiral communication if the linkers are short, indicating that the length of the foldamers also has significant impact on intramolecular helix chiral communication.

Extended enantiopure ortho-phenylene ethylene (o-OPE)-based helical systems as scaffolds for supramolecular architectures: a study of chiroptical response and its connection to the CISS effect

Versatile enantiopure helical systems are described and are of interest owing to their intense chiroptical responses, their attractive architecture for metallosupramolecular chemistry and CISS effect.

Well-Defined Conjugated Macromolecules Based on Oligo(Arylene Ethynylene)s in Sensing

Macromolecules with well-defined structures in terms of molar mass and monomer sequence became interesting building blocks for modern materials. The precision of the macromolecular structure makes fine-tuning of the properties of resulting materials possible. Conjugated macromolecules exhibit excellent optoelectronic properties that make them exceptional candidates for sensor construction. The importance of chain length and monomer sequence is particularly important in conjugated systems. The oligomer length, monomer sequence, and structural modification often influence the energy bang gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecules that reflect in their properties. Moreover, the supramolecular aggregation that is often observed in oligo-conjugated systems is usually strongly affected by even minor structural changes that are used for sensor designs. This review discusses the examples of well-defined conjugated macromolecules based on oligo(arylene ethynylene) skeleton used for sensor applications. Here, exclusively examples of uniform macromolecules are summarized. The sensing mechanisms and importance of uniformity of structure are deliberated.

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.

Enantiomeric Recognition and Separation by Chiral Nanoparticles

Chiral molecules are stereoselective with regard to specific biological functions. Enantiomers differ considerably in their physiological reactions with the human body. Safeguarding the quality and safety of drugs requires an efficient analytical platform by which to selectively probe chiral compounds to ensure the extraction of single enantiomers. Asymmetric synthesis is a mature approach to the production of single enantiomers; however, it is poorly suited to mass production and allows for only specific enantioselective reactions. Furthermore, it is too expensive and time-consuming for the evaluation of therapeutic drugs in the early stages of development. These limitations have prompted the development of surface-modified nanoparticles using amino acids, chiral organic ligands, or functional groups as chiral selectors applicable to a racemic mixture of chiral molecules. The fact that these combinations can be optimized in terms of sensitivity, specificity, and enantioselectivity makes them ideal for enantiomeric recognition and separation. In chiral resolution, molecules bond selectively to particle surfaces according to homochiral interactions, whereupon an enantiopure compound is extracted from the solution through a simple filtration process. In this review article, we discuss the fabrication of chiral nanoparticles and look at the ways their distinctive surface properties have been adopted in enantiomeric recognition and separation.

Influence of different sequences of l-proline dipeptide derivatives in the pendants on the helix of poly(phenylacetylene)s and their enantioseparation properties

Novel helical poly(phenylacetylene)s bearing different sequences of l-proline dipeptide derivative pendants were prepared and used as CSPs in HPLC for enantioseparation.