Deshielding effect

The spectrum of 5-ethylthiothiazole compared to that of thiazole shows a slight deshielding effect on H-Cq (9 versus 9.1 ppm) and a slight shielding effect on H-C4 (7.93 versus 7.88 ppm) (270). 

The most characteristic coupling constant in indazoles is the cross-ring Vs, present both in indazoles and in isoindazoles unsubstituted in positions 3 and 7. 2-Methyl isomers show an additional Vmc.h coupling which can serve to identify an isoindazole unsubstituted in position 3. In 3-azidoindazole, as in 3-azidopyrazole (56), the prototropic exchange is slowed down sufficiently to allow the measurement of a zig-zag /i,4 coupling constant. The deshielding effects observed in A-acetyl derivatives, e.g. 1-acetyl (60) on H-7 and 2-acetyl (61) on H-3, are related to a preferred E conformation (Section 4.04.1.4.3). 

The deshielding effects of electronegative substituents are cumulative, as the chemical shifts for various chlorinated derivatives of methane indicate. 

Methyl ketones, such as 2-butanone in Figure 17.18, are characterized by sharp singlets near- 8 2 for the protons of CH3C=0. Similarly, the deshielding effect of the car bonyl causes the protons of CH2C=0 to appear- at lower field (8 2.4) than in a CH2 group of an alkane. 

The chemical shifts of the H-methyl groups in thiiranes 31a, 31b and 31c were found to be = 1.59,1.44 and 1.45, respectively. The chemical shifts of the -anti-methyl hydrogens (i.e. those of R ) where found to be (5 = 1.25,1.23 and 1.27 in 32a-c compared with <5= 1.74 and 1.64 for syn-R -hydrogens in 32a and 31c, respectively . The consistency of the deshielding effect in accordance with the position of the -hydrogens in ring sulfoxides is thus apparent. These observations validate the applicability of the S—O anisotropy rule to the three-membered ring system. 

In going from a secondary to a tertiary cycloalkyl fluoride, one observes the usual deshielding effect as is exemplified by the isomeric 1 -lluoro-1 -mcthyl-4-t-butylcyclohcxancs. Of course, these two isomers exist essentially in the single conformation given, because of the presence of the 4-t-butyl substituent. 

Substitution of a halogen on the same carbon as that bearing the fluorine substituent gives rise to dramatic incremental deshielding effects (Scheme 3.10). 

The deshielding effects of chlorine and bromine appear to be similar, with the chlorine having a greater deshielding influence in the methane examples above but a smaller influence in the cyclopropane example in Scheme 3.11. 

As was the case for the monofluoro series, halogens attached directly to the CF2 carbon deshield the fluorine nuclei (Tables 4.1 and 4.2). Iodine has the greatest deshielding effect I > Br > Cl > F. 

Unlike the effect of a carbonyl function, a nitrile group attached to a CF3 group will deshield the CF3 fluorines, with a diminished deshielding effect being exerted as the CN is placed farther away (Scheme 5.28). 

Tobey58 has reported further 19F-NMR studies comparing bis(p-fluorophenyl)-cyclopropenone (212) to its dichloride 214 and the cations 215-217. Although the cyclopropene 218 should be a less ambigous reference than the dichloride 214, it can be concluded that the deshielding effect of the cyclopropenone system is related closer to the covalent cyclopropene derivative than to the cationic species. This is in qualitative accordance with the findings in Chapter 5 (a). 

A strong deshielding effect of the /V-oxide group becomes apparent in the // -isomer of 2-(phenylimino)acenaphthenone /V-oxide (217) (Fig. 2.15) (397). 

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