Diastereotopic methylene protons

For 20- [11] and 40-membered [12] macrocyclic di- and tetra-Schiff bases being derivatives of (R)-BTNOL, the chemical shift differences (AS) measured for diastereotopic methylene protons of BINOL unit in CDCI3 have suggested the presence of the partial rotation around the Ar—O—CH2 bond.27 The splitting of some 1H signals in acetone-d6 was explained as a result of a dynamic interconversion process on the NMR time-scale. 

Anisochrony due to axial chirality of the diastereotopic methylene protons in H2C = C=C(Me)COCHBrR (cf. Fig. 24) has been observed the chemical shift differences may be as high as 0.13 ppm.34) Also related to axial chirality are several cases of anisochronous methyl groups in isopropyl moieties which are diastereotopic through being part of a chiral allene of the type Me2CHCR=C=CR R" 35,49). These cases resemble that shown in Fig. 27 where a prochiral center (Me2CH—C. ..) is attached to a chiral ferrocenyl moiety it should be noted that the ferrocenylmethyl-carbenium ion fragment is chiral only if rotation about the Cp—C+ bond (marked a in Fig. 27) is slow on the NMR time scale 42 . 

The following similar achiral molecules contain diastereotopic methylene protons 3-hydroxyglutaric acid, glycerol, citric acid, diethyl acetal, and a cyclic acetal (respectively, Figure 3.43, structures f, g, h, i, and j) structure j involves the additional concept of magnetic equivalence (Section 3.9). 

The other connectivity can be established by way of the NOE interaction between the aspartic acid N—H and the two glycine H-2 s. There is no ambiguity or overlap in this correlation thus proving the sequence given in Problem 7.6A. An interesting aside worth noting is the NOE correlation between the aspartic acid N—H and only one of the two diastereotopic methylene protons of aspartic acid... 

Supplementary support for the interpretation of the temperature-dependent dynamic H NMR spectra of 33 is presented by additional studies of (A,A,A,A)/(A,A,A,A)-[(EtNH3)4n Mg4(L12)6 ] (34). In 33 and 34, the methylene protons of the ligands exhibit identical VT NMR spectra. Moreover, the diastereotopic methylene protons (magenta) of the ethyl ammonium counterions of 34 display similar temperature-dependent coalescence as the ligand vinylether methylene protons (green). This is due to the fact that, even in solution, the ethyl ammonium groups are fixed to the tripodal calix-like surfaces of the [Mg4(L12)6]4 scaffold and therefore the methylene protons are in a chiral environment and display diastereotopicity. 

NMR other aspects Two other aspects of the H NMR spectrum of hydrogen bonded aggregates are useful in determining structure. Variable-temperature NMR can reveal dynamic processes that are fast (or slow) on the NMR time scale at room temperature [51]. Nuclear Overhauser effect (nOe) studies can be used to determine relative proximity of the hydrogen bonded protons and the diastereotopic methylene protons [40,43,46]. H NMR competition studies where two different hubs (for example, Hub(M)3 and Flex(M)3) compete for only three equivalents of CA allows direct assessment of the relative stability of the competing aggregates [45,55]. 

Compounds 6a, 7a, and 8a were isolated as viscous, colorless liquids. These compounds can be identified with NMR and GC-MS spectroscopy. In some cases unwanted and yield reducing side reactions like oligomerization or monoalkylation of the dichlorosilanes occur. Side products are not detected by NMR specroscopy, but GC-MS studies definitely show peaks at higher masses. In the H NMR spectra of 6a and 7a well-separated signals can be observed for the diastereotopic methylene protons at room temperature. Obviously the ring inversion process is slowed down if bulky substituents are used. 

Both the metallation of sulfoxides and the reaction of a-lithio sulfoxides in THF are known to be highly stereoselective. " It has been found that, on reaction with base, the diastereotopic methylene protons of benzyl methyl sulfoxide exchange at different rates, the relative ratio being... 

Much literature precedent supports the assignment of tacticity in methyl acrylate polymers using NMR techniques [40,41]. In the H-NMR spectrum, the shift of the methylene protons is sensitive to dyad stereochemistry. For example, in an isotactic (meso) dyad 28, the methylene protons are chemically non-equivalent and appear as two separate sets of signals, whereas in a syndiotactic (racemic) dyad 29, the methylene protons are equivalent. The H-NMR spectrum of 27 showed multiplets at 1.89 and 1.5 ppm due to the two diastereotopic methylene protons of the isotactic dyad. The rest of the spectrum is consistent with the structure of the n=4 tetrad 27. A racemic dyad structure would have been expeeted to give resonances of intermediate shift to that of the two resonances observed for the telomer 27. This evidence strongly implies that 27 has the allisotactic configuration shown in Scheme 8-12. 

A number of osmium(III) dithiocarbamates are known. The diethyl complex Os(S2CNEt2)3 is made by reaction of (NH4)2[OsCl5] and Na(S2CNEt2). The molecule contains diastereotopic methylene protons and, from a study of the HNMR spectra over a temperature range, rate constants were calculated from the metal-centred optical inversion of the complex. The magnetic moment is 1.61 BM at 25 °C. The dimeric species Os2(S2CNMe2)sCl is made from (NH4)2[OsCl ] and Na(S2CNMe2). ... 

---