Diastereotopic fluorines

NMR signals for difluoromethylene moieties range from -85 to -111 ppm (excluding difluorocyclopropyls), with viunal 7jjp values similar to the primary cases More compbcated sphttmg patterns arise from diastereotopic fluorines when a chiral center IS present in the gemmal difluonde Diastereotopic fluonnes may differ in chemical... 

Although Fig. 4.4 showed the AB system of 1,1-difluorocyclohexane emerging, Fig. 4.5 provides a classic example a CF2 AB system, that which derives from the two diastereotopic fluorines in -butyl 2,2-difluorocyclopropanecarboxylate. In this case, 2JAB = 164 Hz. 

The 13C spectrum of 1,1-difluorobutene (Fig. 4.12) exhibited four signals, a doublet of doublets at 156.2 ppm for the CF2 group bearing diastereotopic fluorines, with almost identical one-bond F—C coupling... 

The fluorine substituent at the 2-position of a trifluorovinyl group is much more highly shielded than the other two fluorines, and its presence gives rise to an enhanced split of the diastereotopic fluorines at the 1-position and enhanced coupling constants, both geminal and vicinal. 

At room temperature the two chair forms average and the average symmetry is that of a planar molecule (C2v) 83) in which the fluorine atoms are related by the C2 axis and hence equivalent. Thus the room temperature spectrum of 1,1-difluoro-cyclohexane displays a single (except for proton splitting) chemical shift for the two fluorine atoms, as shown in Fig. 46 84,85). In contrast, at —110 °C the spectrum shows the expected AB pattern for the diastereotopic fluorine atoms expected from the individual structures shown in Fig. 46. As the temperature is gradually raised, the two doublets broaden and merge into two peaks which on further... 

Two-bond F-F coupling constants between diastereotopic fluorines in a CF2 group can be quite variable. They can be as small as 14 Hz for some vinylic C=CF2 groups (see Section 4.7.1), of moderate magnitude ( 150 Hz) for cyclopropyl CF2 groups, or as large as 240-285 Hz for diastereotopic, acyclic CF2 groups. 

Further evidence on the possible dichotomy of mechanism is provided by study122 of the compounds 62 and 63, in which an additional chiral centre is present at the carbon atom adjacent to the dimethylamino group. At low temperature the 19FNMR spectrum of 62 shows the three absorptions expected for one axial and two diastereotopic fluorine atoms, and the NMR spectrum the absorptions of two diastereotopic N-methyl groups. 

To show that the idea of diastereotopic groups is quite general, let us examine one final example in which there are diastereotopic fluorine atoms. The H proton spectrum of l-bromo-2-chloro-1,1,2-trifiuoroethane shows two doublets of doublets, one pair of doublets at about 5.7 ppm and the other at about 6.5 ppm. To save space, the spectrum is not included, but Figure 5.65 shows the splitting pattern and the Newman projection of this molecule. Remember that this is a proton spectrum, but the single proton is split by the three fluorines (spin = ). 

FIGURE 5.65 An analysis of the sphtting caused by diastereotopic fluorine nuclei in the NMR spectrum of l-bromo-2-chloro-l,l,2-trifluoroethane. 

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