Search

In a paper addressing the problem that the majority of chiral compounds crystallize as racemates instead of conglomerates, Jack Dunitz coined the expression 'Wallach's Rule'. The paper gives credit to Wallach's publication in Liebigs Annalen der Chemie in 1895, in which, among other work, a comparison of crystal density of racemates and their homochiral analogues was reported. Of the nine compounds investigated, eight showed higher densities for the racemates. Although Wallach was the only author of the Annalen paper, it was not Otto Wallach, but Theodore Liebisch, Wallach's colleague from the mineralogy institute in Göttingen at that time, who performed the experiments and reported these results. Short overviews about the CVs of the contemporaries Wallach and Liebisch, as well as Liebisch's postdoc Sommerfeld, are presented. It is discussed whether Wallach or Liebisch had the idea to study crystal densities and which one of the two scientists should be properly credited with proposing 'Wallach′s Rule'. [ABSTRACT FROM AUTHOR]

Chiral polymers and chiral polymeric particles have emerged as a new and exciting field of research in recent years mainly due to their possibly applications in chiral chemistry. This paper reviews the present state of the art regarding production techniques for the synthesis and applications of chiral polymeric particles. The main methods for preparing of chiral polymeric particles such as: direct polymerization, emulsion, precipitation, and suspension polymerization of chiral monomers, are reviewed. Moreover, in this article we also present the use of chiral polymers as chiral templates for the synthesis of chiral mesoporous materials. In this review we highlighted the properties and parameters involved in the preparation of these chiral polymeric materials. The present review focuses mainly on the use of chiral polymer and chiral polymeric particles for enantioselective crystallization and enantioseparation. References of the most relevant literature published by various research groups are provided. Anyway, it is clear that chiral polymeric particles are a distinctive type of chiral nanomaterials that can find many new application in other fields like, chiral drug delivery systems, enantioselective catalysis. We hope that this review article will inspired new researchers in this field and will boost the research dealing on chiral polymeric particles especially in their implementation in new areas in chiral chemistry. [ABSTRACT FROM AUTHOR]

Recent investigations of supramolecular polymers based on chiral triphenylene‐2,6,10‐tricarboxamides (TTAs) showed how temperature‐induced changes in solvation can greatly influence the preferred helical conformation of the supramolecular polymers formed. Here, we combine chiral TTA with achiral copolymerization partners to further investigate temperature‐dependent solvation effects. Systematic variation of the system's composition shows clear impacts on the temperature window where the conformational change occurs. Further, simple chain length variations in the achiral comonomer greatly affect the ability to influence the conformational change in the copolymer, while the differences in the properties of the individual homopolymers are rather small. We herein highlight how subtle changes in the monomers can impact the observed copolymer properties drastically; an effect arising from the emerging complexity of multicomponent interactions in supramolecular copolymers with solvent‐solute interactions being more important than typically thought. [ABSTRACT FROM AUTHOR]

This paper is a choral personal account on a longstanding and fruitful collaboration of organic and inorganic chemists to develop new hybrid polyoxometallic systems. .We explain our motivation and the context of our work that led to polyoxometalates with highly functional and reactive organic groups. Our approach yielded catalytically, biologically, and photochemically active systems. [ABSTRACT FROM AUTHOR]

In solid state physics, any symmetry breaking is known to be associated with emergence of an order parameter. However, the order parameter for molecular and crystal chirality, which is a consequence of parity and mirror symmetry breaking, has not been known since its discovery. In this article, the authors show that the order parameter for chirality can be defined by electric toroidal monopole G0 ${{G}_{0}}$. By this definition, one becomes able to discuss external field that can distinguish two different enantiomers only by physical fields. In addition, dynamics and fluctuations of the order parameter G0 ${{G}_{0}}$ can be discussed, with which one can obtain fruitful insights on a spin filtering effect called CISS (Chirality Induced Spin Selectivity). Emergence of time‐reversal‐odd dipole Mz ${{M}_{z}}$ by time propagation of G0 ${{G}_{0}}$ quantities is discussed to explain the enantioselective effect (chiral resolution) at a ferromagnetic surface. [ABSTRACT FROM AUTHOR]

Chirality is a fundamental chemical property that can be found in almost all aspects of life. Generally, in nature chirality exists in only one of the possible enantiomeric forms. Bitter experience showed that chiral drugs having the same chemical composition but opposite chirality may have extremely different biological effects. It is therefore that detecting and quantifying chirality is important in multiple fields ranging from analytical and biological chemistry to pharmacology, biotechnology, and fundamental physics. To date, the most widely used analytical methods for chiral detection, remain the traditional approaches of measuring circular dichroism and optical rotation. However, these methods suffer from low signal‐to‐noise due to large time‐dependent backgrounds and require complicated optical setups. Recent works associate circular dichroism measurements with the Chiral Induced Spin Selectivity (CISS) spin current measurements. The CISS effect relates the probability of electron spin transmission through chiral molecules to chirality. Depending on the handedness of the molecule, electrons of a certain spin can traverse the molecule more easily in one direction than in the other. It is therefore that the CISS effect could be utilized to electronically measure chirality using spin currents and spin induced dipoles. The review summarizes the different approaches for utilizing the CISS effect for electrical measurements of chirality. Starting with a Hall device that can measure the chirality of the lowest energetic CD band of a monolayer in dry or wet systems. Presenting an enhancement of the effect as well as achieving a wider CD spectrum using electrical gating. Going down to 100 molecules limit with full spectrum response utilizing electro‐optical nano floret devices. [ABSTRACT FROM AUTHOR]

Inspired by biological systems, supramolecular polymers have emerged over the past three decades as an important way to form functional structures with potential to tune the internal order and dynamics and to create materials with the potential for responsiveness, reconfigurability, and recyclability. We focus here on several recent examples of ordered supramolecular polymers with a significant extent of internal order that results in crystalline molecular packing, or with dynamics that allow them to undergo structural reconfiguration. These examples highlight the spectrum of molecular mobility that the noncovalently bound constituents can experience. These materials have found interesting functions in their interaction with chiral biological systems and in their ability to form materials with interesting optoelectronic and photocatalytic behavior. [ABSTRACT FROM AUTHOR]

Glycine is a small molecule. It cannot change its conformation and is achiral. Despite the apparent simplicity, glycine shows endless diversity in its behavior over many phenomena. It was the first amino acid for which polymorphism was reported, first on crystallization and then on hydrostatic compression. The polymorphs differ in their physical properties and their biological activity. Glycine clusters persist in solution, leading to "solution memory". Phenomena at interfaces are critically important for crystal growth, dissolution, and for physical properties, which can be at times unexpected, like polarity in centrosymmetric α‐polymorph. It is a great pleasure to remind of these remarkable phenomena in a special issue honoring professors Meir Lahav and Leslie Leiserowitz, who pioneered the study of the behavior of this unique molecule in many respects, and showed how complex and non‐trivial phenomena can be at interfaces: between phases and between research fields. [ABSTRACT FROM AUTHOR]

In reflecting the author's personal perspective, this review demonstrates how chirality is a unifying theme in modern science. Using the discrete symmetries of parity P, time reversal T and charge conjugation C to characterize chiral systems, the concepts of true and false chirality that emerge provide an extension of Lord Kelvin's definition to dynamic situations This clarifies the nature of physical influences able to induce absolute enantioselection, emphasizing the distinct behaviour of processes under thermodynamic and kinetic control. Parity violation and CP violation are suggested to be manifestations of true and false cosmic chirality, respectively, with possible roles in abiotic enantioselection. Truly chiral systems support time‐even pseudoscalar observables which endows chiroptical spectroscopies with a special sensitivity to absolute stereochemistry, exemplified by the application of vibrational Raman optical activity to small chiral organic molecules and intact viruses. [ABSTRACT FROM AUTHOR]

A few decades ago, the enantiomeric purities of chiral compounds were usually determined by comparison of experimental optical rotations or by preparation of diastereomeric derivatives followed by analysis of their 1H NMR spectra. This situation changed when Emanuel Gil-Av, Binyamin Feibush and Rosita Charles-Sigler achieved the separation of single enantiomers from racemic α-amino acids by means of a chiral stationary phase. Indeed, chiral chromatography allowed a direct comparison of chromatograms obtained from enantiopure samples with those recorded with their racemates. With the aim of celebrating the relevance of Gil-Av's achievement, this review also presents several applications of chiral chromatography in the area of asymmetric synthesis of β-amino acids. [ABSTRACT FROM AUTHOR]

Structural chemistry began with Louis Pasteur, who first saw the configuration of molecules manifest in the shapes of crystals. Here is reviewed forgotten, century-old efforts to make molecular configuration vivid in polycrystalline ensembles. Resorcinol, and a variety of other simple molecular crystals, when grown from the melt in the presence of resolved chiral additives, such as tartaric acids, form spectacular bull's-eye patterns evident in the petrographic microscope. Concentric, rhythmic optical bands in so-called banded spherulites are tell-tale signs of the twisting of helicoidal crystalline lamellae. The sense of twisting was assayed by correlating displacements of extinction bands on rotation with additive configurations. Thus, the arrangements of atoms in space were made manifest in crystal form - not in the appearance of hemihedral facets, but rather, in the chirality of nonclassical morphologies with Gaussian curvature. The far reaching consequences of these experiments for molecular crystal growth and form, long ago foreseen, are evaluated with the hindsight of an extra century of experience and sophistication. The narrative aims to draw a continuous thread through the work of Pasteur, Grigorii Vyroubov, Frédéric Wallerant, John Monteath Robertson, Hélène Metzger, and Jack Dunitz. [ABSTRACT FROM AUTHOR]

Beyond the universality of the symmetry concept, there are different emphases on its application in different branches of science. Chemistry, being between particle physics and astrophysics, represents a bridge in, and a utilitarian approach to, the application of the symmetry concept, which has proved immensely fruitful in twentieth-century science. Some pivotal discoveries, especially in structural chemistry, molecular biology, and materials science, emerged by relaxing some of the stipulations of the classical teachings about symmetry. This highly personal presentation relies on ideas expressed by a number of notable individuals in recent science, among them J. Desmond Bernal, Francis Crick, Ronald J. Gillespie, Aleksandr I. Kitaigorodskii, Alan L. Mackay, George A. Olah, Linus Pauling, Roger Penrose, Dan Shechtman, James D. Watson, Steven Weinberg, and Eugene P. Wigner. [ABSTRACT FROM AUTHOR]

Emanuel Gil-Av (Zimkin) was born in 1916 and passed away in 1996. The introduction of chemical selectivity into the chromatographic separation process constitutes his main scientific contribution. This approach culminated in the first enantiomeric separation of racemic amino acids by gas chromatography on an optically active chiral stationary phase in 1966, together with Binyamin Feibush and Rosita Charles-Sigler. Thus the year 2016 marks an important date to remember and to appreciate the invaluable achievements of Gil-Av in the realm of chirality carried out at the Weizmann Institute of Science, Israel. [ABSTRACT FROM AUTHOR]

Of the many hypotheses on the origin of biological homochirality on the Earth, the combination of the notion of subatomic particles composing a homochiral pool of primary matter, on the one hand, with the suggestion of the synthesis of chiral organic molecules in the plasma torch generated by super-high-velocity impacts (SHVI) of meteorites, on the other hand, was chosen as the working assumption. [ABSTRACT FROM AUTHOR]

The separation and analysis of chiral compounds is an important topic of research, because these analytes are involved in various processes, e.g., biological, pharmacological, human health, etc. Their separation has also been recently obtained utilizing microfluidic techniques, such as capillary electrophoresis (CE), electrochromatography (CEC), and capillary/nano-liquid chromatography (CLC/nano-LC). Enantiomeric separation is mainly obtained by applying the direct resolution method, where a chiral selector (CS) is present in the separation system, either added to the mobile phase (MP) or bonded to the stationary phase or to the capillary wall. Among the several CSs used in microfluidic techniques, cyclodextrins, proteins, peptides, glycopeptide antibiotics, and modified polysaccharides are the most employed. Microfluidic techniques can offer several advantages over conventional analytical tools, e.g., high efficiency, high mass sensitivity, and reduced volumes of MP and sample. Optimized methods have been applied in different fields, such as biomedical, food chemistry, environment, pharmaceutical, etc. Excellent results have been achieved in CE and nano-LC. Although CEC can offer higher efficiency due to the use of a strong electroosmotic flow, its use is less popular than the other. This limitation can be explained with the limited number of the stationary phases (SPs) commercially available. [ABSTRACT FROM AUTHOR]

We are the first to investigate the relationship between protein handedness and the rate of protein folding. Our findings demonstrate that small three-helix, left-handed proteins are less densely packed and should result in faster folding than that of right-handed, three-helix proteins. At the same time, right-handed, three-helix proteins have higher mechanical stability than the left-handed proteins. Moreover, from our analysis we have revealed that bacterial three-helix proteins have some advantages in packing over eukaryotic right-handed, three-helix proteins, which should result in faster folding. [ABSTRACT FROM AUTHOR]

A range of unique properties have been extensively studied in colloidal semiconductor nanocrystals (quantum dots, QDs). The possibility to manipulate the properties of these systems by controlling size and composition is attractive for many different applications, as well as for fundamental studies. Recently, the properties of chirality and chiroptical activity induced by surface ligands at QDs were considered. It was shown that chiroptical activity can be induced in the exciton transitions of quantum dots by capping them with chiral molecules. In this work, chiroptical spectroscopy (mainly circular dichroism) is used to study induced effects in CdS, CdSe and ZnSe QD systems. Some fundamental differences are encountered between CdS or CdSe samples and ZnSe samples. The results are analyzed to account for the mechanism of induction of chiroptical activity in the inorganic core by chiral capping molecules. Two electronic mechanisms of interaction are considered and discussed in light of the results. It is also suggested that chiroptical spectroscopy might serve as an efficient tool to characterize electronic (or electron-hole) level spectra of nanocrystal systems by analyzing circular dichroism intensity and lineshape. [ABSTRACT FROM AUTHOR]

Antiferroelectric liquid crystals (AFLCs) are a recent class of materials in the family of liquid crystals which have high prospect of application in displays with better results as compared to conventional nematic displays. Beside technological applications, AFLCs are very interesting from the point of view of basic studies in soft condensed matter systems as these materials are showing various new sub-phases, viz. SmCα*, SmCβ*, SmCγ* and many others with distinct macroscopic properties in a narrow temperature region in addition to the most common wide-range low-temperature SmCa* phase. In the present article, we report the structure of various sub-phases of AFLCs, their dielectric behaviour and the electro-optical response of some of the materials studied by our group. [ABSTRACT FROM AUTHOR]

This review documents the evolution of chiral MOF catalysts from simple Lewis acid and Lewis base catalysts that afford limited selectivity to more recent advancements that exhibit high stereo- and regioselectivities. MOF-based catalysts provide unprecedented opportunities for interrogating structure-activity relationships at high structural precision and for controlling catalytic activity by using external stimuli. The most notable disadvantage of MOF-based catalysts has been the inherent instability of the materials. This hurdle, which effectively limits the reaction scope of MOF catalysts, is slowly being overcome by the advent of new binding moieties and secondary building units. [ABSTRACT FROM AUTHOR]

The efficient, high-yield approaches to two novel regioisomeric salicyl aldehyde analogs, 4-formyl-13-(2-hydroxyphenyl)-[2.2]paracyclophane and 4-formyl-12-(2-hydroxyphenyl)-[2.2]paracyclophane ( iso -FHPhPC and pseudo -FHPhPC, respectively), constructed on the basis of an aryl[2.2]paracyclophane backbone are described. The key stage of the backbone formation is the Suzuki cross-coupling of paracyclophanyl halides with arylboronic acids. Efficient procedures for the resolution of the racemic hydroxy aldehydes into enantiomers via Schiff bases with enantiomers of α-phenylethyl amine were elaborated, and the absolute configurations of enantiomers were established on the basis of X-ray analysis of diastereomeric imines. Starting from these chiral hydroxy aldehydes the first representatives of bi-, tri-, and tetradentate phenoxy-imine ligands belonging to an aryl[2.2]paracyclophane family were obtained. The induction power of the ligands was tested in the Et2Zn asymmetric addition to aldehydes. [ABSTRACT FROM AUTHOR]

[2.2]Paracyclophane offers an exciting framework for the synthesis of planar chiral phosphines. Its rigid structure and unusual electronic properties have led to the development of a range of mono- and di-phosphines including the commercially available ligand, PhanePhos. This review discusses the synthesis and use of [2.2]paracyclophane-based phosphines and aims to give an insight into their success and failures. It is hoped that this review will encourage the design of new planar chiral phosphines for use in transition metal-mediated catalysis and organocatalysis. [ABSTRACT FROM AUTHOR]

The development of planar-chiral hydrogen-bond donors based on the [2.2]paracyclophane scaffold is discussed. General strategies to access functionalized enantiopure [2.2]paracyclophane derivatives are briefly reviewed, with the focus on suitable precursors for the synthesis of planar-chiral thiourea derivatives. The synthesis of fourteen hydrogen-bond donors is described. The interaction of four thiourea derivatives with hydrogen-bond acceptors (DMSO and tetramethylammonium chloride) was investigated by 1H NMR spectroscopy and X-ray crystallography. A selection of enantiomerically pure planar-chiral derivatives was applied in asymmetric hydrogen-bond catalysis. [ABSTRACT FROM AUTHOR]

The synthesis of planar chiral carbazoles bearing a [2.2]paracyclophane backbone is described. Starting from readily available 4-bromo[2.2]paracyclophane planar chiral carbazoles are obtained in a two-step synthesis by Pd-catalyzed CN cross-coupling and subsequent oxidative cyclization. [ABSTRACT FROM AUTHOR]

A novel set of chiral self-assembling discotic molecules has been prepared based on two previously studied self-assembling C3-discotic systems, namely alkyl substituted benzene-1,3,5-tricarboxamides (alkyl-BTAs) and 3,3′-bis(acylamino)-2,2′-bipyridine substituted BTAs (BiPy-BTAs). By replacing one of the three bipyridyl side chains by an alkyl chain we have prepared a hybrid system that shows properties of both parent compounds. Due to the different packing parameters and mechanisms of self-assembly of the parent BTAs, one might expect a lack of ordering in such a hybrid system. However, we observe columnar mesophases and unique self-assembly behavior in solution. We propose that the solution self-assembly passes through three stages. First, disordered aggregates are formed that evolve into ordered aggregates via an isodesmic process. Finally, these ordered aggregates rearrange in a cooperative fashion into a higher ordered structure. Through the interplay between π-π interactions and H-bonding, the hybrid molecules combine the cooperative self-assembly behavior of alkyl-BTAs with the isodesmic behavior of BiPy-BTA discotics. This work shows that merging two systems with seemingly incompatible requirements can lead to new and complex self-assembly behavior. [ABSTRACT FROM AUTHOR]

The genesis of enantiopurity in nature, as exemplified by terrestrial amino acids and sugars, has fascinated scientists for more than 150 years. This is a field where we know what we do not know. Starting from theoretical treatments postulated more than 50 years ago, numerous hypotheses, theories, and experiments have been formulated to claim niches of logic and truth. Such arguments vary from purely speculative, often bizarre, to sound enough in terms of reproducibility and feasible generation of mirror symmetry breaking. Although we cannot exclude a biotic origin (i.e,. homochirality was an a posteriori event where one enantiomer tripped and the other survived and evolved, in close resemblance to a Darwinian-like process), most data and models available are consistent with an abiotic origin that amplified small and stochastic imbalances to produce enantiopure homogeneity prior to life. This analysis revisits recent theoretical background and experiments, which appear to be prebiotically credible, and focuses on primordial molecules such as amino acids. This by no means implies a recreation of the origin of life on Earth, as this task will likely be a surmise forever. These strategies reveal a high degree of robustness and prove how home-made enantiopurity can be accomplished. Such routes should, in addition, be of enormous practical interest for the production of chiral materials and drugs. [ABSTRACT FROM AUTHOR]

This article begins by describing the synthesis and recognition properties of the cucurbit[n]uril homologues CB[5], CB[6], CB[7], CB[8], and CB[10]. Subsequently, we describe the state-of-the-art in understanding the mechanism of CB[n] formation. We describe the experiments that establish that glycoluril ( ) undergoes condensation with formaldehyde by a combination of chain-growth and step-growth polymerization processes. Chain-growth processes deliver methylene bridged glycoluril oligomers - as intermediates that may undergo macrocyclization to nor-seco-CB[n] when the oligomer is long enough (- and subsequently form CB[n]. Step-growth processes allow oligomers to condense to give longer oligomers connected by a single CH-bridge that undergo macrocyclization to deliver (±)-bis-nor-seco-CB[6] and bis-nor-seco-CB[10]. Lastly, we describe some of the exciting new recognition processes of the newly formed members of the CB[n] family. For example, bis-nor-seco-CB[10] undergoes homotropic allostery during ternary complex formation, (±)-bis-nor-seco-CB[6] exhibits moderately diastereoselective recognition processes (d.r. up to 88 : 12) with chiral ammonium ions in water, and nor-seco-CB[6] functions as an aldehyde reactive CB[n] synthon that can control the folding of alkanediammonium ions into a backfolded conformation in water. [ABSTRACT FROM AUTHOR]

Abstract. A new copper(II) fluoride-chiral diphosphines catalytic system was developed. This one is very efficient and selective for the hydrosilylation of several substituted or unsubstituted aromatic ketones in so far as moderate to excellent enantioselectivities were obtained. An oxygen acceleration effect was observed that led us to propose a practical protocol with a low amount of catalyst. [ABSTRACT FROM AUTHOR]

Abstract. A new synthetic strategy for the construction of an inherently chiral, sulfur-bridged calix[4]arene molecular framework was presented by oxidation of one of the four epithio bonds of a tetrathiacalix[4]arene derivative of 1,3-alternate conformation: Tetra(carboxymethyl) ether of p-tert-butyltetrathiacalix[4]arene of 1,3-alternate conformation (7) was converted to the tetra(l-menthoxy-carbonylmethyl) ether (8). Oxidation of the tetra l-menthyl ester 8 by treatment with NaBO 3 (1.1 molar equiv.) afforded a pair of diastereomers of the monosulfinyl derivatives (9) in 58% yield, which were readily separable by silica-gel column chromatography. Removal of the l-menthyl moieties followed by methyl esterification gave enantiomerically pure samples of inherently chiral (-)- and (+)-mono-sulfinyltrithiacalix[4]arene derivatives ((-)-10 and (+)-10) in 62 and 56% yield, respectively. [ABSTRACT FROM AUTHOR]

Abstract. Starting from either (S)-4-tert-butyl-2-ferrocenyl-2-oxazoline or (S)-S-ferrocenyl-S-4-tolyl sulfoxide, new planar chiral hydroxamic acids have been synthesized employing the directed ortho-metalation strategy. The ferrocene-derived hydroxamic acids are capable of serving as ligands in the vanadium-catalyzed asymmetric epoxidation of allylic alcohols, furnishing the epoxide of 2-methyl-3-phenylprop-2-en-1-ol with up to 34% ee. [ABSTRACT FROM AUTHOR]