In-Situ Monitoring of Enantiomeric Excess During a Catalytic Kinetic Resolution

Is the E/Z Iminium Ratio a Good Enantioselectivity Predictor in Iminium Catalysis?

Developing new enantioselective reactions is an important part of chemical discovery but requires time and resources to test large arrays of potential reaction conditions. New techniques are required to analyse many different reactions quickly and efficiently. Mass spectrometry is a high-throughput method; when combined with ion-mobility spectrometry, this technique can monitor diastereomeric reaction intermediates and thus be a handle to study enantioselective reactions. Through this technique and others, it was noted before that in the organocatalytic 1,4-addition to α,β-unsaturated aldehydes, the abundance of initial diastereomeric intermediates correlates strongly to that of the final enantiomeric products. This work determines isomeric abundance for various catalysts and aldehydes and uses it to predict the enantiomeric excess of two control reactions. The prediction matches well for one reaction but does not predict the obtained results for the second. This finding confirms that the E/Z ratio of the iminium intermediates can be used as a predictor for some reactions, but the kinetics of the following steps can dramatically change the true enantioselectivity.

Chiral Monitoring Across Both Enantiomeric Excess and Concentration Space: Leveraging Quantum Cascade Lasers for Sensitive Vibrational Circular Dichroism Spectroscopy

Recently, high-throughput quantum cascade laser-based vibrational circular dichroism (QCL-VCD) technology has reduced the measurement time for high-quality vibrational circular dichroism spectra from hours to a few minutes. This study evaluates QCL-VCD for chiral monitoring using flow-through measurement of a changing sample in a circulating loop. A balanced detection QCL-VCD system was applied to the enantiomeric pair R/S-1,1'-bi-2-naphthol in solution. Different mixtures of the two components were used to simulate a racemization process, collecting spectral data at a time resolution of 6 min, and over three concentration levels. The goal of this experimental setup was to evaluate QCL-VCD in terms of both molar and enantiomeric excess (EE) sensitivity at a time resolution relevant to chiral monitoring in chemical processes. Subsequent chemometric evaluation by partial least squares regression revealed a cross-validated prediction accuracy of 2.8% EE with a robust prediction also for the test data set (error = 3.5% EE). In addition, the data set was also treated with the least absolute shrinkage and selection operator (LASSO), which also achieved a robust prediction. Due to the operating principle of LASSO, the obtained coefficients constituted a few discrete spectral frequencies, which represent the most variance. This information can be used in the future for dedicated QCL-based instrument design, gaining a higher time resolution without sacrificing predictive capabilities.

Multicomponent Chiral Quantification with Ultraviolet Circular Dichroism Spectroscopy: Ternary and Quaternary Phase Diagrams of Levetiracetam

Chiral molecules are challenging for the pharmaceutical industry because although physical properties of the enantiomers are the same in achiral systems, they exhibit different effects in chiral systems, such as the human body. The separation of enantiomers is desired but complex, as enantiomers crystallize most often as racemic compounds. A technique to enable the chiral separation of racemic compounds is to create an asymmetry in the thermodynamic system by generating chiral cocrystal(s) using a chiral coformer and using the solubility differences to enable separation through crystallization from solution. However, such quaternary systems are complex and require analytical methods to quantify different chiral molecules in solution. Here, we develop a new chiral quantification method using ultraviolet-circular dichroism spectroscopy and multivariate partial least squares calibration models, to build multicomponent chiral phase diagrams. Working on the quaternary system of (R)- and (S)-2-(2-oxopyrrolidin-1-yl)butanamide enantiomers with (S)-mandelic acid in acetonitrile, we measure accurately the full quaternary phase diagram for the first time. By understanding the phase stabilities of the racemic compound and the enantiospecific cocrystal, the chiral resolution of levetiracetam could be designed due to a large asymmetry in overall solubility between both sides of the racemic composition. This new method offers improvements for chiral molecule quantification in complex multicomponent chiral systems and can be applied to other chiral spectroscopy techniques.

Rapid chiral analysis based on liquid-phase cyclic chemiluminescence

Rapid chiral analysis has become one of the important aspects of academic and industrial research. Here we describe a new strategy based on liquid-phase cyclic chemiluminescence (CCL) for rapid resolution of enantiomers and determination of enantiomeric excess (ee). A single CCL measurement can acquire multistage signals that provide a unique way to examine the intermolecular interactions between chiral hosts and chiral guests, because the lifetime (τ) of the multistage signals is a concentration-independent and distinguishable constant for a given chiral host-guest system. According to the τ values, CCL allows discrimination between a wide range of enantiomeric pairs including chiral alcohols, amines and acids by using only one chiral host. Even the chiral systems hardly distinguished by nuclear magnetic resonance and fluorescence methods can be distinguished easily by CCL. Additionally, the τ value of a mixture of two enantiomers is equal to the weighted average of each enantiomer, which can be used for the direct determination of ee without the need to separate the chiral mixture and create calibration curves. This is extremely crucial for the cases without readily available enantiomerically pure samples. This strategy was successfully applied to monitoring of the Walden inversion reaction and analysis of chiral drugs. The results were in good agreement with those obtained by high-performance liquid chromatography, indicating the utility of CCL for routine quick ee analysis. Mechanism study revealed that the τ value is possibly related to the activity of the chiral substance to catalyze a luminol-H2O2 reaction. Our research provides an unprecedented and general protocol for chirality differentiation and ee determination, which is anticipated to be a useful technology that will find wide application in chirality-related fields, particularly in asymmetric synthesis and the pharmaceutical industry.

Real-time determination of enantiomeric and isomeric content using photoelectron elliptical dichroism

The fast and accurate analysis of chiral chemical mixtures is crucial for many applications but remains challenging. Here we use elliptically-polarized femtosecond laser pulses at high repetition rates to photoionize chiral molecules. The 3D photoelectron angular distribution produced provides molecular fingerprints, showing a strong forward-backward asymmetry which depends sensitively on the molecular structure and degree of ellipticity. Continuously scanning the laser ellipticity and analyzing the evolution of the rich, multi-dimensional molecular signatures allows us to observe real-time changes in the chemical and chiral content present with unprecedented speed and accuracy. We measure the enantiomeric excess of a compound with an accuracy of 0.4% in 10 min acquisition time, and follow the evolution of a mixture with an accuracy of 5% with a temporal resolution of 3 s. This method is even able to distinguish isomers, which cannot be easily distinguished by mass-spectrometry.