Atropisomers are molecules whose stereogenicity arises from restricted rotation about a single bond. They have garnered significant attention due to their applications in catalysis, medicine and materials science. The archetypal examples are axially chiral biaryls such as BINAP and BINOL, which represent some of the most important ligand and catalyst architectures available for asymmetric catalysis. However, atropisomerism is also increasingly being studied in other molecular scaffolds, including axially chiral heterobiaryls, amides, diarylamines, and sp3 systems, with a variety of elegant synthetic approaches now available for the stereoselective preparation of such compounds (Figure 1).
Figure 1 Representative examples of important atropisomeric scaffolds
The distinguishing characteristic of atropisomeric molecules is the fact that their stereoisomers may interconvert through bond rotation. For example, given sufficient thermal energy, a single enantiomer of a generic biaryl (M)-A can undergo rotation about the biaryl axis, enabling interconversion with its enantiomer (P)-A (Figure 2). This process causes an enantioenriched sample to decay to a racemic mixture, and the rate at which this occurs is typically expressed as a racemization half-life (t1/2rac), dictated by the magnitude of the associated the free-energy barrier for enantiomerization (ΔG‡). Racemization rates can vary widely, from rapid exchange of conformers (class 1 atropisomeris) to highly configurationally stable molecules which racemize on the timescale of years (class 3 atropisomers). Therefore, assessing the configurational stability of atropisomeric molecules (i.e., the rate at which a single enantiomer converts into a racemate) is crucial for any research conducted in this field.
Figure 2 Enantiomerization of a generic atropisomeric biaryl and arbitrary definitions of atropisomerism according to racemization half-lives (t1/2rac) and free-energy barrier for enantiomerization (ΔG‡)
Several experimental approaches are available to determine racemization rates, and our aim in creating this article was to bring together detailed experimental protocols for the most important methods, namely:
1. Kinetic Analysis: This technique involves studying the racemization of a small quantity of enantiomerically pure material, by performing HPLC analysis on a chiral stationary phase at different time intervals. Plotting the degree of enantiomeric enrichment over time allows the rate of racemization to be determined (Figure 3i). This technique is suitable for atropisomers undergoing comparatively slow racemization (ΔG‡ ≥ 95 kJ/mol), and racemization can be studied at a variety of different temperatures. To illustrate this method, we have selected a worked example of axially chiral enol ether B, which we recently reported can be prepared via cation-directed O-alkylation of tetralones.
2. Dynamic HPLC: This method relies upon the unusual lineshapes that can be observed when an atropisomeric sample is analysed by HPLC on a chiral stationary phase. A stable chiral, racemic compound would give two baseline separated peaks, but in the case of atropisomers, if the enantiomers are able to interconvert on the HPLC timescale, a distinctive plateau will be observed between peaks (Figure 3ii). The kinetic parameters can be extracted either by manual calculation, or more conveniently using the freely available software package DCXplorer developed by Trapp and co-workers. This method is most useful for atropisomers undergoing racemization on an intermediate timescale (ΔG‡ ≈ 80-95 kJ/mol). The use of the method is illustrated by a case study of atropisomeric diarylamine C.
3. Variable Temperature NMR: Variable temperature NMR is a versatile method to determine the rate of conformational and chemical exchange processes. For atropisomeric molecules, a diagnostic feature is the coalescence of diastereotopic signals, which occurs when the rate of the racemization process is matched with the frequency difference between signals (Figure 3iii). The procedure is typically suitable for molecules in which racemization occurs relatively rapidly (ΔG‡ ≤ 85 kJ/mol). The rate of racemization can be calculated based on empirical determination of the coalescence temperature, or alternatively, lineshape analysis of spectra close to the coalescence temperature can be used to extract kinetic information directly. Our article discusses both of these techniques, using exemplar data from class 1 atropisomeric diarylamine D.
Figure 3 Three experimental techniques to measure the rates of racemization in atropisomeric molecules
Overall, these three techniques are sufficient to allow determination of racemization rates for the vast majority of atropisomeric molecules. The overall aim of the article is to bring together practical information about which technique to use, and exactly how these measurements can be performed, that will allow non-specialists to carry out such experiments.