Introduction
In the field of organic chemistry, understanding the stereochemistry of molecules is crucial. Stereochemistry deals with the spatial arrangement of atoms within molecules and significantly influences their physical and chemical properties. When chemists synthesize a new molecule by asymmetric synthesis, they need to determine the ratio of enantiomers. And when they discover a new optically active natural product, they need to determine the absolute configuration (i.e., which enantiomer is present). One powerful tool for probing the stereochemistry of chiral compounds is Mosher's ester analysis, developed by Harry Stone Mosher. This method has become a cornerstone in stereochemical studies due to its precision and reliability.
What are Mosher's Esters?
Mosher's esters are derivatives of Mosher's acid (α-methoxy-α-trifluoromethylphenylacetic acid, MTPA). They are used to determine the ratio of chiral alcohol (or amine) enantiomers, and also the absolute configuration. The method is based on the premise that while enantiomers are indistiguishable by NMR, diastereoisomers have different (but similar) NMR spectra. Thus, the analysis relies on the creation of diastereomeric esters from a chiral alcohol and Mosher's acid. The resulting esters are then analyzed, typically using NMR spectroscopy, to deduce the stereochemistry of the original compound.
Using Mosher's Ester Analysis to Determine Enantiomeric Excess
Preparation of MTPA Esters:
A mixture of alcohol (or amine) enantiomers in an unknown ratio is reacted with (R)-MTPA to form a mixture of two diastereomeric esters.
The reaction typically involves the activation of MTPA with a coupling agent, such as dicyclohexylcarbodiimide (DCC), in the presence of a nucleophilic catalyst like 4-dimethylaminopyridine (DMAP). Alternatively, Mosher's ester can be formed from the Mosher's acid chloride (MTPA-Cl)
NMR Analysis:
The diastereomeric esters are analyzed by NMR spectroscopy. The trifluoromethyl group in MTPA provides a convenient handle for 19F NMR spectroscopy, as it will appear as a lone singlet for easy integration.(r emember that 19F is 100% abundant with a spin value of ½).
We can now convert the enantiomeric ratio into the enantiomeric excess (ee).
At this stage, we do not know the identity (absolute stereochemistry) of the major alcohol enantiomer.
Using Mosher's Ester Analysis to Determine Absolute Configuration
When we have a single, unknown enantiomer, we can use Mosher’s ester analysis to deduce which enantiomer is present.
First, we perform two separate reactions to make both the R-MTPA and S-MTPA esters (we now have two diastereoisomers in separate vessels).
The favored conformation of the R-MTPA ester places R1 on the same face as the Ph group and R2 on the same face as the OMe group. This means that protons in R1 are deshielded and move to a lower chemical shift and protons in R2 are shielded and move to a higher chemical shift.
Meanwhile, the S-MTPA ester places R1 on the same face as the OMe group and R2 on the same face as the Ph group. Thus, protons in R1 move to a higher chemical shift and protons in R2 move to a lower chemical shift.
Therefore, we can determine the identities of R1 and R2 by performing 1H NMR and subtracting the chemical shifts of the R-MTPA ester from the chemical shifts of the S-MTPA ester.
Thus, Δ δSR (δS-δR) will be a positive value for protons in R1 and a negative value for protons in R2. This allows us to deduce the configuration.
Advantages of Mosher's Ester Analysis
Precision: The method provides clear and reliable data on the stereochemistry of chiral molecules.
Versatility: It can be applied to a wide range of alcohols and amines, making it a broadly useful technique.
Non-destructive: The analysis is non-destructive, allowing for the recovery of the original chiral compound if needed. Typically, the analysis can be performed on less than 1 mg of compound, so recovery is not always an issue.
Take Care with the Nomenclature of Mosher's Chlorides!
The conversion of the MTPA-OH (Mosher's acid) to MTPA-Cl (Mosher's acid chloride) changes the relative priorities of the groups present on the stereocenter. In MTPA-OH, the trifluoromethyl group (CF3) has a higher priority than the carboxyl group (CO2H). However, in the MTPA-Cl, the trifluoromethyl group has a lower priority than the chlorocarbonyl group (COCl). Consequently, the R-enantiomer of Mosher's acid (R-MTPA-OH) becomes the S-enantiomer of Mosher's acid chloride (S-MTPA-Cl), which then reverts back to the R-enantiomer upon ester formation. This situation is vice versa for the S-enantiomer of the acid, which converts to the R-enantiomer of the acid chloride, and then back to the R-enatiomer within the ester.
These apparent inversions of absolute configuration are only a quirk of the Cahn–Ingold–Prelog (CIP) system of stereochemical nomenclature, and therefore one should note that none of the four bonds to the stereogenic carbons are broken or formed during any of these reactions. As both MTPA-Cl enantiomers are commercially available, researchers should therefore be aware that the S-MTPA-Cl is required to synthesize the R-MTPA ester, and that the R-MTPA-Cl is required to produce the S-MTPA ester.
Applications of Mosher Ester Analysis
Mosher ester analysis is widely used in the pharmaceutical and agrochemical industries, where the stereochemistry of compounds can drastically affect their efficacy and safety. It's also employed in natural product chemistry to determine the configurations of complex molecules isolated from natural sources.
Conclusion
Mosher ester analysis is an indispensable tool in the toolkit of chemists working with chiral molecules. Its ability to provide precise and reliable stereochemical information makes it a go-to method for researchers and industry professionals alike. By mastering this technique, chemists can gain deeper insights into the intricate world of stereochemistry and develop better, more effective chemical products.
References
Dale J. H.; Dull D. L.; Mosher, H. S. J. Org. Chem. 1969,34, 2543.
Hoye T.; Jeffrey C.; Shao F. Nat. Protoc. 2007, 2, 2451.
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