Carbon chemical shifts electron withdrawing

The data in Table 21 indicate that C(2) is shielded, and C(3) is deshielded, in the Z-isomer relative to the Eav-isomer. It has been found113 that in compounds of general formula XCH=CHY, where X and Y are, respectively, an electron-withdrawing and an electron-donating group, the olefinic carbon chemical shifts of isomeric E,Z pairs are simply related by an equation bz = bE + A. For enamino ketones 343 (and other structurally related compounds113), the relationship for C(2) is... 

The induced field of a carbonyl group (C=0) deshields protons in much the sane way that a carbon-carbon double bond does, and the presence of oxygen makes it even more electron withdrawing. Thus, protons attached to C=0 in aldehydes are the least shielded of any protons bonded to carbon. They have chemical shifts in the range 8 9-10. 

NMR chemical shift data from die protons ortho or para to the electron-withdrawing group can be used to determine the reactivity of the monomer indirecdy.58 Carbon-13 and 19F NMR can be used to probe the chemical shift at the actual site of nucleophilic reaction. In general, lower chemical shifts correlate widi lower monomer reactivity. Carter reported that a compound might be appropriate for nucleophilic displacement if the 13 C chemical shift of an activated Buoride ranges from 164.5 to 166.2 ppm in CDC1359. 

Among the earliest research on the transmission of substituent effects was the pioneering work of Lauterbur (58) and of Spiesecke and Schneider (59). For a wide series of substituted methanes and ethanes they found an approximate correlation of >3C chemical shifts with the electronegativity (E) of the substituents (X). So they concluded that inductive effects through o-bonds play an important role in determining the chemical shifts of a- and -positioned carbon atoms, and that this effect reflects the substituent s electron-withdrawing or -releasing ability. 

The effects of the 1,2,3-triazole and 1-amino-1,2,3-triazole heterocycles on the chemical shift of the carbon atoms in the phenyl ring of compounds (40) and (41) (X = H) are compared with the value for benzene. With regard to the effects of substituent X para and meta) on the 1,2,3-triazole carbons, C(4) is deshielded by electron-withdrawing groups, whereas C(5) is shielded. Reasonable correlations with Hammett a values are observed for the chemical shifts of both C(4) and C(5) of (41) <86MRC53>. The chemical shifts of C(4) and C(5) in 5-amino-l-aryl-l,2,3-triazoles are shifted upheld by 2-5 ppm compared with the corresponding 5-anilino-1,2,3-triazoles. In 5-anilino-... 

The NMR chemical shifts of the C=N carbon in the open tautomeric forms of 2-aryl-substituted 3,4-dihydro-277-1,3-benzoxazines 5 and l,4-dihydro-277-3,l-benzoxazines 6 exhibited a reverse dependence on the benzylidenic substituents X. Electron-withdrawing substituents caused shielding (the shift was reduced), while electron-donating ones caused deshielding <2003JOC2151>. 

The 13C chemical shifts of benzenoid carbons largely depend on the mesomeric interaction between substituent und benzene ring. Electron releasing substituents (e.g. — NH2, — OH) will increase the electron density at the o and p carbons relative to benzene (128.5 ppm), while slight electron deficiencies will be induced by electron withdrawing groups (e.g. — N02, —CN). 

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