Methylenes, diastereotopic

The 2H-NMR spectra of these derivatized carboxylic acids sometimes are a powerful and simple alternative for the estimation of optical purity. Similar analysis of diastcrcomcric derivatives of a-deuteriated primary alcohols with (5)-2-0-acetylmandelic acid and primary amines with (-)-(1 S, 2/ )-camphanoy 1 chloride is also possible. However, in most cases the shift differences in the simple H-NMR spectra of the same derivatives are large enough to obtain the diastereomeric ratio. Shift differences of the two a-methylenic diastereotopic hydrogens are typically between <5 = 0.05 and 0.2. 

The double bond m 2 methyl(methylene)cyclohexane is prochiral The two faces however are not enantiotopic as they were for the alkenes we discussed m Section 7 9 In those earlier examples when addition to the double bond created a new chirality cen ter attack at one face gave one enantiomer attack at the other gave the other enantiomer In the case of 2 methyl(methylene)cyclohexane which already has one chirality center attack at opposite faces of the double bond gives two products that are diastereomers of each other Prochiral faces of this type are called diastereotopic... 

A detailed discussion of the different acidities of the diastereotopic a-methylene protons in sulphoxides, as well as of the stereochemistry of reactions of sulphoxide a-carbanions with electrophilic reagents is beyond the scope of this chapter. A recent review by Wolfe pertinent to these problems is available392. 

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. 

The Ti-FI dimethyl complexes 42 and 43 can readily be activated with B(C6F5)3 or [CPh3]+[B(C6F5)4] to form a methyl cationic species. The addition of ethylene to the cationic species derived from 42 allows room temperature observation of the living propagating species, which displays XH NMR peaks centered at 1.45 and 2.88 ppm that are attributed to the diastereotopic a-methylene protons connected... 

It is important to appreciate that the methylene hydrogens in RCH2N(R )0-will necessarily be diastereotopic if either R or R is a chiral grouping. Under these circumstances the couplings to the two hydrogen atoms will normally be different at all temperatures (e.g. Tordo et al., 1970). 

The diastereomers 251/ewf-251 and 252/ent-252 could be separated and were decom-plexed separately. From the fraction of 251/ewt-251,253 was obtained with 85% ee (e.r. = 92.5 7.5), and the fraction of 252/ent-252 yielded ewt-253 with 88% ee (e.r. = 6 94). A similar situation results from the reaction with tributyltin chloride or alkylation reagents, but the diastereomeric ratio is strongly dependent on the electrophile. The following conclusion is drawn from these and further experiments The enantiomeric ratio is determined by a selection of the chiral base between the diastereotopic methylene groups, since the benzylic carbanionic centres are labile, whereas the diastereomeric ratio results from the relative rate of the electrophile approach syn or anti with respect to the A-methyl group. One question remains—why are opposite d.r. values formed in the alkylation by methyl iodide and ethyl iodide ... 

As seen in Table 1, in the m dyad the methylene protons are diastereotopic and consequently their spectrum consists of an AB doublet of doublets with a geminal coupling constant 7ab = —15 Hz. In contrast, in the r dyad the two protons are equivalent and, therefore, a singlet is observed in the spectrum. 

To prove the correctness of this analysis we shall try to look for alternative explanations. A stereoregular heterotactic polymer. . . mrmrmr. . . could also explain the presence of a methyl singlet and a well-separated doublet of doublets for methylene protons, in agreement with the spectrum of the first sample in fact, there would be present the mr triad and the rmr tetrad with diastereotopic protons. However, in addition, an equal amount of the mrm tetrad should be present, yet the corresponding singlet is not visible with the required intensity. In the same way other hypothetical structures can be rejected. 

At 195 K, the resonances for the Cp ligands, silyl methyl groups and the methylene fragment in 98 exhibit diastereotopic resonances in CD2CI2. As the temperature is increased, coalescence of the pairs of diastereotopic resonances occurs at different temperatures (Cp rings at 245 K, silyl methyl groups at 240 K). All the tern-... 

The benzyl methylene groups in 115 are diastereotopic (J = 13.5 Hz) restricted rotation of the dimethylamino group is apparent in 111. ... 

The spectral data provide information about the structure of a-acetoxydibenzylnitrosamine (VII). The high frequency (1780 cm ) for the carbonyl group in the infrared spectrum (Fig. 9) is consistent with this structure ( ) and the introduction of the chiral center at the benzylic position causes the methylene of the other benzyl ic substituent to be diastereotopic and appear in the nmr spectrum as an AB quartet. The center of the quartet... 

The stereoreactivity of the methylene protons of er -butyl (4-methylphenylsulfinyl)acetate is in sharp contrast with the highly stereospecific behavior of the methylene protons of benzyl methyl sulfoxide. An NMR study of phenylsulfinylacetic acid showed that the reactivity of these two diastereotopic protons is comparable83. These protons are even magnetically equivalent in deuterium oxide solution. The diastereoselectivity of the alkylation of a-sulfinyl carboxylic esters is poor, moreover, the reaction proceeds only when butyllithium, ferz-butyllithium or lithium diethylamide is used as the base in the preparation of the carbanion82. 

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