Diastereotopic proton

TABLE 12. Relative rates of H/D exchange of the diastereotopic protons in benzyl methyl sulfoxide and relative amount of 40 and 41 formed upon quenching of a-lithiobenzyl ethyl sulfone with D20 (after Reference 56)... 

Diastereotopic proton/group A proton (or group) which if replaced by another hypothetical group (not already found in the molecule), would yield a pair of diastereoisomers. 

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. 

Rotation barriers have also been looked at (80TL1553) in TV-substituted 2,4,6-trimethyI-pyridinium cations (267). In addition to diastereotopic protons HA and HB, the a-Me and (8-protons are anisochronous as shown by HNMR, as are the a-Me, a-ring and /3-ring carbons, as shown by 13C NMR. This anisochronism appears to be due to hindered rotation about the pyridinium N—C bond (268), the stable conformation being that in which the hydrogen atom on the sp3 C is in the plane of the pyridine ring. Observation of coalescence temperatures in deuterated acetone or pyridine solvent allows calculation of energy barriers to this rotation, and these are as follows (267 R = Me, R = H), 7.1 (267 R = Me, R = COMe) (267 R = Et, R = H), 8.1 kJ mol-1. 

Table 17. Chemical Shift Nonequivalencc of Diastereotopic Protons in Some 2-Methoxy-2-Phenylphosphonium Bromides77...

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. 

But, you will recall that enantiomers are chemically indistinguishable unless they are in a chiral environment. Therefore we expect shifts of enantiotopic hydrogens to be identical, unless they are in a chiral environment. To summarize, enantiotopic protons normally will have the same chemical shifts, whereas diastereotopic protons normally will have different chemical shifts. 

With the use of a 2D EXSY experiment on a [La(TTHA)]3 sample, an exchange between the diastereotopic protons in the terminal acetate groups has been observed. The activation energy of this process is relatively high (Ea = 69.6 kj) [48], which can be accounted for by a mechanism via decoordination of a terminal N(CH2COO)2 moiety, followed by inversion and recoordination. 

By contrast to its spiro sulfurane analog. 31 in a CDCI, and nitrobenzene solution at room temperature displays a rapid polytopal rearrangement that results in the averaging of the diastereotopic protons of the methylene groups. The increase in the frequency of the polytopal rearrangements on going from sulfuranes to telluranes is in accord with theoretical predictions (77ZOB2011). 

In Table 2 are summarized the shift differences (both H and 13C) between the diastereotopic methyl groups of the compounds46) shown in Fig. 28. (Arguments are adduced in the paper 461 that the conformation shown is by far the preferred one, at least for R = COX.) It is immediately obvious that these differences in shift between diastereotopic protons are much larger for the phenyl than for the cyclohexyl compound presumably because of the much larger differential shielding of the... 

The obvious way to ascertain this is to look at the racemic mixture under the same experimental conditions and see whether the (now externally diastereotopic) protons of the two enantiomers are distinct. The more convenient way is to make this observation for the internally diastereotopic protons in the unlabeled RR CH2 in the presence of the chiral shift reagent52). 

---