Carbon chemical shifts electron releasing

Carbon-13 chemical shifts of representative aldehydes [284] and ketones [285-288] are collected in Tables 4.27 and 4.28. Inspection of the data shows that a, / , and y effects are up to 7, 2, and — 1 ppm, respectively. These increments are significantly smaller compared with those reported for alkyl carbons. Obviously, the electron releasing effect of alkyl groups (( +(-/-effect) slightly attenuates positive polarization of the carbonyl carbons. 

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 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). 

A large number of 13C NMR studies on proline derivatives and proline peptides have appeared in the literature [815-830]. As the electron charge density of cis-proline carbons is different from that of franx-prolinc carbons, these isomers can be differentiated by nCNMR spectroscopy [826, 830]. On the basis of calculations Tonelli [831] predicted four conformations for the dipeptide Boc-Pro-Pro-OBzl, three of which could be detected by 13C NMR spectroscopy [826, 830], In proline-containing peptides the stereochemistry of the proline residue plays an important role for the conformation of these oligomers. The 13C chemical shift data of cis and trans proline derivatives, collected in Table 5.29, are useful to determine the stereochemistry of the amino acid-proline bond, e.g. in cyclo-(Pro-Gly)3, melanocyte-stimulating hormone release-inhibiting factor or thyrotropin-releasing hormone. 

Electron attraction or electron release by substituent atoms attached to the carbon atom. Electron attracting atoms, such as N, O, Hal attached to the carbon, attract electron density from the C-H bonds and thus deshield the proton. This results in movement of the chemical shift to higher 5 values (Table 29.1). Conversely, electron-releasing groups produce additional shielding of the C-H bonds resulting in upfield shifts of 5 values. 

Carbon-13 shifts, measured by the double-resonance technique, of the carbonyl group of weto-substituted benzaldehydes correlate with Hammetts parameter. In the case of the para-substituted benzaldehydes, there was no correlation between the chemical shift and a, presumably because, in contrast to substituted benzenes, resonance contributions for both electron-attracting and releasing groups will not significantly affect the electron density at the carbonyl carbon. 

It has been found that the chemical shift of aromatic ring carbons are a good indicator of the intrinsic electron-releasing or electron-withdrawing capacity of substituents, without any perturbation from approaching reagents. Such perturbation is always present when substituent effects are measured on the basis of... 

The 33S substituent chemical shift can therefore be rationalized in the same way as for the carbonyl carbon in carboxylate groups. When R is unsaturated (CH=CH2 C6H5, etc.) the 33S resonance is generally shielded relative to R = CH3. This was rationalized104 in terms of electron release from the conjugated 7c-system to the sulphur atom. There is still much work to be done in 33S NMR. 

At the methylidene group carbon atom of 3 a relatively high electron density NMRCCDCls) 5 = 8S.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1,4-Addition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THE) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The Si chemical shifts of the solids obtained (CP/MAS-NMR) are presented in Table 2. 

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