Pyridines chemical shifts

Ashe has recently synthesized both arsabenzene (arsenin) (83 )283 and stibabenzene (antimonin) (84).66 The former is very air-sensitive while the latter polymerizes even at —80°. The NMR spectra of these, like that of phosphabenzene,283 show that the a-proton is highly deshielded and Ashe reports the following order of chemical shifts pyridine 5 8.1, phosphabenzene 8.6, arsabenzene S 9.3, and stibabenzene S 10.7. Significantly, the more remote /3- and 7-protons in each compound in the series are each at approximately the same chemical shift, which may imply an appreciable ring current for each compound. J23 = 11 Hz for both 83 and 84, which is greater than the 10 Hz found for phosphabenzene and 5.5 Hz for pyridine. 

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

Table 7.64 Nitrogen-15 Chemical Shifts in Monosubstituted Pyridine 7.115...

Chemical shifts of pyridine and the diazines have been measured as a function of pH in aqueous solution and generally protonation at nitrogen results in deshielding of the carbon resonances by up to 10 p.p.m. (73T1145). The pH dependence follows classic titration curves whose inflexions yield pK values in good agreement with those obtained by other methods. 

For the NH azoles (Table 3), the two tautomeric forms are usually rapidly equilibrating on the NMR timescale (except for triazole in HMPT). The iV-methyl azoles (Table 4) are fixed chemical shifts are shifted downfield by adjacent nitrogen atoms, but more by a pyridine-like nitrogen than by a pyrrole-like iV-methyl group. 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

For another dramatic illu.slration of chemical shifts, have. students calculate the magnetic shielding of nitrogen in pyridine and compare it to its saturated cyclohexane analogue. 

The chemistry of [Rh(OEP)h in benzene is dominated by Rh—Rh bond homolysis to give the reactive Rh(Il) radical Rh(OEP)-. This contrasts with the reactivity of fRh(OEP)] in pyridine, which promotes disproportionation via the formation of the thermodynamically favorable Rh(IlI). ct complex [RhjOEPKpy) ] together with the Rh(l) anion, Rh(OEP)J The hydride complex Rh(OEP)H shows NMR chemical shift changes in pyridine consistent with coordination of pyridine, forming Rh(OEP)H(py). Overall, solutions of Rh(OEP)l in pyridine behave as an equimolar mixture of [Rh(OEP)(py ) and (Rh(OEP). For example, reaction... 

Imamura (11,20,21) synthesized several similar perpendicular dimers exploiting axial coordination of the 4-pyridyl free-base porphyrin to Ru(II)CO (3) and Os(II)CO (4) porphyrins (Fig. 1). The pyridine-ruthenium and pyridine-osmium interactions are much stronger than the pyridine-zinc interaction, and the complexes are perfectly stable in solution and can be isolated by precipitation. One of the ruthenium dimers was characterized by FAB-MS (11). Complexation is accompanied by characteristic changes in JH NMR chemical shift, indicating... 

A phenomenological study was performed to determine the effect of solvent on Sn NMR spectra of these organoraetallic polymers. Samples were dissolved in chloroform, benzene, n-hexane, acetone, tetrahydrofuran, methanol, and pyridine. The Sn NMR spectra in these solvents are given in Figure 1. The appearance and location of the H Sn resonance changes drastically over the range of selected solvents. The chemical shift moves upfield in the order chloroform, benzene, n-hexane, acetone, tetrahydrofuran, pyridine, and methanol. The amount of structural information and, conversely, the broadening of the resonance increases in the same order with methanol and pyridine reversed. 

In the case of pyridine, large differences in chemical shift are observed for fluorines at the 2-, 3-, and 4-positions, with fluorines at the 2-position of pyridines and quinolines being the most deshielded, and those at the 3-position being the most shielded. (Scheme 3.60). 

The fluorines of CF2H groups, attached at the 2- or 3-position of a pyridine ring, appear at approximately -116 ppm, whereas a CF2H substituent at the 4-position appears at -113 ppm. A secondary CF2 substituent exhibits a similar trend in chemical shift (Scheme 4.55). 

Typical proton and carbon chemical shift data for heterocycles bearing a CF2H group and, in the case of pyridine, a CF2R group are provided in Scheme 4.59. 

For commonly encountered heterocycles, the chemical shifts of trifluo-romethyl substituents will depend somewhat upon where in the heterocycle they are located. Examples of trifluoromethyl derivatives for a number of common heterocycles, including pyridines, quinolines, pyrroles, indoles, thiophenes, benzothiophenes, furans, benzofurans, imidazoles, and uracils are given below. 

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 ... 

One other, perhaps even more dramatic and common example concerns compounds like 2 and 4 hydroxy- and amino-pyridines. These compounds exhibit tautomeric behaviour and tend to exist in solution as the corresponding pyridone and imine. This reduces the familiar pyridine-like properties of the ring system, accentuating the effects of these substituents (in terms of induced chemical shifts) and at the same time, radically increasing the expected couplings 2 -3 couplings. 

Nitrogen-containing heterocycles are sometimes basic enough to protonate and form salts in acidic conditions and this leads to substantial changes in chemical shifts of their protons - see Spectrum 5.9 (pyridine alone, pyridine + DC1)... 

We have also noted some strange behaviour with fluorinated pyridines, for example, 3-fluoro nicotinic acid (Structure 6.19 and Spectrum 6.12). The signal for H= (approx. 8.1 ppm) clearly shows couplings of 9.1, 2.9 and 1.7 Hz. The 9.1 Hz coupling must be from the fluorine as it does not appear anywhere else in the spectrum and its chemical shift distinguishes it from either of the other two protons. 

Running a sample in an anisotropic solvent like D6-benzene or D5-pyridine, can bring about some even more dramatic changes in chemical shifts. We tend to use benzene in a fairly arbitrary fashion, but in some cases, there is a certain empirical basis for the upfield and downfield shifts we observe. 

Ql. Run the proton spectrum in a suitable solvent. It s always the best way to begin Stop and think about it. The answer may be right there in front of you. Maybe there is no need for any further experiments. As the state of protonation of the pyridine nitrogen is unknown so chemical shift information may be unreliable but the spin coupling should be relatively unaffected by this. 

Table 2 Calculated and experimental 13C NMR chemical shifts of 1 -boraadamantane framework in isomeric 4-methyl-1 -boradamantane pyridine adducts...

The 13C chemical shifts of 4-aryl-bispyrazolo[3,4-A4, 3 - ]pyridines (CDC13, 30°C, 67.94MHz) show large differences in aryl ring carbon shifts for different aryl substituents, which are predictive for the differing photophysical and chemical properties of these interesting molecules <1996MRC570>. 

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