The Theoretical Basis for Assignment by NMR

The nuclear magnetic resonance (NMR) spectra of two enantiomers are identical. Thus, the first step in using NMR to distinguish between two enantiomers should be to produce different spectra that eventually can be associated with their different stereochemistry (i.e., the assignment of their absolute configuration). Therefore, it is necessary to introduce a chiral reagent in the NMR media. There are two ways to address this problem. One is to use a chiral solvent, or a chiral agent, that combines with each enantiomer of the substrate to produce diastereomeric complexes/associations that lead to different spectra. This is the so-called chiral solvating agent (CSA) approach; it will not be further discussed here [33–34]. The second approach is to use a chiral auxiliary reagent [13–15] (i.e., a chiral derivatizing agent; CDA) that bonds to the substrate by a covalent linkage. Thus, in the most general method, the two enantiomers of the auxiliary CDA react separately with the substrate, giving two diastereomeric derivatives whose spectral differences carry information that can be associated with their stereochemistry. The CDA method that employs arylalcoxyacetic acids as auxiliaries is the most frequently used. It can be applied to a number of monofunctionals [14–15] (secondary alcohols [35–43], primary alcohols [44–46], aldehyde [47] and ketone cyanohydrins [48–49], thiols [50–51], primary amines [52–56], and carboxylic acids [57–58]), difunctional [13] (sec/sec-1,2-diols [59–61], sec/sec-1,2-amino alcohols [62], prim/sec-1,2-diols [63–65], prim/sec-1,2-aminoalcohols, and sec/prim-1,2-aminoalcohols [66–68]), and trifunctional (prim/sec/sec-1,2,3-triols [13, 69–70]) chiral compounds. Its scope and limitations are well established, and its theoretical foundations are well known, making it a reliable tool for configurational assignment. Figure 1.1 shows a summary of the steps to be followed for the assignment of absolute configuration of a chiral compound with just one asymmetric carbon and with substituents that, for simplicity, are assumed to resonate as singlets. Step 1 (Figure 1.1a): A substrate of unknown configuration (?) is separately derivatized with the two enantiomers of a chiral auxiliary reagent, (R)-Aux and (S)-Aux, producing two diastereomeric derivatives.

Assignment of the Absolute Configuration of Monofunctional Compounds by Double Derivatization

The assignment of secondary alcohols can be carried out by using one of several CDAs [13–15]. The most used and most reliable ones are MPA, 9-AMA, and MTPA [35–40]. Figure 3.1 shows their structures, the correlation models, and a summary of the experimental conditions. MPA and 9-AMA esters share the same conformational composition [37, 39] and only differ in the intensity of their shieldings; therefore both auxiliaries present the same correlation between sign distribution and stereochemistry. MTPA has a different conformational composition and correlation model [38]. As shown in Chapter 1, MPA esters of secondary alcohols and other AMAA esters (e.g., 9-AMA esters) are composed of two sp/ap conformers in fast equilibrium [37, 39]. The sp conformer is more stable than the ap conformer, and this allows the NMR spectrum of an AMAA ester to be interpreted as if it had originated from just the sp form: carbonyl, Cα, and methoxy groups in the auxiliary part and a methine group (Cα′-H) at the alcohol moiety are in the same plane. When we consider this conformation in the (R)- and the (S)-AMAA esters, the L1 group is located under the shielding cone of the aryl in the (R)-AMAA ester, while the L2 is shielded in the (S)-AMAA ester (we strongly recommended that the reader builds Dreiding, or similar, models to assist in visualizing this spatial array). A subtraction defined as the chemical shift in the (R)-AMAA ester minus that in the (S)-AMAA ester results in a negative value for L1 and a positive one for L2 (i.e., negative ΔδRS for L1 and positive ΔδRS for L2). Therefore, for any secondary alcohol derivatized as an AMAA ester, the protons showing a positive ΔδRS sign are located in the tetrahedron around the asymmetric carbon (Cα′) as L2 (at the back) while the protons resulting in a negative ΔδRS take the position of L1.

Practical Aspects of the Preparation of the Derivatives

Most of the NMR spectra shown in this book and in the literature have been recorded at 250 or 300 MHz, with a few being obtained at 500 MHz for 1H NMR (the equivalent for 13C NMR). No special pulse sequences are necessary, just standard one-dimensional (1D) spectra although two-dimensional (2D) experiments (e.g., correlation spectroscopy; COSY) may be necessary in some cases in order to get an unambiguous identification of the signals relevant for the assignment. In general, 5–10 mg or less of CDA derivative dissolved in 0.5 mL of deuterated solvent are sufficient to obtain a good NMR spectrum. Temperature, solvent, and concentration used in the NMR experiments should be adequate for each CDA-substrate pair and methodology, because the method is based on the conformational composition of the AMAA derivatives in precise conditions. With the exception of the low-temperature procedure (single derivatization), a NMR probe temperature around 300 K has always been used. In general, the spectra for double-derivatization assignments should be taken in deuterated chloroform. Different NMR solvents are required only in two of the single-derivatization methods. In the assignment by low-temperature NMR, the most convenient solvent is a CS2/CD2Cl2 (4:1) mixture, which allows the use of temperatures low enough (i.e., 213 K) to obtain relevant shifts. In the procedure based on the complexation with Ba2+, the NMR solvent should be deuterated acetonitrile. The barium salt is anhydrous Ba(ClO4)2, which can be added directly to the tube by using a spatula. No weighing is necessary after shaking, as the excess salt will remain at the bottom of the NMR tube and will not disturb the experiment. (R)- and (S)-MPA, MTPA, and Boc-phenylglycine (BPG) are commercially available and can be used without further purification. The first two (MPA and MTPA) can also be purchased as acid chlorides. When using MTPA or the corresponding acid chloride [85] for the derivatization of an alcohol or amine, it should be noted that the Cahn-Ingold-Prelog priority rules assign different R/S descriptors to the acid and to the corresponding chloride; this is due to the different priority order generated by the substituents [i.e., (R)-MTPA generates the (S)-acid chloride and vice versa].

Exercises

This chapter contains fifty exercises in the use of the methods described in the previous pages to the assignment of the absolute configuration of different substrates. They are organized according to the functional group of the substrate and follow the same order as in the previous chapters: alcohols; cyanohydrins; thiols; amines; carboxylic acids; 1,n-diols; 1,2-amino alcohols; and 1,2,3-triols. For each class of functional groups, there are exercises related to all the procedures available for assignment, such as double and single derivatization, and with different CDAs, when this is relevant. The derivatives and their spectra are actual data obtained in our laboratory; they are not simulated. Therefore, in a few cases, the spectra is not of high quality due to a small sample size or impurities that affect the result. In the spectra provided in the exercises below, in most cases, we have labeled the signals relevant for assignment; however, when the structure of the substrate and the spectra are simple, no signal identification is provided. At the end of this chapter, we provide a list of references to the literature in which the solutions can be found.

Assignment of the Absolute Configuration of Polyfunctional Compounds

From a practical point of view, the assignment of the absolute configuration of sec/sec 1,2- and 1,n-diols does not require the separate derivatization (two different steps with the CDA of choice) of each one of the two hydroxyl groups present in the substrate; on the contrary, it can be carried out by simultaneous derivatization of the two hydroxyls (a single step), leading to the corresponding bis-(R)- and bis-(S)-CDA esters [13, 59–61]. The most used CDAs are 9-AMA and MPA [59, 60], although 1-NMA, 2-NMA, and MTPA are also appropriate [59, 60]. This assignment has an important difference compared to that of monofunctionalized compounds [15]; this is due to the presence in the bis-(R)- and bis-(S)-derivatives of two CDA units that produce distributions of ΔδRS and ΔδSR signs that do not follow the trends found in monoderivatized compounds [13, 15, 82]. This means that the NMR spectra of the bis-CDA derivatives cannot be interpreted as if they had originated from two isolated mono-CDA derivatives [82]. Thus, the correlations described for secondary alcohols [35–39] cannot be applied to diols [59–61] because the chemical shifts and ΔδRS values result from the combination of the anisotropic effects—usually shielding—from the two CDA units and not from a single unit, as happens with monoalcohols. A result of the combination of aromatic shielding effects [59, 60] in diols is that the diagnostic protons/signals for assignment are not always the same as in isolated monoalcohols (i.e., L1/L2). For instance, in acyclic syn-1,2-diols, the diagnostic signals [59, 60] are those corresponding to the protons at the alpha positions of the OH groups (i.e., the hydrogens linked directly to the asymmetric carbons) Hα(R1) and Hα(R2) exclusively. On the other hand, in acyclic anti-1,2-diols, the diagnostic signals are from Hα(R1)/Hα(R2) together with those from R1 and R2. As in the case of monofunctional compounds, the assignment consists [13, 59, 60] in the preparation of two bis-CDA derivatives from the two enantiomers of the chosen CDA, followed by comparison of the corresponding NMR spectra and calculation of the ΔδRS (or ΔδSR in the case of MTPA) signs for Hα(R1), R1, Hα(R2), and R2.

Assignment of the Absolute Configuration of Monofunctional Compounds by Single Derivatization

The procedures shown in Chapter 3 allow the determination of the absolute configuration of several classes of compounds (Chapter 1, Figure 1.18), but they require the preparation of two derivatives and the comparison of their NMR spectra. Alternative methods have been developed for secondary alcohols and α-chiral primary amines. These are particularly suited for those cases where the amount of the available sample is low, and they require the preparation of only a single derivative [41–43, 55–56, 165]. There are three different approaches to using only a single derivatization to perform the assignment of those substrates [13, 165]. The first two are based on a controlled conformational change that is produced either by modification of the probe temperature [41, 165] or by selective complexation [42, 55, 56, 165]. The third one is based on the differences observed between the chemical shifts of the free alcohol and those of the 9-AMA ester derivative [43, 165]. In general, these single-derivatization procedures are limited to 1H NMR. Because the shift differences observed in 13C NMR are quite small, they produce insignificant Δδ values, and therefore the signs are not sufficiently accurate to produce a safe assignment [72]. Explanations and examples of applications are presented in the remainder of this chapter. For the assignment of secondary alcohols, a simple approach based on the use of a single MPA ester has proven to work very well [41, 165]. It is based on the controlled shift of the conformational equilibrium between the two main conformers (sp/ap) that were described in Chapter 1 for the MPA esters of secondary alcohols [36, 37]. Thus, for the assignment, it is only necessary to prepare either the (R)- or the (S)-MPA ester and then to compare the chemical shifts of L1/L2 in the spectra taken at room temperature and at a lower temperature [41]. Figure 4.2 presents a summary of the procedure and the graphical model expressing the ΔδT1T2 correlation between the sign and the stereochemistry for the assignment of secondary alcohols derivatized as (R)- or as (S)-MPA esters.