Vinylic proton, chemical shift

Dihydrobenzo[6]furan is of interest as a locked-conformation 2-vinylfuran, the conformation being opposite to that expected for a free vinyl group. Chemical shifts for the vinyl protons of 2-vinylfuran are a-H, 6.48 /3-H, 5.65,5.10 (70T1291). For the corresponding protons in the hydrobenzofuran they are a-H, 6.34 /3-H, 5.72 (77TL151). The difference is very small after allowance is made for the alkylation increments. 

Complete assignment of proton chemical shifts in reaction products of model vinyl sulfonyl reactive dyes with methyl-a-D-glucoside37 39 was published. In ref. 39, two basic types, 3 and 4, were separated and H NMR spectra were measured at 600 MHz and analysed. 

Although proton chemical shifts are influenced significantly by factors other than electron density, they also reflect the polarization of the enamine framework and the degree of n,n interaction. Thus, the chemical shifts of the vinylic protons are modulated by the same factors discussed for the chemical shifts of the corresponding olefinic carbons, such as amine component, steric and electronic effects of the substituents and ring size effects. In particular, the chemical shift of the proton(s) at C(2) is lowered by increasing njt interaction, in parallel with what has been observed for < C(2). No general correlation exists between the chemical shifts of both nuclei probably as a consequence of their different sensitivity to steric, electronic and, particularly, anisotropic effects of the substituents. Nevertheless, for sets of structurally related compounds, reasonable linear correlations can be found between <5C(2) and <5H(2) (see below). Since the XH-NMR data available for enamines are more abundant than those for 13C and 15N, more complete structural information can be obtained for wider sets of compounds. 

The proton chemical shifts of representative acyclic and cyclic dienamines are collected in Tables 14-16. Examination of the Tables show that alkenyl substitution at C(l) or C(2) of an enamine produces also deshielding of the olefinic protons, as seen by comparing the <5H(2) values of compound 149 (3 3.75, 3.85 ppm Table 11) with those of the analogous cross-conjugated dienamine 105 ( 3.85, 4.05 ppm Table 14). The chemical shift of the vinylic protons of dienamines correlate only partially with the charge calculated for the corresponding olefinic carbons . In the linear conjugated... 

Chemical shift nonequivalence can occur when two environments are stereochem ically different The two vinyl protons of 2 bromopropene have different chemical shifts... 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

The chemical shifts for the vinylic protons of some enamines are given n Table 4. 

Much of the difference in chemical shift between aromatic protons (6.5-8.0 8) and vinylic protons (4.5-6.5 5) is clue to a property of aromatic... 

Three sets of nmr chemical shifts for the ethynyl proton have been correlated with eq. (2). Of these sets, two gave significant correlations with eq. (2). Nevertheless, as the most extensive collection of substituents is included in the set which did not give significant correlation, it seems likely that chemical shifts of ethynyl protons are not correlated by the extended Hammett equation. This behavior contrasts with that of chemical shifts for trans- and c/s-vinyl protons and is in agreement with the behavior of geminal vinyl protons. 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

Table 3. Chemical shift of C (10) vinyl proton in different B -derivatives (131-133)...

Table 10. Vinylic C-H proton and a-C-H chemical shift in monosubstituted cyclopropenones and triafulvenes...

Very powerful tools for the study of dienes and, to some extent, polyenes (in particular annular polyenes) are both H and 13 C NMR spectroscopies, which will be discussed in a separate section. As previously mentioned 1,3-butadiene is more stable in the s-trans conformation and in the H NMR spectrum both butadiene (1) and 2,3,6,7-tetramethyl-2,4,6-octatriene (3) display the vinyl proton at a low chemical shift value. In these simple examples the S value can be predicted theoretically. The 111 NMR spectrum of a C25-branched isoprenoid was examined as part of the structural determination for biomarkers and is shown in Figure l6. The other spectral and structure assignments are described later in this review. 

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