Heterocyclic rings carbon-13 chemical shifts

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-... 

Fig. 3. Plot of chemical shifts (in p.p.m. from benzene), of the ring carbon atoms of heterocyclics, against the chemical shift of the attached proton (in t values). (From Page et...

Carbon-13 NMR is often a more useful tool than H NMR for the elucidation of heterocyclic structures in which there are few or no ring protons. For symmetrically substituted 1,2,3-thiadiazoles, the carbon adjacent to the nitrogen atom is expected to have a lower field chemical shift than the carbon atom adjacent to the sulfur atom, as exemplified in CHEC-II(1996) <1996CHEC-II(4)289>. Several examples that follow this rule are illustrated in Table 5. There is now a more extensive body of data available and it is possible to more accurately predict the chemical shift of ring carbons. In the case of monosubstituted 1,2,3-thiadiazoles, the substituted carbon usually has a lower field chemical shift than the unsubstituted carbon. 

C-NMR chemical shifts of the heterocyclic rings of selected bicyclic compounds and of homoberbine-type isoquinobenzazepines (including the alkaloid saulatine, 11) are listed in Tables 9 and 10, respectively. Shifts of a-olefinic and (at higher field) )S-olefinic carbon atoms of enamines 77a, 288, and 292-294 and of the iminium group carbon atom in bicyclic salt 60 are found in the respective regions reported in the literature to be typical of such compounds (82T1975, p. 1986). 

In analogy to cyclopropanes, small II and 13C chemical shifts, compared to acyclic compounds or larger rings, are also encountered in oxiranes. The <5 values of a-protons and a-carbons in oxirane and some related heterocycles are ... 

The pyrones and thiinones show general 13C NMR spectral characteristics similar to the pyridones which reflect charge distributions in the heterocyclic rings. Thus, carbon atoms a or y to the heteroatom are deshielded relative to benzene, while those (3 are shielded. Substituent effects are in general as expected, although fewer detailed studies have been carried out in this area with the oxygen and sulfur heterocycles than with the azines. Chemical shift data for representative compounds are given in Scheme 10. 

The, 3C chemical shifts of 1-arylisobenzopyrylium salts (88) are as expected when a positively charged oxygen atom replaces the carbon atom at the 2-position of the corresponding naphthalene derivative (760MR(8)324). Substitution in the 1-aryl ring has no appreciable effect on the chemical shifts of the carbon atoms of the heterocyclic system. Mesomeric interaction would be prevented if the aryl ring is perpendicular to the plane of the heterocyclic system as molecular models suggest. 

Table 4.66. l3C Chemical Shifts of Heterocycles with sp2 Ring Carbons (<5C in ppm). Part 1 Oxygen and Sulfur as Heteroatoms [416-424]. 

Examination of 13C NMR spectra of type-1 heterocycles (76JA3778, 76JA3783) makes it possible to trace the effects of the nature of a group-16 (VIA) heteroatom on the chemical shifts of the ring carbon centers. Chemical-shift data for various 1-heteracyclohexanes, where the heteroatom is a group-6 element, are given in Table III. 

A 400 MHz n.m.r. study640 of the eight parent heteroyohimbine alkaloids has permitted the chemical shifts of all the protons to be established. Carbon-13 n.m.r. data have also been recorded646 for the alio- and epiallo-oxindole alkaloids containing a heterocyclic ring E. 

A further important general trend in the azines arises on N-oxidation, which results in shielding of the a- and y-carbon atoms, especially the former, and clearly indicates the higher electron density at these ring positions (Figure 16). The corresponding conjugate acids of the A-oxides have chemical shifts very similar to those of the protonated parent heterocycles. 

The 15CH coupling constants (often obtained from 13C satellites in H NMR spectra) of small heterocycles have been listed <1979MI50101>. A 13C NMR study of 1,2-dimcthyl-3-/W/-butyldiaziridinc showed that the population of the 1,2-cis isomer increases with solvent polarity (see Section 2.5.3.8) <1998MC113>. 13C Chemical shifts for the ring carbons of representative derivatives of dithiiranes , azetidines , diazetidines , thiazetidines , dioxetanes , and 1,2-oxathietane S,A-dioxides (-sultones) have been tabulated. 

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