Protons deshielding effects

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

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. 

Molecular models show that in both compounds H-15a must experience a diamagnetic ring current because it is situated under the pyrrole ring. However, in 190 the diamagnetic effet is contrasted by a van der Waals deshielding effect between electronegative 16a-OH and H-15a, and, as a consequence, this last proton occupies an expected position. 

The deshielding effect also depends on the distance from the absorbing protons. As this distance increases the deshielding effect decreases. Thus in the compound the T value of absorption decrease is in the order c > b > a. 

This has been explained above by taking acetylene and benzene as examples. The deshielding effect of protons attached to C = C is higher than can be accounted for by the inductive effect alone. 

The UV spectrum [7max 238, 256 (sh), 293 (sh), 314, 362 (sh), and 390 (sh) nm] of rebeccamycin (337) indicated the presence of an indolo[2,3-fl]pyrrolo[3,4-c]carbazole-5,7(6H)-dione framework. This was also discernible from its IR spectrum. The H-NMR spectrum indicated the presence of an amide NH at 8 11.37 and an indole NH at 6 10.30, in addition to signals for aromatic protons and a sugar moiety. Unlike (+)-staurosporine (295) (see Scheme 2.74), where the lactam function deshielded only the ortho C-4 proton, the C-4 and C-8 aromatic protons in rebeccamycin are both deshielded due to the anisotropic deshielding effect of the phthalimide function. 

The proton NMR spectrum of 2,2 -bipyridine has been obtained and analyzed in a variety of solvents by several authors. ° The full interpretation of the spectra is in accord with the transoid conformation I in solution. The behavior of the chemical shifts of protons at positions 3 and 3 indicates the existence of a strong deshielding effect exerted by the nitrogen atoms of the adjacent rings. Interestingly, the proton NMR spectra of 2,2 -bipyridine taken in various solvents indicate self-association and stacking of the molecules in some cases. The spectra of some substituted 2,2 -bipyridines, - - 2,3 -bipyridine, ° 2,4 -bipyridine, ° 3,3 -bipyri-dine, and 4,4 -bipyridine - - ° have been investigated in detail. It was... 

Studies of the stereochemical dependence of 1H chemical shifts in cyclobutanes, benzocyclobute-nes and /J-lactams have been collected463. In a number of cases the protons or proton groups are shielded if they are cis to vicinal halogen, hydroxy or phenyl substituents, due to a ring-current effect. Carbonyl groups, however, can give rise to deshielding effects. 

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