Possible Case of Halogen Bond-Driven Self-Disproportionation of Enantiomers (SDE) via Achiral Chromatography

Structural Chemistry of C-N Axially Chiral Compounds

In the last several years, atropisomers owing to the rotational restriction around a C-N single bond (C-N axially chiral compounds) have attracted significant attention in the field of synthetic organic chemistry. In particular, the highly enantioselective synthesis of various C-N axially chiral compounds and their application to asymmetric reactions have been reported by many groups. On the other hand, studies on the structural chemistry of C-N axially chiral compounds have attracted scant attention in comparison with synthetic studies. For over 25 years, our group has explored asymmetric synthesis of C-N axially chiral compounds and their synthetic application. In the course of these synthetic studies, we found several notable structural properties in relation to the C-N bond rotation and an association of enantiomers (the relationship between the rotational stability and the structure or electronic effect, the chirality-dependent halogen bond, and the self-disproportionation of enantiomers). Furthermore, on the basis of these structural properties, the development of acid-mediated molecular rotors and the synthesis of isotopic atropisomers possessing high stereochemical purity and rotational stability were achieved. Through this Perspective, I wish to make the chemistry community aware that C-N axially chiral compounds are attractive molecules from the viewpoints of both synthetic organic chemistry and structural chemistry.

Copper (II) Ions Induced Self-Disproportionation of Enantiomers in Capillary Electrophoresis for the Quantification of Atenolol Enantiomers

Despite the fact that the self-disproportionation of enantiomers (SDE) has been found for several decades and has been widely used in crystallization, sublimation and chromatography for the purification or separation of nonracemic compounds, the phenomenon of SDE in capillary electrophoresis (CE) has never been reported up to now. Here, a new approach to separate enantiomers in CE based on SDE was demonstrated by introducing copper (II) ions into the separation media. The enantiomers of atenolol interact with copper ions to produce positively charged complexes with different electrophoretic mobilities from the single molecules. The dynamic equilibrium between homo- or heterochiral complexes (associates) and single molecules of atenolol enantiomers supports the manifestation of SDE. Different mobilities of the single molecules and associates, and different distribution of two enantiomers between the single molecules and associates caused by their different concentrations, produce a net difference in electrodriven migration velocities of the two enantiomers. The relative movement of two enantiomers causes a zone depleted in one enantiomer at the rear end of sample segment, giving a trapezoidal CE curve with a step at the end. Quantification of enantiomers is achieved according to the step height. The analysis does not rely on the use of enantiomerically pure chiral selector and the result agrees with that obtained by conventional chiral CE using a chiral selector.

Asymmetric Synthesis of Isotopic Atropisomers based on ortho-CH3/CD3 Discrimination and Their Structural Properties

N-C axially chiral 3-(2-trideuteriomethyl-4,6-dimethylphenyl)-2-ethylquinazolin-4-ones and 3-(2-trideuteriomethyl-4,6-dimethylphenyl)-2-(1-phenylpropan-2-yl)quinazolin-4-ones were prepared in high enantio- and diastereomeric purities (98% ee). These quinazolinone derivatives are isotopic atropisomers based on ortho-CH₃/CD₃ discrimination and were revealed to possess a slight optical rotation and high rotational stability.

Stereoselective Processes Based on σ-Hole Interactions

The σ-hole interaction represents a noncovalent interaction between atoms with σ-hole(s) on their surface (such as halogens and chalcogens) and negative sites. Over the last decade, significant developments have emerged in applications where the σ-hole interaction was demonstrated to play a key role in the control over chirality. The aim of this review is to give a comprehensive overview of the current advancements in the use of σ-hole interactions in stereoselective processes, such as formation of chiral supramolecular assemblies, separation of enantiomers, enantioselective complexation and asymmetric catalysis.

Recommended Tests for the Self-Disproportionation of Enantiomers (SDE) to Ensure Accurate Reporting of the Stereochemical Outcome of Enantioselective Reactions

The purpose of this review is to highlight the necessity of conducting tests to gauge the magnitude of the self-disproportionation of enantiomers (SDE) phenomenon to ensure the veracity of reported enantiomeric excess (ee) values for scalemic samples obtained from enantioselective reactions, natural products isolation, etc. The SDE always occurs to some degree whenever any scalemic sample is subjected to physicochemical processes concomitant with the fractionation of the sample, thus leading to erroneous reporting of the true ee of the sample if due care is not taken to either preclude the effects of the SDE by measurement of the ee prior to the application of physicochemical processes, suppressing the SDE, or evaluating all obtained fractions of the sample. Or even avoiding fractionation altogether if possible. There is a clear necessity to conduct tests to assess the magnitude of the SDE for the processes applied to samples and the updated and improved recommendations described herein cover chromatography and processes involving gas-phase transformations such as evaporation or sublimation.

Chiral Pd-Catalyzed Enantioselective Syntheses of Various N-C Axially Chiral Compounds and Their Synthetic Applications

Biaryl atropisomers are key structural components in chiral ligands, chiral functional materials, natural products, and bioactive compounds, and their asymmetric syntheses have been reported by many groups. In contrast, although the scientific community has long been aware of atropisomers due to rotational restriction around N-C bonds, they have attracted scant attention and have remained an unexplored research area. In particular, their catalytic asymmetric synthesis and the synthetic applications were unknown until recently. This Account describes studies conducted by our group on the catalytic enantioselective syntheses of N-C axially chiral compounds and their applications in asymmetric reactions.In the presence of a chiral Pd catalyst, the reactions of achiral secondary ortho-tert-butylanilides with 4-iodonitrobenzene proceeded in a highly enantioselective manner (up to 96% ee), affording N-C axially chiral N-arylated ortho-tert-butylanilides in good yields. The application of the present chiral Pd-catalyzed N-arylation reaction to an intramolecular version gave N-C axially chiral lactams with high optical purity (up to 98% ee). These reactions were the first highly enantioselective syntheses of N-C axially chiral compounds with a chiral catalyst. Since the publication of these reactions, N-C axially chiral compounds have been widely accepted as new target molecules for catalytic asymmetric reactions. Furthermore, chiral-Pd-catalyzed intramolecular N-arylations were applied to the enantioselective syntheses of N-C axially chiral quinoline-4-one and phenanthridin-6-one derivatives. We also succeeded in the enantioselective syntheses of various N-C axially chiral compounds using other chiral Pd-catalyzed reactions. That is, optically active N-C axially chiral N-(2-tert-butylphenyl)indoles, 3-(2-bromophenyl)quinazolin-4-ones, and N-(2-tert-butylphenyl)sulfonamides were obtained through chiral Pd-catalyzed 5-endo-hydroaminocyclization, monohydrodebromination (reductive asymmetric desymmetrization), and Tsuji-Trost N-allylation, respectively. The study of the catalytic asymmetric synthesis of axially chiral indoles has contributed to the development of not only N-C axially chiral chemistry but also the chemistry of axially chiral indoles. Subsequently, the catalytic asymmetric syntheses of various indole derivatives bearing a C-C chiral axis as well as an N-C chiral axis have been reported by many groups. Moreover, axially chiral quinazlolin-4-one derivatives, which were obtained through chiral Pd-catalyzed asymmetric desymmetrization, are pharmaceutically attractive compounds; for example, 2-methyl-3-(2-bromophenyl)quinazolin-4-one product is a mebroqualone possessing GABA agonist activity.Most of the N-C axially chiral products have satisfactory rotational stability for synthetic applications, and their synthetic utility was also demonstrated through application to chiral enolate chemistry. That is, the reaction of various alkyl halides with the enolate prepared from the optically active anilide, lactam, and quinazolinone products proceeded with high diastereoselectivity by asymmetric induction due to the N-C axial chirality.At the present time, N-C axially chiral chemistry has become a popular research area, especially in synthetic organic chemistry, and original papers on the catalytic asymmetric syntheses of various N-C axially chiral compounds and their synthetic applications have been published.

Thionation of Optically Pure N-C Axially Chiral Quinazolin-4-one Derivatives with Lawesson's Reagent

The reaction of various optically pure N-C axially chiral quinazolin-4-one derivatives with Lawesson's reagent proceeded without a marked decrease in optical purity to give optically active quinazoline-4-thione derivatives (93-99% ee) possessing a high rotational barrier in good yields.

Halogen bond in separation science: A critical analysis across experimental and theoretical results

The halogen bond (XB) is a noncovalent interaction involving a halogen acting as electrophile and a Lewis base. In the last decades XB has found practical application in several fields. Nevertheless, despite the pivotal role of noncovalent interactions in separation science, investigations of XB in this field are still in their infancy, and so far a limited number of studies focusing on solid phase extraction, liquid-liquid microextraction, liquid-phase chromatography, and gas chromatography separation have been published. In addition, in the last few years, our groups have been systematically studying the potentiality of XB for HPLC enantioseparations. On this basis, in the present paper up-to-date results emerging from focused experiments and theoretical analyses performed by our laboratories are integrated with a descriptive presentation of XB features and the few studies published until now in separation science. Then, the aim of this article is to provide a comprehensive and critical discussion of the topic, and account for some still open issues in the application of XB to separate chemical mixtures.

The self-disproportionation of enantiomers (SDE) via column chromatography of β-amino-α,α-difluorophosphonic acid derivatives

This work presents the first study of the self-disproportionation of enantiomers via chromatography (SDEvC) of β-aminophosphonic acid esters, several of which have been synthesized for the first time. Three types of structures were examined, N-acetylated, dipeptide construction with N-Cbz glycine, and a free amine. In the latter case, this is the first time that SDEvC has been reported for free amine amino acids. In all the three types of structures, significant SDE magnitudes (Δee's up to 55%) were exhibited underscoring the ubiquitous nature of the SDE phenomenon. Chemical models of homo- versus heterochiral intermolecular interactions are proposed to rationalize the SDE magnitude differences amongst these new β-aminophosphonic acid derivatives. In addition, the incorporation of additional, competing binding modes to a molecule, was found to lead to a reduction of the SDE magnitude by shifting the intermolecular binding away from the stereogenic center and/or by leading to a convoluted binding system that disrupts the structured and relatively stable assemblies that give rise to the SDE.

The self-disproportionation of enantiomers (SDE) of amino acids and their derivatives

This review covers the phenomenon of the self-disproportionation of enantiomers (SDE) of amino acids and their derivatives in all its guises from phase transformations (recrystallization, sublimation, and distillation), to the application of force fields, through to chromatography including HPLC, MPLC, gravity-driven column chromatography, and SEC. The relevance of the SDE phenomenon to amino acid research and to marketed pharmaceuticals is clear given the potential for alteration of the enantiomeric excess of a portion of a scalemic sample. In addition, the possible contribution of the SDE phenomenon to the genesis of prebiotic homochirality is considered.

The self-disproportionation of enantiomers (SDE): The effect of scaling down, potential problems versus prospective applications, possible new occurrences, and unrealized opportunities?

This commentary discusses an important, though not widely appreciated, chiral phenomenon of molecular chirality that effectively always occurs whenever nonracemic samples are subjected to practically any physicochemical process (e.g., force field, recrystallization, sublimation, even distillation, etc.) under totally achiral conditions external to the sample itself. The phenomenon is termed as the self-disproportionation of enantiomers (SDE) and though ubiquitous, its presence may not always be readily apparent, or workers may be otherwise oblivious to its effects. In the particular case of chromatography, when the SDE is apparent, the enantiomeric excess (ee) of the chiral compound is observed to vary across an eluted peak, with anterior eluted portions either enantioenriched or enantiodepleted relative to the ee of the starting material, and conversely for the posterior eluted portions. Herein, we highlight various aspects of the SDE phenomenon as it pertains to chromatography and, in particular, the effect of scaling down chromatographic systems, the potential risk of problems that the SDE can cause, as well as opportunities for practical applications of the phenomenon, possible new occurrences of the SDE phenomenon to be searched for, and unrealized opportunities.

Chirality-dependent halogen bonds in axially chiral quinazolin-4-one derivatives bearing ortho-halophenyl groups

In the crystals of racemic axially chiral quinazolinones, the formation of intermolecular halogen bonds was detected.

The self-disproportionation of enantiomers (SDE): a menace or an opportunity?

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied.

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied. The SDE has implications ranging from the origins of prebiotic homochirality to unconventional enantiopurification methods, though the risks of altering the enantiomeric excess (ee) unintentionally, regrettably, remain greatly unappreciated. While recrystallization is well known as an SDE process, occurrences of the SDE in other processes are much less recognized, e.g. sublimation and even distillation. But the most common process that many workers seem to be completely ignorant of is SDE via chromatography and reports have included all manner of structures, all types of interactions, and all forms of chromatography, including GC. The SDE can be either a blessing – as a means to obtain enantiopure samples from scalemates – or a curse, as unwitting alteration of the ee leads to errors in the reporting of results and/or misinterpretation of the system under study. Thus the ramifications of the SDE are relevant to any area involving chirality – natural products, asymmetric synthesis, etc. Moreover, there is grave concern regarding errors in the literature, in addition to the possible occurrence of valid results which may have been overlooked and thus remain unreported, as well as the potential for the SDE to alter the ee, particularly via chromatography, and the following concepts will be conveyed: (1) the SDE occurs under totally achiral conditions of (a) precipitation, (b) centrifugation, (c) evaporation, (d) distillation, (e) crystallization, (f) sublimation, and (g) achiral chromatography (e.g. column, flash, MPLC, HPLC, SEC, GC, etc.). (2) The SDE cannot be controlled simply by experimental accuracy and ignorance of the SDE unavoidably leads to mistakes in the recorded and reported stereochemical outcome of enantioselective transformations. (3) The magnitude of the SDE (the difference between the extremes of enantioenrichment and -depletion) can be controlled and used to: (a) minimize mistakes in the recorded experimental values and (b) to develop unconventional and preparatively superior methods for enantiopurification. (4) The magnitude of the SDE cannot be predicted but can be expected for compounds possessing SDE-phoric groups or which have a general tendency for strong hydrogen or halogen bonds or dipole–dipole or aromatic π–π interactions. (5) An SDE test and the rigorous reporting and description of applied physicochemical processes should become part of standard experimental practice to prevent the erroneous reporting of the stereochemical outcome of enantioselective catalytic reactions and the chirooptical properties of scalemates. New directions in the study of the SDE, including halogen bonding-based interactions and novel, unconventional enantiopurification methods such as pseudo-SDE (chiral selector-assisted SDE resolution of racemates), are also reported.