Draw the Ketone Produced From the Oxidation of 2 Pentanol
Anil V. Karnik , Mohammed Hasan , in Stereochemistry, 2021
5.3.4.2 Use of CDA for the assignment of absolute configuration
In this method the diastereomeric species are produced by reaction of a chiral substrate with enantiopure chiral reagent, called as the CDA. Thus, a diastereomeric derivative of the substrate is produced. Nonequivalence of the chemical shift for two diastereomeric species is caused by the dissymmetric influence of the CDA, employed here as an auxiliary. Since covalent bond formation is involved, the chemical shift differences between diastereomeric species are more in comparison to the CSA method. There has been an increasing use of this method, after the pioneering work (Dale & Mosher, 1973) by Dale and Mosher in 1973. The methodology has been well illustrated in a review (Seco et al., 2004) by Riguera et al.
Thus, a chiral substrate is coupled with two enantiomers of CDA separately, facilitating the NMR spectra for the diastereomeric species. Comparison and analysis of the two NMR spectra helps to propose the configuration of the enantiopure substrate. An ideal CDA for the purpose should have a suitable functional group (Z) to react with the substrate; a bulky or a polar group (R) to cause a clear conformational bias; and a group (Y) capable of causing shielding/de-shielding of protons via through-space interactions. A carbonyl group or an aryl ring is generally preferred as the 'Y' group. For example, when methoxy phenylacetic acid (MPA) is used as a CDA for determination of configuration of an alcohol, (−)-2-pentanol, the −COOH group acts as the 'Z' group, to form ester; the −OCH 3 group functions as the polar group (R), and the phenyl group typically causes the anisotropic effect through space, functions as the 'Y' group. Fig. 5.28 shows the formation of the two diastereomeric esters with the (−)-2-pentanol, is assumed to have R-configuration for interpretation of spectra. This R-configuration for the (−)-pentanol accounts for the NMR data obtained for the diastereomeric species, confirming the R-configuration for the (−)-2-pentanol.
Figure 5.28. Diastereomeric esters of (+)-2-pentanol with (R) and (S)-MPA
The 1H NMR spectra exhibit different chemical shifts for the C1 methyl group (appearing as a doublet), C3 methylene group (appearing as a multiplet), C4 methylene group (appearing as a multiplet) and C5 methyl group (appearing as a triplet). For the R,R-diastereomeric ester, the signals for protons on C3, C4 and C5 appear upfield and signal for C1 methyl group appears downfield, in comparison to the signals for the similar sets of protons for the R,S-diastereomeric ester. To account for the difference in chemical shift, conformations for the two diastereomeric esters, R,R- and R,S- are proposed based on normal conformational analysis principles. It can be seen that in R,R-diastereomer, the protons on C3, C4 and C5 experience shielding due to the proximity through space with the phenyl ring. In the R,S-diastereomer, the C1 methyl group experiences the shielding effect as the phenyl ring now is closer to the C1 methyl group. Please note that for the S-2-pentanol opposite NMR pattern would be expected. The success of this method depends on proposing the most stable conformation for the two diastereomeric species and then accounting for the upfield or the downfield signals for the similar sets of protons in the two diastereomeric species due to the anisotropic effects of aryl ring or any other group present in the CDA. Fig. 5.29 shows the relevant NMR spectral data for the two diastereomeric species and the proposed stable conformations for the two diasteromers. The spectral anisochrony, [Δδ] is substantial, for the comparative signals.
Figure 5.29. NMR spectra for two diastereomers and the spectral anisochrony [Δδ] for the relevant signals.
Several CDAs have been employed, several types of substrates have been evaluated and the most widely used CDA for the purpose, is α-methoxy-α-trifluoromethyl phenylacetic acid (MTPA). Use of 19F NMR and 13C NMR has also been done for the purpose. The above method is representative method involving the use of CDA for determination of absolute configuration. Other variation of the method are also reported (Seco et al., 2004).
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