In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis.

Invited for the cover of this issue is Oliver Trapp and his co-workers at Ludwig-Maximilians-Universität München, the Max-Planck-Institute for Astronomy and Ruprecht-Karls-Universität Heidelberg. The image depicts a magic trick representing the autocatalytic process reported in the manuscript. Read the full text of the article at 10.1002/chem.202003260.

In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis.

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine-5-carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high-resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively.

Mapping the dark space of chemical reactions with extended nanomole synthesis and MALDI-TOF MS.

Understanding the practical limitations of chemical reactions is critically important for efficiently planning the synthesis of compounds in pharmaceutical, agrochemical, and specialty chemical research and development. However, literature reports of the scope of new reactions are often cursory and biased toward successful results, severely limiting the ability to predict reaction outcomes for untested substrates. We herein illustrate strategies for carrying out large-scale surveys of chemical reactivity by using a material-sparing nanomole-scale automated synthesis platform with greatly expanded synthetic scope combined with ultrahigh-throughput matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS).

An Asymmetric, Catalytic (4 + 3) Cycloaddition Reaction of Cyclopentenyl Oxyallylic Cations.

Treatment of 2-tosyloxycyclopentanone with substituted furans in the presence of a chiral amino alcohol catalyst and K2HPO4 results in the formation of (4 + 3) cycloaddition products with enantioselectivities that exceed 90% in certain cases.

Overcoming "speed limits" in high throughput chromatographic analysis.

The combination of high speed autosampler technology and ultrafast chromatographic separations enables faster high throughput analysis. With an injection cycle time of 10.6 s, MISER (Multiple Injection in a Single Experimental Run) HPLC-MS analysis of a 96 well microplate can be completed in only 17min. As chromatographic separations in the sub 5s range become increasingly common, even faster autosamplers will be needed to realize further speed improvements in high throughput LC-MS analysis. Indeed with proper hardware sampling approaches, chromatographic analysis of microplates could approach speeds of spectrophotometric plate readers while maintaining the advantage of multicomponent detection and monitoring.

Unusual reversal of enantioselectivity in the asymmetric autocatalysis of pyrimidyl alkanol triggered by chiral aromatic alkanols and amines.

Temperature dependent inversion of enantioselectivity in asymmetric catalysis is an interesting and somewhat unusual phenomenon. We have observed temperature dependent inversion of enantioselectivity in the asymmetric autocatalysis reaction when triggered by a wide scope of enantioenriched alcohols and amines. The addition reaction of diisopropylzinc to pyrimidine-5-carbaldehyde in the presence of enantiopure alcohols or amines affords the pyrimidyl alkanol product at 0 °C with high ee. However, lowering the reaction temperature to -44 °C affords the opposite enantioselectivity.

Ultrafast chiral separations for high throughput enantiopurity analysis.

Recent developments in fast chromatographic enantioseparations now make high throughput analysis of enantiopurity on the order of a few seconds achievable. Nevertheless, routine chromatographic determinations of enantiopurity to support stereochemical investigations in pharmaceutical research and development, synthetic chemistry and bioanalysis are still typically performed on the 5-20 min timescale, with many practitioners believing that sub-minute enantioseparations are not representative of the molecules encountered in day to day research. In this study we develop ultrafast chromatographic enantioseparations for a variety of pharmaceutically-related drugs and intermediates, showing that sub-minute resolutions are now possible in the vast majority of cases by both supercritical fluid chromatography (SFC) and reversed phase liquid chromatography (RP-LC). Examples are provided illustrating how such methods can be routinely developed and used for ultrafast high throughput analysis to support enantioselective synthesis investigations.

Can the analyte-triggered asymmetric autocatalytic Soai reaction serve as a universal analytical tool for measuring enantiopurity and assigning absolute configuration?

An investigation is reported on the use of the autocatalytic enantioselective Soai reaction, known to be influenced by the presence of a wide variety of chiral materials, as a generic tool for measuring the enantiopurity and absolute configuration of any substance. Good generality for the reaction across a small group of test analytes was observed, consistent with literature reports suggesting a diversity of compound types that can influence the stereochemical outcome of this reaction. Some trends in the absolute sense of stereochemical enrichment were noted, suggesting the possible utility of the approach for assigning absolute configuration to unknown compounds, by analogy to closely related species with known outcomes. Considerable variation was observed in the triggering strength of different enantiopure materials, an undesirable characteristic when dealing with mixtures containing minor impurities with strong triggering strength in the presence of major components with weak triggering strength. A strong tendency of the reaction toward an 'all or none' type of behavior makes the reaction most sensitive for detecting enantioenrichment close to zero. Consequently, the ability to discern modest from excellent enantioselectivity was relatively poor. While these properties limit the ability to obtain precise enantiopurity measurements in a simple single addition experiment, prospects may exist for more complex experimental setups that may potentially offer improved performance.

Visualizing small differences using subtractive chromatographic analysis.

Subtraction of chromatograms coming from two different samples collected under identical conditions can highlight small variations, serving as a useful tool for visualizing differences between experimental and control groups. While the basis for this general approach has been known for decades, the technique is seldom used in modern chromatographic analysis. We report an investigation into the application of subtractive chromatographic analysis in several areas of pharmaceutical research where detection of small differences between samples is important. Our investigation found that elimination of artifacts caused by peak misalignment was often necessary, especially for extremely sharp chromatographic peaks obtained in rapid injection MISER chromatography. Alignment of individual peaks prior to subtraction, combined with fast detector sampling rates, or data interpolation in cases where this is not possible, was found to afford convenient visualization of small differences (∼1%) among samples, suggesting potential utility in high throughput screening of process adsorbents or other applications in pharmaceutical research and development.

Using chromatogram averaging to improve quantitation of minor impurities.

Averaging of chromatograms can lead to enhancement of signal to noise ratio (S/N) in proportion to the square root of the number of measurements. Although the general principle has been known for decades, chromatogram averaging is almost never used in current pharmaceutical research. In this study we explore the utility of this approach, showing it to be a simple and easily accessible method for boosting sensitivity for quantification of minor components and trace impurities, where current techniques deliver insufficient S/N.

Toward structure-based predictive tools for the selection of chiral stationary phases for the chromatographic separation of enantiomers.

ChirBase, a database for the chromatographic separation of enantiomers containing more than 200,000 records compiled from the literature, was used to develop quantitative structure activity models for the prediction of which chiral stationary phase will work for the separation of a given molecule. Constructuion of QSAR models for the enantioseparation of nineteen chiral stationary phases was attempted using only analyte structural information, leading to the producton of self-consistent models in four cases. These models were tested by predicting which in-house racemic compounds would and would not be resolved on the different columns. Some degree of success was observed, but the sparseness of data within ChirBase, which contains enantioseparations for only a subset of molecules on a subset of columns under a variety of conditions may limit the creation of effective models. Augmented data sets gleaned from automated chromatographic method development systems deployed in academic and industrial research laboratories or the use of models that take other factors such as solvent composition, temperature, etc. into account could potentially be useful for the development of more robust models.

Are fluorine-rich pharmaceuticals lost by partition into fluorous phases?

The recently developed technology of droplet microfluidics has demonstrated great potential for many applications such as biochemical assay, high throughput screening, cell culture, directed evolution, and chemical synthesis. Intrigued by its capabilities for miniaturization, flexible manipulation, rapid reagent mixing and high throughput experimentation and analysis, the pharmaceutical industry has begun to investigate droplet microfluidic implementation in medicinal and process chemistry. Segmented by an immiscible secondary phase, usually perfluorinated oil, aqueous or organic droplets serve as individual micro-reactors without suffering cross-contamination. As many drug molecules contain fluorines, it is necessary to investigate whether such compounds will be preferentially extracted into the fluorous phase via fluorophilic solvation, which could lead to erroneous analytical results. In this work, we chose drugs with up to 10 fluorines to probe their partition into perfluorodecalin (PFD) from a variety of organic solvents. A fast and straightforward MISER (Multiple Injections in a Single Experimental Run) LC-MS method was applied to measure the loss of drug after mixing with PFD. We found that no significant partition occurred, with the concentration of drugs in the 'experimental' group measured as ±10% of the 'control' group. The RSD% of multiple injections is <5%. The finding was further validated by the conventional LC-MS approach.

MISER chiral supercritical fluid chromatography for high throughput analysis of enantiopurity.

MISER chromatographic analysis (Multiple Injections in a Single Experimental Run) using supercritical fluid chromatography (SFC) with pressurized carbon dioxide-based eluents is well suited to the high throughput analysis of enantiopurity. SFC is currently the preferred method for fast enantiopurity analysis, with analysis times of only a few seconds achievable in some cases. Injector programming using both the Agilent Infinity and Shimadzu Nexera UC instruments permitted MISER SFC experiments to be performed. Several case studies are presented, showcasing the power and versatility of the technique, with 'plate analysis times' (the time required for analysis of enantiopurity of 96 samples) of less than 33-34 min achievable in the best cases.

A Combined Experimental and Theoretical Study on the Stereodynamics of Monoaza[5]helicenes: Solvent-Induced Increase of the Enantiomerization Barrier in 1-Aza-[5]helicene.

Helicenes and heterohelicenes are attractive compounds with great potential in materials sciences to be used in optoelectronics as ligand backbones in enantioselective catalysis and as chiral sensors. The properties of these materials are related to the stereodynamics of these helical chiral compounds. However, little is known about features controlling stereodynamics in helicenes; in particular, for heterohelicenes the position of the heteroatom could be relevant in this respect. Herein the complete stereodynamic characterization of monoaza[5]helicenes is shown by enantioselective dynamic HPLC and DFT calculations. At variance with previous theoretical calculations, 1-aza[5]helicene shows a surprisingly high enantiomerization barrier, which is triggered by specific solvent interactions.

Chiral 1,2-dialkenyl diaziridines: synthesis, enantioselective separation, and nitrogen inversion barriers.

trans-1,2-Disubstituted diaziridines form stable enantiomers at ambient conditions because of the two stereogenic pyramidal nitrogen atoms. Functionalized trans-1,2-disubstituted diaziridines can be utilized as a chiral switching moiety between two enantiomeric states in more complex molecular structures. However, the synthesis of functionalized diaziridines is quite challenging, because of the limited tolerance of reaction conditions that can be applied. Here we present a strategy to make trans-1,2-disubstituted diaziridines accessible as versatile building blocks in C-C-bond formations, i.e., the Heck reaction, and therefore introducing aryl substituents. The synthesis of trans-1,2-dialkenyl diaziridines with terminal alkenyl substituents and their stereodynamic properties are described.

From stereodynamics to high-throughput screening of catalysed reactions.

Characterising chemical reactions by kinetic analysis is of fundamental importance to experimentally obtain insights into reaction mechanisms. Based on such investigations reactions can be optimised and improved catalysts designed. Enhanced reaction conditions may drastically increase the performance of the reaction in terms of yield and (enantio-) selectivity. Understanding reaction kinetics in more complex systems involving adaptive chemical and dynamic systems on a molecular level as shown here is even more challenging. Here we review recent developments in monitoring reactions including the dynamic interconversion of stereoisomers by integrating (catalysed) reactions and chemical analysis in on-column reaction chromatographic devices. These recent developments allow rapid screening of reactions in great detail and are a central tool in determining reaction pathways and to understand how to control the stereodynamics of chiral molecules.

Coulomb explosion imaged cryptochiral (R,R)-2,3-dideuterooxirane: unambiguous access to the absolute configuration of (+)-glyceraldehyde.

The absolute configuration of (R,R)-2,3-dideuterooxirane, which has been independently determined using Coulomb explosion imaging, has been unambiguously chemically correlated with the stereochemical key reference (+)-glyceraldehyde. This puts the absolute configuration of D(+)-glyceraldehyde on firm experimental grounds.

Imaging the absolute configuration of a chiral epoxide in the gas phase.

In chemistry and biology, chirality, or handedness, refers to molecules that exist in two spatial configurations that are incongruent mirror images of one another. Almost all biologically active molecules are chiral, and the correct determination of their absolute configuration is essential for the understanding and the development of processes involving chiral molecules. Anomalous x-ray diffraction and vibrational optical activity measurements are broadly used to determine absolute configurations of solid or liquid samples. Determining absolute configurations of chiral molecules in the gas phase is still a formidable challenge. Here we demonstrate the determination of the absolute configuration of isotopically labeled (R,R)-2,3-dideuterooxirane by foil-induced Coulomb explosion imaging of individual molecules. Our technique provides unambiguous and direct access to the absolute configuration of small gas-phase species, including ions and molecular fragments.

The first example of enamine-Lewis acid cooperative bifunctional catalysis: application to the asymmetric aldol reaction.

(-)-Sparteine directed lithiation of N-Boc-pyrrolidine, alkylation with chloromethylboronate pinacol ester and acid-based deprotection provides homoboroproline HX salt in 94% ee, which is then an efficient enamine-type pyrrolidine catalyst in an asymmetric aldol reaction when neutralised and especially when esterified in situ with a tartrate ester, for example, providing 90% ee of the aldol adduct derived from acetone and p-nitrobenzaldehyde.