Chemical Shifts in Substituted Pyridines

Table 7.54 Carbon-13 Chemical Shifts in Substituted Pyridines 7.105...

Fig. 4. Schematic drawings of the electron orbitals making the largest contributions to the principal components of the chemical shifts in pyridine and N-substituted (R = H, alkyl) pyridine.

The C chemical shifts in multiply substituted pyridines can be estimated using the C-chemical shifts listed on p. C140 and C145 for monosubstituted pyridines. 

The chemical shifts in multiply substituted pyridines can be estimated using the... 

Most of the multifluoro-substituted pyridines were prepared more than 30 years ago in the Birmingham fluorine group using CoF3 technology. Scheme 6.34 provides the multitude of fluorine and proton chemical shift data that were accumulated at that time. It will be seen that, all other things being equal, fluorines at the 2-position are most deshielded and fluorines at the 3-position are the most shielded. Scheme 6.35 provides a few examples of substituted tetrafluoropyridines. 

They are applicable to compounds in the common NMR solvents - but not in D6-benzene (or D5-pyridine). The substituent effects are additive, but don t place too much reliance on chemical shifts predicted using the table, in compounds where more than two groups are substituted next to each other, as steric interactions between them can cause large deviations from expected values. Note that Table 5.4, like all others, does not cater for solvent shifts, etc ... 

The dramatic effect of nitrogen substitution on carbon-13 chemical shifts was illustrated at the beginning of this section by the fact that the a/p/ia-carbon resonance of pyridine moves upheld upon protonation at nitrogen. A further demonstration that nitrogen substitution does still more extensive violence to charge density-chemical shift correlations is seen in Figures 19 and 20. Thus, in... 

The question of additivity of SCS is difficult to probe, because ortho interactions, notoriously difficult to account for, are bound to arise in even disubstituted pyridine rings, apart from those with the 3,5-disposition. To test this, proton chemical shifts have been measured for six series of substituted methyl pyridinecarboxylates (71), (72), (73), (74), (75) and (76) <750MR(7)4l), and SCS ortho and para can be accounted for by assuming additive substituent, ester, and nitrogen effects. However, for protons meta to the substituent, particularly for the case where both the proton and the substituent are adjacent to N, the additivity breaks down, and there is evidence of substituent-nitrogen interactions. 

Chimichi et al. have reported the synthesis and spectroscopic data for all four thiazolopyridine isomers in order to examine how the differently substituted fused five-membered heterocycle influences the chemical and spectroscopic properties <1997MRG601>. The unambiguous assignment of the chemical shifts of these systems was reported and coupling constants showed that azole ring fusion on the pyridine system is responsible for a significant... 

Even though at present various obstacles prevent a more extensive application of NMR spectroscopy to the study of 77-electron distribution, it seems that it will be possible to make use of the interrelation between the magnitude of the chemical shift of a proton attached at a certain position and the 77-electron density at that position. Among the studies so far published attention should be drawn to the papers dealing with pyridine,111-112 substituted pyridines,113 and pyrimidine and its derivatives.114 Table VII presents data (taken from ref. 112) on experimental and theoretical 77-electron densities in pyridine and the pyridinium cation. The NMR spectra of some azines are briefly discussed by White.114"... 

---